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Old 3rd February 2008, 09:53 PM   #1
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Default 12AU7/EL84 Headphone Amp

I'm trying to design a headphone amp intended for relatively higher impedance headphones, 200 ohms at least, so I don't think transformer coupling should be essential. The person this amp is for has a large collection of NOS etc. 12AU7's and EL84's so would prefer amps of this type.

For the output stage, I'm considering using trioded EL84's in a White cathode follower, with B+ 300V, at around 30mA of current. Ra would be 88 ohms (going off the EL84's transconductance of 11300 umhos), Rk 130 ohms. Bypass cap would probably be a Black Gate, and the output capacitor would in all likelihood be >50uF Solen.

The input stage would probably be a 12AU7 grounded-cathode, direct-coupled to the output stage. Probably no cathode bypass cap to reduce distortion a bit.

I'm a bit rusty on the principles of direct-coupling and how it affects the next stage's biasing. On paper at least, what needs to go on in order to make this work? Thanks!

I'd post a schematic but things are too up in the air right now. It's not essential to use both 12AU7's and EL84's, and they might not be the tubes I would choose, but that's what the guy has the most of...
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Old 3rd February 2008, 10:21 PM   #2
pftrvlr is offline pftrvlr  United States
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The output impedence of EL84 WCF might be still too hign for 250ohm phone. EL84 sounds really good with tranformer.

Or you make an all 12AU7 Aikido headphone amp. I tried 12ax7-12au7 Aikido, it works, but 12BH7 in the output is much better.
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Old 3rd February 2008, 10:45 PM   #3
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An Aikido was another option. Anyway with the EL84's in a WCF, the output impedance is only about 64 ohms, so I think it should be all right. Not the best damping factor in the world, but with headphones, I've heard it matters less...?
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Old 3rd February 2008, 11:13 PM   #4
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John Broskie currently has a new PCB specifically for 24V aikido using 6GM8 with a discrete transistor output. I know your friend may have the EL84, but 6GM8 are only $2 ea so if he sold a few of the EL84 he could nearly cover the cost of the aikido build.

I bought one, haven't got it yet, but do have an aikido 24V as a pre and it sounds bloody good. I did have a buzz issue, but that was a ground loop from another amp I have.

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Old 4th February 2008, 01:19 AM   #5
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The thing is, someone willing to build an extensive (telefunken, amperex, mullard, etc.) tube collection probably won't care about the cost benefits of using cheaper tubes. It's all about the sound differences between the tube types, etc. I'm not sure how much of that I buy into myself (pardon the pun) but it's his choice, and I respect that.
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Old 4th February 2008, 08:24 AM   #6
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Ditch the 12AU7; a bad 6SN7 is almost certain to sound better.
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Old 5th February 2008, 02:55 AM   #7
renfrow is offline renfrow  United States
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Try Glass Jar Audio. They have kits for several different head phone amps, one, SOHA, is for a 12AU7. I have not purchased from them, but, plan to do so in the near future.

Now, we can do this the hard way, or... well, actually there's just the hard way.
-- Buffy Summers (Buffy the Vampire Slayer)
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Old 5th February 2008, 03:36 AM   #8
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I'm interested in designing and building something on my own, point to point.. I'll keep researching, but thanks for the advice so far!
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Old 5th February 2008, 04:44 PM   #9
kevinkr is offline kevinkr  United States
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Hi Soren,

All you need is a simple common cathode stage using a single 12AU7A and a pair of 6BQ5 which can be battery, fixed or cathode biased.

I'd use triode connection with the 6BQ5 and would recommend one of the Edcor economy transformers with say a 5K primary.

Run the 12AU7A at about 5mA for better linearity, and note that some Mazda, Mullard and Amperex types do a lot better on linearity than most made here in the USA. While it isn't the best tube out there it is actually capable of quite decent sound. (I based a commercial line stage on a pair of these, and its overall noise, distortion performance and bandwidth were all in the range typical of comparably priced SS gear at that time.)

Try to keep the plate voltage up on that 12AU7A up there I'd use something like a 27K resistor for plate load or even a ccs load - run it directly off of the 300V for the 6BQ5.

I had actually planned at one time to commercialize such an amplifier, but the business turned turtle and sank rapidly before I had the opportunity.
"To argue with a person who has renounced the use of reason is like administering medicine to the dead." - Thomas Paine
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