About the common Boss SD-1 bypass problem

[grrrunge]

Guys, this is not that hard. It's not magic, it's not mysterious.

The distorted signal leaks because the JFET in the distorted signal path is not turning off fully. Period.

This in turn is because the JFETs do not have a high enough reverse Vgs to fully turn them off.

This in turn is because the sources of the JFETs are not high enough for the ground voltage on their gates to fully turn them off for devices that are within the guaranteed ranges on the device datasheets.

The answer is to either (a) get JFETs with lower Vgsoff or (b) raise the bias voltage on the sources of the JFETs you do have or (c) both of the above.

You may, as analoguru suggests kill the gain of the distortion section while in bypass, but that's an extra credit thing. That just lowers the leakage through the distortion path so it is below the distortion threshold and although it still leaks, it leaks the same signal you're listening to, so you don't notice.

Specifically, Q1 is the problem. Its source is sitting at the bias voltage of 1/2of the battery. This may be as low as 8V easily, and its gate is pulled down by Q4's collector in the discrete flipflop. The series diode means it can't get pulled lower than a diode drop, so you have 4v-0.5V = 3.5V  of back bias on Q1's gate.

The 2SK30A is listed as having a Vgsoff of -0.5V to -5.0V. If you get a -5.0V device, it will not turn fully off in the SD-1 circuit.

You can do a couple of things. One is to stick in a J201. The J201 is notable for having a Vgs0ff of -0.1 to -1.5V. The 3.5V of the existing circuit will turn this one off very well, in all cases. The J201 is often used for a replacement for the 2SK30A in some synth circuits.

The pinout of the 2SK30A is SGD with pins down, flat facing you. The J201 is pinned out DSG in the same orientation.

You can also find other JFETs, such as the 2N5292 or 2N5485, which will work almost all the time, as their maximum Vgsoff is -3.5V.

You can raise the bias voltage from half the battery to some higher voltage by changing R18 and/or R19 to get a bias voltage of maybe 5V or 5.5V. This will probably work just fine. The lowest effort version of this is to change R18 to a 22K or to parallel it with a 51K or 68K resistor.

While I have not done either of these, the schematic and Mother Nature say they have to work - unless there is something even more subtle that Mother Nature is trying to teach me, which does happen from time to time.

Just to report in on this: I replaced the 2SK30's in my SD-1 with J201's with no noticeable result. I tried raising R18 to a 22K, resulting in a gated muffled square wave fuzzy tone. Probably useful in some context, but not very overdrive-like Good thing i use it in a looping system most of the time...

[xterkrs]

Hi,

I'm sorry because I'm reviving an old but usefull thread, I think that there is a lot of useful information being said here and would be better to keep in the same thread all information regarding to this matter for people to consult it (like me).

Because our country situation and the hard-to-find things is a present element, I have to solve with the things I can find. About the analog guru fix, I cannot find either a j201 or a SK30a or any other replacement suggested here. I've found some j113 in an older DOD pedal I was going to dump, and according to the specs needed, I would like to know if these fets could work to do the bypass bleed fix with the cap and the 4148. Here are the specs I find for the j113:

Source and drain are interchangeable
Gate to source breakdown voltage of 35V
Gate to source cutoff voltage of 3V
Zero gate voltage drain current of 2mA
Power dissipation (pd) of 625mW
Operating junction temperature of -55°C to 150°C
Drain source on resistance of 100 Ohm


Other site states a more detailed info in the following PDF:

h t t p://pdf.datasheetcatalog.com/datasheet/calogic/J111.PDF (remove the space in the http)


Thanks in advance for all the help/info you can share.

Erick.

[PRR]

Welcome.

> our country situation

Say where you are. Someone may know where/how to buy parts.

> I've found some j113

Vgs(off) is really 0.5V-3V, and the low end is a bit low to handle guitar levels cleanly.

But try it.

[xterkrs]

Thanks for the reply PRR,

I live in Venezuela so, you can imagine...

Also thanks for the clarification, I'll try with the j113 and report back. May I suppose that the components are away from risk of damage trying with that j113, right?

Which would be the most important values that I need to check if I find some other jFETS?

With gratitude,
Erick.

[xterkrs]

Hi folks,

I'm back to report... Looking for more alternatives in the web before buying the other components I needed, I found the cctsim bleed fix which uses a 2N5457. So, I decided to search a little further and ask many friends and stores as possible here looking for the J201 alternatives (even in SMD) and now this new other FET (2N5457). Finally, one friend that works building amplifiers and that kind of stuff found me a 2N5457, so I trade with him my j113 for his hard-to-find-here FET.

At first, I tried the cctsim mod, it completely shut down the signal and it gave me a total clean sound but.. when playing too hard, the pedal behaved as some kind of compressor and cut the signal intermittently so, as it didn't worked for me I reverted the pedal back to its initial state.

Now I tried the other popular fix with the cap and the diode using this other FET (2N5457), and it worked flawlessly. Conclusion: if you can't find the J201, the 2N5457 is an alternative that works (the vgs(off) is -1.5 - 6, right?)..

Thanks for the help!

Take care,

Erick.