QSC Series Three 3500

[Gmorris]

I originally picked up a QSC Series Three 3500 (300 watts into 8ohm, 625 watts into 2ohm, 0.1% THD) for driving a pair of sonosubs. I found out this amp sounds quite good driving my main speakers as well. This series was QSC's top of the line from the 80's and is dual mono, fanless, complementary push-pull. Dynamics are very good and the only drawback seems to be a barely perceptible hum from the large transformers.

Besides swapping out the input opamp (a NE5532) which may or may not improve much, can anyone recommend any other modifications? Should I by-pass the power caps or by-pass any of the protection circuitry?

A link to the manual which includes many design considerations:
http://www.qscaudio.com/pdfs/discontinued_products/SeriesThree.pdf
Design description begins on page 32 and there is a schematic on page 50.

Thanks for any insight.

 

[taj]

Indeed these are excellent amplifiers. We've got dozens of them still in service and have had very few (if any) problems with them since we bought them in the late 80's. We also use 3800's for driving concert bass. The 3500s handle the mid-highs.

Just curious, which opamps did you use in place of the 5532's? OPA2134 or OPA627, or ??

..Todd

 

[djk]

The current limiting will not function properly if you use another opamp.

 

[Gmorris]

I have an opa2227 in there right now that seems to work fine...it has slightly lower DC offset than the 5532. The opa2107 seems to be ok too. I can't remember if the 2134 worked or not....one dual I put in there ended up with higher than optimal DC offset. The higher offset opamp was either the 2134 of maybe a AD865.
I'm currently not using the amp in a high power situation where the current limiting function would be necessary (I hope...).

 

[djk]

"I'm currently not using the amp in a high power situation where the current limiting function would be necessary (I hope...)."

The current limiting requires a selected 5532 to work properly.

Even a short at small signals could happen in home use, use a speaker fuse for safety's sake.

 

[tonskulus]

Old thread but..

I just picked up two 3500's. They are working but running very hot!
Reason: all trimmer resistors are pretty bad -> bias too high.
I changed them but I have no idea how much bias this amp needs? I just adjusted for 20mA of idle current / output transistor. Amp is now running cool.

Some QSC service manuals (similar grounded collector circuit) are saying to use distortion analyzer and adjust bias so that crossover distortion disappears. If analyzer is not available, measure voltage accross drive transistor emitter resistors.

I cannot find service manual for 3500. Only schematic is available. Another problem is those current limit trimmers (positive and negative). Had to change them too but I have no idea what's the correct setting and how to adjust for this particular model.


Any help would be welcome!