Overcoming interaction between control settings in FMV tone stack

[SoulFetish]

***So this post ended up branching off into looking for insight into better controlling frequency shaping throughout amplifier stages.**

I'm becoming frustrated at the interaction between bass and treble frequencies when adjusting the potentiometer values in my tone stack. I'm really looking to trim a fair amount of the bass, I guess. (without simultaneously increasing the highs). It would be great to have a little more isolation in the tone controls, but once I find a good setting, I really don't change them to be honest. So, finding a solution without having to redesign the tonestack is fine.

I like the touch response I'm getting. The overdrive is tight and I'm not getting blocking. I guess my concern is, if I alter the values of interstage coupling caps too drastically, it may upset some of the circuit dynamics–excursion during overdrive, etc.
Also, I'm using a 15" speaker with a massive alnico magnet. It's a 1960's Jensen P15L. From what I could find out, it's a full range speaker that jensen designed as part of their most efficient line. But, I'm wondering if this might be a place to start in cutting some of the bass (or, should I be looking at the front end of the amplifier??)
For those with experience in the frequency response of a range of speakers, I was able to find these low res plots. I was hoping you could way in on whether or not this is a practical choice for guitar.

[Enzo]

Have you downloaded Duncans Tone Stack Calculator? If not, go to duncanamps.com and get it.

[SoulFetish]

Have you downloaded Duncans Tone Stack Calculator? If not, go to duncanamps.com and get it.

yes, (it's awesome) and I've run a hundred sims. I just don't seem to be able to create any decent isolation. Plus, I've run so many sweeps with different combinations, I'm trying to translate visual data into what something familiar I can "recognize" in a practical sense. Does that make sense?

[Enzo]

Well I was kinda thinking that watching the classic Fender stack, there is no way to make them not interact, suggesting a completely different stack might be the solution.

[bob p]

What Enz said.

The problem is that there's no way to avoid interaction with an FMV tonestack. There is no isolation -- interaction is the nature of the beast.

If isolation is what you're after, then there really isn't any other option other than to go to one of the "one knob" bass and treble controls, or the combination controls that are built by combining the one-knob bass and treble circuits. Although the term "tonestack" gets used in a generic sense to describe any tone control circuit, those tone control circuits aren't actually tonestacks, because they are not "stacked" like the FMV configuration. They also don't suffer from the FMV's lack of isolation.

Before I'd start worrying about the speaker or adding filters elsewhere in the amp, I'd consider changing to a less interactive tone control arrangement, like a James, Baxandall or Voigt. The Voigt is a particularly nice arrangement; it only uses one more resistor than the old Brown Pro/Bandmaster setup, but markedly improves isolation. I don't know why the Voigt arrangement isn't more popular.

[SoulFetish]

What Enz said.

The problem is that there's no way to avoid interaction with an FMV tonestack. There is no isolation -- interaction is the nature of the beast.

If isolation is what you're after, then there really isn't any other option other than to go to one of the "one knob" bass and treble controls, or the combination controls that are built by combining the one-knob bass and treble circuits. Although the term "tonestack" gets used in a generic sense to describe any tone control circuit, those tone control circuits aren't actually tonestacks, because they are not "stacked" like the FMV configuration. They also don't suffer from the FMV's lack of isolation.

Before I'd start worrying about the speaker or adding filters elsewhere in the amp, I'd consider changing to a less interactive tone control arrangement, like a James, Baxandall or Voigt. The Voigt is a particularly nice arrangement; it only uses one more resistor than the old Brown Pro/Bandmaster setup, but markedly improves isolation. I don't know why the Voigt arrangement isn't more popular.

I've been thinking about experimenting with "unstacking" the tone circuit into different stages. I don't know, this isn't very encouraging. I've already built the amp.
I'm going to think about if I can convert it without having to tear into that side of the amp.
On a side note, is the eq circuit in the Lab Series an active tone control? There is a lot of control you get throughout the full range of the eq. Any analysis on the circuit?

[uneumann]

I've been thinking about experimenting with "unstacking" the tone circuit into different stages. I don't know, this isn't very encouraging. I've already built the amp.
I'm going to think about if I can convert it without having to tear into that side of the amp.
On a side note, is the eq circuit in the Lab Series an active tone control? There is a lot of control you get throughout the full range of the eq. Any analysis on the circuit?

A relatively simple extension to the FMV stack is the slope control - see Blencowe's preamp book for an example. The idea is to shift the scoop up/down the mid-range spectrum. While it doesn't decouple the controls, it extends the range of possible EQs greatly. Personally, I find shifting the scoop down towards the 200-400Hz range is a very useful way to control bass. Since that's also the range of spkr impedance dips, it can help control those effects too. Blencowe shows a pot control, but even simpler (and taking less panel space) is a 3 position sw to select among 3 scoop options.

[R.G.]

The real bottom line is as stated: the tone/volume stack inherently is interactive between the treble and bass controls and a mid control if that resistor is made into a front panel control. And "unstacking" with buffering between controls will work, at the cost of complexity. Other control formats can be made to work, with various degrees of interaction.

Removing the interaction requires removing the interaction - by separating the filters with either circuit elements or buffers.

[pdf64]

...Also, I'm using a 15" speaker with a massive alnico magnet. It's a 1960's Jensen P15L...

My understanding is that vintage alnico magnets will now have lost a significant degree of their original magnetic charge, and that may affect the bass resonance magnitude and Q, and the general tightness of the bottom end, as well as the overall sensitivity.
So a suitable ceramic type may be a better fit to your goals.
But if those plots are representative of their current state, and they compare well with similar speakers, then it may not be an issue.
And my EV SRO12 probably isn't much younger, yet it's still monstrously sensitive.

[Mike Sulzer]

If you separate the filters, remember the usual tone stack response with all three controls on 5 emphasizes both bass and treble. The second is most important: an amp can sound dead without it.

[benito_red]

What Enz said.

....... I'd consider changing to a less interactive tone control arrangement, like a James, Baxandall or Voigt. The Voigt is a particularly nice arrangement; it only uses one more resistor than the old Brown Pro/Bandmaster setup, but markedly improves isolation. I don't know why the Voigt arrangement isn't more popular.

hi Bob, finding a less interactive tone control is what I am after as well. I searched the schematics of VOIG in the web but I did not find any. Could you pls provide the schematic of such tone control?
Thanks
benito

[bob p]

Voigt wrote about it in the 1930s. It was designed for use with old record players. I could not find it with Google either -- I guess you have to be an old radio guy to know where to find it.

From: Wireless World, April 1940:

[eschertron]

If some HPF is needed, add a fixed filter somewhere in line to cut bass. The AC15 comes to mind (with a 4-pole rumble filter!) and IIRC the fixed filter shows up in Ampeg and some other designs.

[Chuck H]

Reading the original post and hoping I interpret your desire correctly... You like the character of the tone you have now, but not the EQ of the final sound.?. So this isn't a matter of what the amp is doing because that would change the character of the feel/modulation of the amp. With this in mind I say don't alter anything with the circuit. Now, you have that neato 15" speaker which I'll guess you hope to keep in the circumstances. Because I'd ordinarily say "try different speakers". But, would it be possible to put the 15" you have into a shallower cabinet? Or, if it's a closed back cabinet, maybe you could modify it into an open back. I know the speaker cabinet, in this case, is probably the hardest component in the scenario to change, but it WOULD allow you to keep other aspects that you like.

[bob p]

a schematic might help.