Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Preamps and Effects => Topic started by: joecool85 on April 07, 2006, 10:50:13 AM

Title: Chipamp preamp
Post by: joecool85 on April 07, 2006, 10:50:13 AM
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
Title: Re: Chipamp preamp
Post by: joecool85 on April 07, 2006, 11:00:36 AM
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!
Title: Re: Chipamp preamp
Post by: teemuk on April 07, 2006, 02:11:37 PM
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.
Title: Re: Chipamp preamp
Post by: trevize on April 07, 2006, 09:27:17 PM
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.
Title: Re: Chipamp preamp
Post by: joecool85 on April 07, 2006, 09:45:51 PM
Yes, please share.
Title: Re: Chipamp preamp
Post by: 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.
Title: Re: Chipamp preamp
Post by: trevize on April 08, 2006, 09:43:32 PM
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.
Title: Re: Chipamp preamp
Post by: teemuk on April 09, 2006, 08:40:57 AM
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.
Title: Re: Chipamp preamp
Post by: joecool85 on April 09, 2006, 05:45:22 PM
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?
Title: Re: Chipamp preamp
Post by: RDV on April 09, 2006, 08:54:28 PM
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
Title: Re: Chipamp preamp
Post by: joecool85 on April 09, 2006, 09:31:07 PM
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.
Title: Re: Chipamp preamp
Post by: Stompin_Tom on April 10, 2006, 10:28:05 AM
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?
Title: Re: Chipamp preamp
Post by: joecool85 on April 10, 2006, 11:11:57 AM
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.
Title: Re: Chipamp preamp
Post by: Stompin_Tom on April 10, 2006, 01:20:19 PM
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?
Title: Re: Chipamp preamp
Post by: joecool85 on April 10, 2006, 01:22:49 PM
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.
Title: Re: Chipamp preamp
Post by: teemuk on April 10, 2006, 05:30:18 PM
The preamps and stompboxes might work as is, then again they might not. In order to know that you have to know the input sensitivity of the power amp and the output voltage of the "pre" circuit. You also have to match up the input and output impedances so that you will not have any undesired frequency losses. The concept behind choosing the right amount of gain is actually quite simple to understand: The more gain you have the more closer to it's clipping point the amp stage operates. Amplify too much and you clip the signal. Distortion. Some circuits also become unstabile with too high gain - which usually shows up as oscillation.

Noise is a trickier thing since it comes from everywhere. Everything adds up a little bit of it. Unfortunately, the mean amplitude of guitar signal is quite low; only few tens of millivolts. Run this signal through an attenuating part of the circuit (basically any passive part ie. a tonestack or a resistor) and you will effectively increase the amplitude of noise in relation to signal. This is why you need to amplify the signal before processing it in a passive circuit: To increase Signal to Noise Ratio (SNR).

These two things (SNR) and gain go together: If you plan to process the signal, you need to amplify it in order to keep the SNR acceptable. Typical guitar preamps do not really add up much gain to the signal: Most of the amplification is done just to keep noise to the minimum while processing the signal passively. If processing is done in an active circuit the total gain of a preamp might be close to zero. Many power amps take a maximum input signal amplitude of 1 V RMS to reach the full power, some guitar power amps seem to need even less since they are typically quite high gain. So what is enough gain? If you're still uncertain read the first paragraph. After that read the next one to confuse you even more:

Guitar power (and pre) amps are quite tricky to design. Why? Consider this: In the worst case, an efficient guitar pickup might put out a transient of 2 volts. If the power amp operates at a gain of, say 30, that already equals an output voltage of 60V! You need some serious power to produce a voltage swing like that to a few ohm load. Gladly it's only a transient and your amp might even make it - for a small period of time - but only if the supply voltage and the amp stage's capability to swing close to rail is high enough. Tube amps have inductors at the output and can therefore swing higher than the supply voltage - modern transistor circuits can not do this. In an opamp based preamp circuit, (that can typically swing to about +-14V), a transient as high as this causes some serious distortion! Since the transient voltages can be quite high the gain has to be very low in order to retain a reasonable headroom. Tricky? Not yet, but here's the catch: The typical mean amplitude of a guitar signal is only about 20 mV! With a gain of 30 it means only 600 mV at the output. Quite pathetic, huh?
Title: Re: Chipamp preamp
Post by: Stompin_Tom on April 11, 2006, 11:04:47 AM
Wow. Thanks teemuk.

Here's a question that might be impossible to answer... It'll definately show how little I know.

What determines the output impedance of a circuit?

I know RDV used an old vox preamp design for one of his chipamp projects... did he just get lucky in matching impedance and getting appropriate gains? Or are these things fairly standardized? I guess I'm asking RDV that...
Title: Re: Chipamp preamp
Post by: RDV on April 11, 2006, 11:54:56 AM
The Thomas/Vox preamp includes a gain recovery/limiter stage at the end that has mucho gaino. It will get loud with the standard chipamp setup. It is a little different than anything you'll play through though as it has a fairly unique midrange boost circuit(MRB) that requires a wah inductor. I like mine quite a bit.
(http://fatboy.ssguitar.com/index.php?dir=schems/&file=voxtbpRDV.GIF)
RDV
Title: Re: Chipamp preamp
Post by: teemuk on April 11, 2006, 06:40:21 PM
What determines the output impedance of a circuit?

There are many ways to measure it and Google provides a good help - as usual. One way, but not neccessarily the best, is this: Just measure the resistance between ground and the output while the device is off. From a schematic i would look for resistance between signal node and ground/supply. If you want very reliable results use better ways: Google will list at least few of them with a search term "output impedance". I think that the authors of these sites do a better job in describing the methods than i would.
Title: Re: Chipamp preamp
Post by: willy1usa on April 14, 2006, 06:56:05 AM
Hey guys - great site - gonna be good forum - my 1st post.
For the simple reason that I joy in junk, and have a couple good repair shop sources, I deal mainly in transistors. I have tried for years to build a simple guitar amp., but simplicity is almost impossible. They require so many stages, especially when you include a reverb section. I've seen commercial amps with as many as 11 stages. However, while attempting to repair an old 70s Peavey amp for a friend, I ordered the schematics. These schemayics turned out to be the answer to a lot of my problems - simplicity. In these schematics there's a simple preamp, gain stage, distortion, reverb driver/reciever, and all the tone, vol., eq. etc/ circuits. All of these are 1 to 3 transistor circuits. I've recently built all these circuits individually with common 3904/3906 transistors, and they work great.
Also, I just recently run across an actual "simple" reverb circuit by Forest Cook. It consist of 3 - one transistor preamps, and one LM386 for the driver.
I'm hoping to finally get my guitar amp on the road to completion with 3 stages: Preamp, a gain stage, and power amp, plus the reverb circuit of course.
Maybe I can learn something here, and try a chip amp next ??
Happy soldering and keep the good info coming.
Willy
Title: Re: Chipamp preamp
Post by: joecool85 on April 14, 2006, 07:20:56 AM
Hey, I love transistor circuits!  Let's see those schematics!  And welcome to the board!

Wow, I seem like a dork using all those exclamaition marks eh?