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PostPosted: 08 Oct 2008, 14:38 
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The PA100 circuit is attached.

If I need to build this, I'll probably have to make some changes: I can only get 1% resistors, and those 0.1 3W will have to be removed cause I cant get those around.
Oh, and my speakers are 8Ohm, and not 4.


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PostPosted: 08 Oct 2008, 17:57 
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I just tried rewiring my tranny to 220V taps, and that gave me 11.5V on the secondary.
Crackling is gone, but now the sound starts to distort too much too soon. Maybe the 3875s are underpowered (14.5V when playing).

So, up to now the best solution was the 8R in series with speakers. Good sound, no distortion and no crackling.

What else to try?

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PostPosted: 08 Oct 2008, 20:21 
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Hi Bernardo, that is good that you tracked down the problem. The easiest change would be to get a new lower voltage transformer and use this one for a different project.

Cheers

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PostPosted: 08 Oct 2008, 20:58 
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What about a parallel circuit like the one above? I think 3875s dont handle 4ohms too well, like in my bridged circuit.

I tried different taps in this same tranny (giving me ~12V and ~18V). With 12V distortion was too high and at 18V the crackling was back.

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PostPosted: 08 Oct 2008, 21:34 
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The chips can handle 4 ohms, you just need to keep the voltage down.

The parallel circuit looks fine too and it is very simple. The reason they specify such a tight tolerance on the resistors is that you need to match the gain of the two chips. This needs to be done to share the current equally in both chips. So you will want to match the resistors by hand.

An 18VAC supply for an 8ohm load should work well. With 18VAC, it the chip running hot? If it is, you need more cooling.

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PostPosted: 09 Oct 2008, 06:14 
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gmilitano wrote:
An 18VAC supply for an 8ohm load should work well. With 18VAC, it the chip running hot? If it is, you need more cooling.

Hi Gio!

Well, at 18VAC the chips are not hot at all, but still, I get some crackling (muh less than with 23VAC as before). Thats the weird part. I dont actually know how SPIKE works exactly, but it doesnt seem to be a temperature only thing. To my hands, both heatsinks are cool (only a little bit warm) but sill crackling comes in now and then :confused:

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PostPosted: 09 Oct 2008, 13:10 
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I just tried rewiring the right channel to parallel chips and something is wrong. 1 second after turning the amp on the 3875s get extremely hot. I rechecked the circuit and all seems well... Im missing something here and cant seem to find it...

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PostPosted: 09 Oct 2008, 14:11 
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Even more news:

Disconnecting the input of the 3875s (tube buffer output) fixes the heat problem... but, something is still wrong. Theres 1.6V DC at the input of the 3875s! I mean, theres 1.6V from the input (non inverting pin) to GND.

Whats this?

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PostPosted: 09 Oct 2008, 15:13 
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Even more and more news :mrgreen:

Just rebuilt the whole right channel using new parts. Still same heat up! Argh, this gotta be some bad taste kinda joke....

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PostPosted: 09 Oct 2008, 15:29 
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And the last news because Im flooding the thread!!!!

Each section works fine alone. The problem is when I tie both amplifier sections in parallel. Then theres only heat and no music.
I followed the schematics perfectly and all resistors are 1% hand matched, except the 0.1R... those are 5% unmatched (my DMM wont measure 0.1R). Are they the ones to blame? If so, I reached a dead end now :?

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