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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 20th May 2017, 08:42 PM   #1
garybdmd is offline garybdmd  United States
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Default signal chain in guitar preamp, boost before eq or eq before boost

Hi there, thanks for looking. I am building a guitar preamp and I was wondering if the EQ/toneshaping goes before or after signal a gain stage. I'm not talking about gain in the sense of gain control on distortion. I'm just talking about boosting the signal.

The tones shaping/EQ section was going to be passive, so I was thinking you would first take the guitar signal, boost it, then to the passive tone shaping.

If I first ran the signal into passive tone shaping first , I was thinking there would be some signal loss, (by definition since its passive), then having "make-up gain" after this would give more noise, (lower signal to noise ratio).

Do I have this right? I have included a rough sketch of how I was going to lay things out in the schematic. Assume that all the stages are built so that impedance matching is not an issue, I will design the stages that way.
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File Type: png signal chain guitar preamp.png (13.8 KB, 257 views)
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Old 20th May 2017, 08:48 PM   #2
garybdmd is offline garybdmd  United States
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Default Help or advice on best signal chain for bass, mid and treble filters

Hi, I was wondering in general, if I were to build a tone control with bass, mid and treble in a sequential fashion, what would be the best order to put the controls in. I was going to use passive controls.

I was thinking bass cut first, then treble cut, then mid cut. Is there a best order or does it really matter?

Assume the stages would be impedance matched or buffered appropriately.
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Old 20th May 2017, 09:00 PM   #3
rayma is online now rayma  United States
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Originally Posted by garybdmd View Post
I was going to use passive controls.
I was thinking bass cut first, then treble cut, then mid cut.
Usually they're in parallel.
Otherwise, high filtering first would give more headroom in the buffers.
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Old 20th May 2017, 09:03 PM   #4
DF96 is offline DF96  England
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It is best not to use passive controls. By choosing passive you have already decided to accept signal degradation.
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Old 22nd May 2017, 03:39 AM   #5
PRR is offline PRR  United States
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> Do I have this right?

So far as you write, yes.

But the drawing shows gain after distortion. Most distortion schemes make BIG output, but need gain *before* (or included) to do the big dirt. Gain after distortion is rarely needed.

But plagiarize plagiarize plagiarize ! You are not the first down this trail. Study ALL the schematics you can find. There are many variations, sometimes for no good reason; it may come down to stealing one you know works good and adapting it.
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Old 22nd May 2017, 06:25 AM   #6
tataylino is offline tataylino  Philippines
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It is not a good idea to connect them in series especially for passive networks, you will have big amount of attenuation there. Why not try this active type:
3 band tone control design – tataylino.com
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Old 22nd May 2017, 08:40 AM   #7
milkshake is offline milkshake
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When your circuit behaves linear, then it doesn't matter where you put the EQ.

But when it behaves non-linear, it matters a lot, a whole lot.

In practice guitar amp distortion stages have high pass filters just before them or build in. In tube circuits that could be the cathode bypass cap having a small value, and its worth experimenting with these values to suit your taste.

This is an interesting read on build-in EQ of distortion circuits: The Technology of the Tube Screamer

Most amps follow this: Signal - high pass filter - distortion - EQ - output.

When producing music I always EQ before and after distortion, that gives the most control.
I don't have to think!
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Old 23rd May 2017, 02:54 AM   #8
thoglette is offline thoglette  Australia
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Originally Posted by milkshake View Post
When producing music I always EQ before and after distortion, that gives the most control.
+1 this. You're doing it any way, so do it consciously
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Old 23rd May 2017, 04:25 AM   #9
AquaTarkus is offline AquaTarkus  Canada
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Best to have EQ after distortion.

Sent from my phone. Please excuse any typpos.
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Old 23rd May 2017, 06:05 AM   #10
Gnobuddy is offline Gnobuddy  Canada
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Originally Posted by DF96 View Post
It is best not to use passive controls. <snip>
Originally Posted by tataylino View Post
Why not try this active type: <snip>
As I found out a few years ago, passive tone controls are almost universally used in (valve) guitar amps. At first, this seemed bizarrely primitive to me, considering how poorly these tone controls work, and also considering that "Baxandall's bass and treble circuit, when made public in Wireless World (1952), 'swept all others before it', as Wikipedia puts it. Nineteen fifty-two was a long time ago!

But on reflection, it turns out that there are some good reasons for using these primitive and lossy passive tone controls, all hinging on one key concept unique to guitar amps: overdrive, deliberately overloading one or more gain stages until they clip, and clip severely.

Think for a second about that lovely 1952 active Baxandall tone control circuit. What happens if you overdrive it, and cause the output to clip?

Well, once into clipping, the output signal gets clamped at a more or less constant voltage. This causes the negative feedback signal to no longer track the input signal. Essentially, all negative feedback disappears.

And when that happens, so does the "tone control" action, which functions only because of those frequency-dependent negative feedback networks Baxandall designed.

Worse, the removal of negative feedback unleashes most or all of the open-loop gain around the signal zero-crossings; the result is extremely harsh and abrupt overdrive.

So reason number one to avoid active tone controls in a guitar amp is that you cannot overdrive them at all, otherwise they stop working as tone controls, and simultaneously create unpleasantly harsh distortion. And this must hold true even at the most extreme tone control settings.

It is possible to do this - you have to put a large amount of signal attenuation between the previous (valve) stage, and the input of the active tone control, so that even when the preceding valve is heavily overdriven, the signal to the active tone control is sufficiently small, to prevent it from ever being overdriven itself.

But what we've now been forced to do is attenuate the heck out of the signal - exactly one of the biggest failings of passive tone controls, and we've been forced to introduce it into our design because we were trying to use a superior active tone control!

There is a second reason why passive tone controls are preferred in guitar amps. This is simply that, when overdriven, valve stages put out very large-amplitude signals - hundreds of volts, peak-to-peak. Often we want to follow this overdriven stage with another valve gain stage, but that stage can only accept a few volts peak-to-peak at its input. Certainly not hundreds of volts.

So we have too much signal, and need to attenuate it heavily before we feed it to the second stage. As long as we have to attenuate the signal, why not use a passive tone control circuit to do the job? We get the attenuation we need, and, as a bonus, get the tone controls that we also need.

Once I realized these things, my focus shifted away from introducing active tone controls into my own guitar amp circuit designs. Instead, I started to look for a better passive tone control design, something nearly as good as that lovely active Baxandall, if possible.

After lots of reading and LTSpice simulations, I started with the Voight tone control circuit from Merlin Blencowe's guitar preamp design book, and tweaked it until it gave me something very close to what I was looking for: very little interaction between bass and treble controls, smooth and relatively linear EQ changes with potentiometer rotation, and a nearly perfectly flat setting somewhere in the middle. See attachments for some of my results.

The second image shows that there is some interaction between controls, but it is quite slight compared to the awful "Fender-Marshall-Vox tone stack" that has been so wildly popular for sixty-odd years.

The second of the two attached circuits has been built and is in use in one of my guitar amp designs. I haven't measured the frequency response curves to confirm that it matches the LTSpice simulations, but my ears and brain say that it works well. It is now my favourite guitar amp tone control circuit.

(Note that the apparently obvious solution - the "passive Baxandall" - did not work well at all in my simulations. EQ boost and cut curves were lopsided and wildly assymmetrical, both with log and linear pots.)

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File Type: png Tweaked_Voight_1.png (35.5 KB, 214 views)
File Type: png Tweaked_Voight_2.png (100.6 KB, 209 views)
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