Overcoming interaction between control settings in FMV tone stack

[SoulFetish]

One thing to note is that sfter some experimentation, it seems that the location or stage in the circuit you add filtering is critical. For instance, by decreasing the coupling cap value of the proceeding stage right after the tone stack, and increasing the corner frequency of the HPF, it resulted in an overall sterile sound dominated by high frequency content. I’m going to try and partially bypass the input stage and also try cutting some of the low frequencies right before the open stage and see what kind of results I get.

[eschertron]

Maybe you went too far with the HP calculation? Double the size of the "new" coupling cap and hear how that behaves.
My reaction to your statements that you liked the character, etc., of the sound was that you'd want the tone shaping as late in the chain as possible. Coupling caps to the power tubes, even. +1 to Chuck's suggestion to make it even later than that.

[benito_red]

thanks a lot bob, sure i'm going to give a try to VOIGT tone stack , and I am happy that I learned a new thing

[bob p]

that control was originally used in order to "lift bass on records". it wasn't designed with guitar in mind, so you'll have to tweak the values of the components to get the controls into the range you like.

[Gnobuddy]

that control was originally used in order to "lift bass on records".

A year or two ago, I was searching for guitar tone controls that didn't suck, i.e., weren't so interactive. I found a brief description of the Voight tone control in Merlin Blencowe's preamp design book. I had never heard of it before, but it turned out to be my favourite guitar tone control so far. Only two knobs, though, if that matters to you.

you'll have to tweak the values of the components to get the controls into the range you like.

I don't know if the version in Blencowe's book is the Voight original, or he (Blencowe) already tweaked it. In any case, the printed frequency responses for the Voight tone control in Blencowe's book are no good for guitar - there is very little bass control range above 80 Hz, and that is as low as a guitar in standard tuning gets (okay, 82 point something Hz, if you must be picky!)

So I put together the Voight circuit in LTSpice, and tinkered away until I found a set of frequency responses I liked. Then I built it into a preamp I was designing for a little 2-watt 6AK6 guitar amp. I liked the tone control immediately, in actual use the knobs feel quite independent, and you don't have to hunt back and forth between them to get the EQ you want. Tweak the bass to taste, tweak the treble to taste, play guitar, enjoy. Happiness is not having to futz with FMV tone controls!

I'm attaching two images, both of LTSpice simulations of the circuit. The first image shows the effect of simultaneously stepping both controls from minimum to maximum, and gives you a general idea of the EQ curves you can get out of this circuit. Note that there is some interaction - the insertion loss at mid-band changes a little bit as you boost bass and treble. But the interaction is much less than what you get from Fender's monstrosity.

The second image shows the effect of stepping each control through its full range while leaving the other control at one position, then stepping the second once, repeating the same thing, and so on, until both controls have been stepped through all positions. The resulting frequency response plot is messy, but reveals a lot if you stare at it. For instance, the lines bunch up near full cut, telling you the (log) controls are less effective near their full cut positions. Most telling, though, is that the lines do fall into pretty distinct groups - the treble control doesn't yank the bass lines all over the map, and vice versa. Not much control interaction, in other words.

If I had one criticism of my version of the Voight, I think I would prefer a lower frequency crossover point - it would be nice to have a little more control range at the treble end. I might tweak it some more and try it again. But your ears may or may not feel the same way, it depends so much on your guitar, speaker, personal taste, etc, etc.

One of these days I will try out active tone controls in a guitar amp (negative feedback around a frequency-shaping network, like the Baxandall used in Hi-Fi for decades.) The catch is that you have to make absolutely sure you never overdrive the tone-control: if you do, the negative feedback goes away, and with it, the intended frequency response! I think I can solve this by simply inserting a fixed attenuator between the previous gain stage and the input to the Baxandall, set so that when the previous stage is overdriven to full clip, and the Baxandall is at full bass and treble boost, the Baxandall itself still doesn't clip.

-Gnobuddy

[Gingertube]

Minimalist approach is to use the HiWatt tone stack. Only 2 more components and it is quite a bit less interactive than the standard FVM.

Also, if using the FVM then the cathode follower driven version is less interactive than the anode driven version.

Cheers,
Ian

[benito_red]

Gnobuddy, thanks for sharing such interesting info about VOIGHT spice simulations.
For shure I will test the VOIGHT but I think only the passive version, as I am not a fan of active controls (expecially presence controls) because in my modest opinion they suck in tone quality.
My concern is that delay of the feed-back signal vs the incoming signal can be cause of transient distortion and mostly not in a good way.

[Mike Sulzer]

Gnobuddy, thanks for sharing such interesting info about VOIGHT spice simulations.
For shure I will test the VOIGHT but I think only the passive version, as I am not a fan of active controls (expecially presence controls) because in my modest opinion they suck in tone quality.
My concern is that delay of the feed-back signal vs the incoming signal can be cause of transient distortion and mostly not in a good way.

I agree that there is an issue with feedback tone controls for guitar amps, but I do not think it involves time delay, which is relatively small. The issue is overloading. In general, circuits using a lot of feedback overload abruptly. Whether this is objectionable, or of any importance at all, depends, for example, on where the tone circuit is located, but it is something to watch out for.

[MarkusBass]

Gnobuddy, could you please post your simulation? I'd like to tweak it a little bit more. It' s very interesting tone control circuit.

Mark

[Chuck H]

I agree that there is an issue with feedback tone controls for guitar amps, but I do not think it involves time delay, which is relatively small. The issue is overloading. In general, circuits using a lot of feedback overload abruptly. Whether this is objectionable, or of any importance at all, depends, for example, on where the tone circuit is located, but it is something to watch out for.

And I'd add that the perceived "quality" of tone is subjective. WRT guitar amps, which are prone to be intentionally distorted by the user, our sensibilities (if not benito's ) can become accustomed to certain distortions and even grow to prefer them.

[Mike Sulzer]

And I'd add that the perceived "quality" of tone is subjective. WRT guitar amps, which are prone to be intentionally distorted by the user, our sensibilities (if not benito's ) can become accustomed to certain distortions and even grow to prefer them.

Yes, it is subjective, but there are majority preferences, which tend in the direction of low number harmonics and lower order intermod products. And yes, distortion is very important. Even "clean" playing can be highly distorted on the picking transient, affecting the perceived tone.

[uneumann]

If I had one criticism of my version of the Voight, I think I would prefer a lower frequency crossover point - it would be nice to have a little more control range at the treble end. I might tweak it some more and try it again. But your ears may or may not feel the same way, it depends so much on your guitar, speaker, personal taste, etc, etc.
-Gnobuddy

Having struggled with EQ (like the OP), this point resonates. Just my 2-cents - but I'm rarely troubled by control independence as much as control to extend EQ range. Making BMT independent can be useful (certainly can't hurt), but if your ears tell you to shift the midrange dip lower, or raise the start freq of treble emphasis, the EQ circuit needs controls that can achieve that EQ response. Fender, Voight, Baxandal (and others) all have their range tweaked for a particular amp, guitar, spkr, player, and room. When conditions change, the tone stack is often unable to compensate.
I have not found a silver bullet - still lamenting the problem - but I suggest that anyone frustrated with their EQ consider adding controls that extend the range the tone stack can achieve. That's the direction I find helpful. YMMV.

[Gnobuddy]

Gnobuddy, could you please post your simulation? I'd like to tweak it a little bit more. It' s very interesting tone control circuit.

Mark

Sure thing, here you go.

I had to rename the file to a .txt extension (rather than .asc) for the forum software to accept it for upload, so please remember to switch the extension back to .asc before you try opening it in LTSpice.

-Gnobuddy

[bob p]

I sent an email to webmaster@duncanamps.com asking if he'd consider adding the Voigt tonestack to Duncan't Tonestack Calculator.

I'm a little worried as the web site says not to contact them by email, to use the forum instead, but the forum is DOA. Does anyone have better contact info?

[Gnobuddy]

Gnobuddy, thanks for sharing such interesting info about VOIGHT spice simulations.

You're very welcome!

I am not a fan of active controls (expecially presence controls) because in my modest opinion they suck in tone quality.

Honestly, I have never seen active tone controls in any guitar amp. It makes sense from a 1950's commercial manufacturer's viewpoint: passive controls are much cheaper, and we need to attenuate the guitar signal between stages anyway, so what do we need active controls for?

But I don't need to make a profit on my amps (I'm only building for myself). Cost is not an issue, an active tone control would work better with a MOSFET than a tube anyway, and MOSFETs are cheap. Because of the negative feedback, distortion will be too low to hear, so an active tone control would have no "tone" of its own, and sound transparent, only providing EQ. The big advantage is that you can really make the controls independent.

So I'm going to try it, at some point. Maybe I'll like it, maybe I'll hate it. Even if I like it, there is no guarantee that anyone else will. But that's okay too, since I'm not trying to make a product to appeal to anyone else - just build myself an amp that only I will use.

My concern is that delay of the feed-back signal vs the incoming signal can be cause of transient distortion and mostly not in a good way.

Beware of what you hear from today's crop of mostly technically ignorant audiophiles. Many don't have a clue, and believe a lot of nonsense. The nonsense gets repeated over and over, and eventually, there is so much nonsense that it can be very hard to find the reality at all.

How do we know if delay in the feedback signal is a problem? Simple. If there was a delay in the feedback, it would immediately show up as either outright oscillation, or a big peak in the frequency response curve: delayed feedback is mathematically identical to phase lag, and too much lag causes oscillation. If there's not enough to cause oscillation, it causes a peak in the frequency response.

So if you measure a nice well-behaved frequency response, it's a guarantee that there is no mysterious feedback delay lurking in the background.

Measuring frequency response is quite easy to do, all you need is a sine wave signal and some way to measure the strength of the signal. You can even do this with the sound-card in your computer and a few external components.

In reality, today's semiconductors are very fast, and provide accurate feedback far, far beyond the highest end of the audio bandwidth. Of course the feedback is delayed - but the delay is so short that it doesn't matter until you get to frequencies far, far, far, far above audio.

For guitar, we only need things to behave well until maybe 10 KHz. This is trivially easy with the amazing transistors and MOSFETs we can buy cheaply today.

-Gnobuddy