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Old 11th April 2005, 04:44 PM   #1
Fossil is offline Fossil  Singapore
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Default Need help on tda7294 amp

I've just completed my DZ 7294 kit. Everything seemed ok, no thump on power up or even power down.

But a problem occurred a few seconds after powering down, ie, there'll be a LOUD BUZZ for a split second. It scared the hell out of me coz I was testing at night!

How is this so? It's the same for the 2 kitsets which I've built. Is there any way to suppress it?
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Old 12th April 2005, 02:56 AM   #2
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Check whether u have mispalce any components and all your solder join
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Old 14th April 2005, 02:22 AM   #3
Fossil is offline Fossil  Singapore
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thanx Leolabs!

Apparently it came from a poor solder joint after the V+ caps. Now the buzz is gone!

Next have to troubleshoot the low hum I'm getting when I connect both channels from the pre to the monoblk
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Old 14th April 2005, 02:47 AM   #4
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Then it should be ground loop problem
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Old 14th April 2005, 06:50 AM   #5
Fossil is offline Fossil  Singapore
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yeah! definitely ground loop coz the amp was dead silent if only 1 is connected at a time. With both R & L input, there's this hum, even tho the pre is not powered up.

found a really helpful thread and still reading!
An unusual hum problem. Need help.
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Old 20th April 2005, 04:32 AM   #6
Fossil is offline Fossil  Singapore
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Just some follow-up.

My 1st attempt in making an amp was, well, quite a challenge!

I've got:

1) a pop/buzz on power on
2) low hum, audible from 1'
3) delayed buzz (about 8 sec) on power off
4) ringing with one of the signal HOT grounded

Initially, I thought it was a ground loop problem coz the hum only came out when both sides were plugged into the preamp.

BTW, I have 2 PCBs each with 2 channels on board sharing the same power-in. The PSU has a single a single transformer into 2 rectifiers/cap banks to feed the 2 PCBs. They r housed in 3 separate chassis, 1 x PSU and 2 x Amp, and linked by long (umbilical) cords coz I intended to put the amps near the spkrs. Main earth and circuit ground are not linked. Testing was done with single input into each PCBs but jumpered to feed both channels.

Anyway, I managed to solve the buzz problem quite quickly (see earlier post on this thread) but the hum remains.

But after some troubleshooting http://www.diyaudio.com/forums/showt...040#post620040
I started having doubts that the hum was due to ground loop problem.

So I decided to remove all the jumpers and try to isolate the problem. That proved to be VERY COSTLY!!

Not sure what happened but the moment I powered up the modules, the spkrs went crackling and at the same time, the 7294s on one of the PCBs started smoking! Tested the other PCBs and found that the chips r dead too!

Managed to find some replacement but I decided to only replace one channel per PCBs just_in_case! At the same time, I touched up all the solder points. Tested them individually with input from the PC and sounded OK.

Connected them to the preamp….powered on….HUM was even louder then before! Switched them off immediately coz didn’t want to risk killing my spkrs. Put in a couple of 10ohms dummy load instead. If I can hear the hum, I should be able to see it on my DMM (I don’t have a scope)!!

I was thinking…’why should it be louder?’??
Then I remembered something. I did not really clean the PCBs up this time ‘round after all the soldering.

Took the boards out, soaked the underside with IPA and started brushing frantically with a toothbrush. Thinking I did a pretty good job this round coz the boards were quite sticky after the last time I cleaned them (dissolved lacquer?). Also brushing, I waited for it to dry and started scrapping in-between solder pads to remove any last bit of stubborn solder flux residue and soaked with IPA again.

Plugged all the PCBs back and fired up. Checked DMM for both outputs…1mA…I can accept that…or at least, this won’t kill my spkrs!!
Connected the spkrs up again. Powered on…no pop whatsoever…QUIET…turned the volume up…there’s MUSIC!!!

*REJOICE*

BTW, the each cords are about 2.5m long !!
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Old 20th April 2005, 03:53 PM   #7
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U are more lucky than me,as my first project was a opamp+discrete power amp,with self design PCB.Not like yours,chip amp kitset.

I threw away 7 PCBs(all designed and etched by myself)before successfully finished one.
.

First attempt on DIY amps is quite tricky,often ended up with hum,buzz,oscillation,!@#$%^&*().LOL

But when all problems gone,I enjoy everythin very much,and play my WolfET the whole day!
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