Best Post-EQ Clean Boost Stage Option

[BennyBoy]

Howdy folks,

I am working on a Muff-like circuit with James-style amp EQ section replacing the original Muff-style tonestack. I really dig how the EQ sounds with this new James tonestack, but it's lossy and I need a gain recovery stage after this that doesn't alter the tone noticeably. Everything transistor based I've tried so far has noticeably altered things. Anyone have suggestions for a gain stage here that has little to no tone shaping? JFET? Op-Amp? Anything really!

Thanks!

[FiveseveN]

Everything transistor based I've tried so far has noticeably altered things.

The CE recovery stage in the BMP should be completely transparent provided you don't overdrive it, obviously.

[Zveeen]

Hi,
I had the same issue once while modding an old Randall Commander amp to have an Ampeg Tone Stack (yes even the active Inductor based Mid Control). I soon realized that I can't just swap out a Fender Style tone Stack with a Baxandall EQ. My solution was, I took a Gain Stage from an old Peavy Standard 260 circuit and put it after the Stack, I can send you the Schematic if you want. But I also had the correct Supply Voltage of 22V, so maybe if you would tweak it to work with 9V......
But does an LPB-1 after the EQ really alter the sound as much?

Cheers

[BennyBoy]

The CE recovery stage in the BMP should be completely transparent provided you don't overdrive it, obviously.

Which BMP version are you referring to? The dozens of versions all have variations in this stage. Is there a version that is more "transparent" than others?

[BennyBoy]

Hi,
I had the same issue once while modding an old Randall Commander amp to have an Ampeg Tone Stack (yes even the active Inductor based Mid Control). I soon realized that I can't just swap out a Fender Style tone Stack with a Baxandall EQ. My solution was, I took a Gain Stage from an old Peavy Standard 260 circuit and put it after the Stack, I can send you the Schematic if you want. But I also had the correct Supply Voltage of 22V, so maybe if you would tweak it to work with 9V......
But does an LPB-1 after the EQ really alter the sound as much?

Cheers

That sounds interesting, would love to take a look at that!

The LPB-1 post EQ seems to trim off a bit of the low end, which is kind of essential in what I am working with.

[FiveseveN]

Which BMP version are you referring to?

I really haven't kept track. Use a variable emitter R or the usual bypass cap on a pot to set your required gain. As long as all the high-pass corners (coupling caps + loads, bypass cap if used) are outside the bandwidth of interest it should be as transparent as it gets.

[BennyBoy]

Which BMP version are you referring to?

I really haven't kept track. Use a variable emitter R or the usual bypass cap on a pot to set your required gain. As long as all the high-pass corners (coupling caps + loads, bypass cap if used) are outside the bandwidth of interest it should be as transparent as it gets.

Thanks!

I think I found my issue. It seems I AM overdriving this stage, and that's what is causing the change in EQ (more fuzzy high end). How can I rectify this?

[Zveeen]

can you make a quick schematic of what you did?

[ElectricDruid]

Anyone have suggestions for a gain stage here that has little to no tone shaping? JFET? Op-Amp? Anything really!

If you just want clean gain, then an op-amp is pretty much always the best answer (simplest, cheapest, lowest distortion, etc etc). Getting lots of clean headroom is then just a question of getting big supply rails, and most audio op-amps were designed for +/-15V rails, so towards **30Vpp** which is a *lot* of headroom for guitar!
There's a reason analogue mixing desks are full of op-amps...

[Transistor-Transistor]

The LPB-1 post EQ seems to trim off a bit of the low end, which is kind of essential in what I am working with.

I had the same problem a while ago. It's a simple fix really, you just have to change some capacitor values in the lpb1 circuit
As for the overdriving, you should just use a larger resistor tied to the emitter

[bluelagoon]

Try the ZVEX SHO its a nice simple clean boost, that sounds good.

[fryingpan]

Passive tone stacks really like a high input impedance stage after them. Maybe that's your problem?

[Eddododo]

Howdy folks,

I am working on a Muff-like circuit with James-style amp EQ section replacing the original Muff-style tonestack. I really dig how the EQ sounds with this new James tonestack, but it's lossy and I need a gain recovery stage after this that doesn't alter the tone noticeably. Everything transistor based I've tried so far has noticeably altered things. Anyone have suggestions for a gain stage here that has little to no tone shaping? JFET? Op-Amp? Anything really!

Thanks!

A schematic of what you're actually doing would help

If you just inserted the eq stack right before the typical output transistor, and that's the sound you like, then try the bypass gain cap shown above.. if that isn't working, then you need to try something else and just work with it until it sounds good.
Changes the response of the eq? Tweak the eq
Cuts bass? Look for coupling caps to make larger
Just sounds different and not right? Make it sound right   

[amptramp]

You could always go to the duncanamps tone stack calculator and plug in the all-important output load resistor.  The load resistance is the input impedance of the amplifier stage that follows it and I concur with the posters who have said op amps are usually the way to go, especially in a non-inverting connection since the impedance can be calculated easily and does not change from unit to unit.  FET stages may also work but may require select-on-test FET's to meet the bias specifications.

The tone stack program is free for download here:

https://www.duncanamps.com/tsc/index.html

[BennyBoy]

You could always go to the duncanamps tone stack calculator and plug in the all-important output load resistor.  The load resistance is the input impedance of the amplifier stage that follows it and I concur with the posters who have said op amps are usually the way to go, especially in a non-inverting connection since the impedance can be calculated easily and does not change from unit to unit.  FET stages may also work but may require select-on-test FET's to meet the bias specifications.

The tone stack program is free for download here:

https://www.duncanamps.com/tsc/index.html

Thanks! You mind explaining this a bit in my case? Here is exactly what I am working with. Haven't made a schematic yet, but these are



 my clipping stages and then my tonestack.

[ElectricDruid]

So you've got the Matamp EQ stage after those clipping stages, right? What have you got after the EQ currently? Does it go to a volume control, or straight to the output or what?

[BennyBoy]

So you've got the Matamp EQ stage after those clipping stages, right? What have you got after the EQ currently? Does it go to a volume control, or straight to the output or what?

Yep, EQ right after clipping stages. I have just been running it straight to output (with a 1uf output cap) on the breadboard because it is already only at about unity gain. That's why I am hoping to add a clean gain stage. Will have a volume control on the final circuit.

[ElectricDruid]

Yep, EQ right after clipping stages. I have just been running it straight to output (with a 1uf output cap) on the breadboard because it is already only at about unity gain. That's why I am hoping to add a clean gain stage. Will have a volume control on the final circuit.

Ok, cool. Stick something like this onto it:



You won't need R4 and R5, so ignore them. C4 is the input cap, the 1uF you've got at the moment. You can stick another cap on the output of this and add a volume control with no problems. Increase R7/33K for more gain if required.

Oh, and this is shown with half a dual op-amp, but since you only need one, you'd be better off with a single TL071 or similar.

HTH

[BennyBoy]

Yep, EQ right after clipping stages. I have just been running it straight to output (with a 1uf output cap) on the breadboard because it is already only at about unity gain. That's why I am hoping to add a clean gain stage. Will have a volume control on the final circuit.

Ok, cool. Stick something like this onto it:



You won't need R4 and R5, so ignore them. C4 is the input cap, the 1uF you've got at the moment. You can stick another cap on the output of this and add a volume control with no problems. Increase R7/33K for more gain if required.

Oh, and this is shown with half a dual op-amp, but since you only need one, you'd be better off with a single TL071 or similar.

HTH

Cool, will try this! Is it okay to run R8 to ground instead of Vref, or is that necessary for operation?

[ElectricDruid]

Is it okay to run R8 to ground instead of Vref, or is that necessary for operation?

As drawn, it has to be Vref. If you add a cap below R8, you could take the bottom of the cap to ground. I actually did that in the stage that followed that one I just posted:



(All these bits are from my Hard Bargain distortion article if you want to take a look)

R8 at 10K with 220n like the above schematic would give you a highpass at 72Hz, so that's about the minimum value you should use for the cap. 470n would be another octave lower (roughly) so 36Hz, which is already plenty low.