# Amplifier DC Power Supply Problem

#### Jhon Paul Jaspe

Jun 3, 2017
4
I have finished this 60 watt amplifier. It needs 50 V and - 50 V to operate. I created a power supply that has 50 V and -50 V using zeners. Refer to pic. Seperately, they are working as they should. But when i connect them, i don't know anymore. Please help!!! How can I solve this??? Any help would be much appreciated. 1st pic = Working 60 watt amplifier to 8 ohm load. 2nd pic = working DC supply with 50 V and - 50V. 3rd pic = connected together and BOOM! it's broken.

#### davenn

Moderator
Sep 5, 2009
14,215
hi there
welcome to EP

I have finished this 60 watt amplifier. It needs 50 V and - 50 V to operate. I created a power supply that has 50 V and -50 V using zeners. Refer to pic. Seperately, they are working as they should. But when i connect them, i don't know anymore. Please help!!! How can I solve this???

yes they will show +50 and -50V

You CANNOT use zeners because they don't have a hope in hell of being able to provide the current required for the amplifier and that is why it didn't work when connected to the amp part of the circuit
The current through a 2.5KΩ resistor is going to be tiny

You use a transformer that have a secondary output that when rectified and smoothed produces your required approx. ± 50V

so what is the secondary open circuit voltage of your transformer ?
what is it when rectified ?
and then smoothed ?

lets just take an example calculation
say the sec voltage Vs from centre tap to top winding is 25V AC

through a fill wave rectifier you will loose 1.4V, 0.7 per diode
25V - 1.4V = 23.6V (± 0.2 for variations in the diodes) unsmoothed DC

the addition of the smoothing capacitor increases the avg. DC voltage by a factor of 1.4
so ...
23.6V x 1.4 = 33V

so for your case, you want 50V DC smoothed out
50V / 1.4 = 35.7V unsmoothed
35.7V + 1.4 (diode loss) = 37V AC as an original secondary voltage

Last edited:

#### Audioguru

Sep 24, 2016
3,650
Your plus and minus 50V cannot supply anywhere near enough current to the amplifier. Most audio amplifiers do not regulate the voltages to the output stage.

Your output stage does not have any negative feedback so its distortion will be high.

#### davenn

Moderator
Sep 5, 2009
14,215
Your plus and minus 50V cannot supply anywhere near enough current to the amplifier.

Your output stage does not have any negative feedback so its distortion will be high.

good point

#### Jhon Paul Jaspe

Jun 3, 2017
4
how about this power supply? how do i increase the current coming of here? when i connect this to the amplifier the output is very distorted

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#### Jhon Paul Jaspe

Jun 3, 2017
4
Your plus and minus 50V cannot supply anywhere near enough current to the amplifier. Most audio amplifiers do not regulate the voltages to the output stage.

Your output stage does not have any negative feedback so its distortion will be high.
How do I get negative feedback?

#### Audioguru

Sep 24, 2016
3,650
How do I get negative feedback?
You need to learn about amplifier design and opamps to see how negative feedback reduces the voltage gain to a useable amount, reduces distortion a lot, increases the bandwidth and reduces the damping factor a lot.

An audio power amplifier is designed to have a voltage gain without feedback of thousands or millions. Your amplifier has a gain of only about 120 so negative feedback cannot be used.

#### Jhon Paul Jaspe

Jun 3, 2017
4
You need to learn about amplifier design and opamps to see how negative feedback reduces the voltage gain to a useable amount, reduces distortion a lot, increases the bandwidth and reduces the damping factor a lot.

An audio power amplifier is designed to have a voltage gain without feedback of thousands or millions. Your amplifier has a gain of only about 120 so negative feedback cannot be used.
So I don't need negative feedback? my goal is only to produce an output of 60 W to 8 ohm load
But the power supply to be used is custom made. When i use the built in DC POWER it works. 60 W on load. but my problem is the supply i made

#### Audioguru

Sep 24, 2016
3,650
ALL audio amplifiers have plenty of negative feedback because it "reduces the voltage gain to a useable amount, reduces distortion a lot, increases the bandwidth and reduces the damping factor a lot."

60W RMS into 8 ohms has a peak voltage of 31V which causes a peak current of 3.9A. Your original 50VDC has a 2.5k resistor in series so that when the load current is only 10mA (0.01A) then the 50V is reduced to 25VDC.

Your new circuit has an unknown transformer that probably cannot pass 3.9A without its voltage dropping. Measure the "51.1VDC" when the amplifier is trying to produce 60W into 8 ohms.

What is the "built in DC POWER? What is its differences from the new power supply you made that caused distortion? Did you make your new power supply on an intermittent solderless breadboard?

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