Idling adjustment question power amplifier

[nonanche92000]

Hi ,

There is something that i don't understand with an idling adjustment of an amplifier on the service manual (YST-SW320) :

IDLING ADJUSTMENT :

" To stabilize operation of the amplifier, turn ON the power with NO INPUT SIGNAL and wait for 2 to 3 minutes IN NON LOADED CONDITION before the adjustment.
Adjust VR1 (potentiometer) so that the voltage between terminals TP11 and TP12 is DC 50mV to 250 mV. "

What does " IN NON LOADED CONDITION " mean ?

I think at first without the driver / woofer , but i am not sure of it.

Thanks !

[rhomanski]

Re: Idling adjustment question power amplifier

Generally, the only thing plugged in would be the power cord. Volume at lowest setting. Usually your looking at mV, so a self ranging fluke works perfect. That range is very large for yours. Most are like +-2 mV or something.

On one of mine you have to close it up and let it cook for a half an hour for it to settle down. Then open it up and do the final adjustment which is 20 mV +- 2mV.

[redwire]

Re: Idling adjustment question power amplifier

You always set power-amp bias current with no load connected, and zero signal.
Because you only want to measure output-stage current (mV across emitter resistors) not including (idle) current into the speaker, dummy load etc.

I let amps warm up and temperature stabilize for 20-30minutes. That 2-3 minutes figure is for the production line where you are in a hurry.

[rievax_60]

Re: Idling adjustment question power amplifier

50mV to 250 mV seems unusually high. What value are the emitter resistors and how many are there per channel?

[redwire]

Re: Idling adjustment question power amplifier

The emitter resistors are 0.15 ohm each, but you are measuring 50-250mV across a 22R resistor in the what looks like the class-G boost switch. That would be a quiescent current of 2.3-11.4mA, close to cutoff but it's a subwoofer amp. YST-SW320 schematic here

[rievax_60]

Re: Idling adjustment question power amplifier

The emitter resistors are 0.15 ohm each, but you are measuring 50-250mV across a 22R resistor in the what looks like the class-G boost switch. That would be a quiescent current of 2.3-11.4mA, close to cutoff but it's a subwoofer amp. YST-SW320 schematic here

Yes, that makes good sense now.

[nonanche92000]

Re: Idling adjustment question power amplifier

Thank you guys for all your answers.

So if i'm right , after 30 min of warm up (instead of 2-3 min on the service manual) , without input , and without the speaker, value beetween this two terminals TP11 and TP12 should be somewhere between 50 and 250 mV DC on the voltmeter and with the volume set to the minimum?

I did a full recap , but with exactly the same capacitors series ( panasonic M and chemicon SMG )

thanks!

[redwire]

Re: Idling adjustment question power amplifier

Yes that's the best way to check bias.
If you are adjusting it, the loop is to measure bias, readjust, wait, remeasure, readjust, wait ... and so on. Because, if you adjust bias current the heatsink temperature will change too and it needs time to settle at the new temperature. Most amps settle in ~10 minutes.