The below circuit is from the Fishman Loudbox Mini acoustic guitar amplifier, the input section (related thread here). I'm creating a guitar pedal preamp from this schematic, says I need +/- 14v. Most guitar pedals use 0-9v. Having a larger voltage range increases headroom I'm told, although I don't necessarily understand why.

A friend of mine suggested I use a Max1044 as seen in this electrosmash analysis from the Klon Centaur. This has 0-9v IN but creates +18 -9v on the rails. The 16.2v should work, but the I'm not sure the -8.6v is enough. NE5532 can handle +/-22v, and TL072/74 can handle +/-18v.

What's the best way to handle this?

 

The maximum signal into an audio power amplifier is 0dBm (775mV rms), so any more than 2.2V peak-to-peak will clip the output. Some larger amplifiers need +4dBm, so they will need 3.5V, both of which can easily be obtained from a 9V battery.
The only time you will need more headroom is if the next stage is an equaliser, which will could possibly be removing a large signal, and you wouldn't want the large signal to clip the previous stage.

Don't forget that the NE5532 takes between 8mA and 16mA standing current. If you increase the supply voltage by a factor of 3, you increase the power it uses by a factor of 3 and reduce the battery life by the same amount PLUS the amount the step-up converter uses, and the step up converter also might make the supplies noisy.

The voltage on the source of the JFET is likely to be about 2V (the device used is a switching device so the data published for linear use is a bit sparse). You can make a constant current source that works on 2V, or you could replace it by a resistor.
You could then buffer the 4.5V with a unity gain op-amp and use it wherever the triangle 0V symbol is used. Add a coupling capacitor on the output.

Then it should work on 9V.

 

So can I simply apply 9 to the top rail and ground to the bottom? Or do I need to reconfigure op amps?

 

So can I simply apply 9 to the top rail and ground to the bottom? Or do I need to reconfigure op amps?

Yes, but check that the constant current load for Q1 is working properly

 

If you can find a rail-to rail replacement for the opamps, you could get nearly the same increase in headroom.

 

The main source of noise will be the transducer and R1+R3. If the transducer has zero output impedance (unlikely) then the noise from those two unnecessary series resistors (9.6nV/√Hz) will swamp the noise of the NE5532 (5nV/√Hz). You could easily use a slightly noisier op-amp with lower quiescent current without any detrimental effect.

 

Bob, Ian,

I appreciate the comments. Unfortunately I don’t understand most of it, not at that level yet, still learning. But I’ll dig into them and try to figure it out

 

The MAX1044 voltage "doubler" IC used in the Electrosmash circuit oscillates at an audible frequency of between 1kHz to 5kHz. The audio circuit might pickup and produce the sound.

The higher voltage is used so that the opamps can produce the horrible clipping harmonics "overdrive" sounds.

 

The higher voltage is used so that the opamps can produce the horrible clipping harmonics "overdrive" sounds.

So then for a clean pristine acoustic guitar tone from a piezo pickup, where overdriven or clipped signals are NOT desired, higher voltages aren’t necessary?

 

So then for a clean pristine acoustic guitar tone from a piezo pickup, where overdriven or clipped signals are NOT desired, higher voltages aren’t necessary?

The Electrosmash circuit produces severe "overdrive" clipping distortion.
You need a higher supply voltage if your clean circuit gain is too high and/or you strum very loudly.

 

Some recent on-board guitar preamp designs use two 9v batteries .
Maybe use two 9v batteries in series for 18v?

 

Some recent on-board guitar preamp designs use two 9v batteries .
Maybe use two 9v batteries in series for 18v?

Very few musicians use batteries anymore. Many power supplies have 9, 12, 18v options. But 9v is by far the most common.

 

Very few musicians use batteries anymore. Many power supplies have 9, 12, 18v options. But 9v is by far the most common.

I'm a part-time musician. I can tell you musicians never have enough 9v batteries.

 

Two 9V batteries gives you something that a 18V supply cannot - a proper, low impedance earth!

 

I'm a part-time musician. I can tell you musicians never have enough 9v batteries.

Is that because engineers don't concentrate on designing effects pedals with low current consumption?

 

There is a big difference between a Name Brand alkaline 9V battery and a 9V Super Heavy Dooty one from "over there"
I always buy a battery that has no rice in it.

Here is an 18650 lithium-Ion battery from "over there" that is full of rice:

 

Is that because engineers don't concentrate on designing effects pedals with low current consumption?

Batteries in effects pedals generally only last one or two performances, and cords from 9v adapters get in the way.
Some guitarist with many pedals use a power bus cable so they can use one adapter for all on a special pedal holder.

 

So back to the original schematic. Does the triangle ground mean 0 volts? Even with +14 and -14.

So if I only wanted to use a 9V PS and 0 as ground, how does that work?

 

So back to the original schematic. Does the triangle ground mean 0 volts? Even with +14 and -14.

Yes.

So if I only wanted to use a 9V PS and 0 as ground, how does that work?

You need to make an artificial ground at some voltage in between 0V and 9V. 4.5V is generally used, but it doesn't have to be.
See R29, R30 and C18 in post #1.
A better ground would be obtained by buffering the 4.5V with an op-amp.

 

Ah, so a voltage divider. Got it. If so:

Schematic 9v supply only
+14 +9
Ground +4.5
-14 0