Clean Boost or Buffer to mix with BSIAB

[ijustwannastrat]

When I ordered my Big Muff Pi kit from JD Sleep, he sent me a BSIAB (brown sound in a box, they claim it sounds like a cranked marshall) PCB instead of a BMP PCB. So I figured, why not put a BSIAB in parallel with some sort of clean boost of buffer, with a blend pot at the end of the parallel paths.

So my questions:

1. What is your favorite clean boost or buffer?
2. Why is it your favorite?
3. Should I consider putting an overdrive in instead of a clean?

[jcgss77]

What kind of distortion are you looking for ijws?

[ijustwannastrat]

I am hoping for something that could be used for classic rock, with a little more edge, but clarity of notes.

[Yew]

I imagine you would need to have a signal buffer, Then the clean boost and pcb in parallel, and then your blendy thing

[ijustwannastrat]

Jan 11, 2011 7:50:46 GMT -5 Yew said:

I imagine you would need to have a signal buffer, Then the clean boost and pcb in parallel, and then your blendy thing

Why would I need a buffer before it all?

[flateric]

See my build post re mosfet boost and tubescreamer 2-in-1 build - thats a really great combination. The Mosfet boost to me is a brilliant little circuit - very simple, very clean boost, no changing of eq, can use for guitar or bass, and gives up to 20dB of gain. The circuit itself will not distort but it does wonders in front of a tube pre-amp.
the circuits are in series though, not parallel, so you can use the boost on an already overdriven signal for solos that have a lot more presence to them

[ijustwannastrat]

2 in one mosfet boost + tubescreamer

And where did you get the schematic for this mosfet boost?

[JohnH]

For clean boosts and buffers, I just jump straight into my favorite JFET circuits

See first post of this thread:
guitarnuts2.proboards.com/index.cgi?board=schem&action=display&thread=3150

These buffer and gain modules can be tweaked for each application.

Then you need to mix it all together at the end, and I would use this:

guitarnuts2.proboards.com/index.cgi?board=schem&action=display&thread=4595

Now a watchit for mixing parallel active circuits:

Depending on how many inverting active stages there are, each circuit may or may not change the overall phase, If you mix two outputs with opposite phase, you lose bass and maybe other frequencies. It may be
be more tricky than that too, since there may be smaller phase shifts going on at each frequency, differently in the two circuits. None of that matters much when used singly, but it can become very significant when blending two equal-sized signals. So - mock it up first and tweak it before the final build!

cheers

John

[ashcatlt]

Jan 11, 2011 8:52:08 GMT -5 ijustwannastrat said:

Jan 11, 2011 7:50:46 GMT -5 Yew said:

I imagine you would need to have a signal buffer, Then the clean boost and pcb in parallel, and then your blendy thing

Why would I need a buffer before it all?

It's a matter of input impedance and treble response.  With a passive split, the total impedance will be the parallel total of both circuits, which will always be lower than either one by itself.  When a passive guitar is plugged straight in, you'd expect some (relative) loss at the very top end of the guitar spectrum.

If both circuits are 1M, the parallel total is 500K.  That's really not so bad, being right around what you'd expect in a tube amp.  The difference between 1M and 500K is generally quite subtle.  You'd really have to A/B to tell the difference.  If you were talking about switching from one to both, or if this box was True Bypass and followed by a 1M pedal you might be concerned.  

If, OTOH, both circuits were 500K, you'd be down to 250K total.  This is noticeable and likely unacceptable - especially with humbuckers.  

What if they're nor equal?  Say 1M||500K.  That comes to 333K, which probably still a bit low.  Severe mismatches can cause an imbalance at the inputs as well. 

I haven't looked at the schematic for the BSIAB, but it would likely be just one or two resistors that set the in-Z, and probably safe to change if necessary.  

I'm think you might end up wanting a buffer at the end as well.  Depending on how you end up mixing the signals at the end, you'll likely end up with a rather larger or (almost worse) variable output impedance for the box.  This could lead to strange and difficult to predict behavior depending on what's connected downstream.

Anyway I would (did) use an opamp, but I have no rational reason why.

[ijustwannastrat]

So a buffer before and after will keep the total resistance up? Alright. What exactly is a buffer?


JFET, MOSFET, what exactly are these terms?

in-Z? Oh god....

HERE is the schematics for the BSIAB. It is a PDF, that effects you.


From what I am getting from all your help, is that once I pick whatever effect is going to be parallel with the BSIAB, THEN I need to pick the buffers accordingly. I am willing to learn, so if anyone feels like teaching, I will be on my best behaviour!

[gfxbss]

My personal favorite boost(at least that I own) is the Alembic Stratoblaster.

Simple circuit and just simply boosts. Doesn't add a bunch of gain, but will definitely throw your amp into overdrive....

Tyler

[ashcatlt]

Jan 11, 2011 23:00:34 GMT -5 ijustwannastrat said:

So a buffer before and after will keep the total resistance up? Alright. What exactly is a buffer?

In simplest terms a buffer is an amplifier circuit which presents a relatively high Z (= impedance) at its input and a low Z at the output.  It stands between an impedance sensitive section of circuit and either a) an unknown source or 2) an unknown load.  

In this case the input buffer presents a high-Z input to keep the pickups happy.  The output buffer presents a low (and static) Z source so that is most likely to behave correctly no matter what (within reason) follows.  The circuits in the middle can just do their things without worrying about the outside world.  I try to get my wife to do this for me...

Most of the time when you see somebody talking about a buffer for audio, they mean an amplifier stage which provides unity voltage gain.  The output voltage equals the input voltage.  It is perfectly possible, though, to have gain in a buffer stage.

I use opamps because I don't really get transistors and I'm too lazy to figure it out.  Somebody else is going to have to explain that whole MOSFET/JFET thing.  That said, I'm not completely sure how the BSIAB works but I'm fairly confident in saying that the first transistor is a buffer of some sort, and if I'm not mistaken, the in-Z will be right around the value of R1 which is 1M.

[JohnH]

MOSFETS and JFETS are types of Field Effect Transistor, somewhat different from the more common Bipolar Junction Transistors (BJTs). All are basically three terminal devices in which the current flowing between two of the terminals is controlled by the third terminal.

In a BJT, it is the current into the ‘base’ connection that controls the current flowing between the ‘collector’ and the ’emitter’. A small base current controls a much larger current between collector and emitter, hence there is current gain, and when suitably configured, an amplification may be derived.

In JFETs and MOSFETs, terminal have different names, but it is the voltage at the ‘gate’ that controls current between the ‘source’ and the ‘drain’. This is interesting for us because virtually no current flows into the gate, so the input can be very high impedance, which suits guitar signals. Also, this mode of operation is very similar to that of a valve and in fact, the tonal characteristics of FETs when over-driven are somewhat similar to valves. Hence these circuits can produce some valve-like tones which are not available with circuits using only BJTs.

JFETs and MOSFETs are similar in concept, but while a JFET has extremely high input impedance, a MOSFET has almost infinite input impedance. Hence a practical concern is that until wired into a circuit, MOSFETs are subject to damage from static (as are many modern chips, which are based on MOSFETs). As against that issue, MOSFETs have further advantages of gain and other characteristics which I’m not up to speed on myself.

The BSIAB circuit uses all JFETs, in a fairly complex arrangement in which gain is created by pairs of JFETs rather than just single ones. This pair configuration is called a ‘mu’ amp. There are two such stages, separated by a tone shaping and gain control, then a further single stage, then volume and tone at the output, as a passive network.

I have tried these mu amps (but not the whole BSIAB circuit), but personally didn’t like the sounds I got, and I get very nice overdrive with just single JFET stages in series (see my last post).

This is a simple one that is my main overdrive pedal:

guitarnuts2.proboards.com/index.cgi?board=fxschemes&action=display&thread=4987&page=1

John

[thetragichero]

if i wanted to piggyback two fetzer circuits, is it as simple as connecting the output of the first to the input of the second?
what would i do with the gain pot (i want to use one pot for gain)?
do i use the same value? double the value? hopefullly won't need dual gang pot?

[flateric]

its as simple as putting them in series together, no change to pots. Electroharmonix's first ever fx box was the screaming bird treble booster. They connected 2 boxes together to increase the gain and this led on to the design of the iconic little big muff circuit.

[thetragichero]

how do i do it with one pot?
do i wire them both to the pot?

[JohnH]

thetragichero - is your question related to the subject of this thread?

What are you wanting to have this circuit do?

I suggest a new thread.