6U Modules Update

The original 6U channel concept was to have a common motherboard containing uncommitted EQ switches connected to IDC connectors. Specific EQ boards would be connected to the EQ switches using ribbon cable. In this way a common motherboard could be used and a channel could be configured for any EQ simply by fitting the appropriate EQ board.

Here is a picture of the prototype 6U motherboard:

To the left is the mic pre and at the bottom right you can see the EQ switches and their associated IDC headers (two per switch). You can also see, above the switches, a space for fitting an output transformer. This means a module can be completely self contained with transformer balanced inputs and outputs. Unfortunately there are no spare pins on the standard 32 way EZTubeMixer connector for a balanced output so, rather than change the original 32 way pin out,  I have added a second 32 way connector. This will have the same basic pin out as the original but allocated differently. The mic and line balanced inputs can be used as inputs or as outputs. For example, you could have a balanced output from the mic pre which feeds an insert connected to the extra connector mic input pins. The balanced return would be fed to the extra connector balanced line input. You can see on the right hand side of the PCB where I have made provision for an extra input transformer for just this purpose. This would allow you to have a pre-EQ balanced insert or to use the mic pre and EQ independently.

After basic assembly, the PCB looks like this:

The uncommitted EQ switch and IDC headers idea is fine in theory but it did not work out in practice. The capacitance of the ribbon cable seriously upset the operation of the EQ, especially on the high frequency ranges and they also consume a lot of space on the EQ daughter-board. I wanted to retain the mother/daughter board flexibility, so I looked at simplifying the wiring between the two.

At that stage, the prototype 6U PCB had provision for three switches. Each one is tracked for two banks and up to 2 poles per back. Each bank is tracked to its own 26way IDC connector which means there is a total of six connectors.

The first EQ I worked on was the Helios 69 EQ daughter board. As the bass switch uses 2 banks it needs a pair of 26 way connections. These connectors take up a lot of room on the PCB, more than the switch would have done. There are only 26 connections required (two lots of 12 plus two commons) so a single 26 way connector would be sufficient. The only reason to have two 26 way connectors is if a four pole 6 way switch is required.

I therefore decided to review the switches used in the current range of EQs to see if it would be possible to use just a single 26 way connector per switch. The table below shows the switches used in the current EQs.

EQ	LO	MID	HI	OTHER
3B PULTEC	2P6W	1P12W	1P6W
HELIOS 69	2P12W	1P12W	1P12W
REDD	2P12W	2P12W	1P12W	2P6W

It is clear that no more than two poles are required. In the 2P12W cases, the poles are on different banks but for the 2P6W case both poles are on the same bank. So, although we never need more than 2 poles, they can come from four possible places.

However, we never use both poles on the second bank, in fact we only ever use the first pole on the second bank when we use a 2P12W switch. This means we can forget the second pole in the second bank. Also, whatever switch we use, we always use the first pole on the first switch so this can be permanently tracked to the 26 way connector. Now all we are left with is selecting between the second pole on the first bank in the 2P6W. We can do this with a set of three pads and a link.

The 26 pins of the connector have been allocated as follows:

CONN PIN	SW PIN	CONN PIN	SW PIN
1	Bank 2 Pin1	2	Bank 1 Pin 1
3	Bank 2 Pin 2	4	Bank 1 Pin 2
5	Bank 2 Pin 3	6	Bank 1 Pin 3
7	Bank 2 Pin 4	8	Bank 1 Pin 4
9	Bank 2 Pin 5	10	Bank 1 Pin 5
11	Bank 2 Pin 6	12	Bank 1 Pin 6
13	Bank 2 Pin 7	14	Bank 1 Pin 7
15	Bank 2 Pin 8	16	Bank 1 Pin 8
17	Bank 2 Pin 9	18	Bank 1 Pin 9
19	Bank 2 Pin 10	20	Bank 1 Pin 10
21	Bank 2 Pin 11	22	Bank 1 Pin 11
23	Bank 2 Pin12	24	Bank 1 Pin 12
25	Bank 2 Com	26	Bank 1  Com

This scheme reduces the the number of connectors by half and would hopefully not affect the performance of the EQ. Here is a picture of the updated 6U PCB:

Des[ite all these efforts,  the EQ performance was still compromised. So this scheme has been abandoned. Instead, each EQ type will have its own unique channel PCB. This means more PCBs have to be designed but it does mean the EQ will work as advertised. The first of these PCBs looks like this:

The mic pre section is unchanged but the EQ section now includes an almost complete Helios 69 style EQ. The only parts not on the PCB are the two pots, the peak/trough switch and the EQ in/out switches.. These will be connected to a very simple daughter board via a ribbon cable.

In parallel with this I have been developing the twin line channel (TLC) PCBs. These are intended to form the basis of a  line in line out mixer with two AUX sends and a pan pot per channel with two channels to a module Again a mother board daughter board concept is used but there are far fewer connections between mother and daughter board then in the case of the standard channel module.The first of these developed was the REDD EQ. The reason is that these EQ sections can be cut and pasted between Channel and TLC mother boards so I only have to lay out each EQ once. Here is a picture of the prototype REDD EQ TLC:

The top of the PCB is essentially a standard Twin Line Amp (TLA). At the top left of the PCB are three pots; these are the two AUX sends and the pan pot. Below them is the channel mute switch. Below the mute switch are two preset pots that are used to set  the gain of each half of the TLA so it can make up the loss in the EQ. These are arranged so as to be accessible through small holes in the front panel to simplify set up.

Below these is the EQ itself. It is a new version of the REDD EQ with all for switches in line. The top one is the 10KHz shelving EQ, the two middle ones are the mid boost/cut and mid frequency select switches and the bottom one is the 100Hz shelving EQ. The only component not on the PCB is the EQ in/out switch. If required this can be fitted directly to the front panel and wired to pads provided. It can be fitted in between the 10KHz switch and the gain trim pots.

Above and below the EQ are four holes labelled P1, P2, P3 and P4. These are for the pillars on which will be mounted the second identical EQ.  SImilarly, there are two holes above and below the AUX and pan pots on which will be mounted the AUX send, pan pot and channel mute controls of the second channel. The daughter EQ connects to the second input transformer and the second gain make up amp of the TLA section

The EQ has already been commissioned and it works well. The next stage is to check out the AUX send and pan pots then develop the two mother board PCBs.

6U Modules Part 1

The MKIII uses 6U high channel modules built using extruded aluminium 'cassettes' made by Fischer Electronik. First we need to sort out all the basic mechanical details of the cassettes and the key dimensions for determining the position of controls on the front panel. Later sections cover specific modules.

In general there are two distinct types of channel modules. First there is the regular channel amp based on the EZTubeMixer channel amplifier with its four push buttons and stepped gain control. Each one has an EQ section and an optional routing section. The second type is the twin line input channel. These have no mic pre controls but contain two identical line input channels each with a three band EQ and an optional routing section.

Basics

The Mark III 6U modules are based on an extruded aluminium cassette made by Fischer Elektronik in Germany. They have several types of 6U cassette but we need one that has a cut-out at the rear so it can be used with 3U backplane PCBs that have a centre support. The preferred type is therefore the Fischer model T:

Note: this drawing shows that the top surface of the PCB is 14.2 mm from the left hand side of the front panel. This sets the position of all the controls mounted on the main 6U PCB and is crucial in calculating the x-position of holes on the front panel.

There are several rear panel options. Mostly we just need the type with a single connector hole at the top:

All the modules are 14HP (2.8 inches wide). I have obtained a drawing of the Fischer standard dimensions for this size of front panel:

The overall panel width is 70.9mm. The overall panel height is 261.8mm. The front panel fixing holes are 3.4mm diameter, countersunk and are set in 5mm from the edges.

A standard 6U PCB is 233.4mm tall. We assume the PCB is centred on the front panel. This means the bottom of the PCB is 14.2mm from the bottom of the front panel. This is crucial for calculating the y-position of holes on the front panel.

Mic Pre Channel Amplifier

The MK III mic pre channel amplifier is based on a standard 6U motherboard. This houses standard EZTubeMixer mic pre and gain make up amplifiers with the usual push buttons for phantom power, 20dB pad, mic/line selection and phase change and the standard 12 way Grayhill switch for gain setting. The Grayhill datasheet shows that the centre of the shaft of this switch is 9.35mm above the PCB surface and since the PCB surface s 14.2mm from the left hand side of the front panel, the switch shaft is:

14.2 + 9.35 = 23.55mm from the left hand edge of the front panel

The datasheet for the push buttons shows that the centre of their shafts is 5mm above the PCB surface or 19.2mm from the left hand edge of the front panel. The push buttons will take 6mm diameter round push on knobs so the hole diameter for these should be 6.5mm. The table below shows the y-coordinate of the push buttons and the gain switch on the PCB in mil, the equivalent y-coordinate of the holes for them on the front panel in mm and the corresponding x-coordinate in mm.

Control	PCB y-coord ( mil)	Panel y-coord (mm)	Panel x-coord (mm)
Phantom	8401	227.59	19.2
Pad	8007	217.59	19.2
Mic/Line	7614	207.59	19.2
Phase	7220	197.59	19.2
Gain	6433	177.6	23.55

In addition, provision is made for up to three two deck 12 way Grayhill switches for EQ use. Each switch is wired directly to a 26 way IDC connector. The centres of the shafts of the three switches are set at exactly 1.0, 2.6 and 4.2 inches respectively from the bottom of the PCB. In other words they are 1.6 inches apart starting 1 inch from the bottom of the PCB.. Since the bottom of the PCB is 14.2mm from the bottom of the front panel we can calculate the front panel positions of each switch:

Switch	x - position mm	y -position mm
1	23.55	39.6
2	23.55	80.24
3	23.55	120.88

Depending on the shaft size of the switches, the front panel hole diameter needs to be either  0.25 inches (6.35mm) for the one eighth inch diameter shaft and 0.375 inches (9.6mm) for the one quarter inch diameter shaft.

The EQ itself is housed on a daughter board mounted above the motherboard on pillars and connected to the EQ switches using ribbon cables. Ideally we would like the controls on this PCB to be the same distance from the right hand side of the front panel as the motherboard switches are from the left hand side i.e.

70.9 -23.55 = 47.35mm from the left hand side

Most of the controls on the daughter board will be EQ pots and my preferred type is made by OMEG. The shafts of these pots are 12.5mm from the surface of the PCB. Since the PCB is 1.6mm thick, to get the pot shafts in the right position the pillars need to be:

47.35 -12.5 -1.6 -14.2 = 19.05mm tall.

This is just enough to clear the switches but makes no allowance for the legs of components on the daughter board. In addition 19mm pillars are non-standard so I have decided to use 20mm pillars. This means the EQ pots are 1mm closer to the right hand side of the front panel. This means the x-position of the pot shafts on the front panel is:

14.2 + 20 +1.6 + 12.5 = 48.3mm

The daughter boards is supported by four pillars. The bottom pair are 0.252 inches from the bottom of the PCB. The motherboard fits into slots in the enclosure extrusions so the daughter PCB has to be a little smaller so as not to foul the enclosure. So, the bottom pillars on the daughter board are 0.160 inches from the bottom. If we want the the EQ pots on the daughter board to line up with the EQ switches on the motherboard then the pots need to be:

0.252 - 0.160 = 0.092 inches lower down the PCB

so the PCB y- positions of the pots are:  0.908, 2.508 and 4.108 inches respectively.

There is room between these pots for additional controls where required. To summarise, the front panel positions of the pots are:

Pot	x - position mm	y -position mm
1	48.3	39.6
2	48.3	80.24
3	48.3	120.88

Lastly, there is a pre-set potentiometer used to set the EQ gain make up that needs to be accessible through a small hole in the front panel. The screw of the pot is 7.9mm above the PCB so its x-coordinate is 14.2 + 7.9 = 22.1mm.  The centre of the pads of the pot is 5777 mil from the bottom of the PCB. However, the screw is 50 mil below this so the screw is 5727 mil from the bottom of the PCB. Its front panel y-coordinate is therefore:

5.272 * 25.4 + 14.2 = 148.11mm

The hole size needs to be large enough to accommodate a small screwdriver about 3.2mm in diameter so we will make this hole 4mm diameter.

The front panel layout below shows the the basic layout of the controls discussed above:

Specific EQs will be described in subsequent posts.

Virtual Earth Mixing and Alternative Output Transformers

I mentioned in the previous post that the gain setting resistors of the channel amplifiers have been zero referenced making virtual earth mixing a possibility. I have now received prototype PCBs of the V2 Twin Line Amplifier (TLA) which incorporates the relevant improvements added to the channel amplifiers:

• Provision for Molex KK connectors for internal wiring
• Amplifier gain preset pots moved to be accessible from the front panel
• Tubes moved so as not to foul the enclosure
• Ground reference the gain setting resistors so each amplifier can be used as a virtual earth mixer

In addition, a couple of other improvements have been made:

• Inclusion of a series 1N4007 in the HT+ line to protect against accidental reversal of the HT supply
• Addition of a 100nF capacitor across the HT supply where it enters the PCB to prevent the instability that can occur when several modules are connected.
• Moved one output capacitor so as not to be directly above a 6922 which makes it too hot.

The new TLA V2 is shown below:

Apart from the modifications listed above, the new TLA is functionally identical to the original TLA and can be used wherever the original was used. The one thing this TLA can do that the original cannot is virtual earth mixing and this is what I wanted to test out. I used my new test rack (shown below) to feed the output of the new channel amplifier to one of the buses via a 47K resistor and connected one of the new TLA virtual earth inputs to the same bus. I measured the output of the the channel amplifier and also the output of the TLA virtual earth amplifier. They were within 0.2dB of each other. Allowing for tolerances in the two output transformers included in the test circuit, this is a good result, implying the virtual earth mixer gain is very close to unity as expected, However, when I measured the noise, the result was rather disappointing at only -63dBu; clearly not good enough for any serious mixer. I checked the output of the channel amp in case this was the source of the noise but this was much lower so the noise clearly was introduced by the TLA. I then realised I had forgotten to connect the +ve input of the TLA (the one used for passive mixing) to 0V. I added the necessary link and the noise dropped below -80dBu.

This means virtual earth (VE) mixing is now a potential alternative to the normal passive mixing I use in my mixers. The reason I use the word 'potential' is that VE mixing has it own set of problems, the most serious of which is instability caused by bus capacitance. This is particularly problematic where op amps are used. They have a fixed open loop gain but the closed loop noise gain depends on the number of sources feeding the VE. This means the amount of negative feedback (NFB) varies which affects stability. Steve Dove, in his excellent series on mixing console design goes into more detail about this problem. To paraphrase Steve:

"Bus capacitances, from all the cabling and PCB tracks between the channel amplifiers and the virtual earth amplifier, appears across the input of the virtual earth amplifier. This has the effect of eroding the phase margin and can lead to instability and even oscillation. This capacitance reacts against the feedback impedance to to increase the closed loop gain at high frequencies. Even a few pF is enough to tilt up the closed loop response well within the open loop parameters, threatening instability. In a real mixer with cables from many channels, hundreds of pF may be present. This makes ensuring the required phase and response characteristics very difficult. Sometimes a small series limiting resistor can be added to to define just how much this unwanted gain can rise, but this is at the expense of the ‘virtual earth’ now being determined by this resistor (which rather defeats the object)."

There is another solution to this problem and that is to vary the open loop gain as the number of channels varies. Regular op amps do not do this but many years ago a discrete op amp was designed that does exactly this. It is called a Trans Amp (note that the term Trans Amp is also used to refer to other topologies so this can be a bit confusing when conducting an on-line search). The basic idea is that the resistor that sets the closed loop gain also sets the open loop gain. If this is done correctly then the difference between the open loop gain and the closed loop gain (which is the amount of NFB applied) is practically constant which means the stability criteria are the same no matter what the gain.

This is exactly what the EZ Tube mic pre does and for similar reasons. In the mic pre, we want to be able to vary the gain over a wide range ( in this case from 6dB to 40dB) whilst maintaining stability. The EZ Tube mic pre achieves this by gradually shorting out the cathode resistor of the first tube stage of the mic pre. This gradually increases the gain of the first stage as the resistor is reduced and also gradually increases the closed loop gain. The net effect is the NFB is nearly constant and stability is assured. The TLA uses exactly the same amplifier so it also automatically adjusts its open loop gain as its closed loop gain is varied so it should make a stable VE amplifier that is insensitive to the number of channels feeding it. Note that varying the open loop and closed loop gain by gradually shorting out the first stage cathode resistor is not a new idea. The first instance I know of its use is in the V76 amplifier.

Why are we even thinking of using VE mixing when passive mixing already works very well. There are several reasons:

• Improved crosstalk.
• Reduced requirement for low source impedance bus drive.
• No need to ensure unselected bus signals are connected to 0V.
• No need to adjust the mix amp gain depending on the number of channels

The only disadvantage is that you cannot easily include a mix group fader directly across the bus before the mix amp as you can with passive mixing. This means the mix group fader has to come after the the VE amp. As this cannot drive an output directly you need another amplifier to buffer the fader and drive an output transformer. The only alternative would be to have a transformer directly after the VE amp and use a 600 ohm balanced attenuator at the output as the mix group fader.

On balance, VE mixing has a lot to recommend it so I plan to incorporate it into the 8 tracker build,

Which brings us rather neatly onto the topic of output transformers and their ability to drive 600 ohm loads. One of the most expensive items in the EZTubeMixer design is the input and output transformers so I have been looking for lower cost alternatives. Edcor is well known for its low cost transformers so I got a couple of their XSM 2.4K/600 transformers (listed at $12.97 compared to the £25 for the Carnhill VTB2291). The XSM 2.4K/600 is physically smaller than the VTB2291 normally use in the EZTube Mixer but it is rated at 2.5 watts which is well over what we need. The primary inductance measures 28H at 100Hz and the secondary is 9.8H at 100Hz, both satisfactory values. I have given this transformer a thorough test and I am pleased to report it performs almost identically to the VTB2291. In both cases, the EZTube output amplifier runs out of steam before the transformers do so I am happy to recommend this as a cheaper alternative. At +26dBu into 600 ohms the distortion with the Edcor was 0,49% and at +29dBu it became 1.1%. These results are identical to the performance of the VTB2291. Frequency response was -1.8dB at 20Hz but this is entirely due to the 4.7uF output capacitor which is 3dB down at 14Hz with the reflected secondary load.

To facilitate testing of new 6U and 3U modules and PCBs I have built a sub-rack based test rig.

On the left is space for two 6U modules and the top right row can accommodate four 3U modules. Underneath the top 3U section I have tacked a couple of input/output panels. One panel connects directly to bottom connector of  the left most 6U module to which its on board output transformer is wired. At the moment the right hand panel connects to the right most 3U module on the top row. As 3U moduels have no room for an output transformer I have mounted an Edcor one to the sub-rack behind the panel.

Insert Options

All modern mixers provide some means of inserting external devices into the signal path.  These can often also be used as direct outputs and sometimes these are provided separately. When I was at Neve back in the 70s, the provision of inserts was straightforward; wherever there was a balanced output you could add an insert. Direct outs were not common, but if you wanted one you could connect it to the same place. All Neve consoles had channel amplifiers containing mic preamps followed by EQ. These were balanced in and out so the first available option for an insert  or direct out was at the channel output. So this was a pre-fader, post EQ insert. After the mix bus, the group amplifiers were balanced out so there was another opportunity to add inserts. No special arrangements were necessary. All you had to do was bring out the necessary signals to the patch bay

With the advent of project mixers, inserts and direct outs began to appear at other points in the signal chain. The result is that inserts and/or direct outs may nowadays be required or preferred at almost any point in the signal chain.The diagram below illustrates the places where an insert and/or direct out could be included in the Mark 3 tube mixer.

The top most option shows an unbalanced insert directly after the mic preamp and before the fader and EQ. This type of insert was first popularised by the project mixer manufacturers. It can also be used as a direct out by inserting a mono jack so that the internal signal flow is not interrupted. An alternative direct out can be taken after the EQ gain make up amplifier. You could even switch the direct out between the insert point and the output of the EQ gain make up amplifier. The only disadvantage of this approach is that the insert is unbalanced.

The second diagram shows a balanced version of the first example. An output transformer has been added to the output of the mic pre and an input transformer has been added before the fader. This type of insert is often used as a direct out during tracking so the signal can be recorded dry. The fader and buses are used for creating a monitor mix during tracking. At mixdown, the inserts are used to add effects.

The third diagram moves the balanced insert to post the EQ gain make up amp so it is now post fader, post EQ. In its unbalanced form, this is the typical insert arrangement for project mixers like the Mackie CR1604 for example. Again the insert can be used as a direct out for tracking but this time the fader controls the level recorded to the track and the level sent to the mix. This allows EQ to be applied to tracks as they are recorded but prevents the channels being used as a monitor mix.

The last diagram is the same as the third but moves the fader to after the insert. This means the fader can now be used for monitor mixing but the recorded signal can also include EQ. This the classic Neve configuration.


The Mark 3 mixer channel amp can handle the first two configurations unaided using its  two built in amplifiers .

The third diagram requires an additional input transformer, space for which is provided on the new channel amp PCB. There are then two options for the source of post fader signal to be sent to the pan and AUX send controls. One is from the output of the second amplifier which would be post fader but pre-insert. This amplifier is designed to drive buses so this does not present any problem. The second choice is from the secondary of the input transformer. This means any device plugged into the insert would be expected to be able to drive the bus. In most cases this should not be a problem but, of course, the drive capability cannot be guaranteed. Also, if passive mixing is used, the crosstalk via the pan control depends in part on the driving source impedance. As long as any inserted device has a low output impedance this should not be a problem. Provided these limitations are realised, this scheme is fine for simpler mixers like the 8 Tracker which have a limited number of amplifiers.

The fourth option also requires an input transformer but has one added complication. The post fader drive to the buses now comes directly from the fader itself. However, the fader does not have the drive capability needed to drive the buses so an extra post fade amplifier is needed for this scheme to work. In the one metre monster this is not a problem as there is plenty of space for additional Twin Line Amplifiers (TLAs) in the rear facing 3U sub-rack. Only one amplifier is required for each channel so one TLA will do two channels. In the 8 tracker, amplifiers are at a premium so this is perhaps not a viable option. The pre-fade sends can come pre or post the insert. The post insert, pre-fader send only needs to drive AUX send buses so pan pot crosstalk is not  an issue.

Clearly, the one metre monster can be configured to provide any of the insert/direct out options described above. In the 8 tracker block diagram shown in a previous post, there is no provision for inserts although each channel does have a post fader, post EQ direct out. The 8 tracker could be configured to provide the pre fader, pre EQ unbalanced or balanced insert point shown in the first two options above. It could also be configured to provide the post fader, post EQ insert shown in the third option, using the extra input transformer on the new channel amplifier board, provided the limitations noted above are acceptable. It is the last option that it has problems with as this requires additional TLAs. These would have to be housed in the 3U meter bridge. However, all the available space in the meter bridge is currently accounted for, so to include the TLAs needed to do this type of insert, something will have to be deleted. Here's a reminder of the current layout of the 8 Tracker meter bridge:

On the far right is the TLA used for the master buses and on the far left is the one used for the AUX sends. These we have to retain. We could drop the four direct inputs to provide four  of the extra amplifiers we need but then there is no way to bring in FX returns to the mix bus except via a channel line input and we need all of those for 8 track mixdown. Similarly, unless we are talking about a one person project studio where only one track at a time is recorded, then we cannot really afford to lose the PFL or talkback and anyway these provide us only with two amplifiers. For such a project studio we could perhaps compromise and lose two direct inputs as well as the PFL and talkback amplifiers to get four amplifiers and this would still leave two direct inputs for FX returns. However, we still only have half the amplifiers we need. Although the meters and monitor is only a passive panel, experience in building the EZTubeMixer prototype has demonstrated that this cabling takes up a lot of space so we cannot really put any TLAs in this section. However, as this section is passive, it does not have to be in the meter bridge; it could be anywhere, So one possibility is to house the meters and the monitor circuits in a kind of power bump on top of the meter bridge. That would free up two slots in the meter bridge (one would still be needed for all the cables to pass through) which, if fitted with a couple of TLAs would give us the final four channels we need.

At this point in time, I have not decided which type of insert to include in the first 8 tracker I build. However, it will be a fully balanced one so the additional input transformer will be needed on the channel amps.

8 Tracker Meter Bridge

The 8 Tracker attempts to squeeze a lot of functionality into a small space. The larger part of the mixer is taken up with the 8 channel amps and their faders. All the other functions of the mixer need to be contained in the 3U meter bridge running along the top. Here is a suggested layout for the 3U top sub-rack:

There is no room for a master slider fader so a large rotary knob is used instead. As I am right handed I have placed this in the right most module of the meter bridge. This module also contains a Twin Line Amp (TLA) that provides the mix bus amplifiers for the master left and right outputs. Another TLA is used for the AUX mix bus amplifiers. This is shown at the far left of the meter bridge. Next to the AUX sends are the four direct inputs housed in two modules. Each module uses one TLA which can handle two direct inuts. On the right, next to the master fader, is another TLA which provides the PFL bus and the talkback amplifiers. This leaves just three slot widths in which to house the meters and monitor section. A Sifam AL29SQ meter will fit in one module width with enough room below for its switching so the two central module spaces are used for the meters. This leaves one slot to the right of the meters for the monitor section. As there is a lot of cabling behind the meter and monitor panels and some of it is interconnected, it probably makes sense to combine these into a single three module wide panel. The monitor panel will probably need its own small board to accommodate the 10K:600 monitor isolation transformers.

Frame, Module and the 8 Tracker

In the last post I promised an update of the monster frame once the last parts arrived. Here it is:

As it takes up rather a lot of room in my small workshop, it is currently being used as storage space while I work on other aspects of the Mark 3. However, I think you can see the how the 4U fader section works and at the rear there is a 9U section. The bottom 3U of this would house the rear mounted line amps and the 6U above would be used for connectors.

One of the other aspects I have been working on is the module electronics. I have now built the first prototype channel module PCB as shown below:

All the basic enhancements over the EZTubeMixer PCB are included:

• Provision for  Molex KK connectors for internal wiring
• Ground reference the gain resistors so each amplifier can be used as a virtual earth mixer
• Moved the mic line switch to be before the phase switch so the latter works on both line and mic
• Amplifier gain preest pots moved to be accessible from the front panel
• Tubes moved so as not to foul the enclosure
• Output transformer (direct out|) included
• Provision for several EQ types

The next step is to test the board and then finish one of the EQ boards and try that out.

In the meantime I have been giving further thought to the first mixer I will build with the new modules. I said a few posts back that I would use the 500 frame as a test bed for the new modules and I still intend to do this. However, it occurred to me that it might just be possible to make a compact 8 track all tube mixer based on this frame so I have been investigating that possibility further. Also, the 500 frame is pretty simple; there is no room for lots of modules so it might even make a fairly feasible DIY project. Here is by first stab at a block diagram of the 8 tracker:

At the top left are the 8 identical channel modules. These have similar facilities to the modules I am building for the EZTubeMixer demo in that the mic pre, EQ and routing are all in the one channel module. The main changes are:

• A permanently connected PAN control instead of the smart pan
• Addition of pre/post switching for the two AUX sends
• Addition of pre-fade listen (PFL) switch to allow level setting and quality checking
• Addition of a channel mute switch. This mutes the signal feeding the PAN and AUX sends but leaves the direct out unaffected.

At the top right are the audio bus amps for  the two main buses (L & R), the two AUX sends and the PFL. The AUX sends and PFL would normally have simple rotary controls but you would expect the main bus output to be on a slider fader. Unfortunately, as we only have 8 slots, and all 8 are used for channels, there is no sensible place to put a main bus slider fader. So I have decided to use a single large rotary knob in the meter bridge section for the master fader. It will be on the front panel of the Twin Line Amp used for summing the buses so it can easily be a couple of inches in diameter. As I am right handed, I think I will place it in the right most module in the meter bridge. It's a compromise but a small one I think.

At the bottom left is the talkback and direct inputs. The talkback uses half of a Twin Line Amplifier (the other half is used for the PFL bus amplifier). A simple pot sets the gain and I envisage a front panel mounted XLR into which the talkback mic can be plugged (much the same way as we used to do it at Neve). Two momentary action switches control the routing of the talkback. One switch routes it to the L & R buses for slating and the other routes it to AUX1 for artiste communications. When either switch is operated, a dc signal labeled 'DIM' is produced which is used to operate two relays in the monitor section to reduce the level fed to the control room monitors to prevent howl round.

The direct inputs are modelled on the AUX Returns of the EZTubeMixer demo and indeed they can be used for  FX returns. Each direct input has level and pan controls and uses half of a Twin Line Amp. I think there is enough room in the meter bridge section of the 500 frame to house four direct inputs which means there will be 12 inputs available at mixdown.

Lastly, at the bottom right are the monitoring and metering sections. I envisage just a pair of meters placed centrally in the meter bridge of the 500 frame. At present I am thinking of using a couple of Sifam AL20SQ meters for a nice retro feel.

Each meter has its own 5 way switch that selects between four of the direct outs and one AUX send thus covering all eight channels and both AUXes between them. The output from these two switches feeds a second switch that connects the meters either to the 5 way switches or straight to the main bus outputs. This allows the meters to monitor the main output and easily be switched to read other outputs when desired. The signal fed to each meter is also fed to the monitor section.

The monitor section is quite straight forward. It consists basically of a three way monitor select switch and a level control. The monitor select switch selects between the the main L/R buses, a two track playback input or the meters. Normally you would listen to the main buses and occasionally switch to the two track playback to check sound quality. Selecting the meter position allows you to listen to whatever the meters are connected to. The output of the monitor select switch goes via a couple of relays to a 10K:600 transformer and thence to the monitor level pot. The 10K:600 transformer is there to ensure the monitor section applies a minimal load to the signals it monitors. It also means a simple passive 1Kohm level control can be used on the secondary for the monitor level control (this is pure Neve).

The relays affect what reaches the monitors and its level. RY1 and RY2 are the PFL relays. When a PFL button on a channel is operated, the PFL dc operates the relay and automatically routes the output of the PFL bus amp direct to the monitors so it can be heard. RY3 and RY4 are the DIM relays which apply a 20dB attenuation to the monitor level. These relays are operated by the talkback switches and, as mentioned above, are designed to prevent howl round when talkback is used.

As it stands, this design uses 8 channel modules and 5 Twin Line Amps for a total of 39 tubes.

The next step in the design is to sketch out some front panels for the modules and the components of the meter bridge.

Building The Mixer Frame - in 100 taps

The basic frame of the Mark three consists of a number of one metre wide sub-frames attached to a pair of wooden cheeks.  Each sub-frame consists of a par of end plates with standard extrusions bolted between them. The bolts fit into tapped holes at the ends of the extrusions. The standard extrusions you buy are already tapped so assembly is simply a matter of bolting together the end plates and extrusions. The one metre lengths provided by Schroff are intended for people who want to cut them to non-standard lengths and then tap the ends themselves. This means the standard one metre lengths are supplied untapped. Since I had purchased ten front and ten back rails, each needing four taps each and ten centre rails each needing two taps each this meant I was looking at cutting 100 taps. Regular readers of my blog will know that mechanics is not my favourite occupation so the prospect of doing 100 M4 taps by hand was a bit daunting. I don't even have a set of taps or the right diameter drills and the last time a tapped a hole was during my apprenticeship in 1969. I needed someone else to do this for me.

Fortunately I found a small engineering company, North Norfolk Engineering, less than half a mile from my home. Bernie, the owner, was very helpful and soon had a simple jig set up to tap my extrusions which were completed in a few days. I now know what it costs to tap these holes so I have asked Schroff via their UK distributor to quote for supplying one metre extrusions ready tapped. It is interesting to note that Holger told me that Fischer tapped his one metre extrusions for free.

The next step was to assemble a one metre sub rack just to make sure the taps worked, so I built a 3U by one metre sub rack. I attached a 2 slot mother board to the back and plugged in one of the mix amps from the EZTubeMixer project:

Next we need to attach the end plates to the wooden profiles in the correct positions and use them as a template to drill further holes on the wood. The end plates are attached to the wooden checks using insert nuts which look like this:

To use these, you drill a 7mm hole in the wood and screw the insert nut in using an M6 Allen key. The insert nut is 13mm deep and I am using 18mm thick wood so the nut can be screwed in so it is flush with the surface of the wood. This allows the end plates to be fitted snugly against the wooden end cheeks. Once the end plates are fixed to the wooden cheeks, you need to drill holes in the wood everywhere there will be a bolt attaching an extrusion. The really nice thing about having the end plates already fixed to the wood is that the end plates can be used as a template. No measuring required, just drill  a pilot hole through each existing hole in the end plate with a 4mm drill. Then you remove the end plates and counter bore the holes will a 10mm drill which makes the holes wide enough to accommodate the heads of the bolts. After that it is just a matter of assembling each sub-rack and then bolting its end plates to the wooden cheeks. This is the result:

I didn't have enough insert nuts to fit all the sub racks; I have some on order so I'll update the picture in the next post. However, you can see the main sloping 9U section. The upper 6U is for the channel module and you can see I have fitted an EZTubeMixer module to check it plugs in OK to a 2 module motherboard. Beneath the channel module is a 3U space into which the routing module will plug. At the back you can just see the rear facing 3U unit (the one pictured earlier). When I get some more insert nuts the 4U fader nose can be fitted to the front and the 6U rear panel section to the back.