cheap & cheerful amp ?!?!

[1878]

It's late and I can't sleep because my limited knowledge in these areas is gnawing away at me like a rat in a bag.

I've been after a clean, no frills 30w amp to throw in the boot of my car for informal jam sessions etc. I'm not after anything special, just something to make me heard. After a couple of searches here, I've decided on a Maplin 30w TDA2050 kit:

http://www.maplin.co.uk/Module.aspx?ModuleNo=220055

This seems like a good starting point. I've got a few questions though...

1. I take it I'd have to build a preamp too ??
2. Could these be integrated to use the same power supply ??
3. There seems to be no volume control on the kit. Is it easy enough to fit one ??
4. Would I have to change any values to use it as a guitar amp ??

I've got a nice 100w, 12 inch, 8 ohm speaker here that I'll cab up and use for this project. As i said previously, it's just a throw around amp that I can pack with a multi-effects for a sort of Swiss army knife guitar kit thingy, so I don't want to be going to too much bother.

Any help/suggestions would be greatly appreciated.

Thanks in advance.

[Zben3129]

IIRC, I believe you can make an amp from a lm3886 (?) that runs on 18v that is like 60 watts.


Zach

[rnfr]

i'd buy  an old tube powered pa head. like a bogen chb-50.  i have 3 bogens- 1 that is 50w and runs off 6l6's and a 50w Bell Labs that runs off el34's.  i got them all for less than 125 bucks each and they sound awesome.

edit-  didn't know you were goin for THAT cheap!

[km-r]

a TDA2050 is a 20-25w chip amp which is almost identical to the LM1875... i should know, i build gainclones haha!

anyway, a cap has a supply of 12vdc... the TDA2050 requires split supply that means it will be happy with a supply of +/- 25VDC for it to supply about 25w of audio... i DONT think this amp will be useful in plain 12vdc supply... car amps above, say, 20 watts require SMPS[switched-mode power supplies] to provide the required voltages. with an 8-ohm speaker you MAY just have about 8watts of audio.

so,

1. yes, a simple preamp would be nice. a fetzer valve should do.
2. same power supply? you mean, car battery? yes, maybe but power levels may drop to an almost useless level.
3. yes, its a matter of fitting a potentiometer in the input.
4. yes, but only if you supply it with enough voltage.

[km-r]

http://www.maplin.co.uk/Module.aspx?ModuleNo=45185

4x the price, 8x the power, 4x lower output impedance.

[1878]

Thanks for the replies.

I may have been misleading. It's not to run via the car battery, just something easily portable that'll fit in the back of the car and not break my back whilst putting it there. The car stereo amplifier is a route I'll look into though.

I thought the LM3886 had to be run at a much higher voltage to get the full 60w. I could well be wrong though.

[earthtonesaudio]

1. I take it I'd have to build a preamp too ?? Nope, the input resistance to the TDA2050 is 500k.  Ideally it would be 1M but 500k is tolerable.  However for best performance you'll probably still want a preamp of some sort.
2. Could these be integrated to use the same power supply ??  Yes, but... you will either need to run the amp at a low voltage and sacrifice power, or run the preamp at high voltage with special components.
3. There seems to be no volume control on the kit. Is it easy enough to fit one ??  A 1M pot at the input wired as a voltage divider would work.  But first check the circuit of the kit.  If they used a lower valued input bias resistor, you're stuck with that value as the max input impedance.
4. Would I have to change any values to use it as a guitar amp ??  Probably the input bias resistor, maybe some sort of input tone control.

And for what it's worth, the 2050 does not require a split supply.  Check the datasheet.

IIRC, I believe you can make an amp from a lm3886 (?) that runs on 18v that is like 60 watts.


Zach

It will make 68W into 4 ohms at +/-28V



In any case the power supply will require some thought.  You need a decent amount of current to get high power at low voltage.  And if you want to run a separate preamp off the same supply, it's even more difficult.  Probably the easiest way to go would be to use a bipolar supply, maybe +/-18V, and just run the preamp off of half of it.
And if you only need 30W, for simplicity's sake I would recommend the TDA2050.

[petemoore]

Not to throw another wrench in your rat-bag..
  You can probaly pick up a used Valvetronics or [buncha companies putting these type amps out, similar models from Vox, Epi, Crate, Fender, etc. in that DSP and small combo amp vibe thing]. People I've talked to who have these little buggars [my brother has one] swear the job it does defies it's size, offers versatile sounds, good bang for bukk.

[Mark Hammer]

Keep in mind that:

a) 60W is not really going to produce that much greater output than 30W.

b) Greater output in terms of wattage will mean a much bigger transformer and a bigger heatsink.  The chips themselves are cheap like borscht.  It's the bigger capacity transformers that up the cost, and the bigger heatsinks that require you to rethink your packaging.

[1878]

This is the problem with DIY. I've successfully built a few pedals and got carried away a little... or a lot !! I had an idea building something more than a Ruby may result in my brain writing cheques that my knowledge couldn't cash, but at least I know. I'll spend the money for parts on a cheap amp and (possibly) mod it a little to better suit my needs. Space is at a premium 'cos I've built so many effects that I've now got two effects boards. I could always go back to using my multi-effects, but it doesn't sound as good.

Thanks anyway.