BMP output stage

[blackieNYC]

I built a 2 band Fetzer thing, which ends in a hi/low panner much like the Big Muff. It sounded good right there but I needed a little more output. So I inserted a BMP output stage with a 5088 (470k,100k,15k,3.3k). The output stage distorts a little and it sounded bad. So I put a 10k trimpot voltage divider (like Volume) between the tone panner and the BMP output stage. There are 1-2uf caps on the input and the output of that trimmer as it turned out.
The output stage clips still. It is Farty.  It gets better as I turn down the trimmer. It is almost acceptable with the trimmer all the down but a hair, and the output volume at max (100k pot). 
Again, I'm sure it sounds fine before the trimmer and gain makeup stage. I've tried a cap across the 3.3k - it just makes me turn down the trimmer more.  Why is this output clipping? Is a 5088 too high in hfe?

[Plexi]

Hey, Blackie!
The LPB/BMP recovery stage is my battlehorse in every low output design.. and sometimes to push some low input/treble.
I would say that the values used in the BM tend to compress a bit more than the LPB.

- Try 10k to Collector, and 360R from E. to ground; like the LPB.
- Try 100n cap at input as coupling: any lower value will filter less treble, but enlarge that value introduce bass's a bit more as usual.
This input cap, with the 100k resistor to ground, form the high pass filter: so maybe its causing some extra treble and gain there.
Decrease that value (68k?), and see whats happen there.

Do you have any schematic of the Fetzer 2 band eq?

[blackieNYC]

I should say it is the low end causing the "brapp" sounding distortion

I will post a schematic - but I do know it sounded good before I added the make up stage.
Signal comes in to a 5457 Fetzer, then splits like a BMP tone circuit to two more fetzers (all of which are designed according to the article at runoffgroove.)
This pedal is actually a killer sound. The highs have a J201, the lows get a 2nd 5457. The high have a 100k pot in series with shunt Ge diodes. It yields sparkly jangly joy. Bright, but very smooth.

[nocentelli]

I should think lowering the collector resistor to 10k might do it. If it's still clipping, maybe the output of the fetzers is high enough to warrant a buffer instead of a gain recovery stage.

[anotherjim]

As a Skinny Puppy fan, I'd be interested in a Brap pedal

[antonis]

- Try 100n cap at input as coupling: any lower value will filter less treble, but enlarge that value introduce bass's a bit more as usual.
This input cap, with the 100k resistor to ground, form the high pass filter: so maybe its causing some extra treble and gain there.

No passive RC filter causes any gain (extra or not..) 

100nF/100K form a HPF with cutoff frequency of about 16Hz, so we may consider as "untouched" any frequency above 82Hz..

Decrease that value (68k?), and see whats happen there.

What happens there is a misbiased transistor with a 30% lower impedance now.. 

[blackieNYC]

The winner is: Lowering the collector resistor! With the 15k, the collector measured 4.4V - which I've seen on one of the Big Muff sites as being correct. I dropped it down to 12K or so. The voltage is now 5.0v.  and the headroom is much better.  Perfect. Thanks guys.
(This makes me think that the BMP output stage can add some clipping, with the 15k/3k combination.  There's your Brap pedal - get 4.4 V or less on the collector of a BMP output stage and put some boost before it.  If no one likes the sound of it you can blame the dog..., the skinny puppy.)

Eventually I hooked up my scope.  I noticed that the bottom half of the waveform was doing all the clipping.  I wondered this before - when I see a waveform clipping on one half much "sooner" than the other, does that mean I am misbiased slightly? Or does this require playing with the collector/source resistors, as it appeared to do for me in this case? It seems to me that other than many dirt circuits which may depend on asymmetry, that it would be worthwhile to maximize your headroom by getting the thing to clip symmetrically, no?  All these details we share about biasing transistors, is this what we're really after?

[antonis]

Theory is good for theory..
(and practice for practice..)

e.g. you can't base bias a BJT of h_FE=100 and an Emitter resistor of 1k with a voltage divider of 470k/100k and make your calculations IGNORING base current.. 

or set a quiescent Collector current LOWER than maximun signal current.. 

or anticipate for a symmetrical clipping with a Collector voltage set EXACTLY lalf-way between V_cc & V_ee in the existence of any Emitter resistor..   

[Plexi]

The winner is: Lowering the collector resistor! With the 15k, the collector measured 4.4V - which I've seen on one of the Big Muff sites as being correct. I dropped it down to 12K or so. The voltage is now 5.0v.  and the headroom is much better.  Perfect. Thanks guys.
(This makes me think that the BMP output stage can add some clipping, with the 15k/3k combination.  There's your Brap pedal - get 4.4 V or less on the collector of a BMP output stage and put some boost before it.  If no one likes the sound of it you can blame the dog..., the skinny puppy.)

Eventually I hooked up my scope.  I noticed that the bottom half of the waveform was doing all the clipping.  I wondered this before - when I see a waveform clipping on one half much "sooner" than the other, does that mean I am misbiased slightly? Or does this require playing with the collector/source resistors, as it appeared to do for me in this case? It seems to me that other than many dirt circuits which may depend on asymmetry, that it would be worthwhile to maximize your headroom by getting the thing to clip symmetrically, no?  All these details we share about biasing transistors, is this what we're really after?

Great!!
LPB NEVER fail 

Theory is good for theory..
(and practice for practice..)

e.g. you can't base bias a BJT of h_FE=100 and an Emitter resistor of 1k with a voltage divider of 470k/100k and make your calculations IGNORING base current.. 

or set a quiescent Collector current LOWER than maximun signal current.. 

or anticipate for a symmetrical clipping with a Collector voltage set EXACTLY lalf-way between V_cc & V_ee in the existence of any Emitter resistor..   

"Yes, I can.. and I LOVE it" (DBA)