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Old 4th January 2007, 06:41 PM   #1
Gmorris is offline Gmorris  United States
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Default QSC Series Three 3500

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/discont...eriesThree.pdf
Design description begins on page 32 and there is a schematic on page 50.

Thanks for any insight.
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Old 9th January 2007, 07:29 PM   #2
taj is offline taj
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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
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Old 9th January 2007, 07:53 PM   #3
djk is offline djk
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The current limiting will not function properly if you use another opamp.
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Old 11th January 2007, 10:37 PM   #4
Gmorris is offline Gmorris  United States
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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...).
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Old 11th January 2007, 10:59 PM   #5
djk is offline djk
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"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.
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