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Old 11th April 2011, 08:24 AM   #71
godfrey is offline godfrey  South Africa
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Location: Cape Town
Hi Greg

The move was totally chaotic but I'm here now and settling in. Mental health is improving rapidly now I'm out of the city and staying in a small fishing village on the coast (or as one of the locals described Pringle Bay - "a small drinking village with a slight fishing problem")

Anyway...

The bias scheme you showed looks good, so long as you remember to flip polarity since you're using a negative-earth arrangement. One Possible caveat: If your mains supply isn't as stable as it might be, you may be better off with the old circuit, which will tend to track slow changes in PSU voltage, and keep the output biased about half-way between earth and the "voltage-of-the-day".

Cheers - Godfrey
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Old 12th April 2011, 01:26 PM   #72
GregH2 is offline GregH2  Australia
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Thanks Godfrey,

Glad to hear the move is over with. Sounds like you're in a nice area! I recently moved to the country myself. Commuting isn't much fun but the fresh air is good.

Anyway, I will stick with the old bias scheme - mostly out of laziness. I suspect our line voltage is pretty variable given my location anyway.

I have tried to sim the turn on transients - the speaker one is attached. Pretty nasty.

I also looked at the base and gate voltages and there are indeed a few possible issues. The Vbe of the transistors does go up to about 10v I think, which is too much, but it is for fractions of a nanosecond. Not sure if that is significant or not?

I'm trying to get the motivation to redesign the PCB.... I seem to spend all day on the computer at work these days and spending a few additional hours in front of the computer at night is really unappealing at the moment
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Old 17th April 2011, 08:03 AM   #73
GregH2 is offline GregH2  Australia
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Godfrey,

Hope you're well.

I designed a new PCB and sent off the order today. Not going to post the layout here because it's well, embarassingly terrible really. But the critical traces are short at least, and all gate/base resistors are hard up against the fets/transistors. It will be better than my "air sculptures" at any rate.

I went back to the FET ccs as I wanted to keep it small enough to fit on the express PCB miniboard size (2.5" x 3.8"), and I was having trouble getting it to fit with the additional heatsink and power resistors needed for the bipolar version.

Will hopefully post some pictures of a completed channel and some scope shots in a few days.
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Old 27th April 2011, 12:43 PM   #74
GregH2 is offline GregH2  Australia
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Default Update

Got the PCBs back today and assembled one tonight as best I could with the parts I had available (fig 1).

Plugged it in...no smoke.

Adjusted the bias pot...output varied appropriately. Set it to just under 1/2 the rail voltage. Plugged in the sig gen and scope.

First the 20kHz square wave test (fig 2). Rise and fall times not too bad, but still quite a lot of ringing (fig 2 & 3).

I'm not overly concerned about the ringing though, as I'm fairly sure larger gate stoppers would prevent it - the waveforms on at the 240 fet gate is dirty whereas the waveform at the gate stopper is clean. It seems some oscillation is happening in the few mm of lead between the fet and the gate stopper...amazing considering how close I have them.

There probably isn't any point using larger gate stoppers though as audio waveforms would never cause this, right?


Anyway, it sounds quite nice with my little preamp. Pretty happy with it really. The treble is especially sweet sounding, but overall frequency response is very good.

It's running very hot at about 2A per device though!
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