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Old 28th March 2014, 12:46 PM   #1
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Default Connexelectronics TA3020 kit


I have this kit with IRFP4321 output transistors and it acts kind of funny.

Power supply is a linear kit plus 300 VA 2x35V transformer. Auxiliary power is 12V SMPS from a wall adaptor and 7805 5V linear regulator with separate transformer. Mute pin is connected to the ground via jumper (no mute switch).

It's hooked up to Behringer CX2300 (not 2310) crossover as bass amplifier using XLR-to-RCA adapter cables (pins 1 and 3 bridged; shield unconnected in XLR plug - 50 Hz hum if shield connected at both ends). Mid-high amp is this. The units are stacked on top of each other on a table.

The anomaly: after the amp is powered down for more than half an hour or so, it has +30V DC on both output when powered on (speaker protection does its job well). Rail voltages are OK. When restarted it runs correctly after second or third attempt. Sometimes it induces noise (hiss) into mid-hi amp - restarting bass amp or disconnecting-reconnecting mid-hi amp signal cables corrects this. Once the amp runs correctly, it is stable for days of non-stop operation and gives no more anomalies until next power-down. I have swapped out the amp board for an another identical one and that one behaves identically. Could it be the +12V supply? 12V is on the high side for TA3020, but within ratings.

This is the layout of the amp (amplifier board is under power supply board on the right):
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Old 29th March 2014, 03:16 PM   #2
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So no-one has an idea why an amplifier would act like this?
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Old 30th March 2014, 10:27 AM   #3
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Did you tried to ask me about ?
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Old 30th March 2014, 12:34 PM   #4
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Hi Cristi,

No, not really, it didn't occur to me to contact you directly I got the amp board from a friend who once used 3 of those for tri-amped system, but has now moved to fullrange plus supertweeter camp. He never had any anomalies with these boards, though.

But since you've chimed in, maybe you can give some advice? I'd be grateful! Especially regarding aux. power supply - SMPS didn't work well for +5V (switching noise at outputs), linear supply is much better; maybe I should try linear supply for 10V, too, instead of 7 SMPS? This is one difference between my and my friend's implementations - he was using linear regulators for both aux. supplies.
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