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Aikido 9-pin all-in-1 headphone amp problem

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I've just finished building my aikido HP amp using 6CG7's as input tubes and
6DJ8's as output. For the last couple of nights I've been burning in the amp and testing with my 3 sets of headphones. I've got an AKG K240 @ 600ohms, Beyer DT880 @ 600ohms, and modded Fostex T50RP at 50ohms. What I've noticed is the Fostex sound completely sucked out with this amp, very muddy bass and an almost echo-ish sound. The two 600ohm cans sound superb. I know it's not the Fostex cans as I use them daily at work with a Shiit Asgard amp and brought them home just to try with this Aikido.

When building, I chose resistor values for using 32ohm cans optimally, so the 600 ohm cans should be the ones to sound deficient. I wired a 330uf electrolytic parrallel to the other two output caps which is switched into the circuit when using the 50 ohm load. The only departure from Johns design I've made is to leave the 30uf Daytons in the circuit when I switch in the 330uf caps.

Has anyone else had issues with low impedence cans and this amp? Do you think this amp just doesn't like Planar Magnetic headphones? Would it make that much difference if I switch the 30uf Daytons out of the circuit when switching the 330uf electrolytics in, or just leave them in?

I'm thinking dampening factor may be the culprit, if so, what could I do to fix this, output trannys?

I've attached the schematic I built to showing options based on impedence and my choices.


  • Rev-A Schematic.jpg
    Rev-A Schematic.jpg
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Hello there. I built the headphone amp using the 6cg7 inputs and 6h30 outputs based on this link from John Aikido 9-pin Low-Impedance Headphone Amplifier . It seems to be the same as your thumbnail schematic but with the 6h30 setup. I find that my 600 ohm Beyerdynamics work great with virtually no hum while any low impedance phones exhibit hum. So yes, I'm experiencing the same problem. No ready solution for you except to just to listen with the 600 ohm phones or perhaps bring down your Ik a bit so that you can increase R12 to reduce ripple.
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