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Old 25th November 2010, 12:16 AM   #1
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Default Please help me improve my headphone amp

Hello,

A few months ago I built a clone of a headphone amp which I had previously owned. I think everything went ok and the amplifier sounds pretty good. However, after a lot of reading I think the design is far from optimal. Particularly the input stage, where a 6SN7 is driven with too little current and voltage.

I have drawn a couple of schematics using CCS loading and LED biasing of the input tube, one dc coupled as the original and one ac coupled. The first one improves a bit the input stage but still drives the tube and also the LED with too little current. The AC coupled design allows me to increase the current from 2ma to 5ma and to increase the plate voltage from 48 to 90 volts but now I have a coupling cap. Which one is better?

Thanks in advance for your help.

Diego
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File Type: png Extreme 1CH (ccs).png (21.0 KB, 300 views)
File Type: png Extreme 1CH (ccs AC coupled).png (22.6 KB, 285 views)
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Old 25th November 2010, 02:49 AM   #2
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Enhancement MOSFET easy to find.
MJK self-serving gyrator thingie.
Bootstrap back from output cathode.

LED and good caps for sure! If they do
their job well, elkos ain't hurting nuthin.
If they don't, big elko is extra insurance.
C2 might also benefit to be 470+47+1?
Ditch C2 if LED curve don't bother you.

Omitting gate and grid stoppers for unity
followers was a gamble, but probably OK.
Anodes are tied to B+ and don't swing,
further stabilized by cathode feedback.

Accidentally forgot gate protect zenier!
That oversight still needs fixed for sure.

I have not properly mathed any resistor
values, nor plugged in triode models to
run a sim of what the currents might be.

It might be possible to direct couple the
gate of M1 to V2's cathode? If V2's neg
grid bias is higher than M1's positive gate...
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Last edited by kenpeter; 25th November 2010 at 03:10 AM.
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Old 25th November 2010, 03:28 AM   #3
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Not quite sure if this bias would work or not?
Again I forgot to do something with C2.

M1 + D2 could be a BJT just as easy...
Then V2 would need only a very small
bias to exceed 0.66V, and/or the CCS
POT2 could then trim up to a slightly
higher value. What say MJE340?
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Last edited by kenpeter; 25th November 2010 at 03:41 AM.
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Old 25th November 2010, 03:45 AM   #4
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In the amplifier I built without the CCS the currents are 1.8 ma through the 6sn7 and 70 ma through each pair of 6as7 triodes.

I forgot to mention I'm gonna use the amp with Sennheiser HD800s which have an impedance of 300, so that's why I want to use just the biggest MKP I can fit in.
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Old 25th November 2010, 03:58 AM   #5
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Last edit tonight, I promise...
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Old 25th November 2010, 04:01 AM   #6
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I lied... You had a 1M to GND up front. I'm thinking to put that back in.
But on the wiper side of POT1, just incase it loses contact. Bias won't
be as likely to run away...
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Old 25th November 2010, 04:13 AM   #7
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I've lied yet again! D1 LED wants more current to be on best part of the curve.
Bleeder resistor from B+ or something, then maybe you can totally ditch C2.

By the way, gonna elevate heater of V2?

OK, perfect now, go back to sleep...

Last edited by kenpeter; 25th November 2010 at 04:19 AM.
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Old 25th November 2010, 04:26 AM   #8
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Quote:
Originally Posted by diego6339 View Post
In the amplifier I built without the CCS the currents are 1.8 ma through the 6sn7 and 70 ma through each pair of 6as7 triodes.

I forgot to mention I'm gonna use the amp with Sennheiser HD800s which have an impedance of 300, so that's why I want to use just the biggest MKP I can fit in.
The big 47 could and should still be your best MKP. Heck, why not?

Like I said, if even one quality cap does like you think: The others in
parallel won't see any AC, and can't do anything to hurt the sound.
Only if that 47 don't pull its weight, then others are there for support.
Neither 470 elko, nor a 1uF film are likely to eat up a lot of space.

C2 is totally ditchable, if you dump some extra mA into the IRLED.
Want to put that LED on the lowest impedance part of its curve.
I'm assuming some IR has about the 1.2V you are looking for?

Temped to dump R4's 70mA into it, but that might be positive FB.
If V1's heater had its own 6.3VDC supply, that might be a good
place to steal a few mA. Something like 50mA is about optimal.

If its really only gonna be 1.8mA, maybe a NiMH battery could
stand that much continuous trickle charge? They got a catalyst
to recombine water if the overcharge isn't too fast. I do not
know how a battery's internal resistance might actually sound?

Why am I not asleep?

Last edited by kenpeter; 25th November 2010 at 04:52 AM.
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Old 25th November 2010, 05:07 AM   #9
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With trickle charge battery bias.
Not so obviously: I'm thinkin solder tabs, no springs...

I read that Sanyo 2500mAH is built to survive 250ma?
So 1.8ma should theoretically be no challenge at all.
Theoretically, yeah...

---- edit ----

I'm just now looking at 6AS7 curves and figure around
15V drop across POT2. 2K5 is too small, maybe 25K...
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Last edited by kenpeter; 25th November 2010 at 05:31 AM.
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Old 25th November 2010, 05:22 AM   #10
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You may use couple of resistors instead of transistor. The same dynamic load, the same positive feedback from cathode follower, but without additional transistor with it's non-linearities.
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