Bipolar MXR

[anti-idiot]

will be any difference if I use a Distortion+ and MicroAmp w/a bipolar power supply? will that increase the headroom? i've read on GGG T-808, using a BiPoSu will increase it, is that true? will it work w the D+ and/or the uAmp?

[Mark Hammer]

In the case of the Distortion+, headroom is NOT what you want, so nix that one.

In the case of the Microamp, clean headroom IS the sought after goal....maybe.  Depends on what you want it to do for you.  If your purpose is to slam the living daylights out of an amplifier and make it misbehave, then the desperate search for clean headroom seems a little foolish.  Well, maybe not foolish, but once you've reached the point where signal A is hot enough to corrupt performance of device , additional clean headroom doesn't really serve a purpose.

Assuming that one was feeding, say, a McIntosh or Crown power amp directly, is it the case that the Microamp is limited enough in headroom that you'd notice some clipping at max gain?  Not really, under most circumstances.  Looking at the schematic here - http://www.generalguitargadgets.com/diagrams/microamp_sc.gif - you'll see that the maximum gain, as set by R4\R6 is (56k+2k7)/2k7 = 21.7.  Assuming that your pickups put out the typical 50-100mvAC or so when picking or strumming, multiplying that by x22 still leaves a fair amount of room between peak level and maximum voltage swing from the op-amp with a 9vdc supply.  Slamming a hot humbucker with a power chord, probably WILL result in a brief transient that exceeds the maximum voltage swing when gain is set to max.  But, as noted, is that a circumstance under which you are aiming for clean headroom?  You're welcome to say yes, but my gut tells me no.

So, while there ARE some purposes under which a bipolar supply with greater supply voltage improves performance, a single-ended 9v supply is generally enough for the pedals you mention to give their best under normal operating circumstances.

[anti-idiot]

thanx a lot, mark.
well, in fact, i'm using the uAmp as a 'clean channel' in a 2 channel effect (Dist+/uAmp). Each channel with its respectively tonestack EQ. But, when the gain pot is set at 50%, the uAmp starts to sound a lil bit crunchy (too much).

a lil question:
21.7 means how many db?

I have calculated aprox. the gain of several units, like D+, TS, SD-1, DOD250, even the 535Q's booster. how many gain is needed to start distortin' or overdrivin?

[anti-idiot]

nobody?

[R.G.]

db = 20 * log10(voltage gain)

So if the gain is 21.7, the log of 21.7 is 1.3364, and that's a gain of 26.7db.

A gain of 2 is 6db.
A gain of 10 is 20db.

DBs are additive.
A gain of four is 6+6= 12db. A gain of eight is 6+6+6 = 18db. A gain of 20 is 20+6 = 26db. A gain of 100 is 20+20=40db. A gain of 150 is less than 46db (it's actually 43.5db).

How much gain is needed to start distorting?
It depends on what the clipping level is. If you're overdriving diodes, the peak voltage is +/- one diode drop. For silicon, that's about 0.6V, and a 100mv guitar signal starts clipping at a gain of six. If you're overdriving LEDs at 2V peaks, you need 2V/0.1V = 20 to start overdriving. If you are overdriving an opamp that can swing to within 2v of the power supply, that's 5V/0.1V = 50 on a 9V battery power, and 14/0.1 = 140 if you provide the opamp with a +/-18V supply.

Then there's frequency response. The TS in particular does not have flat gain with freqeuncy. So the gain needed to distort varies from treble to bass.

[billings]

You can also omit coupling caps in some cases when using a bipolar power supply, if you're interested in increasing the bandwidth of the device for some reason.

[anti-idiot]

just to finish... does bipolar makes the headroom twice its unipolar value? and when using bipolar, do i need Vref?

[Mark Hammer]

The answer is "Not necessarily".  In the case of a circuit with no other headroom constraints (like diodes), and with only modest gain applied, doubling the supply voltage, whether by using a +/-9v supply, or by using a single-ended +18v supply, will not add appreciably to headroom.  If the gain applied is substantial, then "yes", limitations on maximum voltage swing will be less of an issue with a doubled supply voltage.  On the other hand, not all bipolars are created equal.  If you had a +/-3v supply (two AA cells per side), that would not provide greater headroom, compared to a single-ended standard 9v supply.

[davebungo]

just to finish... does bipolar makes the headroom twice its unipolar value? and when using bipolar, do i need Vref?

+/-9V will allow an op-amp to swing nominally twice the amount provided by a single 9V supply.  So yes, you double the headroom.  Whether your output ever reaches or needs to reach those levels is another matter entirely.

Vref is 0V in this case, so you can do away with the bias circuit.  Still, make sure your supplies are decoupled though.