Bit OT - Have you built a LM1875 chipamp?

[Threefish]

Hi everyone,

I'd like to hear about your LM1875 chip amp and/or ideas -

I've never been completely happy with my LM1875 practice amp, particularly the treble response. It's driven me bonkers with a variety of preamps on and off over the last couple of years. I built it using an Altronics kit based on a project in Silicon Chip magazine in December 2004. Here's the schem for the amp itself, pretty much based on the app note:
http://www.aronnelson.com/gallery/v/diyuser/20w+Amp.jpg.html

My question is - could the input impedance be affecting the treble response? Looking at the schem, is it the 1M or the 22k, or both, that is setting the impedance? And that 330pF cap - would that be sucking much treble away? Would reducing that 2.2uF NP cap to say 1uF help?

Thanks,

Reid C.

[polaris26]

This is weird - I was just looking at that app note last night!  I was approached the other day by a co-worker to make a small stereo amp for his speakers he wants to use at his work station to listen to an mp3 player.  I looked through my stash of audio chips and had some LM1875's in there I thought about using.  I haven't breadboarded anything yet - but looking at that app note, it would appear that the input impedance would be just under 22k, since the 1M is essentially in parallel with the 22k at audio frequencies.  I am not sure what the purpose of the 10 ohm resistor to ground is.  In any case, I would probably not run a guitar into this without some sort of buffer (an op-amp unity-gain buffer, a j-fet source follower, etc.) since 22k is sort of low for typical guitar pickups. 

Just my 2 cents...
Dave

Hi everyone,

I'd like to hear about your LM1875 chip amp and/or ideas -

I've never been completely happy with my LM1875 practice amp, particularly the treble response. It's driven me bonkers with a variety of preamps on and off over the last couple of years. I built it using an Altronics kit based on a project in Silicon Chip magazine in December 2004. Here's the schem for the amp itself, pretty much based on the app note:
http://www.aronnelson.com/gallery/v/diyuser/20w+Amp.jpg.html

My question is - could the input impedance be affecting the treble response? Looking at the schem, is it the 1M or the 22k, or both, that is setting the impedance? And that 330pF cap - would that be sucking much treble away? Would reducing that 2.2uF NP cap to say 1uF help?

Thanks,

Reid C.

[Threefish]

Thanks for your two cents Dave. I owe you. 
I sortof came to the same conclusion as you. That cap between the two input resistors made me a little unsure though.

Something I didn't add is that I've always assumed my preamp output impedances would be less that about 22k. They've been mostly ROG emulator circuits (thanks guys!) and most recently a fetzer valve, with a couple of ordinary transistor ones that I suspect never quite worked properly.....
I know that a 100k pot on the end of a circuit doesn't equal 100k output impedance, but I've never really looked into it. I have to admit I've never tried a low output impedance buffer before the amp. 

Can anyone enlighten me any more?

[geertjacobs]

Have you seen http://www.ssguitar.com/ before?

[petemoore]

Can anyone enlighten me any more?
  RG writes up impedance explanations occasionally. Maybe there's one in 'addition to the FAQ' thread.
  I'm not that good at explaining it ..

[R.G.]

My question is - could the input impedance be affecting the treble response? Looking at the schem, is it the 1M or the 22k, or both, that is setting the impedance? And that 330pF cap - would that be sucking much treble away? Would reducing that 2.2uF NP cap to say 1uF help?

Yes, the 22K is sucking your treble out, big time. You need a buffer to make your guitar see at least 100K, preferably 1M input impedance. The cap is not your problem.

The simplest thing to do is an opamp buffer. TL072, gain of +1, running on a supply of +/- 12V made from either zeners or 3-terminal regulators from the +/-25V will do it nicely. 78L12/79L12 would be good, and small, as they are TO-92 package.

You could mess around with a JFET or MOSFET buffers, but the power supplies here make that modestly tricky. A big-Vbsoff JFET like an MPF102 would be OK, but would always add some distortion. Maybe good, maybe not.

Check out the article on impedance at GEO.

[Threefish]

Thanks R.G., Pete.  . You are gentlemen and scholars.
I'll have a read over at GEO, and get that buffers article out of my file...