6as7 Aikido headphone amplifier
I just moved from the middle of nowhere to the city. In my small loft apartment there is seldom an appropriate time for window shaking, baby waking loud music. In order to keep my music listening up I purchased a pair of AKG 550's, which sound very nice, however they need something more than a PC soundcard to drive them.
I came across John Broskie's article about the Aikido Push-Pull. I noticed his 6AS7 design designed for a 50 ohm load. I have all the parts laying around already with the exception of a chassis. I'm worried because my AKGs present a brutal load of 32ohms. I have thought of two ways to deal with this issue
1. Put a second pair of 6AS7s in parallel (yeilding 4 output triodes per channel).
This would double the current carrying capability, but would it destroy the advantages of an Aikido output stage?
2. Put a ~20 ohm resistor in series with the output (Much less fun). I certainly don't need the amount of power this circuit delivers.
Is it worth the extra trouble to add another pair of output triodes?
I would appreciate any feedback you may have.
Your headphones will work ok. You need not have exactly 50 ohms inpedance. All that will happen is you'll get a little less power, but hardly worth worrying about since you don't need a lot for earphones. BTW, try listening to a pair of Sennheiser HD580 or 600. You'll love them and they're not that expensive any more. And they're 300 ohm and very easy to drive.
That circuit was designed to drive low sensitivity headphones which require a lot of power. The standing current is more than enough for any normal set of 32 ohm headphones. By "optimised for 50 ohm impedance" it means that resistor values have been chosen to give perfect push-pull balance at 50 ohms - see the equations on the Tubecad site. A slight change may be required for 32 ohms.
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