Quasi-Complementary Amp Question.

[mrk229]

I am helping a member @62vauxhall troubleshoot a Rotel RA-1412. Thread Here:

It had two of the four outputs shorted in one channel. There is some concern that the regulated supplies are not working properly. I want to ask him to monitor the regulated voltages while bringing it up with a variac.

I don't have much experience with the Quasi-Complementary (Totem Pole) topology used here.



My question: Is it safe to power the amp board without the outputs installed? I think it is but want to be sure before I advise somebody how to blow up their unit.

Alternately I am quite confident it is OK to power it with only two of the four outputs in place.
Say the odd Q's 1 & 3.

Thanks for any advice you may offer.

 

[cademan]

Yes, you can as the shorted device(s) has been removed from the circuit. Unlike tube equipment when removing tubes, with transistors, the voltage doesn't go up. Also be sure to remove all outputs in the offending channel before powering up.

Solid state runs on low voltage / high current.
Tube stuff runs on high voltage / low current.

With shorted transistors, be sure to check the transistors in front of them in the circuit like the drivers and pre drivers. Also check the emitter resistors and other resistors associated with that bad channel.

 

[mrk229]

Yes, you can as the shorted device(s) has been removed from the circuit. Unlike tube equipment when removing tubes, with transistors, the voltage doesn't go up. Also be sure to remove all outputs in the offending channel before powering up.

Solid state runs on low voltage / high current.
Tube stuff runs on high voltage / low current.

With shorted transistors, be sure to check the transistors in front of them in the circuit like the drivers and pre drivers. Also check the emitter resistors and other resistors associated with that bad channel.

Thanks for the confirmation. I have asked him to check all of those items. Somebody else has been in this unit before and failed to fix it. So need to be extra observant.

 

[fred soop]

If the driver transistors are good, you can get a small amount of power (enough to drive headphones) with the output transistors removed. Also, this circuit should be able to run at reduced rail voltage for testing. Again, power output will be limited. I've run similar circuits as low as ±5 volts.

 

[Blue Shadow]

I...want to be sure before I advise somebody how to blow up their unit.

Yes, we would not want some half done blown up unit. Glad you are looking to do it correctly.

 

[mrk229]

If the driver transistors are good, you can get a small amount of power (enough to drive headphones) with the output transistors removed. Also, this circuit should be able to run at reduced rail voltage for testing. Again, power output will be limited. I've run similar circuits as low as ±5 volts.

Thank you for that helpful advice. That may be the next step.