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Chipamp preamp

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

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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?


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...


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.



Quote from: Stompin_Tom on April 11, 2006, 11:04:47 AM
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.


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.


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