help with jlh

[NickC]

i built the jlh for split supply. Voltage of 23 volts. Using 4*tip3055 for output with 0.1 emitter resistor for sharing. Bd139 for middle stage and sa872 for the input transistor.
the problem is when i change the pot value for the dc error at the output, the current drawn by the amp also changes. Problem is that the dc error is highly dependant on current. at the moment the amp is idling at 1.5 amp with the error voltage of 0.75 volt. This is the first time i came across this problem. What should i do?

 

[John Law]

NickC

This is normal, the quiescent current adjustment and the dc offset adjustment do interact. Allow the amp to warm up for about an hour or so, adjust the current, then adjust the offset. Re-adjust the current then re-adjust the offset. Repeat until you get a stable situation with the desired quiescent current and near zero dc offset.

 

[NickC]

i built it before i don't remember encountering this problem though
but only thing different was that the hfe of the transistor as 110 instead of 30. when adjusting the offset down to the lowest setting the current drawn will be zero.

 

[chris ma]

NickC,

I did too thought that I have had the same problem as you have. But trust me do what John Law says, you must do it slowly and step by step between the DC offset and the Iq current, not adjusting only the DC offset first nor the Iq current first; BTW which version of JLH you are building? I've finished the JLH for the
ESL designed by Nick and Geoff version.

Chris

 

[NickC]

i built the version with the rudy and mathjlis version split supply
well i try as suggested but still the offset is still big