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-   -   A few GC newb questions + probs (http://www.diyaudio.com/forums/chip-amps/26139-few-gc-newb-questions-probs.html)

tifosi_f1 15th January 2004 07:34 PM

A few GC newb questions + probs
 
Hi!
I've just completed a gainclone amp from Nuuk's site (single channel) and to test it I hooked it up to the psu with a cheap small speaker connected and a source of music. All is good for 10 or so seconds - the sound is clear and undistorted. Then it fades slightly distorts a lot!!

Any ideas? I checked all solder connections and everything seems ok. DC offset is 24mv. I have the chip connected to a large heatsink, and it doesnt get warm.

I am also puzzled on one thing - where does the zero volts line come from on the transformer and what is it for?

crown300 15th January 2004 08:18 PM

Your transformer needs three (or four) wires on the secondary side. A secondary with two wires will not power a GC unless you build a 'single supply' version - see nationals LM3886 datasheet if you need that.

See the second post at this link:
http://www.diyaudio.com/forums/showt...590#post303590

Ropie 16th January 2004 08:05 AM

Hi Tifosi,

You may have a resistor that has a power rating that's too low and is overheating (any burning smells??).

The 0V wire is a 'reference' wire for the + & - V that surround it. Think of it as a balancing point between two extremes (the + & - voltage). I like a colour analogy here: how do you really know if a blue, for example, is light(-) or dark(+) unless you have another tone(0) to guage it against?:spin:

If your transformer has dual secondaries (4 wires on the secondary side) join the two central ones together to get the 0V wire. Maybe this link will help: http://www.antrimtransformers.com/colour_codes.htm

tifosi_f1 16th January 2004 09:09 AM

Thanks very much for your replies guys....

I dont have any overheating resistors, everything seems nice and cool and there are no nasty smells (yet anyway ;) )

The zero volts line seems to be the problem then (cheers for the analogy Ropie :) ) - I have a somewhat strange power supply, it doesnt seem to have a zero volts output, but I'll post a diagram of the inputs/outputs later. I have the LM3875 chip - can I build a single supply gainclone version using this?

Thanks again, Pete.

tifosi_f1 16th January 2004 09:39 AM

2 Attachment(s)
and here is my transformer diagram....

tifosi_f1 16th January 2004 11:00 AM

Fixed it!!

In an act of desperation I accidentally/stupidly hooked my zero volt wire to one of the terminals on the output of the trans. (with the red cable attached) - it works!! no sound fading at all and its all very clear! Must have got the right one then...

Now for the next channel..... ;)

Thanks for your help, Pete.

Ropie 16th January 2004 11:14 AM

I've no experience of that type of transformer I'm afraid,

I'm no expert but I imagine if you don't have the 0V wired up you will get the full range of voltage (from -25 to +25, for example, = 50V) ie: too much V!:bigeyes: This may be overloading your chip causing it to go into protection mode whereby it shuts itself off until it feels OK again.:) If I remember from other threads this can cause a very distorted sound, but the chip should be OK.

Unless you can find someone who can help you with that transformer I'd suggest buying or finding a suitable pair of toroidal transformers with dual secondaries, as recommended by Nuuk, which are fairly easily wired up.


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