Help with Overdrive Recovery Stage

[kevinh92]

Hello all. I've recently been working on an overdrive that I've had a fair amount of success with. It is a single transistor overdrive with the modified BMP tone stack found here: http://www.beavisaudio.com/techpages/BigMuffToneControl/ After I added the tone stack, I realized there was a significant loss of output, so I attempted to add a recovery gain stage based on the LPB found here: http://www.beavisaudio.com/techpages/HIW/HIW.png I left out C1 of the booster circuit because I believe that sets the input impedance(?) and I already had a cap at the beginning of my overdrive circuit to do that. I tested all 3 parts of the circuit on a breadboard and they worked by themselves. I designed the layout posted below for putting all three and some mods in an enclosure together. I know I'm going to hear that I need to go through with a multimeter and check but I was wondering if anyone can see anything immediately wrong with the layout I designed. I think something may be wrong with how I'm adding power to the circuit because the LED stays on whether the "effect" is on or bypassed. Thanks in advance.

[nocentelli]

Firstly, your LED wiring is correct in the diagram, so if it doesn't turn off in bypass, your actual wiring doesn't match the diagram. Second, are the two jumpered lugs on the mids/25k pot grounded (they should be)? If they are connected to ground, I suspect the problem is a lack of coupling cap between the tone pot lug 2 and the gain recovery transistor's base. Capacitors help set input and output impedance of a circuit, but they also block DC voltages, which is useful when coupling separate stages. In this case your Q2 base is connected to ground by the 100k resistor (which is fine) but it is also connected to ground by the resistance formed from R4 and the 25k pot. This parallel resistance is going to be lower than the 100k required.

I am not suggesting this is the (sole) cause of your problems, but is a starting point for your debugging.

[antonis]

After I added the tone stack, I realized there was a significant loss of output, so I attempted to add a recovery gain stage based on the LPB found here: http://www.beavisaudio.com/techpages/HIW/HIW.png I left out C1 of the booster circuit because I believe that sets the input impedance(?) and I already had a cap at the beginning of my overdrive circuit to do that.

The "notorius" input impedance of LPB1 is about 25k (depending on BJT's β..) so you have a gain recovery of about 10k/390R but to an already "reduced" signal from the combination of 100k output pot and 25k input resistance...

[kevinh92]

Thanks for the replies. I guess I forgot to draw that in the layout but I did ground the mids pot. So it sounds like I should try a decoupling cap and maybe play around with some resistor values in the future and maybe a cap in front of the booster to help impedance again. On a rather lazy note, is there a recovery stage that anyone prefers to this one? I'm sure there is but I chose this one because many people seemed to use it.

[PBE6]

Check out this article about boosters from Jack Orman's site:

http://www.muzique.com/lab/boost.htm

I prefer opamps (TL072) for most applications because biasing and gain selection is simple to implement and I usually have opamps elsewhere in the circuit so there's always a spare to be utilized. YMMV.

[GibsonGM]

I like using a JFET, currently.  AMZ's Mosfet Boost is awesome, too.   Or the last stage from the Big Muff Pi.   

Each are about the same to set up as the one you currently are using.

[kevinh92]

I liked the idea about using the last stage of the BMP. Makes sense too since I'm using a variation of that tone stack. I found a cool page that breaks down each stage of the BMP circuit:http://www.kitrae.net/music/big_muff_guts.html I'll probably substitute the LPB booster for the BMP recovery stage. Thanks again!

[GibsonGM]

Probably a good call (and that's a good page, we've all had fun with that!).   The recovery stage is meant to do just that, where as I find the LPB to be a little "hot" if you will, and too midrangey for my style.  It seems to add more color than the BMP stage.

Let us know how you make out!

[antonis]

I liked the idea about using the last stage of the BMP. Makes sense too since I'm using a variation of that tone stack. I found a cool page that breaks down each stage of the BMP circuit:http://www.kitrae.net/music/big_muff_guts.html I'll probably substitute the LPB booster for the BMP recovery stage. Thanks again!

If you want a clean boost you'll have to make the bias resistors equal (for a +4.5 Vb) an then set the gain to your desirable level..
(or else you'll have an extra asymetrical distortion)

[PRR]

> I left out C1 of the booster circuit because I believe that sets the input impedance(?)

Capacitors usually do NOT have primary influence on input impedance.

They low-cut, but very often they DC Block.

I can't fully untangle your layout pictoral. But I would assume without this "C1" your "LPB" input Base bias is dragged-down by direct connection to tonestack stuff. (Some DC readings would give A Clue!). LPB transistor is biased stone-dead, don't do any good, not even pass weak signal.

In general-- if not sure about a part, leave it in. The 20 cents is cheaper than hair-ripping if it doesn't work.

(Or leave it out and Learn Something, sure, that's good, but delays the Music.)

Sometimes when putting several plans together we do find "redundant parts". Output cap on one (so there is no DC on the patch-cord), input cap on the next (to block any leaked DC coming in the cord)-- when you direct connect you can probably omit one of these. But using two caps is rarely a big problem, and works without brain-strain.
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> a clean boost you'll have to make the bias resistors equal ....(or else you'll have an extra asymetrical distortion)

You want the stage output to be near half-supply.

In opamps we can usually guide the input to half-supply and make the output DC-follow.

In simple transistor stages the output voltage is "never" the input voltage. As a thought, input bias is lower than output goal by roughly the gain of the stage (but there are many other factors). I would expect the LPB plan to need 0.8V-1.0V at input to hit 4.5V at output.

His plan is not wrong. He just left-out a critical capacitor.