JLH Headphone Amp

[bkercso]

Thanks for it!
I experienced that idle current is more independent from temperature if T5 isn't installed on the heatsink. T12, T13 emmiter resistors are large enough for good temperature stability, and T5 on the common heatsink foul up this.

I wait for compare this amp with my friends Panda amp.

[bkercso]

According the original article, the main concept of this amp is the low TIM distortion because of the low open loop gain (as the first simulation shows too). Similar design concept used by Kevin Gilmore when constructed Dynahi (or Gilmore amp, I don't remember exactly).
CKKIII open loop gain is even less (as it is an open loop amp), maybe for this is the reason of the sound similarities.

[oz7aff]

I newer noticed any problems with the stability og the idle current. It might also have something to do with the size of the heatsink and the thermal connection between T5 and the heathsink.
Yes, they talk a lot about TIM, one have to remember that TIM was the big thing back there in the late seventies and early eighties

[hotiron]

The idle current in this amp is way too high, if you lower it you will not have the same offset issues. Even cutting the idle current by 50% it is still running in class A at ear bursting levels!

[ashok]

Someone mentioned that even very low bias currents can still sound good. I did try that and it really sounds fine. The heatsink is barely warm. Didn't measure the current but it is low compared to normal class A settings. Distortion levels might have gone up it isn't apparent on listening to music. I blew the + power supply. No idea what happened. I think the mains voltage went up and it blew the diodes and cap. So can't make anymore measurements till it's fixed.

My output offset started at about 4.6 mV and dropped to 4.1 mV after 30 minutes and didn't vary much after that. I can trim it down further but haven't done so. However the board is open and not in an enclosed case.
The plan to upgrade it and box it has been delayed.

[xslavic]

Come on,stop it ,its a fake amplifier ,its very noisy ,whatever you do

[ashok]

Not sure which amp the previous post is referring to. The one I built is the JLH circuit from the Net. Transistors BC557/558/547 and BD139 for the output.

I can't measure anything now as the power supply is blown. Looks like the -ve 12 V regulator blew. There was a lot of power fluctuation that morning. Could be the cause.

Anyway I simulated the circuit and with the component values I have and the output quiescent current appears to be 26 mA. Still sounds very good. Instead of two 1n4148's I used a red LED for the bias control circuit. My phones is 60 ohms I think.
The amp is very silent. No background hiss ( or hum ) at all !

[bkercso]

Originally Posted by xslavic

Come on,stop it ,its a fake amplifier ,its very noisy ,whatever you do

Many eBay parts are fake. But amp is correct.
Fake parts can produce noise, hum or damage the amp.
For example: I measured TO-92 transistors beta from eBay JLH headamp kit: every beta was lower than specified!

Grounding/wireing also can produce hum.

[bkercso]

Originally Posted by oz7aff

I newer noticed any problems with the stability og the idle current. It might also have something to do with the size of the heatsink and the thermal connection between T5 and the heathsink.

Idle current mainly depends on the temperature of T5. I used smaller heatsinks than yours, so the change was higher during warm up. After that I disconnected T5 from heatsink and "problem" solved.

[hotiron]

Originally Posted by xslavic

Come on,stop it ,its a fake amplifier ,its very noisy ,whatever you do

Absolute rubbish! What are you you talking about?
Have you built one?
My rendition is stunning, no hiss no hum only mind boggling great tunes!!