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Old 4th January 2006, 12:38 AM   #1
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Default Headphone amp using 6SN7 + 6080?

Apologies in advanced for my noob questions.

I'm looking for a schematic, or possibly assistance in modifying an existing one, to build a headphone amplifier using 6SN7 and 6080 tubes. I have a quite a few NOS RCA 6080's laying around along with a few 6SN7/12SN7's (and a few 6SL7GT's, not sure if they're useful for this).

I've seen various preamps utilizing the 6SN7, and I've seen a tubed headphone amp schematic on headwize using a 6922 + 6080/6AS7 I assume for gain and as a cathode follower, respectively. I'm not sure if I can just plug the cathode follower on to other schematics (I'm still learning/trying to understand tube design.. Please forgive me

I've also thought that maybe paralleling the triodes of the 6080 for more current drive might be desirable, since I'd like the amp to be usable with my grado cans as well has my AKG and Sennheisers.

I know there are a few commercial designs out there using the 6SN7 for the gain and paralleled 6080's for the output (I believe that one of them uses an output transformer as well). I'd like to try it OTL first, and then maybe move on to investing in iron if the performance gains would be worth it.

Any suggestions or pointers or schematics would be most helpful!

Thanks,

M1
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Old 4th January 2006, 03:54 AM   #2
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Here's one... this was commercially available for a while. There is a schematic on the last page that's fairly readable.

http://www.pmillett.com/Wheatfield/manual.pdf

The power transformer was a Thordarson 26R160 - you can use a Hammond 270HX (or 370HX, I used them for export 220V versions) as well. The choke was 5H 200mADC.

I'd suggest using PP-in-oil motor run caps for the output caps, and a BIG wirewound for the 6080 cathode load resistor - no power films, they get noisy over time.

Pete
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Old 4th January 2006, 05:44 AM   #3
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Wow. Thanks Pete! I've already built a few of your hybrids.. Now I'm psyched up to try this one out. I'm guessing it doesn't work too well with low impedence cans based on the specs on page 8?

Mike
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Old 4th January 2006, 08:40 AM   #4
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Depending upon what phones you are driving, you can also use a single 6as7g with a plate load ccs and take the output from the plate. This is Pete's low mu preamp design, and with a 6as7g the output impedence should be low enough (~300 ohms) for sennheisers. You could also AC couple the output to a transformer or a solid state buffer like the szekeres ... or parallel a couple of tubes to lower the output a bit.

I agree with Pete on the caps. You can get ASC's from Alied or percy or partsconnexion. I tried them as output caps and had pretty good results.
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Old 4th January 2006, 09:07 PM   #5
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It actually sounds pretty decent with Grados (32 ohms), especially if you up the output cap from 47uF to more like 100uF to help bring up the LF response. It's a matter of taste: I find Grado's a bit too edgy, and the lower damping factor improves things to my ears.

If you don't mind generating lots of heat, you can also parallel two 6080 sections per channel - just drop the cathode load resistor by a factor of 2 and double the wattage.

Or build it as-is, then swap in a 6528A or 5998... they will lower the Zo. My favorite is the 5998A.

We shipped the "premium" version of the amp with a 6528A. It got very hot, though... 5A filament!

Pete
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Old 4th January 2006, 11:36 PM   #6
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I built an OTL head phone amp using long tail pair input 6DJ8 to a cathode follower, 2 tubes 5687 paralleled (4 triodes) and it drives my 64 ohm Sennheisers louder than I care to listen. And the LTP input keeps the phase right. It's loosly based on http://headwize.com/projects/showfil...ammer2_prj.htm but the PSU is a little beefier and the output section is like the doulbed one with the higher dissapation 5687's.

Work's very busy for the next week, but I could scrach out the schematic if you'r interested.
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Old 5th January 2006, 01:35 AM   #7
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Quote:
Originally posted by pmillett
[B]It actually sounds pretty decent with Grados (32 ohms), especially if you up the output cap from 47uF to more like 100uF to help bring up the LF response. It's a matter of taste: I find Grado's a bit too edgy, and the lower damping factor improves things to my ears.
Grado's, at least their better ones, kill my ears with solid state. On a millett, my HF-1 sounds great. I know it's not the be-all end-all amp, but dang it, I love and and use it all the time on my computer.

Quote:
Originally posted by pmillett

If you don't mind generating lots of heat, you can also parallel two 6080 sections per channel - just drop the cathode load resistor by a factor of 2 and double the wattage.

Or build it as-is, then swap in a 6528A or 5998... they will lower the Zo. My favorite is the 5998A.

We shipped the "premium" version of the amp with a 6528A. It got very hot, though... 5A filament!

Pete
Does doubling the 6080 sections also lower the Zo by half? Or am I mistaken on that?

I think I might go with the paralleled 6080 sections since I have like 14 NOS RCAs laying around here that I'd like to use.

Can you recommend any sources for the caps you listed? Or could I use some large solens from Parts Express?

Thank you all for the help. I'm pretty psyched to build this project now!
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Old 5th January 2006, 02:06 AM   #8
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Quote:
Originally posted by motherone
Can you recommend any sources for the caps you listed? Or could I use some large solens from Parts Express?
I listed 3. You want more?

Solens will work as well, but they will not sound as good.
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Old 5th January 2006, 03:14 AM   #9
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I have two comments:
1) www.tubecad.com
2) For headphones, you don't need squat for gain. In fact, depending on your source, all you really need is a follower. Leave the 6SL7s in the drawer. Use the 6SN7s.

Grey

EDIT: (My brain is addled tonight and I'm forgetting things.) You're playing music from a computer and blaiming the amp/headphones for being bright?
What's wrong with this picture?
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Old 5th January 2006, 03:23 AM   #10
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Oh, blast and bother...forgot something else...
Yes, doubling the outputs will halve the Zout, all other factors being equal. However, given that you'll need very little gain, you'll likely be using some of the gain as NFB and this will drop the Zout too.
But...
Headphone diaphragms being as low in mass as they are, very little damping factor is actually required. Not to mention that damping factor is largely overrated, witness some tube amps (e.g. the old CJ Premier One) that had absolutely butt-kicking bass, whereas some solid state amps with far, far better damping factors sound bloated and uncontrolled by comparison.

Grey
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