Saturday, 19 April 2014


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.

Wednesday, 26 March 2014

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.

Sunday, 23 March 2014

If Only..

I am not a fan of mechanics but there is the odd occasion when things go just right. Tonight I needed to drill the 16 holes for the VTB2291 transformers mentioned in the last post. I discovered I could set them two inches apart and start 3 inches from one end of the panel. Fortunately the ears of the transformer have slots rather than holes so you don't need to be dead on with the positioning of the holes - got to be a good thing with my mechanical skills. I marked it all out with a ruler and a Sharpie and whacked each intersection with my trusty hole punch. I then placed a transformer at each position to make sure all the punch marks lined up with the slots in the transformer mountings and then drilled sixteen 4mm holes. I de-burred them on both sides with my old three eighths inch drill and attached the transformers using a locking nut on each to make sure they held firm. Job done in less than an hour. If only all mechanics were that straightforward. Here's the result, ready to be wired up:

Saturday, 22 March 2014

Quart Into A Pint Pot

The other day I completed wring up the mic and line inputs and the pre-fader unbalanced insert points including the wiring to the four channel faders themselves. The next job was to wire up the direct outs. These are fed from OUT2 of the channel amp, via a VTB2291 transformer to the direct output XLR connectors at the rear. As there are four channels there are four transformers. There are also the two AUX send outputs and the two main bus outputs that need output transformers for a total of eight. Initially, I intended to mount these eight transformers on the bottom of the Rackz enclosure towards the back. The main sub-rack is plenty high enough to fit the transformers in and there are some convenient ventilation slots in the bottom of the enclosure that could double up as fixing holes for the transformers. However, I had not allowed for the mass of wiring I had just put in. Here is a picture of the bottom of the enclosure looking through from the back:

You can clearly see there is no room to fit the transformers on the right hand side because of all the wiring and when the AUX send and returns and the main outs and the 2 track playback all get wired in there will be no room on the left either. Clearly the transformers can no longer fit there. So where to fit them?
Above is a view of the partially completed rear panel. The mic, line and insert connectors are annotated. Above the connector panels is a good sized black panel marked 'X'. This panel can be removed and behind it you can see the top of the rear of the sub-frame:

The bottom of the panel is 80mm from the sub-rack and the top of the panel is 130mm from the sub-rack. This should leave plenty of room to fit the transformers to the rear of this panel. The only downside is that the channel output transformers are now quite near the channel input transformers.Fortunately they are at 90 degrees to each other so hopefully there will be no magnetic feedback! Here is how the transformers will fit onto the panel.

Now all I have to do is drill 16 holes on the steel panel, bolt on the transformers and wire them up. Did I mention I hate mechanics?

Monday, 10 March 2014

Looks a Bit Like a Mixer

It's funny how sometimes you end up doing some things twice. Recently I wired up the PSU to the sub-rack and fitted them both into the 19 inch Rakz box. For the heater wiring I used heavy gauge mains cable. Nice and thick, low voltage drop but not exactly flexible. Thing is, you do need some flexibility in the power wiring so you can move things about for wiring up and testing other parts of the mixer. So you make the power cables somewhat longer than they need to be to provide this flexibility which is just what I did. The trouble is they do not sit very neatly in the bottom of the mixer because the heater wiring is so stiff. What you really need is some high current but flexible wire for the heaters.

This evening I was starting to wire up the XLRs at the back of the mixer and I was looking for the cable to do this. In this search I came a cross some Van Damme loud speaker cable that I had forgotten I had purchased. Examining it showed it had really thick cores, ideal for heater wiring, but the cable as a whole was really flexible. It seems that in a moment of lucidity some time ago I had ordered a few metres of this cable for just this purpose. So that is how I ended up re-wiring the heaters this evening. Here is a picture showing the sub-rack removed from the main chassis and laid on its back on top of the VU meter bridge:

The black and red twisted pair is the HT supply, the blue and white is the phantom power and the thick blue cable is the new heater cable. Next I replaced the sub-rack into the chassis in order to check that the cable curled up neatly and it did.

Lastly I connected to the mains to make sure the supply worked and lastly I fitted all the finished modules and ran a burn in test to make sure the power supply does not get too hot. In fact it just gets luke warm. At last it is beginning to look like a real mixer.

Sunday, 9 February 2014

The Last Two Panels

The last two panels for the demo mixer arrived the other day from Frank Röllen. The first one is the 19 inch wide panel that sits at the bottom of the mixer and houses the channel and master rotary faders. I made the scales nice and big so I can use big fader knobs. Here's a picture with the knobs laid in position on top of it:

The second panel is the new design for the REDD EQ channel amplifier. Originally this mixer was going to have two Helios and two Pultec style EQs but the more recently designed REDD EQ has proven very popular, and it sounds really good too, so I decided to have just one Pultec channel and use the REDD EQ instead of the other one. Here is a picture of the front panel attached to the channel amplifier and REDD EQ PCBs:

The REDD EQ is designed so it can be fitted into a 3U high module if desired. This means the 3 band controls are closer together than on the Helios and Pultec EQs. This makes the legend a little cramped and forces you to use quite small diameter knobs. Maybe I am just getting old and blind but I prefer big knobs and large lettering. The control above these three, the one with the blue shaft, is the frequency control for the mid boost/cut. As this EQ uses 100% stepped controls, the response is flat when the controls are zeroed so strictly speaking you do not need an EQ in/out switch. However, this is very useful for comparing the effect of an EQ so I have included one. You will also notice  I have fitted the Smart Pan controls. All I need to do now is to fit the AUX send pots and then I can wire up the channel.

The REDD EQ is based on two EQs used in the famous EMI REDD 47 consoles as used to record many of the Beatles tracks.  The design came about completely by accident.  Here is a quote from a thread I started at :

"I was recently asked if I could design an EQ that worked like the 'pop' and 'classic' EQ plug ins that used to be used in the REDD47 consoles and predecessors. Using the curves published in 'Recording The Beatles' I came up with a circuit based on a stripped down and modified Helios 69 EQ with a switch to select 'pop' or 'classic'. I was then asked if I could not make it so both the the 'pop' and 'classic EQ curves were available at the same time. In doing this, a very strange thought occurred to me. The Helios treble EQ is virtually identical to the 'classic' EQ curves (the frequency, step size and gain range are identical) as is the bass cut except for the frequency it works at. The bass boost is very nearly the same but tweaked from a shelf to a bell curve. The clincher is that the 'pop' 4.7KHz peaking EQ is a stepped version of one of the Helios 69 mid boost frequencies.

I then realised that Dick Swettenham, who designed the Helios 69 EQ, had previously worked at Abbey Road studios in the service and design departments so he must surely have had a deep understanding of the innards of the REDD EQ.

I am sure you can now see where I am going with this. Is the Helios 69 EQ simply a modified and expanded version of the REDD EQ?

I simulated a cut down version of the Helios 69 EQ (pic attached)  and it is surprisingly easy to get curves very close to those of the REDD EQ.

What do you think/know??"

From this realisation it was a relatively simple step to incorporate the frequencies of the EMI RS127 'brilliance box' into the EQ as additional mid boost/cut frequencies. This was made much easier by a groupdiy colleague Dylan who measured the curves of the Abbey Road plug-in both for boost and cut. The result was a new hardware EQ that incorparates both types of classic Beatles EQ in one package. If you are interested in how the design developed from there you can read it here.

For anyone interested in how the REDD EQ sounds you can listen here. The right channel is a track being played via the REDD EQ and the right channel is me saying what the EQ settings are.