Using LM4780 for Guitar amp - wondering

[Wynand]

Is there any reason why U can't use the LM4780 chip alone for a guitar amplifier? I see that the ESP Guitar Preamp has 2 gain stages at approximately 20 x gain each.

Would it be possible to do the same using the LM chip?

Is there such a thing as too much gain for the chip in this respect?

[Gcollier]

Actually there is no reason at all you couldn't use this chip as a guitar amp. In fact the LM3886 is used fairly commonly in DIY guitar amps, the LM4780 is really just two LM3886's in a single package. As for gain, you could certainly use a gain of 20 with no difficulties, just watch you are not exceeding the limits of your supply voltage. If you are going for two gain stages then you could use a lower gain for each stage and have a preamp in front of the amp chip. IMHO you are better to parallel the LM4780 and use a gain of 20, them put a nice simple pre in front of it, or use something like an OPA134 with a little gain. What would be really nice is to feed it with a tube buffer! The Chipamp.com LM4780 boards would work just fine for your application. If you need tone controls you could build the guitar pre from ESP, I have a PCB layout for this if you want it.

G.

[Wynand]

Is the sky blue!!! Please. I've been looking for a layout and couldn't find what I needed. Will you post it here or will you e-mail it?

What I meant was if you could do a gain of +/- 400 with the LM chip, thus eliminating opamps. I don't know if I'll try it though.

[RDV]

I'm using a paralleled LM4780 as a guitar amp as we speak. I use a Thomas/Vox preamp design that has a mid-boost that uses an inductor and I'm pretty happy with it. I bought the PCB for the amp part from http://www.tech-diy.com/. I perfed the preamp.

RDV

[RDV]

Here's the Thomas/Vox preamp for those interested.

RDV

[ian_carney]

try one of these:

http://www.runoffgroove.com/fetzervalve.html

it uses an FET and is really simple - works a treat..

Ian

[Wynand]

Thank you both for the links.

I like the simplicity of the fender circuit.

BTW. What is the usual gain for a guitar amp in total?
I know that a MIC could have a 1000 gain. (someone told me so)

[sucker4drumsynt]

Gcollier, Do you have a pcb layout for a guitar preamp/poweramp that you have verified as working? PLEASE e-mail me about this. I want to build a pratice combo amp. Thanks! tbrookew@numail.org

[teemuk]

Originally posted by Wynand
BTW. What is the usual gain for a guitar amp in total?
I know that a MIC could have a 1000 gain. (someone told me so)

Average guitar signal is about 20mV (peak) after the initial picking transient has settled so compare this to the voltage output with maximum power and you get the gain (approximately). Some amps use low gain in preamp (2-10 per each stage) and compensate this with higher gain in the power amp. Elliott's guitar has lower power amp gain and higher preamp gain. Unless you are already restricted by a certain power amp or preamp gain figure you can set the gain in each stage quite freely. However, guitars with hot pickups might give out transients from 1 to 2 volts so using a high gain in the preamplifier will likely reduce the headroom with low supply voltages.

Basically there is no such thing as usual gain since the variation is huge. For example:
Crate GX-15R: about 2x2x150 = 600, varies with frequency
LAB SERIES 5: about 385 to 4828 depending on the channel and input.
ESP Mk2: about 16x16x35 = 8960, varies with frequency.

The negative gains of tonestacks are not calculated but they can drop the gain figure greatly for example:
LAB SERIES 5: 21,4x0.1x2.8x29.3 = 175.6 (channel 1 high input). Seems small but this gives 35,12V on 200mV. This amplifier also has a soft clipper circuit and a compressor that limit the gain when the signal amplitude rises too high.

As you see, it's hard to make any generalizations besides the fact that a relatively low input signal has to be amplified relatively high. Volume potentiometer set to half will affect the gain approximately x0.1. Basically the gain and power output do not have to go hand in hand: With higher gain an amplifier will just clip on lover volume potentiometer position than an amplifier with equal output power but lower gain would.

I hope this satisfied your curiosity.

[Minion]

I am useing a LM3886 Guitar amp right now and it puts out a very Loud 50w with a Gain of 33 on the LM3886 Stage....

I am also useing a pretty simple Clean Preamp stage that has Bas/Mid/Treb tone controlls and Volume and quite a bit of Gain...

I also have a basic 4 transistor Overdrive stage before the Preamp stage that can be turned on and off (With a Footswitch or Button) that puts out a pretty kikass distortion...I am also working on adding a Tube overdrive stage useing a couple 6418 Submini Power amp pentode tubes.......

I have all the Different stages on seperate PCB"s so if I have a Problem with one part of the Circuit I don"T have to scrap the whole amp which is a mistake I made when I first stared this amp...

I have played it throught a Pretty crappy speaker and it sounded quite good and Now I just picked up a 12in 50w Speaker for a Fender Princeton which I have to install and it should sound awesome after I get that done......


Cheers