Regulated PSU for Preamp

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bostonflyer

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Joined 2004
#1
I have searched all the threads concerning regulated PSU for preamps used with NI LM3886 chipamps. I am confused by the sheer amount of information. Would someone please post a schematic for a PSU used in conjunction with OPA627/Buf634?

I have a 150VA 12-0-12 toroid, is there any way to use it? I know that the regulators usually regulate down by 3 or more volts; is 9volts enough?; is there an appropriate regulator to regulate from 12 to 9 volts? The speakers I am driving are currently being driven without a preamp and using a volume controlled line-out on my CD player (2.5V max). I would like the benefits of the preamp: switching inputs, modest gain, and clarity enhancement, etc., Any suggestions will be greatly appriciated.

Bob
 
#3
bostonflyer said:


I have a 150VA 12-0-12 toroid, is there any way to use it? I know that the regulators usually regulate down by 3 or more volts; is 9volts enough?; is the

Bob
Click to expand...


Remember that after rectification you will get 12*1.414 volts DC so after regulation you should still be able to get a full 12 volts.
 
#4
Re: Re: Regulated PSU for Preamp

cyteen said:

Remember that after rectification you will get 12*1.414 volts DC so after regulation you should still be able to get a full 12 volts.
Click to expand...


It might be a little borderline, though, except if using low-dropout regulators.

3X7 regulators or similar need at least three volts more to regulate, in to out, and a slight AC line drop might take that away.

Better to get a slightly higher transformer or regulate to a bit less.


Carlos
 
#5
well a 120VA transformer will also have a higher output voltage than 12*1.414 because it will not be loaded down a lot by the preamp electronics. so without accounting for the transformer regulation you'd have about a minimum of 12*1.414=17Vpeak-2V(2 diode drops)=15VDC. say it has about 10% regulation so you'd be more around 12*1.414*1.11=18.7-2V=16.7VDC to work with. don't forget that most of the low dropout regulators are guaranteed a maximum dropout of around 1.5V. i'd say your transformer is more than adequate for your application.

i use one 15VAC secondary of a plitron 120VA toroid in a power supply for an AMP3 (15Wx2 class D amp) and it doesn't cause the voltage to drop at all even around full output.
 
#7
BWRX said:
well a 120VA transformer will also have a higher output voltage than 12*1.414 because it will not be loaded down a lot by the preamp electronics. so without accounting for the transformer regulation you'd have about a minimum of 12*1.414=17Vpeak-2V(2 diode drops)=15VDC. say it has about 10% regulation so you'd be more around 12*1.414*1.11=18.7-2V=16.7VDC to work with. don't forget that most of the low dropout regulators are guaranteed a maximum dropout of around 1.5V. i'd say your transformer is more than adequate for your application.

i use one 15VAC secondary of a plitron 120VA toroid in a power supply for an AMP3 (15Wx2 class D amp) and it doesn't cause the voltage to drop at all even around full output.
Click to expand...

You are considering positive 10% regulation only, when it should be +/-10%, IMHO.

How much wall AC drops might depend on many factors.

Low dropout regulators, as I also said, should be the way to go on this case.


Carlos
 
#9
Well, it looks like I'm on the ragged edge with the tranny. I've looked at some specs on low drop regulators and have not found one with the right input/output voltages, If someone knows of one please let me know. Thanks for all your posts!!

Bob
 
#11

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#12
BWRX

I'm not sure of the voltage requirement. I want to drive a pair of OPA627/Buf634 for a preamp. As I stated in my initial post, I'm looking for a gain of about 5. I thought that a 12-0-12 150va tranny would do it because of the 1.4 multiplier after rectification, but the minimum voltage drop for regulation seems to be the question. I think I know enough just to be dangerous.

Bob
 
#13
Bob

OPA627/BUF634 combo usualy runs on an average power supply of +/-15V.
With the transformr you got at hand you can achieve a rectified voltage of approx. 16,9V (VOut * 1,414). If you you use LT1085CT/LT1033CT positive and negative adjustable regulators you will come close to the desired +/-15V because these regulators only need 1Volt difference between input and output. The LM317 and LM337 regulators on the other hand need a 3Volt difference. The VA rating of your transformer is more than adequate for a preamp of this kind.

Cheers

Walter
 
#15
GeWa said:
OPA627/BUF634 combo usualy runs on an average power supply of +/-15V.
With the transformr you got at hand you can achieve a rectified voltage of approx. 16,9V (VOut * 1,414). If you you use LT1085CT/LT1033CT positive and negative adjustable regulators you will come close to the desired +/-15V because these regulators only need 1Volt difference between input and output. The LM317 and LM337 regulators on the other hand need a 3Volt difference. The VA rating of your transformer is more than adequate for a preamp of this kind.
Click to expand...

To get to the raw DC supply output you have to compute:

(12v x 1.4) - 1.4 = 15.4v

Certainly regulating to 15v is out of the question, except if he adds more windings at the output.

Bostonflyer can use those regulators, or any other low-dropout type, where going to 12.5v or so would be fine.

I think it's risky not considering brownouts or lower AC peaks when you design an AC based regulator.

Those chips will work perfectly with that voltage.


Carlos
 
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