A-80 output stage bias current

[willysan]

I've bought an used A-80 amplifier. The output transistors have been replaced and just to be sure I've checked the output stage bias current. The Sansui manual states the voltage drop on the emitter resistors (0.33 Ohms) should be 1.5 mV. I achieved that figure but at the very end of the adjusting potentiometer. My doubt is the following: 1.5 mV over 0.33 Ohms give me some 4.5 mA. I do believe this current is TOO small for the output stage but this is the the official word from Sansui.
The amplifier has a harsh sound and I don't have a scope to check whether it has crossover distortion.
Does anyone think the bias current should be higher. Let's say 40/50 mA?

Thanks

 

[Lefty]

Yes 4.5ma seems very low to me also. I have no way of helping you verify the specification.

I can share my opinion that while Sansui made some great vintage gear, I have yet to come across a Sansui service manual that was worth much more then paper it was printed on..... Missing data, typos, errors, unreadable, poor translations, etc...

I no longer have any Sansui gear or their manuals, except for a turntable....

Too harsh...maybe, but fustration tends to clouds one outlook.

Again This is no reflection on the actual quality, performance and styling of their products, great stuff mostly.....


Lefty

 

[oldmarantz]

It seems to be too small
for example the bias is
AUX1 .....25mV
AU7900..18mV
BA3000...20mV
if the bias is too small you will have a crossover distortion...

Dom.

 

[willysan]

Thanks oldmarantz. I guess in my case the voltage drop should be 15 mV instead if 1.5 mV, that would give some 45 mA of bias current. Tonight I'll change the bias. Just watch the smoke!

 

[elnaldo]

Hello. Resurrecting this old thread, since EVERY time I face an A-40, A-60 or A-80, I wonder again and again, and never remember the answer, if the voltage stated in the service manual is a typo (and it should be 15mV instead of 1.5mV), but I also know that these amps have small heat sinks, probably designed to run cool, or just warm.

The test points are across 0.66 ohm (both emitter resistors), (not 0.33 as stated by OP), what gives near 23mA with 15mV, and of course just 2.3mA with 1.5mV

What are your experiences setting the bias with 15mV instead of 1.5 ? Any definitive answer?

I do have an oscilloscope to test or check crossover distortion.

Thanks very much !

 

[painttoad]

Hello. Resurrecting this old thread, since EVERY time I face an A-40, A-60 or A-80, I wonder again and again, and never remember the answer, if the voltage stated in the service manual is a typo (and it should be 15mV instead of 1.5mV), but I also know that these amps have small heat sinks, probably designed to run cool, or just warm.

The test points are across 0.66 ohm (both emitter resistors), (not 0.33 as stated by OP), what gives near 23mA with 15mV, and of course just 2.3mA with 1.5mV

What are your experiences setting the bias with 15mV instead of 1.5 ? Any definitive answer?

I do have an oscilloscope to test or check crossover distortion.

Thanks very much !

i've got an a80 on the bench that i repaired, yadayadayada.



with idle current @1.5mv(as per procedure) across the emitter resistors, i get no crossover distortion.

i have not checked distortion with the hp8903b yet

HOWEVER,

looking at the schematic for the right channel and we see:





i've got the unit partially disassembled for glue cleanup, new main filter caps and a protect relay.

after it is reassembled, i will check thd with 1.5mv and again with 3.3mv and monitor heatsink temps.

i don't really expect much, if any difference in thd. it's more of a curiosity than anything.

 

[Ingy]

Curious about your thd results at 1.5 / 3.3mv. I recently blew up the outputs on this one. Had <4 ohm speakers connected to it. Apparently you should never do that with this amplifier. Read that replacing the emitter resistors to 1 ohm can help with this. Just don't know if they mean both resistors x 1 Ohm..... Originally they are 2x 0.33 ohm emitter resistors per side. These should then be 2x 0.5 ohm (total 1 ohm)? 2x 1.0 ohm (total 2ohm) ?? Also curious how you will provide the power for the protect relay. Thanks.

 

[elnaldo]

Less than 4 ohm means problem for most of the home audio amplifiers.

I'd stay with 0.33 ohm resistors, or 0.47 if you want to modify the original design, and install beefier transistors. Many modern transistors can withstand several Ampere in that same package, if you choose some 150V 15 A parts they will probably take some extra current without blowing up.

I don't remember if the A80 comes with fuses at the speaker output, but the Pcb is usually prepared to install those fuses. They will protect your speakers in case of a short at the amplifier.

 

[Ingy76]

Thank you Elnaldo. When i measured the speakers in my livingroom i was really surprised that it measured 3.6-3.8 ohms. They are B&W DM301.
I'm going to replace them with other 8 ohm speakers.
About the Sansui A-80: i stayed with 0.33 ohm emmiter resistors as you suggested. Already replaced the right channel output transistors (2sc4467 2sa1694) and it plays fine with 8 ohm speakers.
Bias current is set at 1.6mv for safety. (very difficult to tune with the potentiometers) Stil a bit confused because service manual says 1.5mv in description and indeed 3.3mv in schematics....