Improved Tube Headphones Amp

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ruffrecords

Well-known member
Joined
Nov 10, 2006
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Location
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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

 
> none of them could I get much more than 125mW output without a spray of harmonic distortion.

That's not (just) topoogy.

That's not enough tube.

> 3V rms into 32 ohms (280 mW)

Round-up. Near 5V peak in 32 ohms. That's 150mA peak. (4.2V would be 131mA.)

Take something in every repairperson's kit: 12AU7. 6K plate impedance. To get 150mA peak you need 6K*0.150A= 900 volts across the tube, plus the 5V across the load, plus 10% for bias control. Only 32/6,032 or 1/188 of the input power gets to the load. Dependng on assumptions you could have 40W dissipation in a 3W plate.

ECC99 will pass near 40mA at 100V, 65mA at 150V, perhaps 130mA at 224V. With 500V supply and cheating the heater insulation rating 280mW is possible. You still get 1/50th of input power to the load.

You want a large-signal Rp MUCH-MUCH closer to 32 ohms.

6AS7/6080/ECC230 will pass 130mA with less than 50V drop. It is a fat pig which needs huge drive voltage.

Some of the TV H-sweep tubes will do it with much less drive, but now two fat pigs per channel.

There is an opportunity for positive-grid operation, but you can't be there with the toploguy and devices you outline.

Be curious to see how you pushed ECC99 to 280mW in 32r at small distortion.
 
PRR said:
> none of them could I get much more than 125mW output without a spray of harmonic distortion.

That's not (just) topoogy.

That's not enough tube.

> 3V rms into 32 ohms (280 mW)

Round-up. Near 5V peak in 32 ohms. That's 150mA peak. (4.2V would be 131mA.)

Take something in every repairperson's kit: 12AU7. 6K plate impedance. To get 150mA peak you need 6K*0.150A= 900 volts across the tube, plus the 5V across the load, plus 10% for bias control. Only 32/6,032 or 1/188 of the input power gets to the load. Dependng on assumptions you could have 40W dissipation in a 3W plate.

Which is precisely why I am NOT trying to do OTL but in fact have a 12:1 transformer between the tube and its load (the Sowter 8665A I mentioned). So, using your own figures, 5V peak on the secondary becomes 60V peak on the primary and 150mA peak becomes a mere 12.5mA peak, neither of which is a problem for an SRPp ECC99 running at 20mA quiescent.

ECC99 will pass near 40mA at 100V, 65mA at 150V, perhaps 130mA at 224V. With 500V supply and cheating the heater insulation rating 280mW is possible. You still get 1/50th of input power to the load.

You want a large-signal Rp MUCH-MUCH closer to 32 ohms.

6AS7/6080/ECC230 will pass 130mA with less than 50V drop. It is a fat pig which needs huge drive voltage.

Some of the TV H-sweep tubes will do it with much less drive, but now two fat pigs per channel.

There is an opportunity for positive-grid operation, but you can't be there with the toploguy and devices you outline.

Be curious to see how you pushed ECC99 to 280mW in 32r at small distortion.

I used a transformer!

Cheers

Ian
 
I haven't seen SRPP with NFB this far. Nice trick. I have built that very same SRPP circuit as a line amp/line driver and it sounds good alone, except I used 6N6P which is essentially the same tube. And yes the distortion is of the "invisible" 2nd harmonic variety.

that 220pF bypass cap is strange, a high passed SRPP. I guess you are shaping the NFB harmonics with it, but I can't quite understand why it would work.
 
Here's my first shot at the PCB. It is 3.5 inches wide and 5.1 inches high. I have yet to work out how to get the tracks in for the 12AX7 heaters and I might add a decoupling option on the 12AX7 HT.

Cheers

Ian
 

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I mis-read "nearly 200GBP worth of transformers" as a desire to avoid such a rich-man solution.

> I used a transformer! ... 12:1

OK, 4K6 is an excellent load a 2K5 tube.

A ponder: not all HPs are 32. 60 ohm would improve performance, but there are some hungry 250 ohm cans.

Personally, while there are many fine hobbyist tube can-amps, for pro use I lean to line drivers or even small loudspeaker amps. (Had a Fisher 2*20W which, with NFB tweak, was very smooth and robust; also a sweet tuner-tube all push-pull 300mW I hand-built from war surplus servo chassis.)
 
PRR said:
A ponder: not all HPs are 32. 60 ohm would improve performance, but there are some hungry 250 ohm cans.

Indeed and one reason I like the Sowter with its differing secondary taps. I have run the numbers through for five different headphones:

AKG K701 which are 63 ohms with a sensitivity of 93dB/mW
Audio Technica ATH-M50 which are 38 ohms with a sensitivity of 99dB (presumably dB/mW)
Etymotic ER4S which are 100 ohms with a sensitivity of 100dB/mW
Sennheiser HD455 which are 52 ohms impedance with a sensitivity of 94dB/mW
Beyer DT150 which are 250 ohms with a sensitivity of 97dB/mW

Theoretically the first four can all be driven to over 115dB SPL by the 12:1 tap and the DT150s will need the 6:1 tap

Personally, while there are many fine hobbyist tube can-amps, for pro use I lean to line drivers or even small loudspeaker amps. (Had a Fisher 2*20W which, with NFB tweak, was very smooth and robust; also a sweet tuner-tube all push-pull 300mW I hand-built from war surplus servo chassis.)

This amp is for inclusion in a all tube mixer for a client (the first three headphones mentioned above are his) so it needs to be relatively compact and not too power hungry otherwise I would agree small loudspeaker amps can do a first rate job.

Cheers

Ian
 
PRR said:
Personally, while there are many fine hobbyist tube can-amps, for pro use I lean to line drivers ....
A similar thought had occurred to me but in reverse. This circuit will happily drive 280mW into whatever load best suits the secondary tap. A 5K:600 transformer would make it a pretty good line diver capable of +24dBm into 600 ohms at low distortion.

Edit. I meant also to mention that I think people tend to forget that +26dBm is 400mW.

Cheers

Ian
 
I was thinking maybe your layout can accommodate an option for a wcf config.
Pretty minimal changes I think.

And then it could be used for a whole range of things.

In a dual fender style guitar pre, I added a pp wired srpp line drive to one channel and a wcf to the other
(for various gain staging reasons -- too much gain or to little!).
Was very interesting to listen to the differences though the parts were virtually identical.


Something like this would have been a great utility pcb.

Cheers
 
alexc said:
I was thinking maybe your layout can accommodate an option for a wcf config.
Pretty minimal changes I think.

And then it could be used for a whole range of things.

One of the early configuration I tired was a WCF but, despite its incredibly low output impedance, is drive capability was not especially good. However, it did have the advantage that it could be directly coupled to the first stage thus making NFB a whole lot simpler.

I'll have a look at tweaking the PCB to accommodate that.

Cheers

Ian
 
I have been very busy lately but I have some time now to finish this PCB. What has prompted me is a direct enquiry about the availability of the PCB by a group member so I told him I would revive this thread.

So, if there's enough interest I'll finish the layout and get some quotes. PCB price will depend on quantity but it is looking like it can be done for about £20 if there is enough interest.

Any interest?

Cheers

Ian
 
I have finished the PCB layout now. I am happy to do a run of these if there is enough interest. Price will of course depend on quantity but if we get more than 10 the price will start to fall below 25GBP.

Cheers

Ian
 

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