Pro Audio Design Forum

Guys - Roger just sent me an image of the layout and it's very compact. I think this is going to make a nice little kit. Thanks for doing this!

Does it fit the same layout? Is it bigger or smaller than the original one? I'm confused.

OK, so it's not as tall. Cool.

Cool!

The way I set the 1K output bias current was to measure the emitter current of either the top or bottom transistor via the I*R drop across Re. Depending on the size of your heatsink you can set the class-A current from maybe 65 mA to 85 mA IIRC. WIth 3R3 emitter resistors that becomes ~ 0.21 V to 0.28 V across the resistor. If you go to the high end of the scale the THD is very low but you might monitor the current it may take a few minutes to stabilze.

Less bias current is with the trimmer towards it's maximum value. You might want to start there and work your way up.

Nice work Roger! I'm looking forward to getting the boards. I posted your pics over at HeadWize.

When you get the chance, measure the output Vos at the junction of the emitters for both channels. I think you'll find it to be pretty low, <10 mV.

You might want to lower the series resistors in the supply leads. They look to be 10R(?) and lowering them would stiffen the supply. At 10R there may be quite a bit of I*R drop. Since they're fused they could be a slow as 1R or reduced to jumpers.

I think your K240's may be the older "vintage" ones that are notoriously hard to drive due to their voltage requirements. I remember them being 500-600 ohms/channel. The newer ones IIRC are ~50R. Funny you chose the Sony MDR-7506's - that's what I used for my tests. They are 63 ohm and at 106 dB/mW quite loud. I really like those and they sounded so much better than my old Koss pair.

Roger, your measurement method sounds right. Left and right weren't in series were they? Are yours the new or vintage variety?

We used to have K240s at DSL (may still do) and I'd have to change the transducers by the truckload. Someone would be running a mix or working in the control room without talent using the cue system. I'd walk into the big room and those AKGs would be blaring 'cause they'd forget to mute the cue system. Sometimes they'd be so loud that the music stands they were hanging on were rattling. To get that loud you need at least 30V p-p with the AKGs. I never really cared for them but talent loved them. They are comfortable and chicks dig 'em due to the hair thing.

Speaking of the "hair thing", you can wear them while wearing Optivisors! I seem to be using those magnifiers more every day

My wife gave me one of those LED headlamps. Handy if you don't mind looking like a cyclops or coal miner.

raf wrote:

Very nice job !!

I had a chance to check the PC board a few weeks ago and run a few measurements.

The output Vos was <5mV. The orginal slew rate was thought to be ~4V/us but when re-measured turned out to be ~8V/us. This is consistent with the THAT1646 when driven in the "common mode" configuration. The RMAA results are here:

http://www.ka-electronics.com/Headphone_Amp/HPA_1.htm
The load was 32R at about 25 mW IIRC.

I'm considering asking Roger to modify the layout so that the amps are side-byside. That way, the heatsinks can be mounted vertically, as shown, or horizonatally in the same plane as the PC board. This allows a 1U height build and the chassis to be used as the heatsink.

I looked back at this just recently. Nice to hear it's still in mind.