Author Topic: which component makes a guitar compressor work  (Read 235 times)

mgalicki

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which component makes a guitar compressor work
« on: June 09, 2021, 07:19:18 AM »
OK tech guru's, which component makes the compressor compress?

Zerro

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Re: which component makes a guitar compressor work
« Reply #1 on: June 09, 2021, 07:53:02 AM »
Mostly electric details, especially those in list of parts :@)
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midwayfair

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Re: which component makes a guitar compressor work
« Reply #2 on: June 09, 2021, 08:29:59 AM »
Well, the short answer is "all of them."

Typically guitar compressors are a feedback compressor. A feedback compressor takes the input signal after amplification and compressor to generate a control signal and adjust the gain of the signal. (Feedforward takes the input signal before amplification and compression, meaning that the dynamic control of the final signal does not depend on the performance of the compressor.)

Such a compressor consists of two main blocks: An amplifier and the control voltage.

The amplifier is typically not anything special, but it is usually designed to be clean even if the signal isn't being compressed, and there will be some sort of device among the amplifier's parts that can control the gain. Typically it's a variable resistor of some sort, like a transistor or a light-dependent resistor, but there are other methods of changing the gain (for instance, if your amplifier is a FET or a tube, you can change the voltage on the input pin to produce different levels of gain).

Every method of activating the variable element will involve DC. The audio signal is AC, so you need to rectify it. This is done with a diode; one diode is half-wave rectification, two is full-wave. Half wave means that every crest of the wave cycle is a burst of DC, full-wave means every peak and trough are a burst of DC, so it's smoother. There's a compressor after the rectification that acts as a reservoir to smooth out the DC bursts by storing up charge and discharging it slowly. There will also usually be a resistor in parallel with the compressor to control the timing of this bleedoff, and the time can be calculated with normal RC filter equations. A second resistor -- even if it's hidden -- will be forming a low-pass filter with the reservoir compressor for the attack time. (Just for funsies, you can see the exact same practices of smoothing out DC in the power section of every pedal.) The rectified DC signal is then fed into something that will control the variable gain element, such as lighting up an LED or wiggling the gate of a FET.

I know this is a lot of technical information, so my suggestion is to grab a schematic of a low-parts-count compressor and trace the signal to see if you can find each of the parts I mentioned. In particular look for the rectifier circuit, and a variable resistance element attached to the audio amplifier.
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jkokura

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Re: which component makes a guitar compressor work
« Reply #3 on: June 09, 2021, 09:09:03 AM »
I'm so grateful for Jon's knowledge. He's pretty bang on.

I was going to give a cheeky answer and say, "all of them" but someone beat me to it.

I'll give a secondary answer as a question - which circuit? There are multiple 'genres' of guitar pedal compressors. If you want to choose one, some people (they're all smarter than me) can tell you what each component in the circuit is doing, and then you'll know more precisely what they do.

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Aentons

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Re: which component makes a guitar compressor work
« Reply #4 on: June 09, 2021, 12:58:14 PM »
OK tech guru's, which component makes the compressor compress?
The power jack...  8)

Zerro

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Re: which component makes a guitar compressor work
« Reply #5 on: June 09, 2021, 01:41:07 PM »
Very long path, to choose which of them all you would like. Really! So, any aproximations? Ratio, sensitivity, switches, bypass, attack-release-decay, tone??? I have some schematics, including here well known Bearhug, so, something can be done...
« Last Edit: June 09, 2021, 01:44:00 PM by Zerro »
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matmosphere

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Re: which component makes a guitar compressor work
« Reply #6 on: June 09, 2021, 01:58:43 PM »
Squirrels, they are the key element in most compression and modulation pedals, though strangely, not fuzzes.

mgalicki

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Re: which component makes a guitar compressor work
« Reply #7 on: June 09, 2021, 02:28:06 PM »
No kidding, I'm wondering if there are for example, as a delay pedal uses the 2290 chip. Does the compressor use various chips to achieve the compression effect? 

matmosphere

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Re: which component makes a guitar compressor work
« Reply #8 on: June 09, 2021, 03:40:10 PM »
No kidding, I'm wondering if there are for example, as a delay pedal uses the 2290 chip. Does the compressor use various chips to achieve the compression effect?

Some delays use the pt2399 but that isn’t the only way to get delay. There are a couple analog ICs people use and stuff like the FV1.

The compressor thing is also complicated. Some compressors use ICs some use transistors, some use optocouplers. So asking what part does the work, is far to broad a question.

Find a particular circuit you are interested in and start there. The engineers thumb and the bear hug (by midwayfair ^^^) are both great DIY options, very different circuit wise and sound wise.

Zerro

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Re: which component makes a guitar compressor work
« Reply #9 on: June 09, 2021, 03:41:47 PM »
This is allways an analog system. Some vendors are producing opamps which are complete compressors, and some of them are in delays as a pre-comps and past-expanders for delay lines, which need only certain level of signal, without peaks. NE570-571, in ManOWar for example, or MC33111... SSM2166 for mics...
« Last Edit: June 09, 2021, 04:00:10 PM by Zerro »
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Aentons

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Re: which component makes a guitar compressor work
« Reply #10 on: June 09, 2021, 08:57:34 PM »
No kidding, I'm wondering if there are for example, as a delay pedal uses the 2290 chip. Does the compressor use various chips to achieve the compression effect?

Some delays use the pt2399 but that isn’t the only way to get delay. There are a couple analog ICs people use and stuff like the FV1.

The compressor thing is also complicated. Some compressors use ICs some use transistors, some use optocouplers. So asking what part does the work, is far to broad a question.

Find a particular circuit you are interested in and start there. The engineers thumb and the bear hug (by midwayfair ^^^) are both great DIY options, very different circuit wise and sound wise.
This would be a good place to start the compressor journey.

https://www.electrosmash.com/mxr-dyna-comp-analysis