Author Topic: Question regarding "automatic" level adjustment (AGC?)  (Read 1401 times)

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tss

Question regarding "automatic" level adjustment (AGC?)
« on: March 08, 2012, 09:34:26 PM »
I want to build a audio circuit that runs on 5V. How can I make sure that incoming signals from unknown pedals will be adjusted to the right voltages so that no signal will be clipped due to supplies. I saw many effects run on 18V sometimes...

Gurner

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #1 on: March 08, 2012, 09:59:47 PM »
5V AGC circuit on this page ...

http://www.holmea.demon.co.uk/Spread/Spread.htm (about 3 or 4 schems down from the top)

tss

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #2 on: March 09, 2012, 02:20:12 AM »
thanks for the link, however I also want to know if this is what normally done in effects with digital parts to scale the input? I am wondering if this will give a compressor/expander tonal effect which is not wanted normally...


5V AGC circuit on this page ...

http://www.holmea.demon.co.uk/Spread/Spread.htm (about 3 or 4 schems down from the top)

cpm

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #3 on: March 09, 2012, 03:40:45 AM »
my guess is that you usally state a maximum input level, and beyond that, just a hard limiting (such as diode clamp) for safety purpose...
additionally can be added some kind of switching between two input level sensitivities, eg. instrument vs line

also It depends if your maximum power availabe is 5v, that is a hard maximum for every input stage, unless you attenuate passively.


tss

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #4 on: March 09, 2012, 10:33:46 AM »
my guess is that you usally state a maximum input level, and beyond that, just a hard limiting (such as diode clamp) for safety purpose...
additionally can be added some kind of switching between two input level sensitivities, eg. instrument vs line

also It depends if your maximum power availabe is 5v, that is a hard maximum for every input stage, unless you attenuate passively.

Absolutely true. However I am asking also in the perspective of guitar effects. Lets say that I have a modded 18V tube screamer that runs into a digital delay which I don't want to overdrive. The "headroom" of the delay is 5V for example.

artifus

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #5 on: March 09, 2012, 10:42:22 AM »
most pedal effects have input and/or output level pots. those that don't usually follow the convention of unity gain, ie what goes in is what comes out.

cpm

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #6 on: March 09, 2012, 11:05:16 AM »
most pedal effects have input and/or output level pots. those that don't usually follow the convention of unity gain, ie what goes in is what comes out.

The problem comes when placing the box in an amp effect loop, when they dont have control for send/return levels, and their output can be quite hot.
So there can be a switchable pre-attenuation/post-amplification to scale the signal to your valid levels, and usually it will be a static setting enoug to be inside the box in the form of switches or trimmers.
Another solution can be companding, since it helps to even the singal that goes through, but it adds another level of complexity...

Gurner

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #7 on: March 09, 2012, 11:53:11 AM »
Absolutely true. However I am asking also in the perspective of guitar effects. Lets say that I have a modded 18V tube screamer that runs into a digital delay which I don't want to overdrive. The "headroom" of the delay is 5V for example.

Perhaps I'm missing the point, if you are running an 18V tube screamer (wjhich I'm assuming can potentially give you a peak to peak AC signal of near 18V)  & want to run it into a digital delay running @5V rail...then all you need is a potential divider placed before the digital delay, with a 18V to 5V drop pre-calculated  (for example a potential divider with top resistor value of 13k & a bottom resistor of 5k (note: I just used those values for simplicity) - take the signal from the junction into your digital delay & the signal cannot be any more than 5V AC there & therefore your digital delay won't clip.
« Last Edit: March 09, 2012, 01:43:51 PM by Gurner »

PRR

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #8 on: March 09, 2012, 05:54:51 PM »
If it sounds bad, turn it down.

anchovie

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #9 on: March 09, 2012, 09:26:29 PM »
Absolutely true. However I am asking also in the perspective of guitar effects. Lets say that I have a modded 18V tube screamer that runs into a digital delay which I don't want to overdrive. The "headroom" of the delay is 5V for example.

So what happens when you bypass the delay? Do you still want a signal at triple the delay level coming out of the TS?

The only reason for using a pedal to boost the signal to that kind of level is to ensure that you overdrive the next stage, and the only reason to put a delay/modulation/non-gain-y effect immediately after a booster is because you want to deliberately exploit what happens when it receives too hot a signal.
- Guitar built from parts
- Random strings to make a full set
- Any pick that I can find on the floor
- Some pedals that I built
- My all-tube amp head that I built out of parts in my spares box
- Cheap Marshall 4x12 with better speakers fitted

boogietone

Re: Question regarding "automatic" level adjustment (AGC?)
« Reply #10 on: March 10, 2012, 12:32:00 PM »
What you are looking for does not exist. The "correct" way to deal with this is through proper gain staging by the end user. This means that unless the signal needs boosting from the original source or you intentionally want to output a hot signal, to overdrive the next effect or amp, each pedal should be set to unity overall gain. This generally means that the effect output should be approximately the same loudness as the bypassed signal. When you setup your pedalboard, do this and all will be well with the world and you will have ultimate control over your sound. Use the power amp to get your final volume. Everything before that is for shaping the sound.
An oxymoron - clean transistor boost.