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Chipamp preamp

Started by joecool85, April 07, 2006, 10:50:13 AM

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joecool85

Chipamps are designed so that they don't need a preamp, however, that is for when you are running a lineout from a computer, CD player etc.  This means that you need to get guitar signal up to that strength by way of a preamp.

So, here is what we need in a preamp:

- Gets us up to linelevel
- basic EQ (probably 3 band)
- protection, ex-someone pluging in AC power to the input
- input buffer of around 1m
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

joecool85

In talking to brian, we might be able to just increase the gain on the chipamp circuit, then we'd just need a 1:1 buffer/EQ setup in front of it!
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

teemuk

Sounds simple but 1:1 buffer is not the best circuit though. What you haven't considered are the losses caused by the EQ circuit which can be quite big even with efficient marshall-type tonestack. I'd say it's best practice to amplify the input signal slightly before the EQ circuit in order to improve SNR. There's really no point of feeding the PA stage with a noisy signal.

trevize

i received yesterday the schematics of my solid state amp from the italian distributor. it's a fender m80 amplifier with tl072 based pre and darlington final. i do think that fender eq sounds better on ss amps than marshall eq. i can scan and post the schematic if you want. it's a clean pre (it's easy to get rid of second channel) with reverb circuit.

joecool85

Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

joecool85

Looks like earlier when I had posted stuff saying about 60v for input on the lm3886, I was right, but not for the way the chipamp is setup.  We only need 1vRMS if I'm correct.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

trevize

i'll post it as soon as i can. there are some interesting things: the reverb circuit is great and can be reused. for the gain it can be calculated and modified.

teemuk

Quote from: joecool85 on April 08, 2006, 08:30:05 PM
Looks like earlier when I had posted stuff saying about 60v for input on the lm3886, I was right, but not for the way the chipamp is setup. We only need 1vRMS if I'm correct.

Sort of. The typical supply voltage for the chip is about 60V when the negative rail is used as the reference: That´s +- 30V per rail when referenced to the ground. However, the chip's input sensitivity, which is a whole another thing, is totally dependant on the circuit's gain set. Now if i remember right, the LM3886 required at least a gain of 20 to operate stabile. With a one volt that should equal 20 V on output, which gives 100W of power to 4 ohm load - way more than LM3886 can even handle. If you have a notably lower input signal amplitude just raise the gain. This is easy since the chip basically operates just like a high power opamp. I think there is a limit for the maximum gain too (as there is such for opamps as well), which is the point where the chip starts to operate unstabile. Let's just say that i wouldn't go higher than a gain of 100 for example. These two factors limit the input sensitivity to a sensible range. I'd say something from 400mV to 1V should be good.

joecool85

#8
Alright, I've been talking to brian, and stock out of the box the lm3886 kit he has is at roughly 33 for gain.  We could easily raise it to 48 by replacing R3 (680 ohm) with a 470 ohm resistor.  He also said it should have no problem with even higher gain and that the only big problem people had was too low of gain (below 10).

Gain = 1+ Rf/R3

Stock Rf is 22.1k and R3 680 ohm

**edit**
I'd like to see someone try it out.  I can eventually, but it will probably be a while before I get a chance to as I've been pretty busy.  RDV, weren't you building a test bed for stuff like this?
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

RDV

I've not even gotten started yet due to some personal issues(don't ask). I may socket the FB resistor when I finally get around to it. When I build my preamp though I plan on quite a bit of headroom from the preamp itself(for slaving purposes). I will get back on track soon.

RDV

joecool85

It's cool RDV, I just remembered you had mentioned building one.  Good idea socketing R3.  I think if I decide to mod mine at all I will socket that as well so I can play with it easier.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Stompin_Tom

I'm sorry, I wish I understood this stuff better...

What's the advantage of increasing the gain of the power amp? Would this somehow make it easier to design a preamp?

joecool85

Its ok, its great to ask questions like that!

If we increase the gain of the poweramp, we won't need as much gain in the preamp section.  If we can increase the gain enough on the poweramp, we may even be able to get by using something like an LPB1 booster for a preamp.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

Stompin_Tom

Oh, I see. So, in theory, you could just stick a tone stack and a clean boost in a 9volt stompbox instead of doing a proper preamp... or use something from ROG... if it would work, that'd be cool. You could easily have a bunch of very different amps at your fingertips.  sort of like an analog amp modeler, I suppose.

But, this is what teemuk said would be noisey, right?

joecool85

Right.  I'm assuming what teamuk was saying is that if you put too much amplification (gain) in one section (poweramp or preamp), it tends to lead to noise and distortion.  But I could be wrong.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com