Overcoming interaction between control settings in FMV tone stack

[Gnobuddy]

Having struggled with EQ (like the OP), this point resonates.

Don't forget that there is a pretty easy way to slide the Voight tone control curves up or down in frequency - just scale all capacitors (there are only two!) in the circuit by the same percentage.

For instance, if you make every capacitor 50% bigger (i.e. it is now 3/2 of its old value), every frequency on the graphs will slide down to 2/3 of its former frequency. If you had a "knee" at 6 kHz before, it will now be at 4 kHz.

You can usually find capacitor values in the full E12 series, so it is relatively easy to shift the frequency curves up or down to suit your taste.

I had a lot of trouble when trying to scale the resistor values, mainly because of the limited potentiometer resistances available, and because I was trying to do more than just slide Blencowe's curves up or down in frequency - I wanted differently shaped curves, and more control range. It took me a lot of tinkering to come up with the curves and values I posted here.

But if you like the curves in my version of the control, it will be pretty easy to simply slide them up or down in frequency to suit your personal preferences.

-Gnobuddy

[Chuck H]

I only mentioned it because, I think, many players actually like presence controls.

[Gnobuddy]

I only mentioned it because, I think, many players actually like presence controls.

Good point! I've never had a guitar map with a presence control, so it never even crossed my mind that these are actually active tone controls of a sort.

-Gnobuddy

[Gnobuddy]

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.

Agreed. As mentioned in my earlier post, the problem with an active tone control is even worse than just experiencing abrupt overload: the frequency response depends one the existence of negative feedback, and if you clip the output, the open loop gain goes to zero. Which means the feedback goes to zero as well. Which means the intended tone-control frequency response goes to he@@ in a handbasket! No more tone control, just some wildly unpredictable and quite unintended frequency response!

Also as mentioned in my earlier post, I think there is a simple solution: use an attenuator ahead of the tone control, set so that it is completely impossible for the tone control to ever clip, even when the stage driving it is at full clip. The idea here is that the preceding stage acts as a limiter, ensuring that the tone control stage itself cannot be clipped. Problem solved!

I have already used this approach successfully in a different situation: I have a valve guitar amp that uses a little MOSFET as a "sourceodyne" phase inverter. It works better than a triode phase inverter in every way, except it will probably sound nasty if clipped. So I put a fixed attenuator between the (valve) stage that drives it, and the MOSFET gate. Attenuation is set so that, with the valve at full clip, the MOSFET is still happily linear. (Easy to do because MOSFETs only need a couple of volts between source and drain to keep operating perfectly, unlike a 12AX7 that wants a hundred volts or more.)

This is an approach that Leo would not have liked at all: gain costs money! Who would throw away gain?

But of course, inter-stage attenuators to throw away gain are quite routine in high-gain amps of the sort Leo never imagined. The concept isn't new today, and has a long history.

-Gnobuddy

[Gnobuddy]

I don't know if this is of any interest to anyone else, but in case it is...

I am working on a small, low-budget, solid stage guitar amp as a gift for a friend. I want to keep the controls very simple for him, so it needs a one-knob tone control. Most of the circuit is already designed and built; it has a class-D power amp module, running off a 24V DC switching power supply. So that's what I have available for preamp power, something a bit less than 24V DC (after additional RC filtering).

So: I need a one-knob tone control that works off a bit less than 24V DC.

I'm going to try the active circuit in the attached LTSpice simulation. It's an active version of the "tilt" tone control used in the Big Muff Pi, etc. This active version has no insertion loss (good thing, with only 24V B+, I can't build up a huge signal and then chop it down by 25 dB the way the Fender tone stack does.) After some tweaking it has lots of control range and very smooth, symmetric, frequency response curves.

This circuit has a gain of about +12 dB (that's four times) at full boost. It will be driven by another MPF 102 JFET stage. If I attenuate the output of that JFET by a factor of four (or five, for safety) before it gets to the input of this tone control circuit, then it will be impossible for the tone control circuit to ever clip.

What will these EQ curves sound like for guitar? I have no idea! But usually if the curves look smooth and even, it will also sound good. My friend only ever uses clean tones, so I think he can use the full treble boost if he wants without feeling like his ears are being stabbed with needles. I'll try and remember to update this thread once it's built and I've had a chance to listen to it.

Oh, I may use an MPF 102 JFET instead of the LND150 MOSFET shown in the schematic, I think it will be quieter (less noisy).

-Gnobuddy

[MarkusBass]

Thanks for the simulation. If you replace the resistors with audio taper pots, the schematic is a little bit more clearer.
I also wasn't sure about the expressions to calculate the resistance of the voltage dividers. Why there is "1 m" at the end of the expression? Is it 1 Ohm, or 1 miliOhm?



If you want, I can post my version of the simulation.

Mark

[eschertron]

What will these EQ curves sound like for guitar? I have no idea! But usually if the curves look smooth and even, it will also sound good. My friend only ever uses clean tones, so I think he can use the full treble boost if he wants without feeling like his ears are being stabbed with needles. I'll try and remember to update this thread once it's built and I've had a chance to listen to it.
-Gnobuddy

I have used a tilt control similar to yours on a couple amps for a 'pre-emphasis' type of control. They work well! Historically, however, guitar amps notch out a little midrange to get the sound that we're all familiar with. I like one-knob controls, so using the tilt/big muff pi circuit I've split the poles just enough to get a modest midrange scoop along with the high/low balance.

[Gnobuddy]

If you replace the resistors with audio taper pots, the schematic is a little bit more clearer.

I couldn't find a trustworthy LTSpice model for a logarithmic pot, so I made my own. I agree, it doesn't look as pretty on the schematic, but at least I know for sure that both the voltage ratio, and the Thevenin source impedance, are correct at all settings of the pot!

(I say this because I found one online LTSpice model for a linear pot that seemed to get the Thevenin source resistance wrong - it didn't simulate properly when I loaded the pot. It produced the right unloaded voltage, though. Whomever created that model didn't understand that the equivalent circuit of a potentiometer contains not only voltage attenuation, but also a source resistance which varies with pot position.)

I also wasn't sure about the expressions to calculate the resistance of the voltage dividers.

I worked out those equations for a log pot from scratch a couple of years ago. The equations describe what is sometimes called a "10%" audio taper, i.e., one-tenth resistance at one-half rotation. "k" is the fraction of the total rotation of the pot: k=0 at one end, k=0.5 at half rotation, k=1.0 at full rotation. If you stick in the values 0, 0.5, and 1.0 for "k", you'll find the resistances scale appropriately.

Why there is "1 m" at the end of the expression? Is it 1 Ohm, or 1 miliOhm?

This is a kludge to keep LTSpice happy. LTSpice has a hissy-fit if either resistance goes to zero in the pot. To avoid that problem, I just added one milli-ohm onto each resistor. It is negligibly small and won't affect accuracy at all, but it lets you use the "step" command without worrying about LTSpice throwing errors at the zero end of the range.

LTSpice is weird about the prefix "m". If you enter "1m", it means one milli-ohm. If you enter "1M", it still means one milli-ohm! You have to enter "1Meg" or "1meg" if you want 1,000,000 ohms.

If you want, I can post my version of the simulation.

Sure, why not? The more the merrier.

Can you tell us more about your LTSpice potentiometer model? Did you make it, did you find it? Have you checked the source impedance is correct at all pot settings?

-Gnobuddy

[Gnobuddy]

I have used a tilt control similar to yours on a couple amps for a 'pre-emphasis' type of control. They work well!

Good to know, thanks for sharing!

Historically, however, guitar amps notch out a little midrange to get the sound that we're all familiar with.

Understood!

I didn't tell the full story of my amp project in my previous post. There is a lot to tell, but the short version is that the first attempt - guitar, buffer, class-D amp module, thrift-store-refugee boombox speakers - sounded absolutely harsh and horrible to me. So the next thing I tried was to put a 5-band graphic EQ pedal (a Danelectro Fish-n-Chips) in the chain, to see if EQ could make things better.

Experimenting with that, I found that a notch at 800 Hz took most of the nasty harshness out of the guitars sound. I was using a pair of 6.5" woofers salvaged from an old boom-box for budget reasons, and they were still harsh even with the 800 Hz notch. Rolling off the treble above 2 kHz, as well as having the 800 Hz notch, worked wonders, though. Adding a little bass boost below about 400 Hz improved the sound even more.

Once I had fine-tuned the Fish-n-Chips to get the best sound I could, I did a stepped-sine frequency response measurement to see exactly what the Fish-n-Chips was doing. Then I threw some resistors and capacitors and JFETs and BJTs into an LTSpice model and tried to come up with a simple circuit to reproduce the same frequency curve as the Fish-n-Chips. I combined a mistuned twin-tee (for the 800 Hz notch and LF bass boost) with a peaky second-order low pass (for the high treble roll-off above 2 kHz) and came up with a close match to the Fish-n-Chips' best-sounding frequency response.

Then I built that circuit on a scrap of proto-board, and tested it. It took so much of the "nasty" out of the sound of the guitar amp that I started referring to it as a "de-nastifying filter". (I have a thread on diyAudio about the project, oddly titled "Tube Emulation & EQ", but that is another story. )

So this amp already has a notch in the frequency response, tuned to suit my ear (hopefully my friend's as well). The notch and the rest of the "de-nastifying filter" is fixed, and not user-adjustable. The active tilt tone control I'm talking about will be in addition to that, and the only user-adjustable part.

I like one-knob controls, so using the tilt/big muff pi circuit I've split the poles just enough to get a modest midrange scoop along with the high/low balance.

Looks good! I might try that in a (valve) preamp one of these days. If the rest of the amp already sounds good, it might be just what the doctor ordered.

As mentioned, I am on a very tight budget with this project, so I had to use the $5 boom-box speakers and $5 thrift-store 24V switching power supply. But I was determined not to give my friend a crappy-sounding amp, so I've put quite a lot of effort into making sure it sounds decent. The "de-nastifying filter" worked wonders, and then throwing in a JFET input stage helped some more, and the final step will be the tone control.

The amp is set up for clean tones only. My friend never uses anything else, so that let me avoid the huge pitfall of trying to get good-sounding overdrive out of a solid state guitar amp!

It still won't sound as good as a great valve amp, I'm sure, but I think it already sounds better than most of the affordable solid-state guitar amps on the market. It doesn't have that awful harshness that most of them have. My ears don't cringe when I play through it.

-Gnobuddy

[Enzo]

Active tone controls used in:
Peavey-
Rockmaster preamp 1991
Ultra 1991
Triple XXX 2001
TNT2000 2000
Kilobass 1995
and a number of others.

Bugera 333 2007

Carvin X 1990

[Gnobuddy]

Active tone controls used in:
Peavey-
Rockmaster preamp 1991
Ultra 1991
Triple XXX 2001
TNT2000 2000
Kilobass 1995
and a number of others.

Bugera 333 2007

Carvin X 1990

Thank you!

Do you know of any deliberate precautions the designers took to avoid overdriving those active tone control circuits?

-Gnobuddy

[Enzo]

No, I never analyzed the things beyond what I need for service.

[Chuck H]

But it's worth noting that all of those amps (even the old Carvin) are designed for preamp gain, then EQ, then power amplification. So it's likely they are designed to avoid overdriving the EQ since that would nullify the intended design advantages.

[eschertron]

It took so much of the "nasty" out of the sound of the guitar amp that I started referring to it as a "de-nastifying filter".
-Gnobuddy

Excellent! I understand now how the tilt is used for emphasis, with the 'character' already built in.

I'll have to slide over to DIYaudio to find your discussion on the de-nastifying filter.

[Gnobuddy]

I'll have to slide over to DIYaudio to find your discussion on the de-nastifying filter.

The thread starts here: Tube Emulation & EQ - diyAudio

It was split off from another thread by the moderators, so you might find some odd references or apparently off-topic bits at the start. (And the usual thread-drift near the end, particularly because my project has been going very slowly lately because of my having to deal with the usual "stuff" that the universe likes to throw at us now and then.)

-Gnobuddy