Some Discussion on Amp EQs

[vgbob]

Nick, thanks for taking the time to put all this together.
It explains, in a way that finally makes sense, why I've always
liked Ampeg or Baxandall style eq vs. FMV. To me the Ampeg
eq is just more intuitive in working the way you expect the
controls to work. That is not to say the other is bad, just that
it works in a different manner.

 

[dunwichamps]

Nick, thanks for taking the time to put all this together.
It explains, in a way that finally makes sense, why I've always
liked Ampeg or Baxandall style eq vs. FMV. To me the Ampeg
eq is just more intuitive in working the way you expect the
controls to work. That is not to say the other is bad, just that
it works in a different manner.

yes the James/Bax is definitely more representative to what you think is happening. Also my favorite

 

[two fingers]

Nice post! Thanks for the info!

 

[dunwichamps]

Nice post! Thanks for the info!

you're welcome!

 

[J-O]

Awesome post, thanks for taking the time to put all this info up. I can see why I love my v4 so much.

 

[dunwichamps]

Awesome post, thanks for taking the time to put all this info up. I can see why I love my v4 so much.

no problem. The V4 i believe has the same james/bax as the ampeg but the component values might be tweaked. The scheme does not show the part values unlike the SVT schemes which do. The mid control is the same circuit but I think they changed the inductors and caps around a little to get slightly different center frequencies on the bandpass.

 

[Passinwind]

I had originally posted this as a series of posts in the Doom/Stoner thread in the Bassist forum but I figured others might enjoy this discussion about classic amp EQs. I get a lot of questions every now and then about different amp EQs so I figured I could do a big overview of a bunch of popular ones. I wrote most of this from my memory and also using simulations in LTSpice. Please check it out and let me know if I have made an error or something you think should be added to this post.

The following EQs I will discuss.

1. A James/Baxandall EQ with the example being the one from an Ampeg SVT.

(Note: Technically speaking I am going to discuss a James EQ which is not the same as a Baxandall but these 2 have been so mixed together when talking about amps that I am calling them the same thing. This is just a semantic point IMO)

Very nice job. For completism I would just like to post Mr Baxandall's original article here so interested people can decide for themselves if calling the James stack "Baxandall" is appropriate. Personally, I think the distinction between an active stack and passive one is hugely important, but at the same time common vernacular makes it just amount to whizzing into the wind at this point...

Baxandall Negative Feedback tone control article.

 

[dunwichamps]

Very nice job. For completism I would just like to post Mr Baxandall's original article here so interested people can decide for themselves if calling the James stack "Baxandall" is appropriate. Personally, I think the distinction between an active stack and passive one is hugely important, but at the same time common vernacular makes it just amount to whizzing into the wind at this point...

Baxandall Negative Feedback tone control article.

thanks for the extra info! I am probably super guilty of spreading the improper terminology in my posts. I learned most of the difference from Merlin/Valve Wizards book and articles like the one you posted.

 

[Passinwind]

thanks for the extra info! I am probably super guilty of spreading the improper terminology in my posts. I learned most of the difference from Merlin/Valve Wizards book and articles like the one you posted.

Nearly everyone just calls the passive version Baxandall or James/Baxandall these days, so as you say it's basically down to semantics really.

I recently got around to building my own active stack in my last DIY tube bass preamp. It took a lot of digging around to find even a handful of interesting variants, and I ended up doing a ton of brainstorming in LTspice before I even touched a soldering iron. It really is a great tool for stuff like this.

 

[dunwichamps]

Nearly everyone just calls the passive version Baxandall or James/Baxandall these days, so as you say it's basically down to semantics really.

I recently got around to building my own active stack in my last DIY tube bass preamp. It took a lot of digging around to find even a handful of interesting variants, and I ended up doing a ton of brainstorming in LTspice before I even touched a soldering iron. It really is a great tool for stuff like this.

nice and LTSpice is my best friend. I pretty much simulate every amp in there to get an idea of whats going on. Hours spent on phase inverter design.

The baxandall in the article would be nice to try out someday, although I might try it with a triode instead of a small signal pentode.

 

[4Mal]

I'd like to see the FMB set at 666 and at 2-10-2. 666 is sort of considered the sweet spot for fans of the fender Telecaster guitar. I've used it myself but pushthe mids higher. (My tele has a Charlei Christian in the neck position and it justifies more detail all on it's own.

2-10-2 is of course the commonly agreed on 'best approximation' of flat with the RC/Fender tone stack.

As Passinwind knows all too well I'm a big fan of that stack. For all it's flaws, I find it musical in a way that many others aren't. I'm currently rockin a Music Man Rd100 which as I gather is not far off. Lovely amp in my rock'nroll life! Need not apply when doing my normal acoustic thing though...

 

[dunwichamps]

I'd like to see the FMB set at 666 and at 2-10-2. 666 is sort of considered the sweet spot for fans of the fender Telecaster guitar. I've used it myself but pushthe mids higher. (My tele has a Charlei Christian in the neck position and it justifies more detail all on it's own.

2-10-2 is of course the commonly agreed on 'best approximation' of flat with the RC/Fender tone stack.

As Passinwind knows all too well I'm a big fan of that stack. For all it's flaws, I find it musical in a way that many others aren't. I'm currently rockin a Music Man Rd100 which as I gather is not far off. Lovely amp in my rock'nroll life! Need not apply when doing my normal acoustic thing though...

I could do those sims. There is definitely a segment of people who love the Fender 60s clean tone and that gets some of its feel from the strong mid cut on the tone stack so it can be quite musical. As always if you dig then its good.

 

[Passinwind]

2-10-2 is of course the commonly agreed on 'best approximation' of flat with the RC/Fender tone stack.

That depends on actual pot tapers and some other things that make not all Fender tone stacks the same. You already have the Duncan Tone Stack Calculator widget, don't you Mal?

Here's an actual measurement I took of my old Alembic F2B at 2-10-2:

Link Removed

 

[dunwichamps]

I should have mentioned Duncan Tone Stack Calc. It is fairly useful if you wanna play with the common tone stacks. I find LTSpice better as I can do more pots and more controls with more accurate sims

 

[oinkbanana]

what kind of eq is used in OR120 orange amps?

and from what i'm reading, vintage SVT will use type 1&2?

 

[dunwichamps]

Matampas and Oranges uses James/Baxandalls, so #1

yes SVTs use 1 and 2

 

[jungleheat]

Interesting stuff. I have a MM 65-112 and eventually I want to get the tone stack modded so it's a bit more useful. Not necessarily wider response, but just shifting the frequencies that are emphasized to be more conducive to the tones I like. The bass on the "FMV's" is usually too tubby, the mid's are too boxy, and the highs are too nasal. So it would be great to push down the bass and mids, and push up the treble (frequencies). My ultimate goal would be something close to a Roland JC-120, but maybe swapping out component values to be more Marshall-esque would be a good start. Disappointed though that the presence control on most Marshalls wasn't included, as that is a big advantage to their design over Fender.

 

[dunwichamps]

You can mod the TS around to get a better response, You can use Duncan's Tone Stack Calc to see how it mods the EQ around as you change components. Google It!

 

[JimmyM]

Don't know which is correct, but I like saying "Baxandall" more than "James" so I stick with it

Very interesting thread as a longtime SVT/B-15 lover. Were your results directly from the preamp of an SVT or were they coming out of the output transformer? The reason I ask is that most folks say that the SVT won't be flat sounding with knobs at noon. I don't know how to test mine but I have compared it to an amp that has pretty flat response with knobs at noon and ended up with a couple knobs pretty far off noon on the SVT to match tones. But from what I've heard out of the post-EQ DI on SVT's, they do come across to my ears as flatter than through the cab.

 

[Passinwind]

Don't know which is correct, but I like saying "Baxandall" more than "James" so I stick with it

Very interesting thread as a longtime SVT/B-15 lover. Were your results directly from the preamp of an SVT or were they coming out of the output transformer?

From the OP:

I will be showing a # of graphics which show the input and output of these EQs in decibels as a function of frequency. This will show the frequency response of the circuit. For some added realness I will be including the actual tube driver stages so that the freq response plots reflect the loading of the tube stages for a more accurate plot. I will not include all of the gain stages in the amp tho, just the preceding tube driver stage and in the case of the active mid range control on the Ampeg, the 3 triode sections which comprise it.