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.