Inter stage Frequency Response and Transfer function

[Vivek]

I am interested in the Inter stage Frequency Response and Transfer function of stages in a Distortion Pedal or Amp

Please share your knowledge.

Transfer function of the DIGICRUNCH Amp model in DIGITECH MFX



Pre-distortion and post distortion equalisation curves of the Dual Rect model in DIGITECH MFX

[Steben]

As I mentioned earlier: pre EQ looks like low bass - full mids - low treble. Actually, a classic treble booster circuit like rangemaster does just that: high pass filter combined with the lowpass loading of the inductive guitar pickup.
Transfer function looks close to a triode stage.

The pre + post curve only really makes sense when there is no clipping. The many mids normally get clipped. So they form a driven sound without low flabby clipped frequencies and without harsh clipped high frequencies.


A typical Fender blackface stage has much less mids, rather mid cut. Again, one of the reasons crancked, those Fender amps sound less "rocky". An EQ transforms the sound.

[Digital Larry]

Very brief approach to understanding transfer functions.

a) Bends in the transfer function create harmonics.  Sharper corners create higher harmonics.  The sharper the corners, the more high order harmonics you get.
- a straight line transfer function has no corners at all and does not introduce any new harmonics - it's "linear"

b) Asymmetric transfer functions result in even order harmonics.
- a completely symmetric transfer function will have only odd order harmonics

[Vivek]

A typical Fender blackface stage has much less mids, rather mid cut. Again, one of the reasons crancked, those Fender amps sound less "rocky". An EQ transforms the sound.

Do you have graphs of the EQ of each stage in different amps or pedals ? That's what I am after

[teemuk]

I have some but can't access my archives for a moment. In practically all "high gain" designs the pre-emphasis is similar (for obvious reasons) but there are differences in at which mid-range frequency the peak actually is (i.e. 1kHz - 2kHz), Q of the peak, etc.
"Gull wing" response is typical mid-emphasis and again you can expect differences in frequency ranges, in amount of boost and attenuation, etc.

[willienillie]

Do you have graphs of the EQ of each stage in different amps or pedals ? That's what I am after

Not sure, but maybe the Duncan Tone Stack Calculator will be of some use for you:

http://www.duncanamps.com/tsc/

That one you download and install, very small program.  Somebody put up an online version, but I don't have a link.

[Vivek]

Not sure, but maybe the Duncan Tone Stack Calculator will be of some use for you:

http://www.duncanamps.com/tsc/

That one you download and install, very small program.  Somebody put up an online version, but I don't have a link.

The online version is here : https://www.guitarscience.net/tsc/info.htm

[Vivek]

I have some but can't access my archives for a moment. In practically all "high gain" designs the pre-emphasis is similar (for obvious reasons) but there are differences in at which mid-range frequency the peak actually is (i.e. 1kHz - 2kHz), Q of the peak, etc.
"Gull wing" response is typical mid-emphasis and again you can expect differences in frequency ranges, in amount of boost and attenuation, etc.

Wow, what an honour to have a reply from the Guru Teemu K.

Thank you for your insights, posts and the Audio Bible book!!!

I am trying to design a multistage Amp in a box.

I am looking for details of the pre-distortion frequency response and the post distortion Frequency response of some of the famous sounds like "American", "British" etc. Then I will try to implement that in hardware.

(The graphs I posted in the original post were from some MFX software that I wrote about 15 years ago. It would run on DIGITECH MFX pedal)

[teemuk]

I won't access my archives anytime soon (currently I have no computer, just a mobile phone) but "American" and "British" are quite broad definitions.

You could do what I did: download SPICE software and simulate circuits of dozens of amps you see most fitting to those categories, or just in general. Simulate, simulate, simulate...

It's an arduous and very time-consuming task, I know, but what you're basically asking is that other people do your homework and research. ;-)

You will likely find some general design patterns from amps of specific categories. I know I did.

I assume...
American? = Fender-ish, gullwing pre-emphasis pre distortion (tonestack preceding distorting stages, broad bandwidth towards high and low ends of spectrum, mid-range notch). Fender-derived circuit architectures.

British? = Mid-range heavily pre-emphasized, notch filtering in varying degrees post emphasis (distinct treble boosting in preamp before distortion, tone controls post distortion). Marshall derived circuit architectures.

[ElectricDruid]

Do you have graphs of the EQ of each stage in different amps or pedals ? That's what I am after

I've got details up on my website for the Digitech PDS-1150 and the Boss Metal Zone:

https://electricdruid.net/digitech-pds-1550-distortion-pedal-analysis/
https://electricdruid.net/boss-mt-2-metal-zone-pedal-analysis/

There's similar stuff for some other pedals up on Electrosmash. For example, the TS808:

https://www.electrosmash.com/tube-screamer-analysis

HTH,
Tom

[Vivek]

I've got details up on my website for the Digitech PDS-1150 and the Boss Metal Zone:


HTH,
Tom

Thanks Tom

You were kind enough to email me your MT2 spice files about a year ago !!!! Thanks !!!!

I had a project at that time, to enter the frequency curves of the MT2 into a digital MFX, to emulate the MT2. I did a curve fitting and generated best biquad filter coefficients to approximate the frequency response of the MT2. I posted my data on your site.


Now I will pull up your files again, and study Transfer Functions of each stage.

However this time, I am aiming at a preamp or a Multistage Distortion for rock. The MT2 and the Chainsaw are too rad for my current application.

I entered the Pete Cornish CC-1 crunch pedal into LTSPICE, and was contemplating writing an article similar to your style. Can we actually collaborate on this ?


Vivek

[ElectricDruid]

Aha! That was you! Sorry, I hadn't made the connection.

I agree with Teemuk. Simulation is the way to go with this. And it doesn't just have to be amps. Get the circuits for some well-regarded drive pedals and do a sim of them. You can study the frequency responses at each stage, analyse the amounts of gain they use in each stage, etc etc. That'll give you a sound basis of material and ideas to use to put your own design together.

[Vivek]

I agree with Teemuk. Simulation is the way to go with this.

Thanks for the sage advice !!!

So far, I analysed about 10 distortion pedals after looking at the schems of 250 pedals at pedalpcb.com and Aion.

But I dont know how to analyse Tube Amps, and that's the concept I am after for my next pedal. Lots for me to learn.

[Rob Strand]

The Boss OD3 and Boss BD2 are examples of pre-post eq.

The voicing of a pedal is different to an amp, since a pedal feeds into an amp.
The amp's voicing adds to the post EQ.