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Voltage regulator circuit

G

Gustavus

Golden Member
OK folks, I need your help. I recently purchased a Creek OBH-8 MM preamp to couple my Shure V15 MR cartridge to an Audiophile 2496 A/D audio card for a major project of transferring a large collection of classical LPs to CD. The power supply that came with the Creek Preamp is absolutely lousy -- an oscilloscope shows almost 1 volt of ripple on the 24 volt output. Plus which it is an incredible antenna for RF noise, That is no surprise since it is a wall wart supply like you might expect with a battery charger. I have many 12v batteries that go into my several UPS's. They are 11 amp/hour so a pair of them should supply the 24v the preamp requires for several days of continuous recording. Unfortunately the voltage out of a pair of them in series is 27 volts -- not 24 as it should be for the OBH-8.

I have searched on net for a circuit that would very accurately regulate the 27 volts from the batteries to a quiet and super stable 24v out -- with no success. I would assume such a regualtor circuit would use a zener for voltage reference. Do any of you know where I can find such a circuit? A switching DC to DC converter will not work since it would add noise. I need a linear circuit that is very noise free.

Thanks for your help.
 
This contains some information on voltage regulators.
http://www.national.com/appinfo/power/files/f4.pdf

A voltage regulator can be a complicated circuit.

This is a 24V regulator datasheet.
http://www.datasheetarchive.com/datasheet/pdf/88/884686.html

More info on the LM78xx series.
http://www.eidusa.com/Electronics_Voltage_Regulator.htm

Putting those batteries in series is not a good idea. If one of them discharges before the other one, which is a reasonable possibility, the outcome may not be pleasant.
 
Navid,

Thanks for the leads. I may use a 7824 but had hoped there was a simple circuit that took it's reference from a precision zener. I don't think there is any problem with putting batteries in series -- my three APC Smart UPS's all have four of the 12 volt batteries in series. The problem is when batteries are in parallel and one or more of them dumps through another that has gone bad. I bought an expensive Lambda L219 regulated supply (22v - 32v) but it has too much noise on it to use directly with the preamp. I will use it to charge the pair of 12 volt batteries. VERY expensive battery charger. Never thought it would be so difficult to get a clean 24v.
 
WAIT. WAIT. You don't need a regulator.

Two 12v batteries connected IN SERIES is all you need. It will be the ULTIMATE clean power for a pre-amp. No AC power supply (short of spending a lot of money) wiill be better.

The voltage is only 27 volts because it's not under full load. Voltage is always dependent on load. That's okay. Also 3 volts over voltage for a pre-amp is no problem. It's an analog circuit - it doesn't require regulation. The power supply that came with it is likely even farther out of spec than that.

It'll work fine, trust me, I've been a sound engineer for 20 years.

If you REALLY want to knock off the 3 volts (and I tell you, it really isn't a problem) then put TWO 1N4001 diodes (frankly ANY silicon power rectifier diode rated for more than 24 volts will do) IN SERIES with the positive lead of the battery in this order in proper polarity:

Positive battery lead to diode #1 anode, diode #1 cathode to diode #2 anode, diode #2 cathode to the + on the pre amp.

Each diode will knock off about 1.25 volts if I remember correctly.

 
I agree with FlyingPenguin. It is very unlikely your preamp will care about the 3 extra volts. Putting diodes in series will drop about 0.7 volts per diode, but this should be, as he said, totally unnecessary. Go for it.
 
Originally posted by: Gustavus
Navid,

Thanks for the leads. I may use a 7824 but had hoped there was a simple circuit that took it's reference from a precision zener. I don't think there is any problem with putting batteries in series -- my three APC Smart UPS's all have four of the 12 volt batteries in series. The problem is when batteries are in parallel and one or more of them dumps through another that has gone bad. I bought an expensive Lambda L219 regulated supply (22v - 32v) but it has too much noise on it to use directly with the preamp. I will use it to charge the pair of 12 volt batteries. VERY expensive battery charger. Never thought it would be so difficult to get a clean 24v.
Click to expand...

There must be a voltage reference integrated on those LM78xx parts.

You can build your own simple circuit. The problem is that you have to make sure that it is stable. A voltage regulator has negative feedback. Any circuit with negative feedback may become unstable (due to poor design) and oscillate. In fact the sine-wave you have seen on the scope may be the result of such a circuit.

That is why I suggested an integrated regulator. An integrated regulator has everything integrated, and tested. So, as long as you follow the data sheet, it should be stable.

As far as noise goes, have a look at the data sheet and see what it is specified to be.

Do you know how much current your pre-amp draws from the 24V supply?
 
Thanks for the suggestions. I found a site devoted to audiophile electronics that has actual tests of a number of 24v supplies, plus designs for a some super quiet power supplies for audio preamps.

http://tangentsoft.net/elec/psu-tests/

The OBH-2 in the test is the $90 "top of the line" Creek power supply for the preamp I have that came with the OBH-1. I can't imagine why Creek even ships the OBH-1 since it is so noisy and susceptible to RF pickup. As I said it is a wall wart like you would expect with a cheab battery charger. Lots of reports on web of peoples bad experiences with it.

The preamp only draws 30ma. Since it is such a simple circuit I may build the regulator based on a LM317A to try. It would be nice to simply use the batteries since I can put them in a Bud box and ground the shield on the power cable to it to shield from any pickup. The turntable is well separated from any other electronics so the only interference would be from the drive for the Technics 1200SL itself.

Again thanks for the suggestions and responses.
 
Without a doubt, a battery is the lowest-noise solution.
I just don't know if the amplifier can tolerate a voltage higher than 24V.
 
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