Final Touches and a New Home

It is amazing how many small details need to be attended to in order to complete a mixer. You think it is just about done and then you find a whole host of small jobs to do. For example, there is the need to label all the connectors at the rear of the mixer. If you have money to burn you can design your own rear panels complete with engraved labels. For those of us with less money we can buy some pre-punched but not labelled panels and label them ourselves. But how to make the labels? They need to be both neat and durable. At first I looked at some strips intended for 19 inch rack panels with the idea of cutting them down to size but you still have to make the labels to fit into them. In the end I decided to invest in a thermal label printer. I bought a Brother P-Touch 1005 which was half price in a sale a W.H. Smiths. The good thing about this printer is it can do vertical printing an one or two lines. This turned out to be ideal for labelling groups of four XLRs. Here is the result:

It is a pity I did not get them exactly vertical but the beauty of these labels is you can easily do them again. The two columns of print are very handy, especially when the left and right connectors have different functions.

Another job is deciding on a colour scheme for all the various knobs and fitting them all. Although this is only a 4 into 2 mixer, it has 60 knobs! I decided to select colours for each of the three bands of the EQs, for the pan controls, the AUX sends and the main gain switch. Here is the result:

As you can see I picked yellow for the pan controls, red for the high EQ controls, blue for the mid and grey for the low. The AUX send and returns are red, the channel gain is blue and all the faders and monitor controls are black.

The next job was meter illumination. The meters I fitted have lamps built in. Fortunately they need a 12V supply so I just hooked them up as an additional load to the heater supply. Unfortunately, the lamps in one of them did not work so I had to change the meter - at some point I may take the non working meter apart and fix the lamps.

Now I need to let everyone in an a little secret. About a year ago I was approached by a guy in Canada about building him a custom four channel mixer. I told him about this mixer and he decided to buy it if I would add some specific mods for him. This I agreed to do. One of the mods was the unbalanced inserts after the mic pre and pre the fader and EQ.  Another modification was the addition of a trim control on the main bus outputs. The idea here is to allow the bus amplifiers to be driven harder so as to produce more 'tube tone'. The trim control is a simple fixed range level pot using the 'Neve trick' to allow a single pot to alter the level on a transformer balanced output. I used a twin 1K pot to allow both left and right outputs to be altered together. A series resistor in the cold leg limits the pot range to about 12dB. With a nominal +4dBu output, this means the bus amp can be forced to output up to 12dB higher i.e at +16dBu. Since the output transformer has a 2:1 ratio, this means the output stage actually runs as high as +22dBu which will cause the amplifier to produce a small amount of 2nd and 3rd harmonic distortion. even at this level, the amplifier has plenty of headroom left so distortion will increase with level for quite a way before clipping occurs.

And of course, now I have sold the mixer, there was one more job to do - write the user manual including how to fit all the tubes and get it going once it had arrived. I am pleased to say it took only five days to reach Canada, it arrived all in one piece and, apart from one cable that came loose from its connector and at tube that needed to be wiggled in its socket to stop some scratchiness, the mixer seems to be working fine.

Headphone Fun

I had always planned that the EZTubeMixer would include a headphones amplifier which would of course be an all tube amplifier. In fact, the development of the headphones amp preceded the development of the EZTubeMixer. I had been working on the design of an all tube headphones amplifier for quite some time but with little success. The problem area was the output stage. Here is the post I wrote to diyaudio.com back in 2011:

"For many months I have been trying to develop an all tube headphones amp that can drive a wide range of headphones to a high level with low distortion. The designs have been based on a transformer output using a Sowter 8665A to match the tube stage to headphones of different impedances. With nearly 200GBP worth of transformers in the design it was more in the audiophile price bracket than the budget one. I have tried cathode followers, mu followers and White followers, open loop and and with NFB but with none of them could I get much more than 125mW output without a spray of harmonic distortion. So I abandoned the task for a while.

Then, back in May, I came across Pete Millett's SRPP design using the ECC99. I had not tried an SRPP design because of their reputation for high distortion so it was almost in desperation that I knocked up a prototype. Sure enough it had quite high distortion but I noticed two interesting things. First it was quite capable of delivering 3V rms into a 32 ohm load (280mW) which is enough to drive just about any headphone to an almost painfully loud level. Secondly, although it produced 3% THD at this level, the harmonics fell away rapidly, most of the distortion being 2nd and 3rd harmonic. This was in stark contrast to other designs I had tried.

That's as far as Pete Millett's design goes. There is a preceding amplification stage but no NFB so although it can provide a high output, the distortion is rather high. It seemed to me it might benefit from some NFB, so I designed a single triode stage based on one half of a 12AX7 and closed the loop from the SRPP output back the the 12AX7 cathode. Unfortunately, because there is a dc blocking capacitor in the feedback loop the closed loop gain rises at very low frequencies and it is not possible to apply enough NFB to reduce the distortion significantly without instability.

The classic way to ensure unconditional stability in tube NFB circuits is to ensure there is only a single low frequency pole in the loop which as often as not means the NFB network has to operate down to dc, i.e no series caps in the NFB loop. This can often be problematic from the point of view of setting the dc conditions in the tubes and this case was no exception and a compromise had to be made in slightly unbalancing the SRPP stage to achieve it. Despite that, the results are good:

2V rms into 32 ohms (125mW)

2H = 0.18%
3H = 0.032%
4H = 0.006%

Higher harmonics were immeasurable

3V rms into 32 ohms (280 mW)

2H = 0.28%
3H = 0.063%
4H = 0.014%

Other harmonics immeasurably low.

For 3V rms output into 32 ohms an input of 0.46V rms is required.


The prototype was built on a die cast box as a chassis but I am now well on the way with a PCB layout which looks as though it will fit onto a board 3.5 inches by 5 inches (with the transformers external)."

And here is the schematic:

As you can see there is NFB at dc from the output of the SRPP stage back to the cathode of the preceding 12AX7 stage. This works because the closed loop gain needs to be quite high to compensate for the relatively large step down ratio when driving 32 ohm headphones (22dB), so even with36dB of closed loop gain the overall gain is only about 16dB. The open loop gain is about 56dB so there is about 20dB of NFB, enough to reduce the distortion tenfold.

Since the SRPP really needs elevated heaters in order not to exceed the Vhk of the ECC99, I soon realised I could raise the cathode voltage of the 12AX7 and reduce the closed loop gain to as little as 6dB. Some alteration to the bias of the 12AX7 is necessary but other than that the modification is quite simple. An interesting property of this topology is that as the closed loop gain is reduced, so is the open loop gain because the 12AX7 stage gain drops as the un-bypassed cathode resistor is increased. The net effect is that the amount of NFB is fairly constant as are the stability margins. I then realised that by selectively bypassing the 12AX7 cathode resistor you could vary the closed loop gain, at the same time altering the open loop gain and maintaining stability. This became the basic topology of the Eurochannel mic pre and the Twin Line Amp. The only difference is that the ECC99 is replaced by a 6922. This has a higher mu than the ECC99 and increased the open loop gain to just over 60dB allowing for closed loop gains of up to 40 dB with low distortion. The addition of a 2:1 transformer allows the Eurochannel to easily output in excess of +22dBu into 600 ohms.

So, the headphones amplifier design had been around for a long time. I had designed a PCB and I had built and tested a PCB prototype. I also tested it with some low cost Edcor transformers and it performed very well. All I had to do now was to fit this into the EZTubeMixer enclosure.

That's when all the problems began.

As I have mentioned in previous posts, there is not a lot of room in the Rackz enclosure.  Just about all the available space has been used already and the only possible place to fit the headphones amp PCB and transformers is in the meter bridge behind the VU meters. Unfortunately, the meter bridge has only two surfaces available onto which to mount the PCB; the top and the back. I tried it in the top but, because the ECC99s get quite hot, they heat up the other components on the PCB to over 60 degrees Celsius. Mounting on the rear was a little better but the heat from the tubes still rose right past the output caps raising their temperature to over 60 degrees Celsius. The ideal solution would be to mount the PCB on the bottom surface of the meter bridge but it does not have one. So I had no choice but to indulge in some mechanics to provide the necessary surface (and you know how little I like mechanics)

To cut a long story short, I found the lid of an old aluminium box was just the right size and also had the necessary fixings, so I attached it to the meter bridge (see picture below):

I then fitted the PCB to the new bottom surface:

Then added in the transformers so I could test it:

And lastly fitted it into the EZTubeMixer:

As you can see, the heat from the ECC99s rises away from the PCB so the components remain relatively cool. The rising heat does, however, warm the top surface of the meter bridge and after a couple of hours of running the top of the meter bridge reaches a little over 40degrees Celsius. This is quite warm to the touch but definitely not hot.

Monitor Matters

The next stage of the mixer build is the wiring and commissioning of the monitor panel. As I mentioned in a previous post, it is surprising how much wiring there is even in a small mixer and half of it seems to be associated with the Monitor Panel. The monitor in this mixer is quite straightforward. It consists first of a 6 way rotary switch that selects one of the four direct outputs or the two AUX sends. The output of this switch feeds a three way switch that selects between the 6 way switch, the main stereo bus  and a two track playback input. The output of this switch then feeds the two VU meters and two pots. One pot is the monitor level control which feeds the monitor out sockets at the rear of the mixer and the second one is the headphones level pot.

Nothing really complex but it turns out to be a lot of cable. There are six cables going to the 6 way switch plus four more for the bus and  two track playback going to the 3 way switch. Then there are two outputs to the meter, two to the headphones amp and lastly the two monitors outputs themselves for a total of 16 cables. All these need to be run under the main mixer sub-rack to the rear of the mixer and the headphones and meter cables need to reach even further to the meter bridge. Here is a picture of the completed wiring of the Monitor Panel:

To help route the cables neatly to the rear of the mixer they are gathered together in bundles using tie wraps. To make sure I could identify the cables when they reached the back of the mixer I colour coded the tie wraps as follows:

• The bundle of 6 going to the 6 way switch were coded using the resistor colour code to represent 1 through 4 direct outputs with 5 and 6 repenting AUX 1 and 2 respectively.

• The remainder are all stereo pairs so each pair was colour coded and a the far end of the cable, the right hand channel had a red tie wrap added to it.

Commissioning the monitor section actually highlighted a number of other faults which I was then able to correct. In fact the monitor section is very useful for overall mixer  and channel commissioning. I first checked the inputs to the 6 way switch. I used the known working Helios 69 module in these tests. First I checked the four direct outputs by placing the Helios 69 module in each of the four channel slots and making the appropriate selection of the 6 way switch. Then I checked the two AUX sends.

Next I checked the main left and right buses. This was useful because it showed up several problems with pan controls. It turned out the pan on the Helios and one other channel amplifier and also on one of the AUX returns were not working as expected. In all three cases a simple wiring error was found and corrected. Checking the AUX returns was also useful as it highlighted another issue. The AUX return is completely passive. The signal passes through a transformer, a level control and lastly a pan pot before being fed to the left and right buses. There is perhaps one dB of loss in the input transformer so with the pan hard left or hard right, the level on the bus might be a dB or so lower than the level from a channel. However, it turns out to be over 9dB less. The reason for this is the step down transformer used for the direct outputs. This steps down by 6dB so to get a nominal 0dBu level from a direct out means the channel amplifier is outputting +6dBu and this is the level it feeds to the bus. If we allow a 1dB loss in the direct out transformer then this, plus the 6dB extra level and the 1dB loss in the AUX return input transformer, accounts for 8dB of the 9dB difference.

The solution is to use larger value bus feed resistors for the channels. Currently the bus feed resistors are set at 47K. If we increase them to 150K then the bus level from the channels drops by 9dB. Normaly this would make the noise level about 9dB worse, but since the channel level is already 6dB higher, the overall effect is only a 3dB worsening of noise. The extra signal loss on the bus is made up by the bus amplifier which has plenty of gain in hand as this is only a 4 channel mixer.

The next problem to be highlighted by the Monitor Section was with the AUX sends. Each one worked fine on its own but, with the amplifier gain set to 60dB and with both AUX sends turned up full, some sort of oscillation occurred as could be seen by the VU meter trying to wrap itself around the end stop. Checking the wiring from the AUX send pots to the bus resistors in the channel amp revealed the wires were laid flat along the top surface of the PCB. There was a good chance this would couple the signal from the AUX sends to other parts of the amplifier circuit. Simply re-routing all the bus feed signals in the air an inch or so above the PCB cured the problem. In future it would probably be best to use screened wires for the feeds from the pan and AUX send controls to the bus resistors. This modification has been applied to all the channels amplifiers.

The last problem identified by the monitor section was with the meters themselves. I tested the 2 track playback input which is simply a direct connection from the connectors at the rear of the mixer via the switches to the meters. Feeding the same level into both the 2 track playback inputs should give identical readings on the meters but it does not. One meter is about 1.5dB below the other. I checked this again by monitoring the direct and AUX sends where the one signal is sent to both meters and got the same result. Clearly one meter is faulty so the next step is to feed in exactly +4dBu and see what each meter reads.

Hissssss and Pinggggg

When designing a tube mixer you always aim for as little noise as possible from the microphone pre-amplifier. The  inherent noise in tubes makes it very difficult to design a low noise mic pre-amplifier without an input transformer. Even with a 1:10 ratio input transformer, it is difficult to achieve the very low noise levels that can be achieved by the best semiconductor designs. One reason for  this is that the noise generated by tubes can vary a lot from tube to tube due to small  manufacturing differences. For this reason it is usually necessary to select the input tube of a mic pre for lowest noise.

In addition to their inherent noise, tubes have one other drawback when used for mic pres and that is microphony. When you tap them, an audible sound can sometimes be heard. Again, the degree to which this happens varies a lot from tube to tube of the same type so again it is necessary to select tubes for low microphony.

Now I have cured the hum issues with the demo mixer I am in a position to measure the noise of the mic pre and also to test various tube types for noise and microphony. I used the Helios 69 input channel with the EQ switched out. First I fed in a signal at -60dBu into the microphone input , set the gain to maximum (60dB), the channel fader fully up and then adjusted the second stage gain for 0dBu at the channel direct output. This means there was then a total of 60dB of gain from the mic input to the direct out. To measure noise I then plugged in a shorting XLR with pins 2 and 3 shorted,into the mic input. I then used my Lindos audio test set plugged into the direct out to measure the noise levels at the output.

A brief aside is necessary here to understand the results that will be presented. Noise in microphone preamps is usually evaluated relative to the thermal noise generated by the resistance of a typical microphone that might be plugged into it. A typical mic has a source resistance of around 150 ohms and this seems to be the standard by which mic pres are measured today. The rms thermal noise due to a 150 ohm resistor at 20 degrees C in a 20KHz bandwidth is close to -131 dBu. If we amplify this by 60 dB then the noise we measure at the output should be 60dB higher or -71dBu. The noise measured will in fact be higher than this due to the additional noise generated by the mic pre itself. If you measure the rms output noise and subtract the gain then you get the noise level at the input of the mic pre. This noise level is called the equivalent input noise or EIN for short. In a perfect mic pre with a 150 ohm source, the rms  EIN should be -131dBu. In a real mic pre you would expect it to be a few dBs higher.

Notice I said the 'rms noise' from the resistor. Most measuring systems cannot directly measure rms noise. The simplest are average reading and are calibrated to read the correct rms value when presented with a sine wave, but noise is not a sine wave so these can produce inaccurate readings for noise. In addition, weighting curves are often added to the output signal supposedly to more accurately reflect the response of the ear. The A weighting curve is one of the most popular and is often favoured by marketing departments because it tends to improve the EIN figure by a few dB. There is an international standard for measuring noise (ITU-R 468) which includes a curve that adjusts for the response of the ear and also includes a quasi-peak detector to account for the ear's sensitivity to short, spiky bursts of noise that simply would not show up on an rms reading device.

The bottom line is that the value of EIN you get depends a lot on the type of measuring device you use and the weighting you apply to it. This article by the Institiute of Sound and Communications Engineers lists the various methods in common use and the differences in the the results obtained. Many manufacturers quote EIN figures without giving any further information about how the measurement was made and what weighting was applied. It is not surprising that many of these quote an EIN close to -131dBu. The more honest ones achieve figures in the -128dBu region. For comparison, back in the 70s, Neve used to quote the EIN of its mic pres as -126dBu.

The tests I conducted used my Lindos test set which meausres noise to ITU-R 468 including the quasi-peak reading meter. This gives a more realistic figure for EIN but it can be anything from 4 to 7dB worse than the figure obtained using the simpler and conveniently weighted measuring methods. If you look at the published specifications for classic tube mic pres you will generally find that they quote an EIN figure of 'better than -120dBu' but usually without specifying how the measurement was made. We would like to do better than this. In the EZ Tube Mixer mic pre, the noise is determined principally by the 12AX7 tube used at the input. I tried three different types of 12AX7 in my tests:

• 12AX7LPS made by Sovtek in Russia. I tested 8 of these.
• 12AX7EH by Electro Harmonix and also made in Russia. I tested 9 of these.
• 12AX7WA another variant made by Sovtek. I tested 11 of these.

All three types of tube are in current production. Each tube was tested in the same channel amp at 60dB of gain. To beat the -120dBu EIN of the classic tube mic pres we need to achieve an output noise level of better than -60dBu. After measuring the noise level I tapped each tube with my finger nail to see how microphonic it was. The Lindos test set has a built in loudspeaker preceded by a 50dB amplifier when it is making  noise tests. So there is a total of 110dB of gain before the speaker. This means you can clearly hear the hiss when making noise measurements and you can easily hear your finger tapping on the tube. With this test I divided tubes into three categories.

• Microphonic - these tubes were judged to be so badly microphonic as to be unusable. With tubes in this category you got a very loud pinging noise when they were tapped and you could clearly hear your finger gently rubbing the glass envelope of the tube.
• Good - these tubes produced an audible ping but it was less than 20dB above the noise level
• Very good - these tubes produced a barely audible ping which was less than 10dB above the noise level

All of these are of course subjective measurements which is why I did  not feel able to split them into more than three categories. Note that none of the tubes was completely lacking in microphony. Every tube tested was microphonic to some degree. The results were as follows:

12AX7LPS - 6 of the tubes measured -63dBu for noise; one measured -62dBu and one was -58dBu; 2 tubes were microphonic, 5 were good and 1 was very good.
12AX7EH - 2 of these measured -63dBu for noise and  7 measured -62dBu; 8 had good microphonics and 1 was very good
12AX7WA - 9 of these measured -63dBu for noise and 2 measured -62dBu; 1 was microphonic, 2 were good and 7 were very good.

In conclusion, the 12AX7WA was the best all round with many measuring -63dBu noise level and plenty of very good from the microphonic point of view.  Both the 12AX7LPS and the 12AX7EH turned in  good noise performances at around -62dBu to -63dBu but their microphonic performance was not as good as the 12AX7WA.

I think the 12AZ7WA is the best choice from the noise and microphonics point of view. This is interesting because the internal construction is much shorter than the EH and LPS versions so perhaps this makes them more rigid and accounts for their better microphonic performance. What is also interesting is that I had previously discounted them for use as mu followers because they had rather higher levels of intrinsic distortion compared to other versions of the 12AX7, the LPS in particular producing very low levels of distortion.

The EZ Tube Mixer mic pre topology uses an SRPP output stage which I expect to be the main contributor to the overall distortion level. The 20dB or so of negative feedback in the mic pre is designed to reduce the SRPP output stage distortion by tenfold. This means I do not expect the distortion of the input stage 12AX7 to make much of a contribution to the overall mic pre distortion. Just to make sure, I measured the 1KHz distortion for the WA, LPS and EH types fitted in the first stage. The distortion measurements were made with the gain set to 30dB and with an input level of -20dB. This means the input to the 12AX7 is 0dBu and the output from the direct out transformer is at +10dBu. This means the SRPP output stage is running at +16dBu since the transform steps down by 6dB.

The results were remarkably consistent. The LPS type produced 0.094% distortion, the EH type produced 0.096% and the WA produced 0.094% distortion. This simply confirms that the vast majority of the distortion occurs in the output stage as expected.

It seems that the best noise, microphony and distortion is obtained with selected tubes of the WA type. The typical -62dBu measured output noise represents and EIN of -122dBu. This meets our target of better than the classic tube mic pres and if we add only the minimum 4dB to convert this measurment into its rms equivalent we are achieving an EIN in the region of -126dBu which is as good as a classic Neve.

Hummmmmmmmmmm

There comes a time in the build of almost any audio DIY project when you encounter a hum problem and the EZTubeMixer is no exception. After fixing channel 3 it was time  to make some measurements of each channel. I fitted channel 1 and channel 2 with the two complete channel amplifiers and tested them using my Lindos test set. Frequency response and noise level were both good. I then plugged in the other two channel amps. As soon as I powered up and the tubes had warmed up there was a loud buzz at about -30dB from the channel I had last tested. Looking on the scope the buzz was at exactly 100Hz but half the waveform was dead flat and the other half was a high frequency. I could not trigger well enough on that portion of the waveform to see exactly what frequency. I disconnected the HT supply and I could plug in all 6 modules and it was fine which at least demonstrated it was probably not the regulated 12V supply hooting. I took out all the first stage tubes (12AX7), plugged in all six modules and there was no oscillation so it was either getting into the first stage or the first stage was creating it. With four modules fitted and only one fitted with its 12AX7 the oscillation occured. I added a 10K across the HT to pull another 30+mA from the HT supply and repeated the above test. No oscillation. Adding a 12AX7 to a second module the oscillation returned.

Grounding is always a potential cause of hum problems. The power supply for this mixer is situated at the front and the dc supplies travel by cables to the motherboards at the rear. I needed to make a connection from the HT 0V at the power supply to the mains safety earth tag which is next to the mains inlet connector at the rear. My normal rule is to take a wire direct from the HT- to the  safety earth tag but, as the end of the HT run through the motherboards is right close to this tag I decided to connect from the motherboard HT 0V to the safety earth tag. I did not think it would make any difference. However, when checking the heater elevation voltage at the heater supply I connected a DVM from the heater -ve to a different earth connection that is used for the screen between windings in the HT transformer. With just two 12AX7s fitted, connecting the DVM to make this measurement stopped the interference dead in its tracks. Aha, I thought, grounding problem. So I disconnected the lead from the motherboard HT 0V to safety earth and made a connection direct to the the PSU PCB HT 0V. Switched on and it was dead quiet. So I added a couple more 12AX7s and tried again with 4 modules. I was very disappointed to find the buzz had returned.So, just to make sure it was not a problem with my Lindos test set, I unplugged it and tried again. With all four channels fully populated with tubes there was no buzz. Plugging in the signal source from the Lindos increased it a little but plugging in the output brought it back fully. I disconnected the mixer output from the the Lindos and plugged in a passive VU meter - and the buzz returned. With both the input and the output connected there was full buzz but with just one there was only a small buzz. This happened to all four channel amp line inputs and direct outputs. Puzzling but at least I was getting some sort of a feel for the cause.

I then added the last two modules (the bus amps) but was again disappointed to find that with absolutely nothing connected to the mixer, the buzz was there. So it was nothing to do with anything connected to the mixer, it was the mixer itself. So, thinking it was probably the channel amps themselves that were oscillating, I ordered some 33pF capacitors to place across the 47K feedback resistor to see if that got rid of the high frequency oscillation.

Just to be absolutely sure it was not the linear heater supply causing the problem, I temporarily replaced it with a switched mode power supply intended to supply LED lamps. It is rated at up to 15 amps so it should be able to cope with the heater inrush current. Sure enough, it coped perfectly well with the heaters from all six modules but unfortunately the buzz was still there. At least I now know that this little SMPSU, which only cost £15, is perfectly capable of supplying the heater power for a small mixer.

 In the meantime I contacted my friend Holger and told him about the buzz problem. He very quickly got back to me to say he had had a similar problem which he had cured by adding 10nF decoupling capacitors across the HT supply on each two channel backplane PCB. I did not have any 10nF 400V capacitors to hand but I did have some 220nF ones. I quickly attached one across the HT supply on each of the motherboards, replaced all six modules and switched on. To my great relief, all sign of the buzz and high frequency oscillation had vanished. I plugged in my Lindos test set and still there was no sign of buzz.

I then re-connected the linear heater supply and checked that was OK. I also added a safety ground link from the panel on which the power supply is mounted to the mains safety earth. Lastly, I refitted the power supply to the mixer and repeated the tests. I am pleased to say that even with the PSU inside the mixer there is no sign of hum or buzz. It is so free of hum that I was able to measure the EIN of the mic pres and I found one of the 12AX7s was quite microphonic - looks like these will need to be selected by hand.

The only unanswered question is what caused the buzz in the first place? Since decoupling the HT supply at the motherboard cured the problem this suggests  it is an HT supply impedance issue. Possibly the inductance of the HT cabling and the power supply smoothing caps is to blame. The silly thing is, very early on in my career (over 40 years ago) I learned the importance of decoupling power supplies where they enter a PCB. I even had it on my standard list of things to look for in design reviews. What did I not do on the EZTube mixer mic pre board?? Fortunately the motherboard decoupling does the trick but for future versions of these boards I think there is going to be on board decoupling.

Output Transformer Fault

Now I have all the output transformers wired up it was time to wire them the the XLRs, plug the Molex KK connectors into the backplane PCBs and test each channel's direct out. A couple of hours soldering and it was time for testing. Channel 1 was fine, as were channels 2 and 4, but channel 3 had a very low output. A quick check of the secondary dc resistance revealed it was very different from the other channels. So I removed the transformer panels to check if I had made a wiring error, but channel 3 was wired exactly the same as the others. Further dc resistance checks on the primary and secondary windings revealed that the VTB2291 transformer was wired backwards. I had wired to the correct numbered tags so it looked like the tag panel had been put on back to front by the manufacturer. Fortunately I had a spare transformer so I removed the faulty one and replaced it with the spare one. Channel 3 now functioned as expected.

I removed the links from the faulty one and again checked the winding dc resistances which confirmed the tag panel had indeed been fitted the wrong way round. I contacted Colin at Audio Maintenance who quickly supplied a replacement transformer.

Transformer Panel Wiring Complete

After fitting the large output transformers to the panel, the next step was to wire them up. First you need to wire the links on both the primary and secondary windings to set the correct ratio (2:1 in this case). Two links are required on the primary as the two windings are wired in parallel. They are the red and black wires shown on the picture below. The secondary needs only a single link to wire the two windings in series. This was done with yellow wire. The primary sides are connected to unbalanced amplifier outputs so these need a single core screened cable terminated in a two pole Molex KK connector to plug into the backplane PCB. These are the grey cables you can see.The secondaries are balanced and use a twin core screened cable (black cables). Only the hot and cold are connected to the transformer. The screen is not connected at the transformer end but only at the XLR end. The XLRs are not connected yet. They will be connected after the panel has been mounted in the mixer.

The four cable pairs on the right are the the direct outs of the four channel amps. The four on the left are the master bus L/R and AUX1/2 send bus outputs which come from the two Twin Line Amps fitted next to the four channel modules.

I have a simple convention I use to identify the hot and cold wires in the Van Damme twin screened cable I use. The two wires have white and blue insulation so I make the white hot  and the blue cold for obvious reasons.