Tone Stacks.....why/what/whuuu?

[John_C]

Why on earth are the tone stacks of most common amplifiers really strange affairs with controls that all rely on each other and basically just don't do what they say on the knobs?

For example, turning down the treble on some marshalls doesn't just turn down the treble, it turns up the bass and low mids as well! Why not just have a much more user friendly carefully tuned parametric eq?

How come we don't see so many tone stacks like this

that actually makes sense to me.....

 

[TheWinterSnow]

Because the original tone stack was made by Leo Fender back before the technology of linear equalization existed. The design has stuck and been a part of that signature guitar amp tone ever since while hifi audio amps became more linear as the technology advanced.

Amp designers today are afraid of straying too far away from the fender/marshal tone stack because many people like that coloration and expect near identical values of such tone stacks.

 

[Wolfeman]

Because it's hard to find inductors in that range that aren't the size of transformers. And you have to be careful in how those EQ's load down the amp.

Some amps do use those, like the Mesa graphic EQ and in some Fryette amps.

The FMV tone stack has just become a standard, something everyone is used to. I personally like how the tone controls interact with one another.

An even better method, IMO, is to use a james tone stack with a seperate mid control thrown in between a gain stage-

 

[TheWinterSnow]

Oh shit I didn't even see that, a 20H inductor would be huge (twice the size of a typical choke).

 

[Wolfeman]

Oh shit I didn't even see that, a 20H inductor would be huge (twice the size of a typical choke).

Yeah it's nuts, mouser doesn't carry any in that range in a small audio size.

The way a lot of builder get around it, is to use one half of a small transformer. All you have to do is check out the specs on them, do a little math to figure out what the inductance will be, and you are all set. When I had my Herbert apart, it looks like that is what Peter Diezel did to find a small inductor for the mid cut.

 

[John_C]

good spot about the inductor. Thanks for explaining this stuff for me, as you can tell i'm just setting out into trying to understand how amps really work and it's a great help!

Eric, would you mind explaining the little schematic you posted?
I can see that for example with the wiper all the way to the right you'd get got a high pass filter on the right and a (slightly less effective?) low pass on the left, but wouldn't that produce the same effect each side of the centre of the pot? Feel free to make me look like an idiot as i really have no idea!

 

[Wolfeman]

good spot about the inductor. Thanks for explaining this stuff for me, as you can tell i'm just setting out into trying to understand how amps really work and it's a great help!

Eric, would you mind explaining the little schematic you posted?
I can see that for example with the wiper all the way to the right you'd get got a high pass filter on the right and a (slightly less effective?) low pass on the left, but wouldn't that produce the same effect each side of the centre of the pot? Feel free to make me look like an idiot as i really have no idea!

No the scheme I posted is pure midcut, centered at 500hz. It's hard to explain the Q, but figure it to start and end at 100/1000hz.

 

[John_C]

mmhmm, it filters the highs and lows out, leaving the mids to go to earth. Or am i still looking at it wrong?

 

[Wolfeman]

mmhmm, it filters the highs and lows out, leaving the mids to go to earth. Or am i still looking at it wrong?

Eh, it's a simple bridged T filter, leaves the highs and lows alone and cuts mid.

Maybe I can explain it better this way. When the pot on top is at one extreme, it bypasses the whole network (flat response, input goes straight out the wiper).

Turn the pot the other way, and it starts to output from the right side of the filter network.

 

[John_C]

ah ok, i finally get you! I didn't realise where the input to the circuit was (top left of the T?). Seems to give -3dB points of 125Hz and 2kHz.

Thanks for the help, with that and a lot of research I'm learning slowly

 

[Wolfeman]

You got it. Framus likes to use that, they put that exact circuit smack dab in front of a James stack. Works pretty good IMO. I may try it out in my next build.

 

[Wolfeman]

ah ok, i finally get you! I didn't realise where the input to the circuit was (top left of the T?).

Yeah sorry I forgot to label the input