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Old 23rd September 2006, 05:16 PM   #1
aerius is offline aerius  Canada
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Default The Surplus Store Headphone Amp

First off, please excuse the messy handwriting, hope you can all read it.

Anyways, the story begins a few years ago when I needed a tube amp to drive my Sennheiser HD580 headphones, and later on my AKG K340. I was poor at the time and couldn't afford to buy an amp, so raided the local electronics surplus stores and came out with some tubes & parts. Many months of fiddling, experimentation, and learning later and I eventually came up with this.

Click the image to open in full size.


And the power supply
Click the image to open in full size.
The store let me have a drawer full of chokes for cheap so I felt compelled to use them.


A few notes. It was made to drive high impedance headphones so there's too much gain for Grados & other low impedance 'phones unless you a)listen real loud or b)don't mind being stuck near the bottom of the volume control range. My headphones are also wired with separate grounds on each channel so I just float the secondaries on the transformers. The PSU puts out 290V with 6SN7's and about 280V with 6BX7's and 6BL7's.

Though not shown on the schematic, there's a switch on the 1k cathode resistor on the output which switches it to 630R for use with 6BL7's. It sounds better that way.

Building it has been good if messy fun, and I'm really happy with the way it sounds.
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Old 24th September 2006, 03:41 AM   #2
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One note on a possible mistake..
Perhaps the 50uf 330V cap should have the negative end connected to ground,instead of the 6SN/BX/BL7 cathodes?
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Old 24th September 2006, 06:25 AM   #3
DougL is offline DougL  United States
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Quote:
One note on a possible mistake..
Actually, there is a name for that. Its called a "Western Electric"
Bypass. Itís a valid, if somewhat exotic and unusual design choice.

Here is Lynn's presentation

By the way aerius, Congratulations on completing your Headphone Amp.
Looks like a very workmanlike effort.

Doug
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Old 24th September 2006, 06:49 AM   #4
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Hi Aerius

Nice looking schematic! It's almost something like a small power amp, could drive some 8R horns or other high efficiency speaker.

About the OPT you are using....i just checked google and found out that it is a toroidal with 2X115V primary and 2x7V secondary. This is, with secondaries in parallel, about a 8k to 8R trafo (ratio^2 of ~1000). The HD580 is 300R headphone, that implies that the plate to plate resistance for the output tubes is about 300k. Am I right? Isn't that too high a value, which eventually drops output wattage? have you tried wiring the secondaries in series for use with the sennheiser? Well, maybe not necessary, if you get enough power this way it's fine, and the high plate to plate resistance helps in reducing distortion (to what amount...)

I am just asking all this because I have a pair of OPT out of a Bocama (8k to 8R and 16R) and was thinking on using it on a selfsplitting PP stage driving my 120R HD590. This situation is close to yours, so I ask what your experiences are... I will surely still have to check some curves to see what exactly happens when a tube is loaded by a higher than normal load.

Erik
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Old 24th September 2006, 04:05 PM   #5
aerius is offline aerius  Canada
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Quote:
Originally posted by DougL


Actually, there is a name for that. Its called a "Western Electric"
Bypass. Itís a valid, if somewhat exotic and unusual design choice.

Here is Lynn's presentation

By the way aerius, Congratulations on completing your Headphone Amp.
Looks like a very workmanlike effort.

Doug
Thanks! I'm trying to remember whether it was Lynn's page or Gary Pimm's page which gave me the idea. It looks a bit weird but it definitely works. The bypass capacitors are also motor run caps, I have the space so I figured why not.

Quote:
Originally posted by ErikdeBest
Hi Aerius

Nice looking schematic! It's almost something like a small power amp, could drive some 8R horns or other high efficiency speaker.
It can drive speakers, though not very well with the 6SN7 tubes on the output, but with the more powerful 6BL7 or 6BX7 it will indeed drive high efficiency speakers rather well.

Regarding the transformers, your numbers look about right. I have tried putting a 30 Ohm resistor in parallel with the headphones and as far as I can tell it sounds the same. I left the resistors in place since I'm too lazy to desolder them, plus it's handy since I can break in tubes without having my headphones plugged in. I did try putting the secondaries in series but found that it had too much gain that way even for my 400 Ohm K340 headphones, but if one has the AKG K1000 ear speakers I think that would be the way to go.
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