preamp or buffer for 10W solid state power amp?

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I asked about this in the lounge, but I don't think it gets enough traffic to get discussion on the answer... it seems close enough to on topic, so my apologies if it crosses a line.

I salvaged an amp circuit from a powered speaker I found by the side of the road. The chip is a HA13001, which is a 2x5W chip.

The circuit on the board is essentially what you see on the left in this schem - two sides bridged in what I guess is a 10W configuration:



Mind you the amp has no volume control, so any volume adjustment has to happen on the front end (unless someone can suggest a place to insert a master volume control).

I found the input and sent a few different signals into it, with different results:

1. My guitar - works, but the amp distorts very easily and at a quieter volume, ton of 60HZ hum/buzz unless I turn town the tone control on the guitar.
2. Headphone out on my mixer - distorts at a relatively higher level, no hum - completely quiet other than amp hiss.
3. Synthesizer headphone output - just hisses and sputters and thumps.

I am guessing that due to the strange differences in response to different devices, this would have to be some sort of impedance problem, but I don't know what to do about it. I am looking for a pre-amp or buffer circuit that will allow me to plug a guitar, synth, or mic into this amp. The data sheet says it has a 200K ohms input impedance.

Any ideas of what needs to be done or what circuit to use?

[compuwade]

1st, add a 10k-50k log pot at the input. Without this volume control everything you connect to the input will be at full volume and therefor distort as if it were turned all the way up. However, this is not always true because most guitars do not have enough output to push the amp to it's full potential. 2nd, for a test you can connect a buffered pedal like a BOSS or a Digitech without enganging the pedal to see if the buffer helps clear up the distortion. However, my guess is that the speaker was on the side of the road because it no longer worked properly, or the chip is just a piece of junk. I once bought an old CB that had a 5 watt chip in it that was exactly that. It was only good for amping the CB speaker because anything under 32ohms would cause it to distort like crazy. Lucky for me the CB had hundreds of good usable parts.

[Top Top]

1st, add a 10k-50k log pot at the input. Without this volume control everything you connect to the input will be at full volume and therefor distort as if it were turned all the way up. However, this is not always true because most guitars do not have enough output to push the amp to it's full potential. 2nd, for a test you can connect a buffered pedal like a BOSS or a Digitech without enganging the pedal to see if the buffer helps clear up the distortion. However, my guess is that the speaker was on the side of the road because it no longer worked properly, or the chip is just a piece of junk. I once bought an old CB that had a 5 watt chip in it that was exactly that. It was only good for amping the CB speaker because anything under 32ohms would cause it to distort like crazy. Lucky for me the CB had hundreds of good usable parts.

Thanks for the suggestions. Actually that is the same suggestion that someone in the lounge gave... about the pot at the front... maybe I should have listened 

The speaker was actually at the side of the road because it had a wireless receiver in it that had a blown cap (discovered this upon opening it). The amp actually works great given the right variables and once I ditched the cheesy speakers that were built into it.

This one is designed to use 4-8ohm speakers. Sounds great with two 2ohm and a 4ohm in series.

[compuwade]

After reading the datasheet, it seems that 10% distortion is quite high. The usual amount of distortion at normal usage for most decent amps is 0.5%. The only thing that isn't clear is whether or not the 10% is at max power output or typical power output. Anyway, my point is this could be most of your problem.

[PRR]

It's a <1% THD amp. Until it clips.

It is customary in some markets to quote at 10% THD, "because everybody else does". For any recent chip-amp, 10% is FAR into clipping, and the Watts-number is about 15% higher than an "almost clipping" measurement.