Problems with DIY amp and 9V adapter

[Bubesz]

Hi!

Some time ago I built Brymus's amp. It works...well, partly. The preamp is not really clean, and on the higher notes I can hear a octave-down like effect. If I turn the guitar volume down, it becomes clean, but it still has that effect. The other big problem is that when I use both the preamp and the power amp (with an old Russian Hi-fi speaker), it's VERY distorted, even on the cleanist settings.

At the moment I can't really use the amp because my 9V adapter died, a resistor burned out, and I don't know it's value so I just randomly replaced it. Now it gives 8.9 volt without load, but only 3-4 volts when it's connected to the poweramp (I use a separate one for the preamp). And 4 volt isn't enough to power it...

Do you have any ideas how to solve these problems? I want to repair/finish the amp and give it as a gift for Christmas, so it would be important... Thanks.

The schematics:  http://i419.photobucket.com/albums/pp276/Bry928/SolidState%20Amps/12Watt-TDA1519B%20build/12WattTDA1519B_schematic.jpg

I used the improved version of the preamp: http://i419.photobucket.com/albums/pp276/Bry928/SolidState%20Amps/12Watt-TDA1519B%20build/386PreAmp_2.jpg

My layout: http://img86.imageshack.us/img86/1640/kp070.jpg

[PRR]

The power amp pulls a LOT of current, more than many small 9V adapters can provide. Sounds like it already killed a resistor inside the adapter. You say you "randomly replaced it", which isn't much help; I will observe that a series resistor to this power stage must be a very low value, a couple ohms, or it will sag as you describe. You could try rolling the dice again, but I wonder if the next burn-out will be the adapter's power transformer.

Get a good 9V >700mA adapter.

[Bubesz]

I measured the new resistor, it's 436 ohm. I didn't remember the value because I replaced it a few weeks ago and haven't cared about it lately. Should I lower it?

And can you give a solution for the other problems?

[PRR]

> can you give a solution for the other problems?

Use a good power supply. You already burned-up this one, so I don't think it was strong enough.

Amplifiers are never happy on weak power. You may find that with a solid 9V-12V supply, your other problems are cured.