Average pre-amp frequency response curve?

[composition4]

Hi all,

I'm making an effect that I want guitarists to be able to send the output: a) straight into a power amp/effects return or b) into the guitar input on an amp. I want it to sound approximately the same in either case, so on the "to guitar amp input" jack I intend to have an EQ opposite of the guitar amp's clean channel preamp section EQ.

Obviously it'll vary a lot depending on the type of amp, but has anyone already done the research on what an "average" Fender/Marshall/Mesa/Crate/Whatever preamp clean channel EQ curve looks like with all controls mid-way (which I'm aware isn't flat response on most amps). I'm not just thinking about the tonestack response, but also any effect of coupling caps)/fixed filters/miller capacitance/loading etc. I'm hoping someone has maybe looked into this so I can save myself hours of simulation and/or getting hold of amps and measuring!

Thanks in advance
Jon

[GibsonGM]

Maybe this will help?

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

Once you're in the ballpark, tweaking to taste shouldn't be TOO hard, I'd think...

[antonis]

What Sir Mike said but in an ACTIVE configuration.. 
(+/- 12dB should be fine for your need..)

[composition4]

Thanks for the responses, appreciated.  I'm not sure if I'm making my intentions clear enough though.  I already know things like a typical Fender tonestack has a mid scoop around 500hz when all knobs are at 5 etc, however obviously there's coupling caps and other intentional and non-intentional filters in a "typical" preamp.  I'm not looking for suggestions for tone controls, I'm just asking if someone has measured the EQ difference between the preamp input and preamp output of a Fender, or Marshall etc so I can set a fixed EQ on the output of my pedal to counteract that to a degree

Thanks
Jon

[merlinb]

I already know things like a typical Fender tonestack has a mid scoop around 500hz when all knobs are at 5 etc, however obviously there's coupling caps and other intentional and non-intentional filters in a "typical" preamp.

Traditional Fenders/Marshall generally have very broadband coupling (they were based on early hi-fi circuits), so the frequency response of the whole preamp basically is the frequency response of the tone stack. Easiest thing to do is to simulate an example circuit. Here's a sim of the JCM800 lead (all tone controls at midway point, gain at about 8/10). This amp has more treble boost and bass cut (from the first stage cathode bypass cap) than most, but you can see it adds only a modest downward tilt compared with the tonestack itself. The treble boost becomes more exagerated at lower gain settings.

[composition4]

Thanks Merlin... yes I'm living inside circuit sim world at the moment but just a bit time-poor and hoping someone had done the sims or measurement.  I thought there was a fair bit of other frequency shaping other than the tonestack, but it sure makes my job easier if not.  Would you suggest the same could be said for more modern clean channel circuits on other amps in your experience?  I'm assuming the answer might be yes given that most amp clean channels are fairly derivative of the classics?

[marcelomd]

Maybe this will help?

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

Once you're in the ballpark, tweaking to taste shouldn't be TOO hard, I'd think...

This one, made by our forum friends, has some extra stacks: http://www.guitarscience.net/tsc/info.htm, check it out!

[PBE6]

If the vast majority of the EQ curve is determined by the tonestack of a given amplifier, it seems that replicating that tonestack but placing it in the feedback loop of a non-inverting opamp circuit would get you virtually all of the way there.

[Rob Strand]

Considering only the tone-stack there is a lot of room for variation:  Marshall, Fender, control settings, different "slope resistors".   Add onto that the things merlinb mentioned and you realize the response is all over the place.   The dip in the response is quite different between Fender and Marshall.   One option is to just pick an amp then pick a favourite setting.  If you look at some small modern tube amps some have a preset tone control (like a Marshall or Fender tone control) then they just add a simple one-knob tone control on top of that.

[PRR]

My reference curve for clean Fender blackface would start from:

-3dB @ 50Hz
0dB 100Hz-500Hz
+3dB @ 1KHz
+6dB 2KHz-15KHz, falling past 16KHz

A mid-dip on top of that is very popular.

You need optional top-cut for mellow backing vamps, a bass-cut for screeming leads.

But further: the speaker AND its interface with the cabinet and the amplifier make a lot of difference. Guitar through a good PA system is unconvincing.... the 1930s Magnavox-derived speaker cone's response IS the "soundboard" for guitar tone. And the speaker is responsive to amplifier damping. Even a funky-cone speaker made for guitar will sound different on high-damping all-purpose power amp or low-damping for-guitar amplifier.

[composition4]

Thanks guys, you've given me a pointer in the right direction