Rocktron Big Crush true bypass modification.

[olddawg]

I have a Rocktron Big Crush that I would really like to modify to true bypass switching using a 3PDT switch. I can't seem to locate a complete schematic for the circuit to work it out. Some people are using the Millennium Bypass on these since they come from the factory with a 2PDT switch. Some people are just swapping (modifying the buffer I think) a few parts. But I would prefer just a simple switching scheme that takes the pedal completely out of the chain and still switches the LED. Anyone have a complete schematic? Any suggestions on how to mod it to true bypass? Any and all advice or suggestions will be greatly appreciated.

[Mark Hammer]

The Big crush has a decent input buffer, that is used for the buffered bypass. On the surface, there would appear to be little advantage gained by TB-ing it, apart from having it be immune to power outages...something which pertains to fewer and fewer players these days. I suppose you could conceivably replace the resistors in the input stage with 1% metal film for a tiny improvement in noise specs, and maybe replace the 1M input terminating resistor with 2M2. BUt these are minor things.

What was your motivation for changing the switching arrangement?

[olddawg]

The Big crush has a decent input buffer, that is used for the buffered bypass. On the surface, there would appear to be little advantage gained by TB-ing it, apart from having it be immune to power outages...something which pertains to fewer and fewer players these days. I suppose you could conceivably replace the resistors in the input stage with 1% metal film for a tiny improvement in noise specs, and maybe replace the 1M input terminating resistor with 2M2. BUt these are minor things.

What was your motivation for changing the switching arrangement?

There is a huge volume drop and bass roll off when not engaged. It's a common complaint and a well documented problem. I have other buffered pedals in the chain that are not problematic.

[Jazz P Bass]

This gentleman seems to have done it rather well.
Rocktron big crush | What's wrong with this circuit?

Although, a simple mod it is not,

[olddawg]

This gentleman seems to have done it rather well.
Rocktron big crush | What's wrong with this circuit?

Although, a simple mod it is not,

[ATTACH=CONFIG]32060[/ATTACH]

Yeah... I saw that Jazz . It is a Millennium Bypass application. Not difficult really. But he seems to have some LED issues with it. I was just trying to use the simple 3PDT switch method if I can figure it out, have a true bypass and not have to mess with perf boarding FETs and such. I have a 3PDT switch.

[olddawg]

I guess I could wire the new switch exactly like the old one and use the the extra gang to switch the input directly to the the output jack. The output circuitry would still be hanging off off it though. Not sure what effect that would have. Maybe RG will give me a clue?

[Mark Hammer]

Maybe the cure is to simply change the values of the buffer output. 10k and 47nf gives a bass rolloff around 340hz. The 10k/10k pair drops the signal by half. Maybe the 10k to ground ought to be higher, such as the 100k found in the actual compressor output. That will increase bypass output, by not attenuating as much, but it won't fix the bass rolloff issue. For that, I'll simply suggest tacking on a 100nf cap in parallel with the stock 47nf.

The Big Crush also uses TWO buffers in series for bypass, which is unnecessary. The output should probably be taken from the first buffer, with the second buffer hardwired to the volume pot. The switch would then select from either the buffer, for bypass, or the output of the compressor.

[olddawg]

Maybe the cure is to simply change the values of the buffer output. 10k and 47nf gives a bass rolloff around 340hz. The 10k/10k pair drops the signal by half. Maybe the 10k to ground ought to be higher, such as the 100k found in the actual compressor output. That will increase bypass output, by not attenuating as much, but it won't fix the bass rolloff issue. For that, I'll simply suggest tacking on a 100nf cap in parallel with the stock 47nf.

People have done this mod to the buffer:
Rocktron Big Crush - the bypass mod that makes it usable - Telecaster Guitar Forum

I was hoping to conserve real estate on my pedal board because it is a big pedal to begin with, but I'm thinking now maybe a 4PDT switch might be in order or build a looper pedal to simply take the damn thing in and out.

[J M Fahey]

Agree with Mark Hammer.

The problem is not in the buffer at all but signal attenuation and bass cutoff are there by design

I guess they found their still not fully developed compressor stage worked well but had 6dB LOSS when engaged.
The proper solution was to correct that or worst case, leave as is but add a 6dB (2X) gain stage after compressor, a klunky solution.
What they did was even klunkier, they padded 6dB the direct unprocessed signal and to boot, since the padding resistor lowered net resistance to 20k , the formerly good .047 coupling cap now was too small, cutting all bass below 160 Hz
The Forum software is barking at me for using way too many [eek] emoticons , but they are needed here.
True bypassing it is also klunky, sorry, since there is a solution..
What must be done is to correct the original error:
a) lower R4 to 4k7 or 2k2
b) rise R25 to 100k or 220k . They are only needed to provide a ground path to an earlier capacitor.
c) rise C40 to 100nF or even use an electrolytic 470nF or 1uF (x25 or 50V).
+ end towards the input buffer.

Rocktron is a good Company but this kind of mistakes happens when they are pressed to finish something NOW!!! for Frankfurt.
In fact the disease is so well known in the Industry, that it's called "Frankfurteritis".

If they ordered 50000 boards in advance to cope with expected sales, they were stuck with them and sold the pedal as is .

[olddawg]

Agree with Mark Hammer.

The problem is not in the buffer at all but signal attenuation and bass cutoff are there by design

I guess they found their still not fully developed compressor stage worked well but had 6dB LOSS when engaged.
The proper solution was to correct that or worst case, leave as is but add a 6dB (2X) gain stage after compressor, a klunky solution.
What they did was even klunkier, they padded 6dB the direct unprocessed signal and to boot, since the padding resistor lowered net resistance to 20k , the formerly good .047 coupling cap now was too small, cutting all bass below 160 Hz
The Forum software is barking at me for using way too many [eek] emoticons , but they are needed here.
True bypassing it is also klunky, sorry, since there is a solution..
What must be done is to correct the original error:
a) lower R4 to 4k7 or 2k2
b) rise R25 to 100k or 220k . They are only needed to provide a ground path to an earlier capacitor.
c) rise C40 to 100nF or even use an electrolytic 470nF or 1uF (x25 or 50V).
+ end towards the input buffer.

Rocktron is a good Company but this kind of mistakes happens when they are pressed to finish something NOW!!! for Frankfurt.
In fact the disease is so well known in the Industry, that it's called "Frankfurteritis".

If they ordered 50000 boards in advance to cope with expected sales, they were stuck with them and sold the pedal as is .

Thanks for your input Juan. Makes me wonder why they didn't correct this problem since this pedal has been in production for the better part of a decade. Truthfully, in real life application, I love this pedal. I've never noticed any gain loss at all when it was on. As a matter of fact it's more of a boost. It makes a damn EP VJ head (for bigger rooms I used an 18 watt Marshall variant) so loud I have to use an attenuator in some clubs with a live drummer. I just left it on all of the time. People were always asking me how I could get all of that volume and thickness out of a VJ. The answer was the Big Crush and a Bad Monkey after it. The problem comes when I switch it off. Wimpy City. I've recently changed projects and am using a larger 2 channel amp with a 3 x 12AX7 preamp. So I want to turn the Big Crush off sometimes without the volume loss. But honestly....... it's one of the best/most usable compressors (half done or not) I have ever used. I will look into your suggestions. It's not a big deal to swap a few components. I just got to remember to take a picture of the board before I start. Lol.

[olddawg]

So ... I thought I would resurrect this. After trying a couple of other compressors and a bunch of mods on this one, I picked up another unmolested Big Crush off of EBay for $25. I put the first one back to the original configuration. AND... Since both input and output on these units are common grounded to the chassis, I installed a $7 4PDT switch, cut a trace at the input and the output, and hard wired it true bypass. Works great! Great bang for the buck with these units with a very simple mod. I guess I should mention it isn't for the ham handed. 12 connections on the switch in very close proximity.