A few GC newb questions + probs

[tifosi_f1]

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]

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]

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?

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]

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]

and here is my transformer diagram....

[tifosi_f1]

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]

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! 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.