Saturday, 24 November 2018

Serendipity

I think it it really wonderful when several disparate threads come together and provide inspiration as happened to me recently. The first thread was a request for four channels of tube mic pre-amplification. We initially discussed four tube mic pres in a rack mountable enclosure but it soon became apparent that what was required was a true mixer, with EQ, pan pots, rotary faders with big knobs and a monitor section. I sent the prospective customer pictures of various tube mixers from which he selected this one as being closest to what he had in mind.


It uses the old EZTubeMixer modules and rotary faders with big round knobs. And this was the start of the second thread. I had built this mixer several years previously and used a ready made steel enclosure made by a company called Rackz. So I searched the web for Rackz hoping to buy another enclosure. Unfortunately I discovered Rackz had gone bust a few days previously and their web site was no more which was a bit of a blow because they made some really nice fully screened enclosures. An extensive web search revealed absolutely nobody making anything similar so it was back to square one.

This got me thinking about another recent enquiry, this time for a rack mounted mixer, but in this instance, the prospective customer already had a custom built 19 inch rack mounting enclosure made of wood. All he needed was a rack mounting tube mixer to go in it. So I thought, why not get someone to build a wooden version of the Rackz enclosure? Fortunately there are quite a few people who make custom wood enclosures for 19 inch racks. I chose to try out Sound Desks if for no other reason than they are are in the UK and they already make something not too far away from what I need (sounddesks.co.uk).

So I drew up a very rough sketch of what I needed and sent it to them:

There is a 10U sloping spacing split into a 4U space that will contain the faders at the bottom and a 6U space above that for the channel amplifiers. At a steeper angle above that is a 3U space for the meters. At the back there is a 6U space which should be plenty for all the connectors. All in all very similar to the Rackz box. Sound Desks responded quickly with a quote which I passed on to the prospective customer who was quite happy with it.

My only concern was that the wood provides absolutely zero screening so you have to make sure the the sub-rack is fully screened, Fortunately this is not really a problem because most sub-rack manufacturers supply them for use in harsh EMC environments (like electric trains) and provide kits that allow the entire sub rack to be screened.

And this is where the third thread came in. Back in the mid 70s when I was at Neve, I once had a meeting with the man himself, Rupert Neve. When I joined Neve, Rupert had already left. The company had run into financial difficulties and to save it Rupert had sold it. The new owners kept Rupert on as a consultant but the result was he was rarely at Neve itself. When I met Rupert he had come to see my boss Tony Cornwell and after they had talked for a few minutes Tony invited me into the meeting. Rupert had brought in his his latest design for a compact radio console (known as the CRC). It was a radical departure from normal Neve build practice. The complex metal channel module enclosures were gone; modules were little more than a plug in PCB with an attached front panel. Screening between modules was to be done by steel sheets that slid in on card guides between modules. The aim was to make a Neve that was affordable by local radio stations.

I have no idea what became of the CRC as I left Neve shortly after but I never forgot the idea of a compact console which is when the three threads coalesced and I realised I had the basis of a low cost compact tube recording console. It would use:

  • Standard sub-racks with full screening which, being made in quantity are reasonably priced.
  • A wooden enclosure similar to the Rackz proving a sloping space containing slider faders and channel modules and a meter bridge that could also house 3U modules like the Twin Line Amp (for use as bus amps etc) as well as meters
  • Be populated by modified versions of the MarkIII 35mm wide modules which meant here would be space for 12 modules instead of just the six of the EZTubeMixer
For the channel module I started with the REDDPLUS channel amp. I needed to find space for a couple of Aux send controls with pre/post fader selection, a pan control and a mute, solo and pfl switches all of which are normally in the routing module in the standard Mark III layout.. The only space I could make available involved deleting the HPF (we can use the EQ instead) and the insert switch which is a bit of a luxury anyway ( there is still a per channel insert but it is unbalanced, again to simplify the mixer and keep costs down). But this space was not enough so I decided to have one Aux permanently post fade (for FX) and just the second one switchable pre/post fader (for foldback or FX). But there was still no room for this switch so the pre/post switch becomes global and Aux 2 of all channels is relay switched by a switch on the Aux master module. Even then there was not enough space so I ditched solo but kept PFL (so you can gain stage and check quality) and combined it with Mute on a single switch because you never need both at the same time. Putting the two Aux send pots side by side completed the transformation and it does fit in the space availabel (see below).

For the groups you have to have a compressor available but it would be nice to have it available when tracking. So there is a compressor for each of the Left and Right buses. They can be linked for stereo, they can be switched in and out. But they can also be disconnected from the bus and routed to TRS sockets at the rear so they can be patched into any channel using the Patch switch.

There are 8 channels and two buses which leaves two channels for Aux and monitoring. The Aux master module has master level pots for Aux 1 and Aux2 as well as the Aux 2 global pre/post switch. There is plenty of room left on this panel so I added a couple of Aux returns with individual level and Pan controls.

The final module is the monitor one. No serious mixer is complete without talkback so I included that in this module. There is a simple talkback level control and a switch to route it to the master L/R buses (slate) or to Aux 2 (foldback). When talkback is operated, the monitors are dimmed to prevent feedback and the Dim indicator lights.

Below the talkback section is the monitor section. This has a simple three position selector switch. In the centre it connects the monitor to the master L/R buses. To the right it monitors a 2 track playback input and to the left it monitors the two Aux sends outputs. The level send to the monitor amps/speakers is set by the monitor level control. PFL overrides any monitor setting and feeds the PFL bus output to both L and R monitor outputs (pre the level control). The meters are connected direct to the monitor outs pre the monitor level control so they show actual levels.

The basic layout of the mixer is shown below:


Below the channel amps  are the full throw faders and above them is the meter bridge. As shown, two meters are fitted which leaves space on either side for a pair of Twin Line Amp modules which act mainy as bus amps.

As shown, the mixer is 8 into 2 but if the Aux master, talkback and monitor functions were moved to the meter bridge it would be possible to add two more channels making it a 10 into 2. Including the two Aux returns there would be 12 inputs at mixdown.

Monday, 1 October 2018

You've Been Framed

As anyone who has read this blog will know, I am not mechanically gifted; you might even say I am mechanically dyslexic. If there are five different was to put a 19 inch enclosure together I will try all the wrong ones before finally hitting on the right one even though I have read the instructions. Fortunately I have not passed this gene on to my children. My son is the flat pack king. He can assemble any Ikea product in less than 30 minutes without the instructions.

So when it came to the frame of the Mark III tube mixer I decided to give the detail design and fabrication job to the professionals. I get all my sub-rack components from SRS here in the UK, not simply because I like to support local industry, but because they are a management buyout keen to build their business and they realise a successful business is built upon happy customers. So I sketched out what I wanted; a 30 degree sloping panel with a 3U space at the top (mostly for metering and ancillaries), a 6U space (for channel amplifiers) and a 3U space (for routing). Below this would be a horizontal 4U space for the faders. At the back would be a 9U space split into three 3U spaces. The top two are for the connections to the mixer and the bottom 3U space is for additional line amplifiers (Twin Line Amps).

The construction of the frame is really simple. It consists of a pair of side plates connected together by a bunch of standard length extrusions. The extrusions are placed in such a way as to provide the spaces required for the modules and to provide mountings for the back plane PCBs that the modules plug into.

From the users point of view the basic layout looks like this:



Because the new modules are 7HP wide (~35mm) you can fit exactly 12 of them into a standard 19 inch sub-rack that uses standard sized extrusions. So a basic mixer section is 12 modules wide. You can certainly make an 8 into 2 mixer in this format and possibly even a 10 into 2. If you want something bigger then you bolt a couple of sections together. This will give you a 24 modules wide mixer in which something like a 16 into 4 would be viable.

The two side plates that are connected together by the extrusions look like this:

Not very exciting to look at but the guts of a mixer never are. Once this has been clad in a nice looking bit of timber it will look a lot more attractive.

The hard bit is designing the side plates. It would take me simply ages to work out exactly where all the holes need to go plus one plate needs to be a mirror image of the other so you can use countersunk screws to attach the extrusions (this is necessary so you can bolt sections together to make larger mixers). So this is the job I got SRS to do. I sent them these sketches and they responded with a fully dimensioned drawing and a very reasonable quotation. I requested a couple of minor modifications and then I ordered two pairs.

I forget how long it took SRS to make them but it was not long - maybe a couple of weeks. When they arrived they looked beautiful. Nice clean countersunk holes and very handy semi-shears on the inside to aid locating the extrusions and to prevent them rotating when you tighten up the screws. I already had all the necessary extrusions so I quickly built one section. I fitted the new back plane PCBs I had designed and started plugging in modules. All the 3U spaces worked perfectly. However, when I tried the first 6U high modules (Channel One of the previous post) it would not fit. The front panel seemed to be too tall. I tried slackening off the extrusions, pushing them apart and re-tightening but this made little difference. In the end I assumed my panel design was too tall and filed down the top and bottom of it until it fitted. To be certain, I found an old blank 6U panel make by Schroff - surely they must be the right size. To my surprise it did not fit either. It was beginning to look like it was not me after all but the SRS frame. A conversation with the supplier of the Channel One front panel and trying the Schroff 6U panel in a standard SRS sub-rack convinced me something was wrong with the frame.

So I contacted SRS and explained what I thought I had found. To their great credit they immediately suggested they send someone out to look at it. Considering I am a one man band working in a shed at the bottom of the garden I was impressed by their willingness to help. I was still unsure if I had done something silly in the assembly (knowing my reputation for building things wrongly) but when Martin from SRS visted me we went though it with a fine tooth comb and he confirmed there was definitely something wrong. He suggested he took the complete assembly back to base where it could be looked at in detail. A few days later I got a call to say they had  managed to tweak it so a 6U panel fitted properly and they sent it back to me.

When it arrived I was eager to get stuck into building the first Mark III mixer but to my disappointment the 6U panels still did not fit. Whatever adjustments they had made had been lost during transportation so this was not a viable solution. After another phone call SRS agreed to subject the drawing to a thorough check. They soon called me back to say the drawings were fine so I expected to be told there was nothing they could do. But instead they said they were going to conduct a detailed examination of the complete manufacturing process to find out what had caused the problem. This took some time; about two weeks, during which time they discovered a number of small tolerances all of which could add up to the problem I had. I do not have a complete list of what they looked at but I know it included checking the length of extrusions because small variations in this parameter could cause one of an adjacent pair of extrusions to become warped when the end plates screws are tightened. I know they also checked the concentricity of the countersunk screws they were using and also the tolerances on their manufacturing machines.. In the end they made some slight changes to the design and added an extra semi shear to ensure all the extrusions are properly placed. They even checked their entire inventory of extrusions and discarded any that were not the correct length.

SRS then made me two new sets of end plates free of charge. One set they built up and thoroughly checked. The sent me this and the second pair of side plates. Again I eagerly tried a standard 6U panel in the 6U section and to my relief and joy it fits perfectly. Now I can get down to the interesting work of building a mixer.

I want to use this blog to publicly thank SRS for their tremendous support in solving this problem for me. After all I am probably their smallest customer.

Thank you  SRS

Sunday, 15 July 2018

Channel One

The channel module is the heart of any mixer. The combination of mic pre and EQ that makes up a normal channel module is what defines the basic sound palette the mixer can provide.  So, having developed the basic 35mm wide building blocks as detailed in the previous post, it is time to put them together to make a full blown channel module.

One of the benefits of having a modular approach to the PCBs that make up a module is that you can reuse them in other modules. For the first channel amp we reuse the front panel board of the the classic mic pre module (see previous post) and the front panel PCB of the REDDPLUS EQ (see previous post). I made a couple of tweaks to both of them but they are otherwise largely unchanged. These two together fit onto a 35mm wide 6U high front panel to make up the channel module. One thing we do need that is new is a 6U main PCB for the module but even this is mostly made up of PCB layouts we already have. The main mic pre section is simply the 35mm TLA stuck on a 6U PCB with some added transformers for input and output. The bottom half of the board just uses the main REDDPLUS EQ board.


One the left you can see the TLA circuit and to its right are the input and output transformers. To the right is a footprint for an additional transformer and just below it is the REDDEQ circuit. The normal signal flow is input transformer, mic pre, passive EQ, gain make up amplifier and finally output transformer. The output transformer would usually be connected to either a direct out or insert point. One advantage of having the output transformer in the channel module is it makes if very easy to build a 6U 19 inch rack containing a bunch of tube channel amps.

As you can see there is plenty of space on the board for later additions.

The red rings around the three tubes are for testing an idea to make the module more rugged. The rings are just thick enough that they are gripped between the PCB and the steel screen and, in combination with the tube sockets pins, hold the tubes firmly in place and also buffer them from external forces such as you might find in a mobile situation. In addition, it is a real pain to have to remove all the tubes from all the modules of a mixer in order to ship it. It also requires the customer to fit all the tubes to the modules before fitting the modules into the mixer. My initial idea was to stuff the space between the tubes with bubble wrap to hold the tubes in place during shipping. All the customer would have to do is remove the bubble wrap before plugging in a module. With the red rings, the tubes are already gripped firmly so none of this is necessary. The big issue is the red rings have to withstand the bulb temperature of the tubes.

The front panel has been attached so you can see the front panel mic pre and EQ boards. If you look to the left, centre and right of the front panel you will see the low cost Ettinger blocks used to connect the front panel to the PCB. You can also see pillars attached to the two mounting holes of the 32 way connector and another pillar in the top right hand corner. These, together with the Ettinger blocks, are used to mount the module steel screening plate.


The other side of the module is screened by the 0V plane of the PCB.

 And from the front it looks like this:


Now we have a proper channel amp we can think seriously about configuring a real mixer.

Monday, 21 May 2018

Come Together

It is nearly five months since my last post but a lot has been happening, mostly in trying to bring together all the design concepts detailed in previous posts and incorporate them all in the design of a real mixer. Let's start with the basic components, the PCBs.

The last post detailed the new 32 way connector pin out and the reasons it was necessary to update the old design. In order to be able to use the new pin out you first need a new motherboard:






Reflecting the new 7HP (35mm) module width, this motherboard has twelve slots instead of the previous six. With this many slots, it is possible to consider building a complete mixer in a 19 inch rack width. With twelve slots you can have eight input channels and still have four slots left for bus amps.

Talking of bus amps, our old friend the Twin Line Amp (TLA) has been update to a 35mm module width:





Notice the three tubes of the original TLA have been mounted vertically so they will fit into a 35mm wide module. Unfortunately there is no room for the pair of input transformers the original TLA had. With a bit of PCB layout tweaking it might just be possible to squeeze in one small diameter mic transformer like the Cinemag CMMI-10PCA which is just under 28mm in diameter but that is for the future. For the present we have a half sized pair of bus amps.

Buses need channels to feed them and channels need mic pres. The Classic mic pre was the test bed for trying out vertical tube mounting so it was a natural to convert to the new pin out:





It also sports a new front panel layout and some extra features. First the four toggles have been rearranged into a square so save space. Secondly a stepped gain control has been added to make it operate more like a conventional mic pre is expected to work. Lastly a simple 12dB/octave HPF has been added.

This module is also the first to benefit from the new module mechanical design. From the left the module looks just like a PCB:


But there is a hint of something going on to the right:



And looking at the module from the right makes it clear the steel screen of the new mechanical design is being used. The steel screen is attached at the front using the same small die castings that are used to fix the front panel to the PCB and at the rear it is mounted to the top of the 32 way connector using a couple of pillars. This makes for a very strong, rigid box construction. You can see how the box is formed in this top view:


You can see the pillars used to connect the steel screen to the 32 way connector and the small die castings used to connect it to the front panel.This picture also illustrates the new front panel controls mounting scheme discussed in an earlier post. Here is a close up of the front of the module from above:





To the right is the front panel, at the top is the steel screen, the main Classic PCB is at the bottom and you can see the two small die castings coupling the front panel to the steel screen and the main PCB. Just behind these die castings is a small new PCB which is parallel to the plane of the front panel. It is attached to the front panel solely by the toggle switches and the rotary switch controls fitted to the front panel. This one has four Molex connectors mounted on its rear surface. Two are for mic and line inputs, one is the output to the mic input transformer and the third is both input and output to the HPF and the stepped gain control. The PCB is just 26mm wide and 100mm tall. If you want to change the controls on the front panel all you have to do is create a new version of this small PCB. In the old scheme, where controls were fitted to the main PCB, you would have had to create a complete new main PCB 100mm by 160mm to achieve the same result. The new scheme makes customising modules a lot simpler and cheaper.

Talking of making things cheaper, the new module mechanics cost a fraction of the cost of the Fischer modules I used to use but there are still some expensive items in it. The worst culprit is the small die casting of which four are used. Right now these cost over £4 each if you buy them from Farnell. They are still more then £2 if you buy 25 of them. Other distributors, such as Digikey, have them at lower prices, where they are just over £1. Even at this price, they contribute over £4 to the price of the enclosure. However, thanks to a groupDIY member, there is an even cheaper alternative. It is a small brass cube made by Ettinger:

These are available from Farnell for less than £0.40 each. They are not 100% compatible with the die castings but they are extremely close. The main differences are first that the tapped hole used to attach the PCB is M3 rather than the M2.5 used on the die castings. Fortunately I slightly oversized the holes in the PCB so an M3 screw will just fit through the same hole.

Secondly, the fixing points on the front panel are different. They are slightly closer to the edge which actually makes a little more space available on the front panel. They are also smaller than the die castings which reduces the amount of front panel area they consume which also helps with front panel layouts.


Lastly, the spacing between the front panel and the main PCB is slightly less using the Ettinger cubes. It is not much, about 0.2mm or 8 mil, but it will mean the 32 way connector will mate 8 mil less with the motherboard connector. I do not expect this to be a problem but I will move the holes on the PCB to compensate for this in the next PCB revision.


The new front panel controls mounting scheme has also been used in the creation of the first 35mm EQ module.  This EQ is based on my REDD EQ design which had fixed frequency high and low controls and selectable frequencies for mid boost/cut. The new design extends this by adding three selectable high frequencies and three selectable low frequencies. I call it the REDDPLUS:


It is a relatively simple EQ. In line with the general thinking of trying to reduce costs, some design decisions have been made which at once simplify the design but also add features. The major simplification is to remove the requirement for cut Q to be the same as the boost Q. This makes the EQ a bit more like the Helios 69 EQ where the cut Q is sharper than the boost. This makes sense since cutting tends to need to be more surgical than boosting. This means we no longer need a 2 pole 12 way switch for the mid section. Instead we can use a much cheaper single pole 12 way switch and we only need one inductor for the mid band instead of two. The second simplification is to remove the requirement for the Q to be the same at each frequency setting (constant Q) and instead use constant bandwidth. This significantly simplifies the frequency selection switching to the point where we can select three frequencies with a single toggle switch. These same concepts have been applied to the Low and High bands to extend them to operate at three different frequencies. The resulting front panel looks like this:


There are just three 11 way rotary switches that provide  up to10dB boost/cut in 2 dB steps and three toggles switches each of which selects one of three frequencies. The next version will include an EQ in/out switch as well.

I mentioned earlier there is not enough room on the TLA board for an input transformer. However, if you build it on a 6U high PCB there is plenty of room. There is also room for an output transformer and room enough  for a line input transformer that can be used as the return from a balanced insert. Not only that, there is also room to fit the above mentioned REDDPLUS EQ which together would make a complete 35mm channel amplifier. There will be more details of this in a future post because I have only just sent off the PCB layout for prototypes to be made. In the meantime here is a picture of the PCB layout:


And a first draught of a front panel layout:


Sunday, 31 December 2017

The Ins and the Outs

One of the perennial problems facing a mixer designer is how many pins to use on a module connector and what to assign to them. You could use a really large connector with many pins but this will add unnecessary cost to many smaller projects. Alternatively, you could use one standard connector and an additional one only for more complex modules. The only downside of this method is you may need two motherboards for the complex modules.

To get a better handle on the problem, let's look at the existing Eurochannel 32 way connector pin assignment.


This is the original hand drawn schematic from 2012 that defined the 32 way connector pinout. The first four pins are assigned to the mic input and the second four to the line input. Notice how the screen of each input uses two pins. The reason for this is that, at the time, I was not using a backplane PCB for the modules to plug into. Instead I was using regular connectors and wiring the backplane by hand and I found it very awkward to wire a mic cable screen to a single pin on the connector. It was much easier if I used two adjacent pins.

Next are the 48V supply and the chassis connection (which is also the 0V of the 48V). Both these use two pins each simply because they are buses and it makes hand wiring a backplane easier.

Next we have the unbalanced OUT1 and its associated 0V closely followed by the FG and FS pins. OUT1 is the output of the first amplifier in the Eurochannel design. It is often fed to an external fader and returned to the module; this is the purpose of the FS (fader slider) and FG (fader ground) pins. Not all modules or applications use this feature but it is included for those that do.

Next is a pair of pins for relay power. Most modules will need some form of auxiliary power for LEDS or relays so these two pins are often used.

Then we have four pins assigned to buses. In many cases this is too few. It allows for a stereo bus and a couple of AUX sends but this means there are none left for a solo or PFL  audio bus and its associated dc bus. We could really do with twice as many bus pins.

This if followed by the main unbalanced output OUT2 and its 0V pin.

Lastly are the power pins each of which uses two pins. The HT needs tow pins so we can ensure there is always a suitable gap between these two. The heater supply needs two pins so we can easily bus the relatively high heater current along the backplane.

As you can see, this particular pin out is determined by a variety of factors, some relevant to the module and some relevant to the process of building a mixer. It has some limitations, particularly in the number of buses and there is no provision for balanced outputs simply because in the Eurochannel design they were external.  There are also some non-optimum assignments from the point of view of PCB layout. For example, all the power supplies are at one end of the connector. From the point of view of PCB layout it would be better if they were near the middle of the connector. Despite its peculiarities, this bus standard has served well for over five years.

The new 6U modules being designed for the Mark 3 include output transformers so they need a way for these to be connected to the outside world. The 6U size board allows a second 32 way connector to be used. This can provide balanced outputs and additional buses but a lot of its pins are not really needed and this solution only works for 6U modules. It is no good for 3U modules.

Holger Classen has also been working on new versions of Eurochannel modules. His approach has been to retain the 3U module size but to make the module deeper. This allows output transformers to be fitted onto the PCB. In this way he can fit two complete mic pres into a single 3U module. He cannot add another 32 way connector so he proposed a new assignment of the existing 32 pins to allow for two balanced outputs. We have discussed this at length and made a few modifications until we were happy the new assignment would work for us both. Here is the new assignment of pins:



Notice first that the way each pin is referenced has changed. This reflects the actual row and column in which the pin is located. It also makes it clear that a three row connector is used but pins are only fitted in rows a and c. This implies a 0.2inch spacing between rows. Also, only even numbered pins are fitted which again indicates a 0.2inch pin spacing.

At the top is the first balanced input and its associated balanced output. Note that only the input has a shield. The balanced output output does not really need a shield pin (its cable screen can be connected at the XLR end) This pinout is duplicated at the bottom of the connector for the second balanced input and output. A chassis pin is located at both ends of the connector.

All the power pins are located about the centre of the connector. This time the 48V and its ground have only a single pin each, just like the utility power (which replaces the old relay power). Heaters and HT supply all have two pins each for the same reasons they did in the original. HT 0V is renamed AGND to more accurately reflect its purpose.

We now have a total of eight unassigned pins, and they are genuinely unassigned. Their purpose will be determined by the design of the backplane PCB. This provides a great deal of flexibility for both Holger and myself (Holger already makes his own backplane PCBs) but all backplane PCBs will have this core pinout. So the supplies and the chassis connections will be bused on all backplane PCBs. In 1 and In 2 will both be brought out to 3 pin Molex connectors in the same way as the mic and line inputs are at present. Out1 and Out 2 will both be brought out to 2 way Molex connectors just as Out1 and Out 2 are at present.

All other pins on the backplane PCB are assignable on a design by design basis. One new version of backplane PCB will bus pins 24a, 24c, 26a and 26c for use as mix buses. Pins 8a and 8c will be brought out to a 2 pin Molex as wil pins 10a and 10c. In many mixers these can be used for external unbalanced connections to faders or EQ. This version of back-lane PCB thus emulates the original Eurochannel backplane PCB. The designs of existing PCBs (Eurochannel, Twin Line Amp, Classic, EQ with gain make up etc) will be migrated to be compatible with this backplane PCB.

All new designs will adopt the new pinout and new backplane PCBs will be designed as and when the need arises.

Tuesday, 19 December 2017

Universal Vertical Tube Preamp (UniVert)

In the previous post I mentioned the first 1.4 inch wide (7HP) module I designed was based on the two tube Classic design. In fact it is a bit more than that. Rather than just repackage the Classic design I thought I would try to broaden its applications. The Classic is basically two identical gain stages. They are normally configured as mu followers for high gain and low distortion, but they can be configured as SRPPs for better drive capability. They are normally simply cascaded with a gain pot between them but there is no reason some overall negative feedback (NFB) cannot be used. This would make them similar to the Eurochannel and Twin Line Amp boards and they could be used in the same kinds of applications. The two stages of the Classic do not provide quite as much open loop gain as the Eurochannel circuit so either the maximum gain must be reduced or slightly higher levels of distortion must be tolerated.  Because this makes it  more universally aplicable I decided to call it the UniVert. Here is its schematic:

V2, the output stage, is configured as a mu follower using a 6922 tube. For better drive capability at the expense of higher distortion, it can be configured as an SRPP stage.

V1, the input stage, is also configured as a mu follower but with a few extras. The key additions are R6 and R18. R6 allows the cathode of V1B to be raised high enough that R18 can provide NFB right down to dc, just as in the Eurochannel, to ensure stability at all gain settings. The closed loop gain is set by R19.

I have not built this configured with NFB but I have simulated it and it does perform well. The first one I built was a straight pair of mu follower stages. Here is the schematic for this with component values added:



As you can see, the NFB network is not used. The 220 ohm cathode resistors bias the mu followers at about 12mA each which is necessary for them to be able to drive a 600 ohm load via a 2:1 step down transformer. The first stage could be biased at a lower current because it usually only has to drive a 10K gain pot. Here is a pic of the PCB:


The PCB has all the usual EZTubeMixer fixtures and fittings including the 32 way DIN connector, mic and line inputs, phantom power, external fader connection, buses and provision for fitting an input transformer. Notice the two tubes are at the top so their heat does not flow past any passive components. This is not always the case in the Eurochannel and Twin Line Amp PCBs. It is made easier of course by having only two tubes. I am still not certain that three tubes could be squeezed into this size PCB but I am pretty certain they would fit in a 6U version, but that is something for later.

 In the centre you can see  the tracking for the pair of 9 way right angled headers that connect the tube mounting PCB to this PCB. And that is the reason this is the V2 PCB. In the first version I managed to get this connector mirror imaged! At least the boards could be used for mechanical mock ups. Talking of mechanics, you will notice there is quite a bit of bare PCB on the left which will be behind the front panel. This is to leave sufficient space for the new front panel mounted controls PCBs which give maximum flexibility in positioning front panels controls. Also, the 100uF 250V capacitor used to couple the gain setting resistor is too tall to be mounted normally so it has to be laid on its side. I could find only one source of a 100uF 250V electrolytic capacitor so I decided to stick with radial leaded types.

In performance terms it seems to be virtually identical to the original 14HP wide version. However, I do need to operate this version ,without the bottom screen, side by side with the  14HP Fischer module version, to measure exactly how good the new mechanicl system screens the electronics.

Monday, 18 December 2017

Fat ones, thin ones, tall ones, short ones - Module Mechanics Revisited

At the start of this blog I mentioned the reasons for using a module enclosure, particularly the mechanical integrity and screening properties and also that the Fischer modules were reasonably priced. Except they are no longer so cheap. SInce I started using them the price has gone up by 40 percent. In addition, after a couple of years experience using them, their shortcomings are beginning to become noticeable.

Tube modules tend to be be quite large to make enough room for the tubes. In turn, this means tube mixers tend to be pretty big (Holgers 12 channel one is one metre wide). To put this in perspective, the modules are nearly twice as fat as 500 series modules. So I have been spending some time looking at ways to squeeze more functionality into a given space and also how to squeeze existing functionality into a smaller space. Put another way, I can get just six tube modules into a 19 inch rack space but you can get eleven 500 series modules in the same space.

The first option I looked at was stereo or two channel modules. These are the same width as the existing modules and fully compatible with the standard mechanics and motherboard interface, but just have two channels of electronics in them rather than one (these were mentioned earlier in the Mark 3 6U modules discussion). They are line level only modules so the active electronics is essentially the Twin Line Amp design. Many people now seen to be moving to line level only mixers and use external preamps for tracking either direct or via the line level mixer. This basically splits the functions of a classic mixer in two. From my point of view, the advantage is I can now get twelve channels in a 19 inch rack space instead of just six. The 8 tracker becomes a 16 tracker overnight.

However, the Fischer modules are beginning to become a limiting factor. The type T 6U module I use places the top surface of the PCB 14.2mm from the left hand edge of the front panel. This relatively large offset makes front panel design more difficult for a two channel module. The controls of the left half are 14.2mm plus their height from the left edge of the front panel. For the twin channel module to be symmetrical about the centre line, the right half controls must be the same distance from the right side of the front panel. So the controls are pushed 14,2 x 2 0 28.4mm towards the centre making the centre look cramped and the edges bare. Maybe a different mechanical scheme could reduce the 14.2mm offset sufficiently to fis these limitations.

These modules operate at line levels so screening is not so critical as it is for microphone preamplifiers. Maybe a simpler mechanical scheme can be used that will be more cost effective?

The second option I looked at was a making modules half the existing width, that is 1.4 inches wide instead of 2.8 inches. This means the tubes cannot be mounted directly on the PCB as they are at present. Instead they need to be mounted vertically which means they use up more PCB space. It turns out there is probably not enough space to house the three tubes used in the Eurochannel mic pre or the Twin Line Amp plus the input transformer and all the required passive components, but there is room for the two tubes used in the Classic mic pre design. So this was the first PCB I laid out after first designing an adaptor PCB to allow the tubes to be mounted vertically.

At this point it became clear that 1.4 inch wide Fischer modules, although available, were not going to have enough room. Most models of Fischer modules are quite wasteful of width. For example, the T types used for 6U modules mentioned earlier, where the top surface of the PCB 14.2mm from the left hand edge of the front panel. In a 1.4 inch wide module we only have just over 35mm to play with. The tubes are 22mm in diameter and if we lose another 14.2mm due to the module type we have already used more than 36mm. There was only one Fischer module that wasted much less width. This was Design I illustrated below:


This version only loses 6mm leaving an available space of 29.56mm above the PCB which is plenty for the tube. The module kit consists of:

  • A plastic insulating panel that fits to the rear of the PCB (good idea)
  • A standard blank front panel
  • A piece of bent aluminium that forms the screen
  • Four steel standoffs
Considering it includes a blank front panel I don't need (all my front panels are custom made), a piece of bent aluminium, four standoffs and a bit of plastic, it is not very cost effective. I did ask but Fischer will not sell any of the component parts separately.

In the meantime I decided to try a simple construction of my own. This starts with the original Eurocard method of fixing the front panel to the PCB which makes the top surface of the PCB just under 4mm from the left hand edge of the front panels (10mm better than current modules). Here is a drawing that shows how this works:



A neat little die casting is used. The PCB is fixed by a screw into a tapped hole in the die casting and the front panel is similarly fixed using another tapping. The result is the top surface of the PCB is just 3.97mm from the left hand edge of the front panel. Here is the fixing in detail:




It occurred to me that by using a longer screw we could fix a tapped standoff to the die casting and use this to attach a side plate at the right of the module. At the back of the PCB is the 32 way connector which is attached to the PCB in a similar manner. Again using a longer screw we could attach another tapped standoff to attach the rear of the other side plate. Here is a picture of the basic assembly:


This early version also includes a screen on the left side. This is held 3mm away from the bottom of the PCB by spacers. The screen is made from 1mm mild steel and is very rigid. A single long screw goes through the left screen, through the spacer and the PCB, through the threaded hole in the die casting and into the threaded standoff. At the back the stand off is fitted similarly using the 32 way connector fittings. On the right is the right side screen, also made from 1mm steel and held on by screws into the tapped stand off. The whole assembly is very rigid and strong, even though the front panel has not been fitted.

This then needed to be mated to the new PCB using vertical tubes, shown here:


You can see the two small boards on which the tubes are mounted. These are fixed to the main PCB using right angled headers. When the mechanics and PCB are mated they look like this:


The up side of this scheme is it is easy and cheap to build. It is also very strong so there is no likelihood of front panel controls being damaged by flexing of the PCB as in the EZTubeMixer design. The down side is there is only screening on three of the six sides of the module. However, the sub-racks I use have top and bottom screens fitted so the only side of the module not screened is the back where the 32 way connector is.

This scheme is easily extended to wider or taller modules. Wider modules simply need taller standoffs. Taller modules simply need larger sheet steel screens. The only concern might be whether the larger steel screen flexes sufficiently in the centre of the screen to be a problem. If it is, another standoff could be used.

In practice we can probably eliminate the screen beneath the PCB. The left most PCB in a sub rack has its PCB right beside the aluminium side of the sub rack. Its right side is screened by its right  hand steel screen. But this also screens the bottom of the PCB of the module to its right and so on up to the other end of the sub rack. This may seem like a further compromise but it is a screening scheme I have seen recently in two professional broadcast mixers, one made by Glendale and one made by EELA, both for BBC local radio. If it is good enough for the BBC it is good enough for me.

The mechanical design could be refined further. For example, the the standoffs at the front could be eliminated if another pair of diecastings were used to mount the screen to the front panel on the right. This may or may not be an advantage. The scheme with stand offs does mean you can test a module without the front panel but it is not clear if this is an advantage or not. The general idea though is definitely worth exploring further.