ruffrecords
Well-known member
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 this was most certainly not a poor man's headphones amp!! 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 base 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.
I'll post a circuit when I have drawn it neatly. I am 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).
Cheers
Ian
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 base 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.
I'll post a circuit when I have drawn it neatly. I am 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).
Cheers
Ian