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Instruments and Amps Everything that makes music, Especially including instrument amps.

signal chain in guitar preamp, boost before eq or eq before boost
signal chain in guitar preamp, boost before eq or eq before boost
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Old 1st June 2017, 09:37 PM   #11
garybdmd is offline garybdmd  United States
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Thanks Gnobuddy and everyone else for the great comments. I think I probably wasn't super clear about the gain and EQ though. I really was getting at distortion per say when I said gain. I was more talking about a clean preamp.

1) Guitar-> buffer-> gain stage of ~3 -> passive tone control -> to power amp
2) Guitar-> buffer-> passive tone control -> make up gain ~3 -> to power amp

As far as the best order for tone controls, as has been mentioned there are a lot of parallel circuitry.

But in the preamp I am currently working on I was thinking roll of bass during and initial stage (bass control), treble roll off (treble control), and mid cut (bridge-t) style. Its sort of easy to incorporate simple tone controls between stages.

You can always have a small capacitor before a stage that you can bypass with a potentiometer for a bass control. You use the next stages input impedance in the RC calculation formula.

You can always place a treble capacitor to ground in series with a potentiometer for a treble control. You have to use the output impedance of the preceding stage in the RC calculation formula.

You can always sandwich a bridged-T filter between stages if impedance matching permits, with a potentiometer to ground.

You can also parallel an inductor in series with a potentiometer as a mid control to ground. In the RLC circuit you use the preceding stages output impedance in the RLC frequency formula and the width can be controlled by keeping the product of LxC the same and raising or lowering L and C.

So really I see that most guitar preamps/amps use all tone controls at once typically, but it seems multistage designs lend themselves easily to individual controls.

In practice I have typically rolled off bass first, then treble, then final mid control, but a few times I did bass control, then mid cut, then treble at end. Just my own meandering experience.

Taytalino asked why not go active (thank you for the circuit you posted). Gnobuddy's explanation is much more elegant than mine. My reasoning was just simplicity, if it seems to work well enough.
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Old 1st June 2017, 11:21 PM   #12
AquaTarkus is offline AquaTarkus  Canada
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It's advisable to reduce bass before any stages that get overdriven, or else the sound will be muddy,
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Old 3rd June 2017, 03:52 AM   #13
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by garybdmd View Post
I was more talking about a clean preamp.
If the preamp is truly clean, then it doesn't matter what order you have the various sub-circuits of your tone control in. There is a very elegant mathematical concept called "linear superposition" which electronic engineers learn, and which proves this.

But once you introduce overdrive, Aqua Tarkus' advice is the way to go. Linear superposition does not apply during (nonlinear) overdrive! So attenuate bass before overdrive, otherwise you'll have a muffled and woofy-sounding amp.

-Gnobuddy
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Old 3rd June 2017, 09:04 PM   #14
garybdmd is offline garybdmd  United States
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Thanks everyone! Thanks Gnobuddy. I am going to look up "linear superposition" and read more.

My clean signal chain is basically going to be:
buffer - toneshaping - make up gain - buffer - power amp
(or I may sandwhich some filters between the stages, still debating)

Distorted section will be:
buffer - bass rolloff/distortion section - tonestack - make up gain - buffer - power amp

I will post a schematic after I finish, which will be a while because I'm still perfecting the distorted section.

I'm switching gears so to speak to the distorted section of the preamp, I've also been working on the distortion section, I looked at the Boss Ds-1, which I kind of liked in general. I thought it was interesting that it has a high gain transistor stage followed by an op amp with a gain of 22. Seems opposite to me I thought the op amp was more suited to a high gain application and transistor to lower gain.

So I built the boss ds-1 but replaced the op amp section with a transistor section and it seems to work fine. I'm getting nearly identical waveforms in simulations and also nearly identical bode plot. If anyone is interested I can post but I've just replaced the op amp gain 22 with transistor gain 22.
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Old 4th June 2017, 03:07 AM   #15
AquaTarkus is offline AquaTarkus  Canada
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Quote:
Originally Posted by garybdmd View Post
My clean signal chain is basically going to be:
buffer - toneshaping - make up gain - buffer - power amp
(or I may sandwhich some filters between the stages, still debating)

Distorted section will be:
buffer - bass rolloff/distortion section - tonestack - make up gain - buffer - power amp
If your power amp is push-pull, you could use a long-tailed pair PI as the make up gain & buffer combined.
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Old 4th June 2017, 07:41 AM   #16
Gnobuddy is offline Gnobuddy  Canada
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Quote:
Originally Posted by garybdmd View Post
I am going to look up "linear superposition" and read more.
The basic concept is quite simple - for instance, if you run three different sinewaves (at three different frequencies) into a linear amp, then the output is still those same three sinewaves; no new frequencies are generated.

This also means that if you put, say, bass cut before treble cut, or switch the order around, it makes no difference; those three different sinewaves remain independent of each other all the way through the (linear) amp. Doesn't matter if you cut or boost one before the other.

But if you run those three different sinewaves into a nonlinear device - an overdriven amp, or a clipping diode, for example - then the output is no longer just the same three original sinewaves. We know about intermodulation distortion, for example - new frequencies get generated.

In the case of an overdriven guitar amp, the lower frequency bass signals from a guitar tend to be much larger than the higher frequency treble signals. When you run this combination of frequencies through an overdriven gain stage, the large bass signals tend to dominate the signal, and the smaller treble signals get squashed into oblivion.

So what comes out of the amp tends to sound like nicely distorted bass, but with very little treble to go with it. Muffled and woofy.

The time-tested way out of this is to cut the bass signals down to size, so they don't dominate the treble so much, before squashing the whole lot through an overdriven stage.

Which is sorta why the typical heavily overdriven guitar amp doesn't have all that much deep bass, and guitar amps that do have deep bass tend to be clean.

Quote:
Originally Posted by garybdmd View Post
I thought the op amp was more suited to a high gain application and transistor to lower gain.
Sometimes the gain-bandwidth product of an opamp limits how much gain you can get out of it, and still have a wide enough frequency bandwidth. The (now antique) 741 opamp had a gain bandwidth product of 1 MHz, and a DC open-loop gain of 100,000. That enormous gain only went from DC up to 10 Hertz (not kilohertz) though!

If you used negative feedback to cut the gain down to only 100, then you got a bandwidth of 10 kHz. Enough for guitar, maybe, just barely, but not for Hi-Fi. (The 741 wasn't suited to Hi-Fi in many other ways as well.)

So here was an opamp with a gain of 100,000, but for audio use, you had to keep gain down to 50 or 100 at most!

By the time the wonderful 5532 opamp came along, gain bandwidth product was up to 10 MHz. You could get a gain of 100,000 times up to 100 Hertz, or 1000 times up to 10 kHz. If you wanted a frequency response out to 40 kHz, say, you are still limited to a voltage gain of 250 times.

-Gnobuddy
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Old 5th June 2017, 02:07 AM   #17
AquaTarkus is offline AquaTarkus  Canada
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Quote:
Originally Posted by Gnobuddy View Post
T
In the case of an overdriven guitar amp, the lower frequency bass signals from a guitar tend to be much larger than the higher frequency treble signals. When you run this combination of frequencies through an overdriven gain stage, the large bass signals tend to dominate the signal, and the smaller treble signals get squashed into oblivion.

So what comes out of the amp tends to sound like nicely distorted bass, but with very little treble to go with it. Muffled and woofy.
In my experience, it's even worse than that, a mushy distorted mess, presumably due to intermodulation products.
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Old 10th June 2017, 01:25 AM   #18
garybdmd is offline garybdmd  United States
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Just wondering ... i know there's probably more than one answer but wheres a typical roll off frequency for the bass before distorting the signal. My amalysis for the boss ds1 was 200-300hz. (I don't think electrosmash analysis got that right). Ive read as high as 700hz for some circuits
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Old 11th June 2017, 03:38 PM   #19
AquaTarkus is offline AquaTarkus  Canada
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Quote:
Originally Posted by garybdmd View Post
Just wondering ... i know there's probably more than one answer but wheres a typical roll off frequency for the bass before distorting the signal. My amalysis for the boss ds1 was 200-300hz. (I don't think electrosmash analysis got that right). Ive read as high as 700hz for some circuits
Without having the math, to hand, I tend to think the 200Hz/300Hz is quit adequate, However personal preference also comes in.If you like the sound of an AC30 Top Boost, that amplifier cuts an extreme amount of low end in its preamp, possibly cloer to you 700Hz
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Old 12th June 2017, 12:29 AM   #20
Printer2 is offline Printer2  Canada
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Anybody have a three pole switch?

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