Plexi-Drive: Alter the bass frq drive?

[lars-musik]

Hi,

a fellow musician has got a Wampler Plexi-Drive that he plays a lot. But he is not really content with the way it overdrives the bass of his (8 *whoa, what is this?* string)  guitar. As my ears are not nearly as fine tunes as his I didn't really get what should be altered.

I don't want to poke  around inside his expensive (original!) pedal, so I  thought I'd make a prototype and would ideally implement something he can turn, flip or switch to alter the bass response and adjust it to his linking.

Where in the circuit do I have to look? Anyway to change it at all that not being a multiband device?

Here's a schematic for reference:



Thanks!

[Pojo]

The low end response is set by c2 and c3, pre-clipping. The larger the value of the caps, the less low end is cut. C2 sets the overall response and c2 sets how much of a perceived 'bass boost' is added when the bass boost switch is engaged.

[Gus]

I don't understand what the person does not like from the first post. 

There are a number of caps that can be changed in value

Is too much bass getting overdriven/fuzzed?              Adjust the high pass filter(s) higher in frequency and/or reduce C5 in value or remove C5 for less 2nd stage gain

Is too little bass getting overdriven/fuzzed?                                                     Adjust the highpass filter(s) lower in frequency

Are too many highs removed or do you want more highs removed                      Adjust the low pass section

You have a number of places to adjust caps, you can also adjust resistors
High pass C2, C3, C6, C7. C4 is a brighten up cap
Low pass C8, C9

[Markw5]

It's not clear exactly what your friend wants, more or less bass, and before or after the distortion. If he doesn't know, it often helpful to have something like a Parametric EQ pedal around for testing. You could put it before or after the distortion pedal to see what effect EQ changes have on the sound at those points. If your friend can figure out what he likes that way, then you have some idea of where to go.

[lars-musik]

Thank you all. This gives me a good idea of what to look for. Yes, it is quite difficult to change something if you don't really know why and what. But tinkering around with circuits is part of the fun, innit?  I'll surely come around with more questions, once the thing is set up.

[J0K3RX]

If I were looking for a pedal to use with an 8 or 7 string the plexi drive would not be on my top 10 list... I doubt that he wants more bass because 7 and 8 string have plenty of that! I look for pedals that tighten up and focus the low end, give it more definition and clarity and a little bit of overdrive.. More bass would muddy it up and cutting the bass frequency will likely make the low end sound thinner.  Is he using it as an overdrive or a distortion pedal? What amp is he using? What pickups? What is the signal chain, any other pedals before/after the plexi drive? All of these figure into the equation.. If using it as an OD then may have him try the standard OD settings that most extended range players use.. Gain all the way down, tone in the middle and level all the way up. It may sound good but, my guess is that it won't. The Wampler Plexi Drive is really not the kind of pedal for 8 string even with some modifications it's still gonna be like driving a Tractor Trailer through a mouse hole... To get it to sound good with 7 or 8 string guitars you will probably have to redesign the whole thing but, if that's what you are wanting to do then good luck. I personally would start with a different design that would not need a total make over to achieve good results. 

[lars-musik]

He's a pro in matters of equipment: He's playing a Brunetti Singleman and has quite something in front of his feet. I can't remember the whole pedal chain, but it comprises a BB Preamp, TC Spark, some Strymon Reverb or Delay, a Plutoneium  Chi-Wah-Wah, some compressor or other and lots more.  Ah, forgot, since last week his pedal board sports one of my nice little 1590a Klons.

In that whole context he likes the Plexi-Drive except, well, its behaviour in Bass clipping/distortion.

Because I thought it would be nice thing to spend an evening or two, I'd like to show some variations on his favoured OD/Dist. And I am interested in the results myself.

[PRR]

> it overdrives the bass of his (8 *whoa, what is this?* string)  guitar.

Playing three, even two, notes together, through distortion, makes ugly non-musical sounds.

I don't want to hear *eight* notes intermodulated together.

Whatever. Here's the approximate frequency cuts of the stock circuit.



The 150Hz/700Hz choice after the first stage is reasonable. Multiple notes spread apart in frequency make the worst sounds when distorted. Many-many fuzzes only let in 700Hz-1500Hz to minimize the ugly-sound.

The output high-cuts above 5KHz, shelving to 1KHz, so you can select how much high-frequency tizzz gets out.

I don't think the 70Hz pole in the middle makes much difference. All other poles are pretty much out of the audio band.

[Quackzed]

i'd want a way to let the low end through without the lows getting all fuzzed out and 'flatulent' 
on the other hand i wouldn't want big clean low notes either... i'd probably either try to eq in a big wide hump in the low lows at the end of the circuit, or try to let those low lows pass around all the filtering and clipping and blend 'em back into the output. 2 different approaches but the same 'basic' idea: get the undistorted lows to the output...
the first approach could be as easy as adding a wide freq gyrator at the end, tuned to @ 60 to 100hz? i like 100hz for a 4x12 cab kinda resonance fwiw, maybe a 2 octave or 1 even? trial and error here... but i'd guess that might work out, cross your fingers for 60 cycle bs to not be an issue... i'd try that. the high pass filters at the beginning make the lows low enough to not get much clipped in the circuit, (maybee make c5 bigger) and youll need a decent amount of gain on the gyrator to get em back to the party, but doable...
   

[lars-musik]

I just went through some gyrator circuits but I think rather change some values in the circuit as it is than to plug a new stage at the end. 

Some questions @Paul regarding the calculated cut-off frequencies and corresponding filters (just trying to understand - I am still in the log phase of my electronics learning curve): The 150/700 Hz filter comprises of the respective cap and the variable 500KΩ resistor allowing the cutoff frequency to be shifted. So the resistor/potentiometer is more of a tone control, isn't it? Why is it called "Gain"?

The second high pass filter (70 Hz) doesn't seem to make any sense, assuming that the first filter already cut these frequencies. Or is there any way, the lower frequencies that passed the first filter in minute amount was boosted significantly by the first gain stage?


And again my breadboard glowers at me from the top shelf where it is gathering dust.
 
EDIT:

Another afterthought: If I want to change the corner frequencies of low/high pass filters: Is it better to change resistor (easier via pot) or the capacitor?

[Pojo]

I just went through some gyrator circuits but I think rather change some values in the circuit as it is than to plug a new stage at the end. 

Some questions @Paul regarding the calculated cut-off frequencies and corresponding filters (just trying to understand - I am still in the log phase of my electronics learning curve): The 150/700 Hz filter comprises of the respective cap and the variable 500KΩ resistor allowing the cutoff frequency to be shifted. So the resistor/potentiometer is more of a tone control, isn't it? Why is it called "Gain"?

The second high pass filter (70 Hz) doesn't seem to make any sense, assuming that the first filter already cut these frequencies. Or is there any way, the lower frequencies that passed the first filter in minute amount was boosted significantly by the first gain stage?


And again my breadboard glowers at me from the top shelf where it is gathering dust.
 
EDIT:

Another afterthought: If I want to change the corner frequencies of low/high pass filters: Is it better to change resistor (easier via pot) or the capacitor?

The 500k pot actually is functioning to attenuate the signal between the first and second JFETs, so being labeled as a Gain control is appropriate. It does factor into the cut-off frequency of the the high pass caps C2 and C3 but it is seen as a static 500k to ground no matter where the wiper happens to be.

And as Paul mentioned, the 70Hz roll off at C6 really doesn't do any tone shaping. There needs to be a cap there to block DC between stages and Brian most likely selected a value just big enough to not lose any more low end.

Anyway, having had a bit of fun messing with this circuit I've got lots more I'd like to add on this topic but I'm already up wayyyyy too late. I'll try to add more later.

[lars-musik]

Anyway, having had a bit of fun messing with this circuit I've got lots more I'd like to add on this topic but I'm already up wayyyyy too late. I'll try to add more later.

Please do! I'd really like to profit from your experiments!

[PRR]

> the resistor/potentiometer is more of a tone control, isn't it?

No.

The "221" cap is "small" at least for bass. Ignore it for a moment.

The "pot" is wired as a Potentiometer. Full-down it is 500K. Full-up it is 500K in parallel with whatever is connected to its wiper. Here this is the Gate of a JFET. In normal operation the JFET Gate is a reverse diode, "infinite" impedance. So the pot is still 500K.

(For pencil checking, try assuming the Gate is 200 Megs. No small JFET gates leak this much. However infinity and 200Meg is all about the same here, 500K versus 499K, so we do not care.)

Oh, the Gate is also a few pFd of capacitance to ground. Miller effect may be significant. Also wiring strays. 100pFd may be a pencil-check value. Just 100K across a 500K pot set at half-up is a 13KHz high-cut. Not a big deal for guitar; but does suggest why a higher pot value was not used. However we also have this "221" (220pFd?) cap top-to-wiper. So in fact at "half pot" the lows-mids will be half, but the highs tend toward 2/3rds. Experience says that a slight (sometimes blantant) brightening at half-pot is "better" than any loss of sizzle.

[lars-musik]

Ahh. Stupid of me. The difference between a voltage divider and a rheostat was there, it just hid somewhere between me ears. Sorry.

But thanks a lot for the extenive explanation, I really need and appreciate it.

Unfortunately there's something I don't understand - again. Does R14 and possibly C11 have anything to do with the power filtering? This looks unusual to me. Wouldn't R13 and the huge C10 suffice? Possibly with another 100nF thrown in for good measure?

And I hope R15 is just meant as a current limiting resistor for the LED? I am inclined to leave R14, C11 and R15 out and just use my standard offboard 3pdt wiring.

[Groovenut]

Ahh. Stupid of me. The difference between a voltage divider and a rheostat was there, it just hid somewhere between me ears. Sorry.

But thanks a lot for the extenive explanation, I really need and appreciate it.

Unfortunately there's something I don't understand - again. Does R14 and possibly C11 have anything to do with the power filtering? This looks unusual to me. Wouldn't R13 and the huge C10 suffice? Possibly with another 100nF thrown in for good measure?

And I hope R15 is just meant as a current limiting resistor for the LED? I am inclined to leave R14, C11 and R15 out and just use my standard offboard 3pdt wiring.

R14 C11 appear to be there to isolate the LED current form the rest of the circuit, mostly to minimize pop at turn on. A Jfet power rail will be much more sensitive to noise than an opamp power rail.

[LightSoundGeometry]

The low end response is set by c2 and c3, pre-clipping. The larger the value of the caps, the less low end is cut. C2 sets the overall response and c2 sets how much of a perceived 'bass boost' is added when the bass boost switch is engaged.

I am looking at C2 and C3, what forms the filter right there? is it R2 and then c2 and c3 in series there ?

[Groovenut]

The low end response is set by c2 and c3, pre-clipping. The larger the value of the caps, the less low end is cut. C2 sets the overall response and c2 sets how much of a perceived 'bass boost' is added when the bass boost switch is engaged.

I am looking at C2 and C3, what forms the filter right there? is it R2 and then c2 and c3 in series there ?

The filter for C2 and C3 is a high pass formed with the gain pot and the R4.

[LightSoundGeometry]

The low end response is set by c2 and c3, pre-clipping. The larger the value of the caps, the less low end is cut. C2 sets the overall response and c2 sets how much of a perceived 'bass boost' is added when the bass boost switch is engaged.

I am looking at C2 and C3, what forms the filter right there? is it R2 and then c2 and c3 in series there ?

The filter for C2 and C3 is a high pass formed with the gain pot and the R4.

right on,  I see it now! after I get my tim all worked out, I am going to give this a shot.

[PRR]

> Does R14 and possibly C11 have anything to do with the power filtering? This looks unusual to me.

Looks unusual to me. R18 C10 don't give a lot of filtering. LED does not need filtering. The JFET stages do, as Lawrence says. But they are 1mA total while LED is 2mA, so why filter all 3mA when you only need 1mA of clean?

> R14,.. and R15 out

You need some resistance with the LED!

Here is another, better filtered, way to do it.



Not that Wampler's circuit is "wrong". It probably works fine in many situations.