Need help for Behringer OD300 input level mod

[nilsomat]

Hello and greetings to the forum (my first post)!

I recently got a Behringer OD300, which i wanted to use with an analogue synthesizer. Apparently the level of the synth is much too high, so i wanted to modify the OD300 for higher input levels. The easiest option would be to use a pair of resistors at the input as a voltage divider, but then probably noise levels would be higher and output levels would be too low. Rather than reducing the input voltage i would like to reduce the internal amplification of the box.
As far as i know there is no schematic for the box available, it is said to be related to the BOSS OS-2 of which i will link to a schematic. I also made a high resolution shot of the pcb of the Behringer, luckily all parts are numbered and with a value.




Now to my question, which are the parts responsible for the internal amplification and how can i track them down on the board?
Any help of information would be greatly appreciated!
Greetings,
Nils

[Quackzed]

if your synth has a volume control, first try and get it low enough ,or at around the level you want and make sure the pedal does what you want... after that i'd say a voltage divider at the input is a good idea. or maybee (if noise is indeed an issue) a -bit- of attenuation at the input as well as some gain reduction...
see the thing is, these distortion boxes run off 9v. so if you hit it with a big signal, like your synth, you'll have to really crank down the gain inside the box and it may still hit the input of the box with too hot a signal... so a bit of 'input attenuation' as well as some gain reduction inside the box might be the optimum solution...
in the boss schem ,if youu look at the dual drive pot- it has a 270k drive pot - one side is in the feedback path of one opamp (top) with a 12k - r22-and the other side of the pot is in the other opamps fedback path with a 1k r37; so with the drive control all the way down, theres still 12k on the top opamp and 1k on the bottom... you 'could' reduce these- but to keep the balance, r37 could be reduced to 0k(jumper this resistor) to keep the balance the top r22 only gets downed to 11k... so theres still a decent amount of gain there...
so to keep the balance you'll still need a decent amount of input attenuation...
a voltage divider at the input will probably be fine...right at the input jack tip and before everything... that way you wont be slamming the input buffers / switching circuit of the pedal... but again this will basically work just the way a volume control on the synth will work.
so i'd try to turn the synth down to see if its gonna work ok that way and if you get it to where you want it that way, you can play with voltage divider values to get it close at the input jack of the pedal.

[nilsomat]

Thanks for the quick reply,

the problem with the output of the synth is the huge noise floor if i turn it down. I can actually reach an acceptable input level for the box when i turn the output volume of the synth way down, but there i have a constant sound which is almost as big as the actual signal. Acceptable S/N levels can only be generated whit the output volume of the synth turned way up.
Thanks for clarifying which resistors to change, i will have a go to find these in the Behringer, and try to bridge or change them.
I understood that lowering the resistance in the feedback path reduces the gain, right?
Thanks a lot again!

[Quackzed]

yes, no prob

[nilsomat]

Just in case anyone is interested: I did some tracing and found out the corresponding parts:
(Pictures available upon request)

D6 (Be) = D7 (BO)
D5 (Be) = D8 (BO)
D4 (Be) = D9 (BO)
C25 (Be) = C26 (BO)
R14 (Be) = R37 (BO)
R22 (Be) = R22 (BO)

The schematics and values are quite similar 

So in the Behringer R14 (1k) would be the resistor to change in the feedback part of the overdrive, R22 (12K) would be the resistor to change in the feedback of the distortion part.
I will try to bridge these and see how it comes out. Also i might change the diodes (all 4148) to germaniums, any recommendations?

[nilsomat]

Instead of installing an additional voltage divider at the input, would it be possible to just change the values of R28 and R34 (BOSS schematic) or R33 and R29 (Behringer PCB)?
Could i just add some resistors, and what values would be good for a start?
Thanks for input!

[Quackzed]

r34 i think is there to bias the first transistor, rather than act as part of a voltage divider, but you could add a resistor to ground between r28 and c22(boss schem) that would make r28 part of a voltage divider... i'd go that route, experiment with the added resistor value, as well ass possibly adding a cap to ground in parallel with the new resistor, to dump some high end noise to ground at the input...maybee 250pf-500pf for starters see if it cuts too much treble or helps enough with noise...
as for germanuim diiodes, i'd think i'd try the other things first. germanium diodes will cut output down and clip more than silicon and if your feeding the box a bigger signal to start with, theyll really clip the hell out of it. might be what you want, but might be too much considering the signal difference of the synth...but you could always try it, so long as the box's volume pot can bring the output back up enough after the clippers....

[nilsomat]

Just wanted to share an early picture of my tracing....
Any input welcome...