I build a headphone amp recently but I find the sound rather harsh. This is the circuit. Its driving some DT559 cans. Just for the record I would not use an OPT again. The result is ugly and the mains transformers couple into the OPT producing hum. Toroid's are better here but if you have the HT volts a WCF circuit would be a solution. The frequency response is fine. So the question is are headphones meant to be driving from low output impedance or are better on higher drive impedance?
Does it sound better with a passive HT filter in place of the series regulator? Or running it open-loop? I know these are audiophool prejudices, but in output stages, they are sometimes justified.
Poor performance in a headphones amp with an OPT is most often down to the transformer itself. Building a phones amp with an OPT is no different to building a power amp - you need to take care with the positioning and orientation of the mains and OPT to eliminate hum pickup.
A WHite CF is not the solution. Although it has an incredibly low output impedance, the WCF has poor drive capability simply because it cannot pass enough quiescent current.
Cheers
Ian
Nick I measured the THD and it was < .1% at 2Vppk which is very loud. In terms of quality the amp is pretty blameless. The bode plot is fine - however the OPT does not contain a screen and I had to reverse all the primary and secondary connections to get stability due to primary to secondary capacitance. I used a toroidal 240V - 12V mains transformer 1VA6 for OPT in the prototype and that did seem to work better. I problem now is the mains transformer is toroidal and the OPT EI. The NFB removes most of the hum once the valves have warmed up.
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The WCF can be made to work but only with +200V HT and ECC99 so throw power at the problem. If I plug my headphones into the audiolab SS amp they sound much more natural. There's a peak at 5KHz to my ears but not on measurement.
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People are missing the point and not addressing your question... which is the key issue. This likely has nothing to do with the circuit topology, power supply, etc.
Source impedance drastically affects the way headphones sound. And there is no "right" answer.
There is an old IEC spec for headphones that specified a source impedance of 100 ohms. That is very high by modern standards. Some EU headphones were designed to this spec... and if you drive them with a low source impedance they sound very harsh indeed. On the other hand many modern headphones - especially planars - NEED a low source impedance or they sound dead. It's all about damping, I guess.
I don't know about your headphones or what the source impedance of your amp is. But just stick some resistance in series and see what happens.
Pete
Source impedance drastically affects the way headphones sound. And there is no "right" answer.
There is an old IEC spec for headphones that specified a source impedance of 100 ohms. That is very high by modern standards. Some EU headphones were designed to this spec... and if you drive them with a low source impedance they sound very harsh indeed. On the other hand many modern headphones - especially planars - NEED a low source impedance or they sound dead. It's all about damping, I guess.
I don't know about your headphones or what the source impedance of your amp is. But just stick some resistance in series and see what happens.
Pete
I wonder...
What is the primary inductance of your OPT?
If the inductance is too low, the fairly high ra of the ECC88 may cause lack of bass response, which subjectively could sound 'harsh' (too trebly).
Perhaps?
You did say the frequency response is fine, so if this is a dumb question, please forgive the intrusion.
What is the primary inductance of your OPT?
If the inductance is too low, the fairly high ra of the ECC88 may cause lack of bass response, which subjectively could sound 'harsh' (too trebly).
Perhaps?
You did say the frequency response is fine, so if this is a dumb question, please forgive the intrusion.
Does that IEC standard specify what the impedance of the headphones should be in this case?
I've swept it down to 10Hz and the response is flat. Ok I get the 2Vppk above 20Hz but that sounds fine. It more likely to do with source impedance. I'll try a series resistor and have a listen. Certainly the WCF will be fine with series resistance.
Added 100R in series with each headphone output and what a transformation - the metallic sound has gone. So thanks. So half decent quality headphones should be driven from 120R or so not 0R. I guess this is what in line with what modern amps do. I did not know this I assumed it was like speakers.
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No - it is just one of the conditions for the standard measurement setup.
I think when this was originally written most headphones were between 100 and 600 ohms, though... 120 ohms source into an 8 ohm IEM probably isn't good.
Pete
So the HD599 are quoted as 50R, they work a lot better with 100R in series. I also tried this off a modern op-amp driver and this gives the same result. I can only assume there's some sort of transducer resonance at around 5KHz which gets damped out with 100R or something like that. When designing the WCF version I'll allow for a 100R output resistor.
That's one of the biggest reasons why people prefer one headphone amp over another. Some headphones like 100 ohm source impedance, some 0.
When designing a DIY circuit with low Zout (like with opamp buffers) I usually put resistors there that can be selected by the builder, by ear, with their preferred headphones.
Tube amps vary all over the place. OPT and NFB gets you pretty low. OTLs (especially no NFB amps) are usually much higher.
Pete
The design in #1 has about 20dB NFB and a 10K to 15R transformer so I would expect only a ohm or two of output impedance.
Maybe a current FB rather than voltage FB would be interesting, so the NFB would be based on the current through the headphones.
OK so rewired for CFA feedback. Remove headphones and replace with 33R resistors to ground. Remove grounds from secondary's of OPT and wire headphones tip and ring to there instead. So now feedback measures voltage across 33R and hence current in transformer and headphones rather than voltage across headphones. I now have most of the power back and it sounds just great. Very interesting.
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Mostly for Newbies . . .
The headphone amplifier that has a series 100 Ohm resistor at the output is "A Mix Mode".
It is not a voltage source (0 Ohms output impedance).
It is not a current source (infinite output impedance).
It is . . . a resistive impedance signal source.
Now you have 3 things to try on your headphones:
A very low output impedance 'voltage source'. (much lower impedance than the headphones).
A very high impedance output impedance 'current source'. (much higher impedance than the headphones).
And a medium impedance amplifier with a resistive output impedance, somewhere less than an order of magnitude impedance different than the headphone impedance.
(> 1/10 of headphone impedance, and < 10x of the headphone impedance).
Your Sound May Vary
Your Mileage May Vary
Have fun, and listen to the music
The headphone amplifier that has a series 100 Ohm resistor at the output is "A Mix Mode".
It is not a voltage source (0 Ohms output impedance).
It is not a current source (infinite output impedance).
It is . . . a resistive impedance signal source.
Now you have 3 things to try on your headphones:
A very low output impedance 'voltage source'. (much lower impedance than the headphones).
A very high impedance output impedance 'current source'. (much higher impedance than the headphones).
And a medium impedance amplifier with a resistive output impedance, somewhere less than an order of magnitude impedance different than the headphone impedance.
(> 1/10 of headphone impedance, and < 10x of the headphone impedance).
Your Sound May Vary
Your Mileage May Vary
Have fun, and listen to the music
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