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45V PSU Final Design

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Frosty_47

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I finaly finished designing a PSU for my Audio Amp.

I apologies in advance for super large Image file...

http://www.freewebs.com/frosty47/Frosty%20PSU%20Final.JPG

Op-amps: U1 & U2 are difference amplifiers. U1 outputs difference across the MOSFETS to U2 which than subtracts the input from the result of U1. The output of U2 is fed into the summing amplifier U3. U3 adds 9Volts to the output from U2. This keeps the gate voltage at 9Volts higher than Source. U5 outputs +55V for U1 and U2.

Transistors Q1-Q4 make-up the current limiting circuit. Rsense limits the output current: Iout = 4*Vbe/Rsense.

Q5 is NPN Darlington Pair with Hfe = 400min.

Zenner Diodes are cascaded in series to provide 65V dc input power to the op-amps.

First LM317 provides 30V reference to the U4 regulating op-amp.

Second LM317 provides 9V reference to the summing amplifier U3.

All Op-Amps are OPA-454.

T1 is a small signal transformer. 0.1 Amp output current should be enough.

Please Coment
 
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audioguru

Well-Known Member
Most Helpful Member
Most audio power amp power supplies have a center-tapped transformer, a bridge rectifier and two filter capacitors. No more.
 

kchriste

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As mentioned by AG, your power supply is much too complex for an AF amp.
Also, simple current limiting (Constant current) is usually done in a much simpler way using the pass transistors to waste the power rather than a second set of devices:
 

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Frosty_47

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As mentioned by AG, your power supply is much too complex for an AF amp.
Also, simple current limiting (Constant current) is usually done in a much simpler way using the pass transistors to waste the power rather than a second set of devices:
Thank you very much!

I will try my best to test this tomorrow in the lab. I like the simplicity of pass transistor to limit the current. However, MOSFETS can be cascaded many times for more power. It is not simple to cool one transistor that will have a lot of power dissipation across it when the supply is shorted and limited at 4Amps. My goal will be either 4 or 5 Amps (depending on how much the Transformer is going to heat up). Further, MOSFETS don't have the thermal runaway problems.

I will test both setup for current limiting. Thanks again for your suggestion, time and interest.

Oh, and why are the days of digital watch numbered?
Not that I like digital watch, I prefer to wear my soviet mechanical watch over the fancy quartz/digital stuff in fashion these days...
 
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Frosty_47

New Member
Most audio power amp power supplies have a center-tapped transformer, a bridge rectifier and two filter capacitors. No more.
Would that not be a source of distortion?
I imagine that some of the ripple from the unregulated PSU will be passed on to the speakers.

If this design works, I will be very happy because that would be the first "true" power supply I designed. By true I mean a proper power supply, not just some quick LM317 that is so commonly overused by newbies such as my self.

Thank you very much for taking time to review my projects!
 
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Frosty_47

New Member
If this works, I would like to build a variable version of this. All I would have to change is reference voltage to U4, and put a couple of potentiometers in place of 94K resistor. Rsense can also be replaced by low resistance Rheostat.
 
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kchriste

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Oh, and why are the days of digital watch numbered?
It is a joke. 2009/6/14 :D
Frosty_47 said:
I imagine that some of the ripple from the unregulated PSU will be passed on to the speakers.
If you look at a common design found on the net you'll see why a little supply ripple won't be a problem. C3 & C4 combined with R13 filter the +33V supply ripple out. C5 & C6 combined with R16 filter the -33V supply ripple out. The ripple doesn't get through Q8 or Q9 because the source voltages follow the gate voltages which are supplied from the extra filtered part of the circuit:

The amplifier above has the same issue as your design in that it won't output full power due to the extra Vgs voltage required. But it is a simple example and that is why I posted it.
 

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Frosty_47

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It is a joke. 2009/6/14 :D

The amplifier above has the same issue as your design in that it won't output full power due to the extra Vgs voltage required. But it is a simple example and that is why I posted it.
Well the extra Vgs is not a problem to supply. This however, tends to increase the idle current by a lot. I am yet to design a circuit that will keep the Vgs at just the right level. However, this will probably vary from MOSFET 2 MOSFET, so an individual "tuning adjustment" will be required.
 
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Speakerguy

Active Member
Push/pull class AB amplifier designs tend to be very immune to power supply ripple. Feedback pretty much destroys it; as such unregulated supplies are virtually universally used in audio equipment that uses your standard CT transformer/bridge rectifier/filter cap arrangement.

Power supply regulation is much more important for things like open loop class D amplifiers where the ripple would show up directly at the output.
 
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Frosty_47

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Thank you all for your help once again!

I wish my school focused more on amplifiers and power supplies. Perhaps I wouldn't be such a noob in the field.

We have never designed/build or even studied a single power supply or an audio amplifier (except for some very primitive class A, B, and AB designs that have no more than two transistors and a couple of diodes).

I learned more from people like you than I did in school.
 

Frosty_47

New Member
Push/pull class AB amplifier designs tend to be very immune to power supply ripple. Feedback pretty much destroys it; as such unregulated supplies are virtually universally used in audio equipment that uses your standard CT transformer/bridge rectifier/filter cap arrangement.

Power supply regulation is much more important for things like open loop class D amplifiers where the ripple would show up directly at the output.
Thanks!

Can you recommend a simple method for keeping the idle current down in the push-pull MOSFET amplifier ? At +/- 30V the idle current seems to sit around 1Amp for me. I can't imagine what it will be at +/- 45V...
 
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kchriste

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I'm not talking about the bias Vgs which sets the idle current. What I'm talking about is the fact that the audio signal output will not swing anywhere near ±33V because the source voltage is always apx 6-10V lower than the gate voltage (Nfet example). Now, if you powered your driving OpAmp from ±43V and the FET drains from ±33V, you'd be able to get close to ±33Vpk audio at full volume instead of ±23-27Vpk. To do this you'd need a separate low current ±43V supply for the OpAmps in addition to the high current ±33V supply for the MOSFETs. (Or you could stack two small 10V supplies on each of the ±33V rails)
 
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Frosty_47

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I'm not talking about the bias Vgs which sets the idle current. What I'm talking about is the fact that the audio signal output will not swing anywhere near ±33V because the source voltage is always apx 6-10V lower than the gate voltage (Nfet example). Now, if you powered your driving OpAmp from ±43V and the FET drains from ±33V, you'd be able to get close to ±33Vpk audio at full volume instead of ±23-27Vpk. To do this you'd need a separate low current ±43V supply for the OpAmps in addition to the high current ±33V supply for the MOSFETs. (Or you could stack two small 10V supplies on each of the ±33V rails)
Well thats exactly the method I used in my PSU design. Except for the voltages of course. Thanks for clearing this up, I will do just that. I got some OPA-454 Op-amps that can be safely operated with +/- 50V supply. And they are very good quality op-amps, perfect for audio amplifiers. However, I am still confused on how I should limit idle current...
 
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audioguru

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Most Helpful Member
Would that not be a source of distortion?
I imagine that some of the ripple from the unregulated PSU will be passed on to the speakers.
No.
Only your circuit feeds the ripple on the power supply to the output Mosfets. A proper amplifier circuit has excellent power supply noise rejection.
 

Frosty_47

New Member
No.
Only your circuit feeds the ripple on the power supply to the output Mosfets. A proper amplifier circuit has excellent power supply noise rejection.
Ouch That Hurts!

Just Kidding :p

I understand that negative feedback at the output will greatly attenuate PSU noise. But if the op-amps themselves are powered from a noisy PSU than wouldn't some of the noise still be passed on ? Although I suppose that this will vary from op-amp to op-amp. Don't most modern op-amps have good power supply rejection ratio.
 
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Frosty_47

New Member
I just checked out PSRR for OPA-452 and it's 40dB(min) at 10Khz, and 80dB at 90Hz. But that's not all!

OPA-454 has 70dB+ PSRR at 100KHZ! Holy SMOKES! That is better than LM317; typically 65dB (without Cadj).
 
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Hero999

Banned
As I said in the other thread, you don't need a regulated power supply for a decent audio amplifier.

Use PTC resistors (polyfuses) for overcurrent protection.
 

Frosty_47

New Member
Design Update

I added U5 to the design. It outputs +55V for U1 and U2. I figured this is better becuase there will be no noise passed on to the gate of the MOSFETS.
 
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