What is the difference between a preamp and a normal effect?

fisshy · April 18, 2009, 02:04:45 AM

Let me say if I want to use a distortion pedal as a preamp, what I have to chang?
My friend said that the output impedance is different, so I have to change it.
How do I change it?

Mark Hammer · April 18, 2009, 10:02:06 AM

Chances are very good that the output impedance of the distortion pedal in question is low enough to be considered "low" in comparison to typical stompbox impedance. Chances are also reasonably good that its impedance will be considered "high" enough too, although likely not as high as devices promoted as high input-impedance (so for example 500k as opposed to 10M).

Conversion from "distortion" to "preamp" may be easy, or maybe not. Some considerations:

1) If it is a circuit intended to use clipping diodes, simply removing the diodes (or lifting one end of them) will clean things up appreciably.

2) Some circuits are deliberately designed to NEVER provide a clean output. Many modders of the MXR Distortion+ have noted that they cannot get it to be clean under just about any circumstance.

3) Cleanliness is a function of what you aim for. There are limits to how much clean gain can be achieved with a 9v battery and the device in question. The newly released Visual Sound Truetone uses a charge pump to turn 9v into a higher supply voltage, which in turn raises the maximum limit on clean headroom. So, following this example your pedal might not work fabulously as a clean preamp if modded, but may come closer to the goal if appropriately modded and fed a higher supply voltage.

4) Depending on the pedal in question (and you haven't specified it yet), the mods required to clean it up and make it simply push out a higher amplitude copy of the input signal may be so substantial that you're simply better off making a clean booster from scratch.

owenjames · April 18, 2009, 12:44:54 PM

A standard Overdrive can be turned into a preamp by just lowering the gain to zero and using the level control as your boost. Most modern ODs will have high input impedence and low output impedence.

The alternative is like the previous poster suggested, removing the clipping diodes, or building one, shouldn't be hard, you really only need one transistor.

aziltz · April 18, 2009, 01:04:10 PM

preamps can provide distortion too.

IMO, what you as asking about really are the same thing. Its a bunch of grey area. Depends on the application.

What did you want a preamp for? If you're thinking between guitar and amp, then it can be the same as a distortion pedal.

If you want a mic preamp that's a different story.

fisshy · April 18, 2009, 02:24:31 PM

Sorry for the bad explanation I had.
I was just curious about the difference.
Recently, I made a high gain stompbox and I tried it by putting it in straight to a power amp. (I know it is not meant to be, but I was just curious)
What I realize was that I didn't have enough volume. Even I turned the volume to the biggest, I only got bit.

aziltz · April 18, 2009, 05:38:19 PM

Quote from: fisshy on April 18, 2009, 02:24:31 PM
Sorry for the bad explanation I had.
I was just curious about the difference.
Recently, I made a high gain stompbox and I tried it by putting it in straight to a power amp. (I know it is not meant to be, but I was just curious)
What I realize was that I didn't have enough volume. Even I turned the volume to the biggest, I only got bit.

right, i think that's a lot of the difference. If you look at the Tech 21 Sansamps, they have a LOT of output, because they can be used as a preamp, a DI straight to mixer board or stompbox.

Ice-9 · April 18, 2009, 06:15:22 PM

I now see what you are getting at. You could look at it like this (simplified) The full amp can be broken down into different stages such as "input stage" " gain stage" "tone stage" preamp stage" "power amp stage" So by putting a dist pedal directly into the power amp you are losing all the gain of the intermediate satges and that is why the volume is lost. In reality there isn't a great deal of difference between an effect pedal and the input stage of an amp except for maybe the amp input will work on a higher voltage giving it more headroom. Also the stages of the amp will be designed to match each other more closely.

I may be wrong or vague but it's only my opinion.

aziltz · April 18, 2009, 08:59:16 PM

Quote from: Ice-9 on April 18, 2009, 06:15:22 PM
I now see what you are getting at. You could look at it like this (simplified) The full amp can be broken down into different stages such as "input stage" " gain stage" "tone stage" preamp stage" "power amp stage" So by putting a dist pedal directly into the power amp you are losing all the gain of the intermediate satges and that is why the volume is lost. In reality there isn't a great deal of difference between an effect pedal and the input stage of an amp except for maybe the amp input will work on a higher voltage giving it more headroom. Also the stages of the amp will be designed to match each other more closely.

I may be wrong or vague but it's only my opinion.

i'd say that's more or less correct.

in the end, preamp is just a word, and can apply to everything before the "power" amp, in stages or as a whole. for a distortion pedal to "act" as a AMP preamp, it just needs enough output to drive the power amp stage.

but, the word preamp could also apply to any distortion pedal on its on, as its just another STAGE in the chain.

fisshy · April 21, 2009, 11:47:27 AM

So, let's say if I want to make the distortion+ into a preamp, what should I do?

Mark Hammer · April 21, 2009, 12:22:36 PM

You start by using a better op-amp. Then you change the biasing resistors. Then you eliinate the diodes. Voila, you have a boster. Look at the MXR Micro-amp and the Distortion+ and compare the two. You'll see what I mean.

harmonic · April 22, 2009, 03:32:16 AM

I'm pretty sure he doesn't just want a clean booster. From what I'm reading, he's looking to boost the output of the distortion box such that it has enough output to drive a power amp. At least, that's how I'm getting his meaning ...

fisshy · April 22, 2009, 05:19:09 AM

Quote from: harmonic on April 22, 2009, 03:32:16 AM
I'm pretty sure he doesn't just want a clean booster. From what I'm reading, he's looking to boost the output of the distortion box such that it has enough output to drive a power amp. At least, that's how I'm getting his meaning ...

yup, that is what I meant

Joe Hart · April 22, 2009, 07:59:08 AM

Interesting! Before I knew better, I would stick a distortion pedal in front of a power amp and could never get enough volume (or anything resembling a good tone!!). So, can anyone answer this: what is the "normal" output of, say, a Distortion Plus and what is the "normal" output of a preamp? I'm just wondering how far off they are.
-Joe Hart

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What is the difference between a preamp and a normal effect?