"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)."
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.
