Voltage regulators for line level circuits

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Rehabilitated?

Maybe it is just that Grey isn't here to tell everyone how evil I am
I have posted schematics and lots of info for a long time.

H.H.
 

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Shunt Regulator

Hello,

I have thought of running my power amp with a shunt regulator before.

Can a shunt regulator reduce output impedance like a series regulator?

The current source has high output impedance of course.

Or would it still need to be fed from a series regulator to get the lower impedance ?

The load is constant at 2A 24V. (single ended)

Kevin
 
Modding for 5V

As Harry suggests, an op-amp suitable for 5V operation is the first thing to find.

AD825 looks promising here.

Then a 2.5V low-noise reference is required. WJ suggests some AD parts, I have a unit running from a TL431, set to min o/p of 2,5V.

In this state the TL431 noise is low, and unamplified by it's own loop gain - about the only good use I've found for TL431's ;)

MY one is using an OP27, which I'm not sure is specified for 5V, but works well nonetheless.

Maybe Jan could offer some devices to try, failing that the important criteria are high open-loop gain over the widest BW and low voltage noise, along with a good element of DC stability.

Andy.
 
This is what I use to feed my amp.

I have found this schematic on the net and slightly modified it.
It's a op-amp, as you can see, and you can modify the gain by working on R5/R4 ratio for the positive rail and on r17/R16 ratio for the negative rail.

The circuit has a very low ripple and a good stability, but if you find some bugs... I will appreciate your advices :)

My 2 cents, bye!
Fabrizio
 

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AD825

Harry if you look at the spec sheet - you'll see it's partially specced to +/-2.5.

It's only fully characterized for +/-5.

Don't forget the reference is filtered quite heavily, and hence even the 431 works well here - still much quieter at the reg o/p that anything off-the shelf, by a tens of dB

A.
 
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Voltage regs

I think if we want to make real progress with these regulators, we're looking in the wrong direction. We keep on looking for *good* audio opamps for regulators. What we need is high gain, low noise, high bandwidth, low output impedance and high drive current.

An interesting family of opamps are those used for xDSL drivers. Just a couple of examples: TI's '6052 and 53, ADI's AD8016/17. These things have specs out of this (audio) world: slewrate 1600V/uSec, bandwidth 300Mhz, output drive of 200mA in a 25 Ohms load, volt noise < 2nV/RtHz.

There are disadvantages: there are only a few spec'd for +-12/24 or +-15/30 V supply, and they invariably are SMD types.

These are CF devices, and in a regulator should give flat output impedance into 100's of kHz, maybe not as low as the traditional regs at 1kHz. Because of the high drive capability, in many cases they can be used without a pass transistor. (Espercially for +5V logic local regulation, because practically all are spec'd for single 5V operation). That means for local regulation you need fewer components, just a central reference distributed to the local opamp regs, or each one a dedicated reference if you want to get really fancy.

It may be worthwhile to check them out.

Cheers, Jan Didden
 
real progress with these regulators

I am afraid that I disagree.

Bufferring the output of op amps has real advantages in signal path and regulator circuits. This particular true when using the regulators output voltage as the supply for the opamp supply.

Regulator circuits use output caps. Fast twitchy op amps do not like large capacitive loads.+5V logic local regulation without output
caps? I think not.

Jung went from the AD797 to the AD825 in his regulator, hmmm....
I wonder if opamps over 100Mhz are such a good idea. There comes a point where asolute low impedance value start to have real trade offs with stability considerations as well as load transient response. This is especially true when remote sensing is used.

H.H.

PS Another 5 volt amp is the OPA228/OPA227
 
CF amps

Jan,

I've modelled some reg's using such devices and the performance specs are awesome, the improvement in reg BW is truly stunning.

I may pursue this on later, but am considering the discrete route at present. Tom Evans has designed some discrete reg's (Lithos) used in the Groove and Microgroove+ phono stages.

The difference in sonic performance between the MG and the MG+ comes solely from PSU performance, and has to be heard to be believed.

A.

P.S. OPA 228/227 model well and I have these on my list of devices to try.

P.P.S. Walt hasn't discarded use of the AD797, but the non-degenerated low noise front end of this device can give rise to non-linearity in high noise applications. This is where JFET and degenerated devices (AD817) start to be applicable. As with everything, there's no universal solution.
 
Real life regulators

"I've modelled some reg's using such devices and the performance specs are awesome, the improvement in reg BW is truly stunning."

Try actually building some regulators with parts that fast......... you might find out that it is a whole new ballgame getting them stable in the real world.

H.H.
 
AX tech editor
Joined 2002
Paid Member
Voltage regs

Hi Harry,

I beg to differ. What's an opamp with a pass transistor other than an opamp with an extra emitter follower? The pass transistor doesn't change the loop characteristics of the combo that much. I have used opamps with high cap load, without problems. If you get to uF's and more, the loop characteristics change so drastically that there will be no instability. Because these opamps can deliver very high current, they can by themselves supply the load current without having to rely on the buffer function of the cap. This very much decreases the phase shift between opamp output voltage and current, which would be a cause of instability.

Yes, Jung went from the 797 to the 825, partly because I urged him to find an alternative to the 797, which is a superp opamp, but not IMHO in these applications.

Jan Didden
 
"The pass transistor doesn't change the loop characteristics of the combo that much"

Sure it does! Open loop Ro is much lower and additional poles are added. For very fast op amps stability and compensation become more of an issue. Plunk a 25Mhz follower inside the loop of an AN 811 if you need convincing...

"I have used opamps with high cap load, without problems. If you get to uF's and more, the loop characteristics change so drastically that there will be no instability"

I guess you could make the same case foran op amp with a pass transistor. Don't forget that capacitor ESR and ESL can enter into this with a lower enough regulated output impedance and popping caps on the output isn't as slam dunk as one might think
with some circuits...

"Because these opamps can deliver very high current, they can by themselves supply the load current without having to rely on the buffer function of the cap. This very much decreases the phase shift between opamp output voltage and current, which would be a cause of instability."

The cap is providing current where it's reactance is lower than the output impedance. You are atually going fro an inductive to capacitive reactance as the frequency increases for a circuit with feedback as to loop gain is decreaing with frequency. So phase shift is occuring in either case. Phase shift in its self is not the cause of instability but the phase margin of the closed loop of the regulator is.

Thank's for such an interesting debate.

H.H.
 
Jung/Didden 5v supply

With a suitable op amps and a coulple of more diode drops ( make D1 into three series diodes) it think one could bootstrap the op amps suppy from Vo. There should be plenty of 2.5 series or shunt references out there or a green LED in series with a diode would work as a shunt refence and be very quiet.

Maybe an op amp from the following list would work:


OPA 228/227
LT6203
LT1812
EL2120C
AD8605

H.H.
 
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