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.

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

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.

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: