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Improved Tube Headphones Amp

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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 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 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.


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).

Circuit of one channel attached.

Cheers

Ian
 

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If you want to try something different, I suggest looking at local negative feedback around the first stage. One example of this can be found in the second stage of this amp. It works quite well and has low distortion as well.

Looks like good work, though. What made you go towards a parafeed (just curious)?

I think my next project will be a headphone amp, hence, I'm exploring the field a bit.

~Tom
 
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Is it wrong place to use local NFB?
Most of the distortion is generated at the output stage.

Some people don't like global negative feedback. If the majority of the distortion is caused by the output stage, then local feedback around the first stage wouldn't help on the distortion, obviously. I threw it out there as a suggestion as it sounded like the original poster was happy with the distortion but wanted lower gain. My brain isn't firing on all cylinders today so I could have misread or misunderstood, though.

~Tom
 
I'm wondering how you might fare with a white cathode follower output stage? I've used one successfully out to several hundred mW, but can't recall much about the linearity. (except that it was reasonably good) I used 5842/417As in the output. No global feedback though..

Of all the other topologies I tried before the SRPP, the White follower was the best, but it was still short on output power and at over 2V rms into 32 ohms it produced a spray of harmonics. Incidentally, the 5842 does not really have a large enough Vhk to be used in a White follower or SRPP circuit.

Cheers

Ian
 
Last edited:
Hello Ian,
While you are messing around try a Single End Triode headphone amplifier.
I tried this transformer EDCOR Electronics Corporation. GXSE10-16-1.7K using a triode wired 6BQ6GTB (cheap) with near 180 volts on the plate. With 32 ohm headphones attached the SET sees ~ 3400 ohms load. The Triode wired 6BQ6GTB has an Rp ~ 650 ohms. The SET 6BQ6GTB will output near 2 watts. At 0.1 watts into the headphones the distortion is much reduced.
I tried this setup on the breadboard and liked it a lot.
DT
All just for fun!
 
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Ian, I had just one other questionably useful thought and I was wondering whether you have been able to characterize the OPT independently of the stages driving it.

Your comments about distortion spectra left me wondering, and I'm assuming that the lower the driving source impedance the lower the measured distortion until the transformer starts to saturate. This is just a SWAG on my part and possibly all wet.

One other probably very ill-advised thought I am is having is to increase the value of the output coupling caps by a couple of orders of magnitude (are they really 4n7 or are they 4u7 - I can't tell, chalk it up to my laptop screen and my eye sight) and include them inside the global feedback loop. This might be a very bad idea, but I had some success with this approach in somewhat less demanding applications. (I was using 4.7uF output caps)

The numbers you are getting in any event look pretty good.
 
If it were me, I'd use half the ecc99, or put the two sides in || if the chassis is cut for two tubes, and load it with a CCS and cap couple it to the transformer like it is. Or, maybe a triode strapped EL84 per side with a CCS load. I'd also probably use a different driver (and CCS load it), but mostly because that seems like a lot of gain, and lose the feedback.
 
Hello Ian,
While you are messing around try a Single End Triode headphone amplifier.
I tried this transformer EDCOR Electronics Corporation. GXSE10-16-1.7K using a triode wired 6BQ6GTB (cheap) with near 180 volts on the plate. With 32 ohm headphones attached the SET sees ~ 3400 ohms load. The Triode wired 6BQ6GTB has an Rp ~ 650 ohms. The SET 6BQ6GTB will output near 2 watts. At 0.1 watts into the headphones the distortion is much reduced.
I tried this setup on the breadboard and liked it a lot.
DT
All just for fun!

That's an interesting approach. I would be interested to know what distortion values you measured.

Cheers

Ian
 
Ian, I had just one other questionably useful thought and I was wondering whether you have been able to characterize the OPT independently of the stages driving it.

I have not done that yet other than to measure the distortion of the output stage with a 5K load and then with the transformer - they were near as makes no odds identical. The Sowter transformer is made with an M6 and mu metal core so its inherent distortion should be low.

Your comments about distortion spectra left me wondering, and I'm assuming that the lower the driving source impedance the lower the measured distortion until the transformer starts to saturate. This is just a SWAG on my part and possibly all wet.

I have not seen any evidence of transformer saturation and the Sowter transformer is rated to 500mW and I am only asking less than 300mW of it so it should not be anywhere near saturation. But you are right, transformer distortion is lower the lower the source impedance. That said, it is only really a problem at low frequencies and I have not made any measurements below 1KHz.

One other probably very ill-advised thought I am is having is to increase the value of the output coupling caps by a couple of orders of magnitude (are they really 4n7 or are they 4u7 - I can't tell, chalk it up to my laptop screen and my eye sight) and include them inside the global feedback loop. This might be a very bad idea, but I had some success with this approach in somewhat less demanding applications. (I was using 4.7uF output caps)

The numbers you are getting in any event look pretty good.

I think it is my poor writing. They are definitely meant to be 4.7 micro Farads. Someone else asked about the cathode bypass cap wondering why it was 220pF when it is really 220uF. My mus and pees look very similar!

Cheers

Ian
 
Kinda has that look with the cap coupling into the transformer. Am I mistaken?

I guess I can rephrase my question: Why cap coupling into the transformer? What's the advantage over, say, a standard SE output stage?

~Tom

The principal advantage is that the transformer does not have to carry any dc current. This means it needs no gap and, for a given core size, can have a much higher inductance with a correspondingly better low frequency response.

Cheers

Ian
 
If it were me, I'd use half the ecc99, or put the two sides in || if the chassis is cut for two tubes, and load it with a CCS and cap couple it to the transformer like it is. Or, maybe a triode strapped EL84 per side with a CCS load.

Am I right in thinking you would use a solid state CCS?

I'd also probably use a different driver (and CCS load it),

Why would you want to CCS load the driver?

but mostly because that seems like a lot of gain,

I am not quite sure what you mean by 'a lot of gain'. Are you referring to the 0.46V in for 2V out or something else?

and lose the feedback.

So, without the feedback how would you keep the distortion down?

Cheers

Ian
 
Hello Ian,
The SET headphone amplifier and AudioTester did not make it onto the bench at the same time. I did not measure distortion.
Speaking of distortion, also trying to blend a subjective and objective approach please tell us what output voltage do you measure for your SRPP when the headphones are on your head playing at a level that will not damage your hearing? Much less than 2 volts RMS?
Have you measured distortion at a comfortable listening output?
I suspect that you have arrived. Your reported distortion is excellent as is.
DT
All just for fun!
 
Hello Ian,
The SET headphone amplifier and AudioTester did not make it onto the bench at the same time. I did not measure distortion.
Speaking of distortion, also trying to blend a subjective and objective approach please tell us what output voltage do you measure for your SRPP when the headphones are on your head playing at a level that will not damage your hearing? Much less than 2 volts RMS?
Have you measured distortion at a comfortable listening output?
I suspect that you have arrived. Your reported distortion is excellent as is.
DT
All just for fun!

I have not done that yet because this design is for a client with three types of headphones none of which I have and not all of which are 32 ohm types - another reason for using the Sowter transformer with tapped secondaries.

The three headphone types are :

AKG K701 with a sensitivity of 105dB/V so 3V will give about 115dB SPL. These phones are also specified at 93dB/mW with a maximum input power of 200mW (another 23dB) or 116dB SPL maximum. They are 63 ohm impedance so I think the distortion at 3V rms will be even lower than I measured at 32 ohms.

Audio Technica ATH-M50. These are 38 ohms impedance with a specified sensitivity of 99dB (no units given) Assuming this means 99dB/mW then 100mW will give 119dB SPL and 100mW at 38 ohms is 1.95V rms so again the current design should be comfortably able to drive them.

Etymotic ER4S (in ear phones). Impedance 100 ohms, sensitivity 100dB/mW and maximum input 90mW which corresponds to 118.5dB SPL. For 90mW at 100 ohms we need exactly 3V rms and since we are driving into 100 ohms rather than 32 then again I think the current design will be fine.

My own headphones, which are all I have listened on so far, are an old pair of Sennheiser HD455. These are 52 ohms impedance, sensitivity 94dB/mW. 200mW would give 117dB SPL and need 3.2V rms. The impedance is slightly above the 32 ohms test load but the voltage required is higher so again I think the current design should be fine. They certainly sound bloody loud to me!

I also have a pair of Beyer DT150 which have an impedance of 250 ohms and a sensitivity of 97dB/mW. 100mW into these will give 117dB SPL and need a voltage of 5V rms. This is clearly beyond the 32 ohm tap but the next tap gives twice the voltage so should be able to provide 6V rms into 128 ohms. 5V rms into 250 ohms is therefore not a problem for it.

The big question is what are normal listening levels? and I guess that is very user dependent. I don't know if there any figures available for that sort of thing. Either way, the distortion is directly proportional to output level so a 10dB reduction in output voltage will give a 10dB reduction in listening level and result in 10dB less distortion so I guess we could typically be in the sub 0.1% distortion range if typical listening levels are under 100dB SPL.

Cheers

Ian
 
AKG K701 with a sensitivity of 105dB/V so 3V will give about 115dB SPL.

* * *

Audio Technica ATH-M50. ... 100mW will give 119dB SPL

* * *

Etymotic ER4S (in ear phones). Impedance 100 ohms, sensitivity 100dB/mW and maximum input 90mW which corresponds to 118.5dB SPL.

* * *

Sennheiser HD455. These are 52 ohms impedance, sensitivity 94dB/mW. 200mW would give 117dB SPL and need 3.2V rms.

* * *

The big question is what are normal listening levels? and I guess that is very user dependent. I don't know if there any figures available for that sort of thing. Either way, the distortion is directly proportional to output level so a 10dB reduction in output voltage will give a 10dB reduction in listening level and result in 10dB less distortion so I guess we could typically be in the sub 0.1% distortion range if typical listening levels are under 100dB SPL.

Of course there are standards. 80-85dB is a normal listening level with headphones. Here are some US government standards for sound exposure: http://www.cdc.gov/niosh/docs/98-126/pdfs/98-126.pdf

As you can see, your 117dB phones can be tolerated on that standard for all of 18 seconds per day. There is no reason to design to that level.

Am I right in thinking you would use a solid state CCS?

Yes

Why would you want to CCS load the driver?

Because it is lower distortion, more stable, keeps the power supply out of the signal current loop, increases PSRR by 100dB or more, and sounds better.

I am not quite sure what you mean by 'a lot of gain'. Are you referring to the 0.46V in for 2V out or something else?

I'm not sure what you are not sure about. The ECC83 has a very high mu which will lead to a hair trigger volume pot at reasonable listening levels.


So, without the feedback how would you keep the distortion down?

With a low distortion circuit. All the feedback is going to do is inject the back EMF into the driver circuit and lead to all sorts of nasties. So THD may be low, but the distortion spectrum is worse that without feedback. If you are adding lots of feedback, you might as well just use an opamp -- cheaper, easier, and will probably sound better too. A CCS loaded triode is a remarkably low distortion circuit. There is no sense mucking it up.
 
Push Pull Symmetry

Hello Ian,
Thanks for sharing the big view of the range of possible headphones to could be used with your SRPP headphone amplifier. For conversation sake it seems like a one size fits all approach. My experience with SRPP’s is that the circuit resistors need to be tuned to the particular load impedance to optimize output voltage and current symmetry. (Symmetry := The top tube should push as hard as the bottom tube pulls and vice versa.)
DT
All just for fun!
 
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