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Old 6th March 2012, 05:08 PM   #1
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Join Date: Jan 2011
Default Power supply for guitar preamp

A lot of guitar amps use power supply similar to the one shown on the picture below:

Click the image to open in full size.

Power regulation for the preamp is achieved with zener diodes.

I need a some help to determine the zener and resistor value.

I followed the example from this page:
Zener Diode Voltage Regulator - Electric Circuit

In my case it would be:

1. Choose a Zener Diode
Zener diode with an 15 Volt zener voltage.

2. Calculate the Maximum Current in the Circuit
Preamp needs 15mA of current, plus at least 5mA for the zener diode, therefore Imax = 25mA to be safe. If you add 10-20% to the load current, this will give you a safe value for the maximum current 30mA

3. Select the Power Rating of the Zener Diode
Pz = Vz x Imax = 15 * 0.030 = 0.45 Watts. Therefore a 1/2 Watt power rated zener diode should be perfect.

4. Select the Resisitor
R = (Vs-Vz)/Imax = (48-15)/0.03 = 1.1 KOhms

5. Select the Power Rating of the Resisitor
P = (Vs-Vz)xImax = (48-15)x0.03 = 0.99W = 1W
Is my calculation right?

I already have two C15HP (BZV85-C15) diodes.
From the datasheet: Izt = 15mA for Vzt = 13.8 to 15.6V; P = 1W
Can I use them? Or 1N5245B is better?

Is it better to use 317, 337 regulators?
Which configuration is less noisier?
What do you recommend?

Last edited by karul; 6th March 2012 at 05:28 PM.
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Old 6th March 2012, 09:12 PM   #2
DF96 is offline DF96  England
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Your calculation is fine, but component selection needs to leave some room. If you calculate a component will dissipate 0.99W you don't use a 1W item unless the circuit will be running in a freezer. 1.5W might be OK if very well ventilated, but 2W would be better.
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Old 6th March 2012, 09:50 PM   #3
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thanks DF96

Which configuration is less noisier? With zeners or with regulators 317 and 337.

I've already build a preamp powered with zener regulation.
Recently I've build new preamp - I'm still tweaking it. I'm using a adjustable power supply during the tests. Judging by ear, I can's say which one is less noisier - to be precise - I'm pleased with the results in both cases. I have no equipment to do some deeper analysis.
I was hoping to get an answer from experienced members about this subject.

Zener regulation is very often used for guitar preamps.
Why is that? Because is good enough and cheaper, or because it's a better solution?
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Old 6th March 2012, 10:18 PM   #4
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I think that we need to look at C50 & C51 connecting AC input power to the chassis.
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Old 6th March 2012, 10:41 PM   #5
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speedskater, did you mean C50, and C61 - capacitors between mains and the chassis ?

There are a lot of guitar amps with capacitors between mains and the chassis. I guess you don't recommend this because safety reasons.

Btw, this is a power supply from a Peavey Bandit 65.

I've should have mentioned before, the preamp is with opamps.
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Old 6th March 2012, 10:44 PM   #6
DF96 is offline DF96  England
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A regulator will be more stable and probably quieter than a zener.

The caps on the mains may be a crude attempt at filtering. Better to use a proper packaged filter.
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Old 12th March 2012, 06:51 PM   #7
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do you have a final schematic for this power supply with all the components values? a component list would be helpful too.
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Old 14th March 2012, 07:11 AM   #8
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Mastercarbit, I don't know what you mean. Can you be more specific?
That's the whole power supply. If you mean the whole schematic of the amp - here it is.
Attached Files
File Type: zip p-bandit-65.zip (387.8 KB, 35 views)
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