Author Topic: 3-Band Tone Control in a BMP?  (Read 557 times)

KarenColumbo

3-Band Tone Control in a BMP?
« on: January 10, 2018, 04:16:20 AM »
Hi @ all and a ripping good 2018 to all!

My learning curve recently guided me to the good old BMP. After buying and reading much material I decided to plug in an effective tone control between stages 3 and 4 (as intended) and tried several of them:

1. AMZ variations of the "classic" one-knobber. Not extreme enough for my personal taste, but working first try
2. A mixture of the AMZ two-knob (Bass - Treble) with a band of R.G.'s parametric eq as sweepable Mids after it: Bass/Treble control was extremely influenced by the parametric Mids, which were mayhem :) Not really usable in this context
3. Zwo bands of R.G.'s parametric - Bass and Mids. Worked fine (had to push up the gain for the Bass part a bit). Only problem: The whole BMP oscillates when switched on, until I wiggle the eq gain controls a bit. Very effective, due to the squealing not really usable in practice.
4. Coming soon: typical B/M/T tonestack with buffers before and after the filter section.

What I'd REALLY love to have is just a switch I'd label "Bass boost/cut" to let it chug or cut through the mix, sweepable Mid Boost/Cut parametric and maybe a treble roll-off pot.

I searched the forum, of course, and Googled a lot - but (not necessarily surprising) haven't found something in this regard.

My question: Did any of you try such a thing? Will it work of is the good ol' BMP just allergic to being Mid-sweeped?
"In the beginning there was nothing - which exploded" - Terry Pratchett

iainpunk

Re: 3-Band Tone Control in a BMP?
« Reply #1 on: January 10, 2018, 05:29:47 AM »
Hey, happy 2k18 to you too!!!

First of all, cool project!!!! I use a mid boost right after my bmp so, yeah, its a sound you either love or hate, not for everyone. It really cuts through with the mids boosted.

Next up, if you want to have a bass cut, you could place it between the 1st and 2nd gain stage, for a tighter distortion sound. Id put a 1nf cap between the 1st stage cap and the sustain pot, with a switch over it to short the cap out.
When the switch is open, you'd have 2 caps in series (cutting alot of bass), and when closed you'd practically shut the smaller cap off and thus don't cut the bass.

The trebble roll off is something i hope you can figure out yourself.

And the parametric mids... Dont be scared off by the stigma but... Search the metal zone mid control... I know about the stigma, but it has a really good powerful tone section!!

Godspeed

Iain
If we don't study the mistakes of the future, we're bound to repeat them for the first time.
-Ken M

R O Tiree

Re: 3-Band Tone Control in a BMP?
« Reply #2 on: January 10, 2018, 06:22:57 AM »
@ iainpunk - The BMP tonestack has a 7dB mid-scoop by default with the tone control centralised.  Once you've scooped that bit out of the response curve, then boosting them again with an Eq pedal works, sure, but at the expense of (some) added noise.  Thankfully, there's a very simple solution:

Download and install this file from Duncan Amps.  It shows you the schematics of a whole bunch of tonestacks (above the left hand pane of the window, you'll see tabs for the different circuits, so scroll all the way right to click on the BMP one).  In the right hand pane, you'll see the frequency response curve.  Now, in the left hand pane, double click on "C1" and change its value to "10nF".  You'll see that the response curve almost flat-lines.  Below the left hand pane, you'll see 3 slider controls.  Bass and Treble are greyed out (only 1 tone knob in a stock BMP).  Click and drag the Mid control left/right to see how the tone control behaves.

I built a BMP for a mate of mine some years ago that had a simple SPDT switch connected to the required 3.9nF and 10nF caps, so he could swap from "scooped" to "flat-line" very easily.

@Andreas - Yes, you can put a more flexible set of tone controls after a BMP with excellent results.  Or (b), do the "flat-line" mod above, then replace the 100k Lin ptoentiometer with simply 2 x 51k resistors (as if you had welded the control knob of the stock tone control in its central position) and then stick a 6-band Eq pedal after the BMP.  Finally (c) remove the tone stack completely and stick a 6-band Eq after the BMP.  There's a proven circuit at General Guitar Gadgets that won't be too hard to build.  With a bit of tinkering, you can convert that 6-band Eq to a 3-band, which will require some thought about how to widen the frequency bands, so they "touch" each other, so you have control across the entire spectrum, not just narrow "humps"/"dips" in the response curve.
...you fritter and waste the hours in an off-hand way...

Mark Hammer

Re: 3-Band Tone Control in a BMP?
« Reply #3 on: January 10, 2018, 07:21:14 AM »
I installed two toggles on a 90's reissue BMP someone gave me.  One toggle shifts the rolloff of the bass filter up, so that full bass can have more note definition, and the other shifts the rolloff of the treble filter down, so that full treble has more body to it.  It's a very simple cap-value change, that provides a fair amount of tonal flexibility.  The only thing it doesn't do is yield a mid-peak without much bass or treble.  But if that's the tone a person wants, I'd suggest a different pedal.  Implementing 3-band EQ would just eat up too much signal.  What I did to my own BMP doesn't reduce signal level.  Indeed, by passing more, it can be said to raise output level at some settings.
« Last Edit: January 10, 2018, 07:22:45 AM by Mark Hammer »

marcelomd

Re: 3-Band Tone Control in a BMP?
« Reply #4 on: January 10, 2018, 07:41:31 AM »
Download and install this file from Duncan Amps.  It shows you the schematics of a whole bunch of tonestacks ...

Just wanted to point out our forum fellows Jarmo and ~arph made a browser friendly version of Duncan's Tone Stack Calculator: http://www.guitarscience.net/tsc/info.htm

Works beautifully anywhere.

mth5044

Re: 3-Band Tone Control in a BMP?
« Reply #5 on: January 10, 2018, 12:39:26 PM »
You could try putting in ROGs tonemender where the tonestack of the BMP currently is. Might not need the input buffer of the tonemender, might not need the makeup gain of the BMP, and whatnot.

R O Tiree

Re: 3-Band Tone Control in a BMP?
« Reply #6 on: January 10, 2018, 12:43:46 PM »
Implementing 3-band EQ would just eat up too much signal.

Looking at the GGG 6-band Eq schematic, R3 (trim pot on the input buffer op-amp stage) is there to provide a "master gain", isn't it?  Additionally, the tonestack in a stock BMP attenuates the output from Q4 in any case?  It shouldn't be too hard to remove the tone circuitry completely, install a 6-band (or 3-band) Eq, break out the trim pot (R3 in the GGG Eq schem) to a front-face control and have the functionality that the OP wants?
...you fritter and waste the hours in an off-hand way...

KarenColumbo

Re: 3-Band Tone Control in a BMP?
« Reply #7 on: January 11, 2018, 02:45:54 AM »
Forgot to add: I actually tried the Tonemender, had a switch between a two-knob-tone control la AMZ and the tonemender (I left the buffer in, just in case). It worked fine, but it didn't exactly hit the spot I was searching for.

And I must add, that I tinkered a lot with the BMP itself until it had the sound I was searching for. Quite un-Fuzz-like, more a tight, a bit compressed distortion, very dry and heavy, broad-banded,sweet, actually. I was surprised at first, then I wanted to give it a chance to develop some "character mids". Maybe I got sidetracked there - but I felt it could be possible.

The reason I went for parametric is that when it sounded nice on my gear (cheap lefty strat with a fat Air Norton humbucker through a H&K clean channel tuba amp) it sounded not quite as good on my friend's equipment (Fender Jaguar through a clean AC30) - simply the wring midband for his (and my) taste.

What worked best was the sweepable Mid - like a stationary Wah on highest gain settings and almost synth-like when turned completely down. But it ate much of the Lows and Highs. A conundrum, it seems.
"In the beginning there was nothing - which exploded" - Terry Pratchett