dumb question: why 9V, 4.5V, and 0V instead of 9V and 0V???

[GonzoFonts]

I did a few searches on the subject, but could not find the answer I was looking for.

Why do pedal designs use 9V, 4.5V, and 0V and not simply 9V and 0V?

What’s the advantage of using 4.5V for the bias?

Can someone give me a design reason for this?

For example, a lot of electronic books will discuss an emitter follower amplifier which is the first stage of most pedals, but the books only use 9V and 0V and I have yet to find a book that does an analysis on an emitter follower with 9V, 4.5V, and 0V.

Thanks in advance.

GF

[jmusser]

I've always assumed (and maybe wrongly) that the 4.5V being exactly in between 9V and ground, was for setting the reference for the incoming signal. As in a sine wave being able to go 4.5 + , or 4.5 - . I believe that anything over this, gets the tops wacked off into more of a square wave. I believe that diodes set this ceiling in a lot of stuff we do, and cause distortion of the wave form this way. I would guess that during the amplification process in a lot of cases, we do not even remotely reach the 4.5 ceiling, so that's why diodes or other methods are introduced to cause distortion. Remember, that we're starting this process out with whatever the pic ups supply from magnetic induction, and that can't be a whole lot. All this may be totally my imagination, but this is how I have it in my head that this process works. Hopefully someone will clarify or denounce this, if I'm way off! I haven't seen any graphed scope pictures to see what voltages we're actually talking about going through various stages.

[Mark Hammer]

AC signals are called "alternating" because they move back and forth, or rather up and down, about a reference point.  Bear in mind that what transistors do is to essentially "permit" current to flow from the power supply so to produce an output that goes above and below some middle point, there has to actually BE a "middle point".  That can be provided by means of a bipolar power supply, OR by using what starts out as a single-ended supply and "faking" a middle point by means of a Vref.  In this use, 0v fulfils the role of the negative supply.

Note that while dividing the supply voltage in half to derive a fake mid-ground provides the most balanced opportunity for headroom (i.e., neither side of the waveform has more "room to move" than the other), that is not always the ideal solution.  In some instances, people will opt for a reference voltage that is essentially asymmetrical.  E.g., a Vref of 4v will mean that the positive half-cycle has more headroom than the negative halfcycle.

In John Hollis' Zombie Chorus design, he used an asymmetrical bias scheme, not because of what the op-amps wanted but because of what the delay chip needed.  The op-amps just happened to be along for the ride.  Since the circuit had no gain and since the delay chip was going to need a modest signal anyways, restricting the possible signal swing on one half of the waveform was not a hardship.

[Narcosynthesis]

i think it is due to the way you would want a transistor to amplify the signal without clipping and the like
a normal guitar signal is an ac waveform, so goes both positive and negative (it alternates between them) - think of a sine wave as a really basic input
if you just have a transistor biased with only 0v and 9v, you can only amplify half of the signal, either the positive or negative half would get cut off
when you bias it with the 4.5v, you are essentially using 4.5v as 0v, where 4.5-9v is essentially positive, and 4.5-0v negative, so you can amplify both halfs of teh signal correctly

if you look at something like the lpb or rangemaster, they have resisters before the transistor to set the voltage up like this (though not always exactly halfway between 9v and 0v

someone please correct me if i am wrong, this is what i understand from doing electronics at uni (and lookig at fcse amps and the like)

David

[transient]

I have a related question:

For the Tillman preamp ( http://www.till.com/articles/GuitarPreamp/ ), Don states that the transistor should be biased to roughly 6 volts. Why?

Is it that the circuit uses 3 volts for some other reason, leaving only 6 volts for headroom, which would explain why the transistor is biased to swing between 3-9 volts? Or is it for asymmetrical clipping?

It's probably none of these, that's why i thought it would be good to ask :lol:

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