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Old 2nd September 2002, 05:37 PM   #11
swede is offline swede  Switzerland
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I think your Velleman kit is pretty unstable and gets into some kind of self oscillation without a speaker load. I believe there was a discussion some time ago about commercial amps that relied on "badly built" powersupplies to remain stable, making it impossible to rebuild it and make it sound better.

Well, good luck though. I hope this isn't your last attempt in the DIY field.

Best regards,
//magnus
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Old 2nd September 2002, 06:47 PM   #12
trwh is offline trwh  United Kingdom
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"Well, good luck though. I hope this isn't your last attempt in the DIY field."

Heh, it won't be - the Velleman amp was only for a very small PA rig I am building up. I shall build a pair of bridged LM3886 amps (yup, it is that small) instead. The reson why I am so keen on chip based amps (the Velleman kit used the hybrid TDA7250), is that they offer built in protection from nasties such as speaker lead shorts, thermal overload, etc. With a portable setup, I figure its only a matter of time until the output gets shorted for some reason, or someone drives the amp into thermal overload. With a limiter to protect the speakers and National Semi's SPIKE protection for the chips, I figure I can make an (electrically at least) indestructable setup for party duty!

Thanks again,
Tim.
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Old 6th September 2002, 09:28 AM   #13
mcp is offline mcp  United States
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Hi Tim

See that you are still having problems with the Velleman kit. Here is my schematic using the tda7250 You may like to cross reference with the kit to see where the problem is.

Regards
Mike
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Old 29th March 2003, 06:49 AM   #14
Ford_V6 is offline Ford_V6  Romania
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What is the meaning of the circuit between the pin 5 of the ic and the negative voltage and ground (2 res. and 1 cap.). I know that pin is used for standby the amp.

I atached the schematic of my amp too.
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Old 29th March 2003, 06:51 AM   #15
Ford_V6 is offline Ford_V6  Romania
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Default schematic

the schematic
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Old 29th March 2003, 07:45 AM   #16
mcp is offline mcp  United States
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Default pin5

According to the datasheet, Pin5 is used for standby, mute or play mode. The way I have configured it, it acts as a delay power up (or what is commonly called a "soft-start"). On powering up, it will wait for a split second before it switches on the amplifier. This overcomes the problem of popping due to sparking at the power switch.
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Old 29th March 2003, 08:59 AM   #17
Ford_V6 is offline Ford_V6  Romania
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Great, thanks
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Old 29th March 2003, 09:12 AM   #18
trwh is offline trwh  United Kingdom
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Hi

I see that this thread has resurfaced - I was having all of these problems with my Velleman kit last year. If you're interested, I never did get the amp to behave, and it is sitting in my junk box until I can find a use for its large heatsink / enclosure.

I have built my LM3886 PA amp though, here's a pic.

I used two of Rod Elliot's P19 boards, one for each channel. The bridged LM3886 amps are running in dual mono configuration, powered from a 25-0-25V 300VA toroidal transformer. There is a limiter circuit mounted on the front panel, built on my PCB whose layout I posted here a while ago. The amp produces 100W into an 8 ohm load before the limiter kicks in, and gives plenty of volume (noise?) with my cheapo 15" 96dB fullrange disco speaker cabs.

There is an 80mm 24V fan running via a resistor and a 50 degree 'cut-in' from one of the supply rails. This cools the main heatsink, and since the LM3886 chips are mounted directly onto the heatsink (which is isolated from the chassis), the chips never activate their thermal protection under normal use.
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Old 29th March 2003, 10:16 AM   #19
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Quote:
Originally posted by trwh
I have built my LM3886 PA amp though, here's a pic.
That's a compact, no-nonsense amp. I LIKE it.
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Old 29th March 2003, 11:19 AM   #20
trwh is offline trwh  United Kingdom
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Thanks, Circlotron!

Here's an alternative view.

Rod wasn't sure about using the chips at their full rated power into what is effectively a 4 ohm load. I used direct heatsink mounting, a large fan, and a limiter to help with this, and the chips do indeed perform OK.

See ya,
Tim.
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