First Gainclone finished

[jarkaa]

Just finished my first gainclone using lm3886t with 2*24 165va toroid.

I just wanted to thank Decibel dungeon (Nuuk) for good instructions and schematic that I slightly modified.

many thanks to all active members here. Your posts have been more than helpful.

The amp is working great. No humming or other problems except the heat.

The chips become very hot when driving at higher volyme levels althought the heat sinks are quite big. Is this normal?? I also want to know is bigger heatsink the only solution??

Jarno

[Nuuk]

Hi Jarno, I am glad that you found the DD gainclone pages of help.

I have noticed with several of my Gainclones that when new the chips and heatsink get very hot. Then for some reason they start operating at a lower temperature.

I will be interested to see if you find the same thing happens with your GC.

[Saurav]

This is interesting. I've had my Gainclones finished for a while, but put them in my system just now because I have the active crossover and bi-ampable speakers ready. The first hour or so, the enclosure got really hot - hot enough that I put a granite slab under the amp, I didn't want it sitting on the carpet. Then I found that one of the resistors in my active XO was really hot too, and realized it was touching another component (the XO is on a breadboard right now, and *very* hacked together). I separated those components, the resistor cooled down, and now the Gainclone doesn't get as hot any more, even at the same volume levels.

It got hot enough that I couldn't hold the enclosure - is that normal? Or is it possible that the XO was oscillating or something, and that's what was heating the amp up? I'm using 96dB speakers, so it shouldn't be sending more than 5W into the speakers even at the peaks. Or maybe it's just what Nuuk described - it gets hot in the beginning, and then doesn't get as hot (though I have no idea why the chip would do that).

[joensd]

Regarding the heat problem it also depends mainly on the supply voltage vs. impedance of the speaker.
24AC secondaries are quite high for driving 4Ohm-speakers meaning you need adequate heatsinking (more than the "usual" aluminium bar") to keep it cool.
(At 96dB efficiency it puzzles me though; either you are deaf or you really kick butX)

A friend of mine used a big 24AC transformer he had lying around and is driving his 87dB Yamaha 4Ohm speakers with it.
He´s using heattunnels with temperature-controlled fans and they still get quite hot (the chips).

[Nuuk]

This sounds more likely to be the problem in the active crossovers causing the problem. What did it sound like while it was playing in the 'hot' condition?

[GregGC]

Originally posted by Saurav
This is interesting. I've had my Gainclones finished for a while, but put them in my system just now because I have the active crossover and bi-ampable speakers ready. The first hour or so, the enclosure got really hot - hot enough that I put a granite slab under the amp, I didn't want it sitting on the carpet. Then I found that one of the resistors in my active XO was really hot too, and realized it was touching another component (the XO is on a breadboard right now, and *very* hacked together). I separated those components, the resistor cooled down, and now the Gainclone doesn't get as hot any more, even at the same volume levels.

It got hot enough that I couldn't hold the enclosure - is that normal? Or is it possible that the XO was oscillating or something, and that's what was heating the amp up? I'm using 96dB speakers, so it shouldn't be sending more than 5W into the speakers even at the peaks. Or maybe it's just what Nuuk described - it gets hot in the beginning, and then doesn't get as hot (though I have no idea why the chip would do that).

The chip shouldn't get hot at all when in Idle state. There must be some kind of oscilations happening or really high DC offset on the speakers. Mine doesn't heat up at all. When driving 90dB/m speakers at normal (to me, 5-10 W) levels it just gets a little bit warm.

/Greg

[Saurav]

This sounds more likely to be the problem in the active crossovers causing the problem. What did it sound like while it was playing in the 'hot' condition?

In terms of sonics, I didn't notice anything wrong, but I'd just fired up my new speakers. There was more of a hum/buzz, which I attributed to ground loops in my funky crossover. Now that I think about it, that points to oscillation too. It wasn't a high pitched squeal, but that could be because my lowpass section was oscillating, and the following high pass section took out the upper frequencies. The amp's DC offset is fine, or was fine the last time I checked, under 10mV.

Later that night I was back up at the same position on my volume control, and the amp stayed cool, so it probably was something in the XO. It just means I need to get it soldered down sooner than I thought. This is what it looks like right now, and I know it's asking for trouble:

[SY]

Saurev: Two questions. Are these high-speed opamps? Have you stuck a scope on the output of the XO?

[Saurav]

Are these high-speed opamps?

LOL... you have a good memory They're AD826s... not quite as high speed as the LM6172s I tried, I think, but they're bipolar inputs so I had to use some tricks to get the DC offset down. The schematic I have was intended for FETs, I think, because the output is shorted to the inverting input on every opamp. I've been replacing that short with (a) resistor(s), trying to figure out the impedance seen by the non-inverting input and using the same value. I think it's working somewhat, I have ~250mV DC offset with the short, and < 10mV with the resistors in place.

Have you stuck a scope on the output of the XO?

Yes, on every output, before I connected the amps up. The low pass circuits were clean, the high pass one had a slight wiggle on it, less than 1mV. I think a cap across the feedback resistor would fix that.

None of the resistor/capacitor leads are cut down to size though (so I can reuse them on the PCB), so I guess I made two components touch each other that shouldn't have touched. That's my best guess at this point. The resistor that got hot was in series with an output, and IIRC inside the feedback loop to the inverting input (though now I can't remember if the feedback loop to the non-inverting input that achieves the filter function is connected before or after the output resistor).

[SY]

It's not for nothing that Pease refers to those sorts of breadboards as "slabs of trouble." Your diagnosis is likely right, BUT... it does help to check the outputs with the cables connected (which you might have done); sometimes, cables are just the right capacitance to turn filters into very high fidelity high frequency sine wave generators.