Roland Double Beat Mods, help needed.

[SprinkleSpraycan]

I have been working on a Roland Double Beat and am very please with most of the results of what i have tried. There some issues i am trying to work out and need the expertise of the forum.

1: when the low end control is fully clockwise the gain control has a limited range but is useful and i am happy.
turning the bass down allows the gain control to have a MUCH LARGER effect, even getting into more overdrive rather than fuzz. This too is desirable. However, once ive done this there starts to be a 'sizzling' sound as notes decay. Doing my best on the breadboard to identify a solution, i think i need to change R6 to something like a 68k. Am i on the right track? what causes this sound and how to i address it?

2: this pedal does not play well anywhere but first in the chain (i have another thread about this). I tried an AMZ pickup simulator but it isnt as useful as id like. Are there better ideas/options? or do i need to suck it up and let it leave at the front of the signal chain?

3:besides the 'sizzle' that i absolutely want to fix i am very pleased with the sound of this pedal. it has many sounds available due to the lowend control and the bone ray tone control. PLease offer anyother suggestions though if there are glaring problems. This originated with the Aion FX Argus schematic. One potential thing i may try is adding another gain stage at the end purely to create more volume. something like the end of a big muff or a sho boost or something.

[duck_arse]

on the breadboard still? you could try moving the Q3 stage to the end of the tone stage. otherwise, just tack on a big muff last stage.

I'm also interested in the sizzle reasons to be given. I'm working at a heath 28 using very low gain power transistors, and it sizzles in the decay. sounds like transistors angry for being starved of current, or something.

[SprinkleSpraycan]

on the breadboard still?

well its boxed up too but i have it on a breadboard again to see if i can solve the problem. i originally attributed the noise to general breadboard noise. i can crack open the box and adjust the stripboard when i have potential solutions. its the never ending apollo 13 "make this fit, fit with this, using only this"

[Mark Hammer]

This is the Double Beat circuit I use.  The original only provides for the top three switch selections.  Since I had plenty of 6-position rotaries, I added three more filter settings that I felt would sound different enough from each other.

[antonis]

you could try moving the Q3 stage to the end of the tone stage.

I shouldn't do it..
(at least, not for R6 = 120k value..)

@OP: Shouldn't you scale down by a factor of 10, or so, all resistor values..??

[SprinkleSpraycan]

@OP: Shouldn't you scale down by a factor of 10, or so, all resistor values..

I really wish I knew what that meant  please elaborate

[antonis]

Noise reduction and succeeding stage drive easiness..

e.g. In your circuit, Q2 output impedance is about 120k (R6) and is called to drive a 50k one (Q3 input impedance - R10 divided with Q3 stage gain..)

[duck_arse]

on the breadboard still?

well its boxed up too but i have it on a breadboard again to see if i can solve the problem. i originally attributed the noise to general breadboard noise. i can crack open the box and adjust the stripboard when i have potential solutions. its the never ending apollo 13 "make this fit, fit with this, using only this"

transistor quantum anger, I think it's called. when the last electron passes through the lattice, all the holes close behind it, causing the noise like falling water spray.

[SprinkleSpraycan]

Noise reduction and succeeding stage drive easiness..

e.g. In your circuit, Q2 output impedance is about 120k (R6) and is called to drive a 50k one (Q3 input impedance - R10 divided with Q3 stage gain..)

Ya....over my head. Let's ask this, what Causes the sizzling sound of the decaying notes?

[Rob Strand]

Noise reduction and succeeding stage drive easiness..

e.g. In your circuit, Q2 output impedance is about 120k (R6) and is called to drive a 50k one (Q3 input impedance - R10 divided with Q3 stage gain..)

Ya....over my head. Let's ask this, what Causes the sizzling sound of the decaying notes?

There's any possibilities.  No magic pill you have to try a few things.

I'm assuming possible oscillation.  Try these one at a time:
- try larger value for C6,  100pF or 220pF use smallest value that fixes problem.
- 1k to 20k in series with input line (located between base of Q1 and C1/C3
- 100pF to 1n from Q3 base to ground
- 100R to 470R in series between C9 and pin 3 of gain pot.
  Limits gain. Use smallest value that fixes problem

If it's not oscillation none of these might work.  It could be some intrinsic issue with the design, like biasing.

 

[SprinkleSpraycan]

There's any possibilities.  No magic pill you have to try a few things.

I'm assuming possible oscillation.  Try these one at a time:
- try larger value for C6,  100pF or 220pF use smallest value that fixes problem.
- 1k to 20k in series with input line (located between base of Q1 and C1/C3
- 100pF to 1n from Q3 base to ground
- 100R to 470R in series between C9 and pin 3 of gain pot.
  Limits gain. Use smallest value that fixes problem

If it's not oscillation none of these might work.  It could be some intrinsic issue with the design, like biasing.

 

Thanks for the suggestions! Heading to the work bench with them now.