Shunt-regulated 24V DC Power Supply for Preamp Modules

[snoopyma]

Hi,

I am using some vintage modules such as Neve, Audix and Audio Developments as preamp and they require +24 volt dc power supply. I am using LM317 series regulated p.s. to feed them. Now I want to upgrade them. Are there any designs which adopt the shunt regulated concept? Any information is greately appreciated! Thanks.

 

[jan.didden]

You mean something like this? Select R2, R3 to get 1.25V at their junction. Add caps at the output as felt necessary.

Jan Didden

 

[snoopyma]

You mean something like this? Select R2, R3 to get 1.25V at their junction. Add caps at the output as felt necessary.

Jan Didden

Hi Jan,

Thank you for your reply. Sorry to say that I am just a layman to electronics only. What I know from shunt regulated design is from Allen Wrights' SuperReg design for the tube amp (high voltage B+). I am currently using his SuperReg boards for my tube preamp. It really sings. What I am looking for is a similar design for supplying 24 volt DC for my vintage s.s. preamp modules.

Please advise whether the design you propose suits my case. If yes, how to play around the values of R2 & R3 to get 1.25V?

Thanks.

Regards,

TC MA

 

[peranders]

You could also take a look at LM431 from Texas, National and others.

 

[Black Heart]

Not sure how sophisticated you wanna get, but I've tried out a lot of different regualtor types and settled on shunt designs myself. Even simple zener regulators sound better to me than your typical high feedback series pass regulator. But they're too noisy for most applications.

In any case, the type I've been using for the last few years uses a AD712 type opamp (required) as the control element and uses a lithium cell for the voltage reference. The shunt element is a mosfet and I use them with LC filters. No electrolytics anywhere, just chokes and quality polyprops. All copper from the diode bridge to the load. Very precise and quiet. The negative regulator is slaved to the positive with ground as its reference. Pretty simple design, and the lithium cell gets rid of the positive feedback loop that most regulators have and makes the regulator much more like a battery since the reference is truly independent. I use them on all my sources and in the power amp too. All simple off the shelf parts. The AD712 or other similar bifet input type opamps are required in order to use the lithium cell as a reference.

If interested, I can post a schematic later. Might not be what you're after, but let me know.

 

[planet10]

f interested, I can post a schematic later.

Yes please...

dave

 

[snoopyma]

Not sure how sophisticated you wanna get, but I've tried out a lot of different regualtor types and settled on shunt designs myself. Even simple zener regulators sound better to me than your typical high feedback series pass regulator. But they're too noisy for most applications.

In any case, the type I've been using for the last few years uses a AD712 type opamp (required) as the control element and uses a lithium cell for the voltage reference. The shunt element is a mosfet and I use them with LC filters. No electrolytics anywhere, just chokes and quality polyprops. All copper from the diode bridge to the load. Very precise and quiet. The negative regulator is slaved to the positive with ground as its reference. Pretty simple design, and the lithium cell gets rid of the positive feedback loop that most regulators have and makes the regulator much more like a battery since the reference is truly independent. I use them on all my sources and in the power amp too. All simple off the shelf parts. The AD712 or other similar bifet input type opamps are required in order to use the lithium cell as a reference.

If interested, I can post a schematic later. Might not be what you're after, but let me know.

Yes, sound very interesting and should be the one that I'm looking for. Kindly post the schematic please. THANKS.

T.C. MA

 

[AndrewT]

Hi Blackheart,
you've dropped yourself right into it.
We all want to learn from your schematic.
Go on, post it.

 

[Black Heart]

Yes please...

dave

Yes, sound very interesting and should be the one that I'm looking for. Kindly post the schematic please. THANKS.

T.C. MA

Hi Blackheart,
you've dropped yourself right into it.
We all want to learn from your schematic.
Go on, post it.

Sorry for the delay guys but I've been kind of busy. I pulled the power supply stuff out of my DAC schematic into another file and am massaging it slightly and then will convert it and post it. Should definitely be sometime today. Didn't really mean to make the offer in my first post here and then be a no show and didn't know about the moderation delay in effect for newbies. Thought that I might have a schematic around of just the power supply that I could easily post, but couldn't find it on my laptop. I do have the I/V output stage of my DAC broken out since it is a separate module, and I do have the schematic for my amp module including the regulator, but the DAC regulator is closest to what snoopyma was asking for since it is set up for about +/-20 VDC. Stay tuned

Here's a simplified schematic of the DAC I/V output stage if you're interested. Power supply details to follow ... click for schematic

 

[Black Heart]

OK, I cleaned up the power supply schematic and converted it to a gif and uploaded it to my site and you can click on it below. I'll leave it up there for a few days or so. I tend to design things a bit unorthodox, because why go to the trouble of designing something that's already been done, so if something doesn't make sense feel free to ask. This was one of my later versions that was a result of my power amp design where it is regulating about 55 VDC, so I reversed the input polarity on the opamps and used them in an inverting comparator configuration so I could power them at a low voltage and use common source shunt devices (N-channel on pos reg, P-channel on neg reg). Earlier designs used the more customary reversed common drain connection on the shunt elements with opamps configured as linear gain stages using negative feedback. Either way works OK. You can use the opamps in the "normal" mode with + to V+ and - to ground for a +24V regulator, and the opposite for -24V side. Lots of options for dealing with different voltage levels. I just wanted to develop a common design for all voltages so went with the dual stage +/-5V opamp power version since I need those voltages in the DAC anyway, and it makes for a very clean and low noise output with the opamp powered by a clean +/-5V supply

Chokes are by Hammond and the MKP caps are Sprague 735P (bought a ton while they were still available - great caps) and paper/epoxy caps are WIMA MP3-X2. Resistors are mostly precision bobbin wound.

Click here for schematic of my shunt regulator.

 

[Black Heart]

I should have probably mentioned earlier, but shunt regulators are harder to design than the much more standard series type. They're akin to class A amps, and require you to know more about the circuit design and your goals. You have to know ahead of time how much current you're gonna use in your amp circuit and how much you want to "preload" the regulator, and you have to account for the full current in the design, because just like in that class A amp, max regulator dissipation would be at no load.

And the design I showed above adds a lot more variables because I go with L-C filtering of the AC. You can see by the voltage rating of the transformers in my schematic that a much higher secondary voltage is needed in this design because of the L-C filter and the shunt regulation, which is done across the choke resistance. So it all has to work together as a total design, which might make it difficult for novices. But I thought it might start a good discussion in any case. There are much more standard designs around. I remember one from a preamp project in the old Audio Amateur. You can also go full out with a discrete design which is what I did at first. Looks almost like a small power amp with a mosfet input and a few gain stages. Kind of overkill for a preamp though. And it needs a trimmer for setting the voltage. And hard to get enough gain for low noise without 3 stages. So I settled on opamps. But the shunt design sounds so much better in conjunction with the L-C filtering that it more than makes up for the small concession to using opamps. A preamp or DAC makes a perfect platform for discovering how much better music can be without electrolytic capacitors. Especially if you have an amp free of them also

I do have a schematic for a more standard shunt regulator design which I'll try to add later with an edit to this post.

Dave

 

[Black Heart]

And here's a more traditional shunt regulator approach, while still keeping the lithium battery as a reference ... the feedback resistor value depends on the output voltage you want to get 3.2V at the -input, and the series resistors depend on the current. The only thing fixed in this design is the use of the AD711 series for the opamp. There are a few other types that can be used but most opamps input circuits will discharge the battery when powered off so if you don't use the AD711 you'll need to test it with a high value resistor in series with the battery to measure if there is any leakage current into the opamp. It should be zero. Some other types might also have trouble with the reference at the same level as the + supply, as it is on the tracking negative regulator.

[IMGDEAD]http://members.mailaka.net/davey/reg.gif[/IMGDEAD]

 

[analog_sa]

Hi Black Heart

Many thanks for publishing your designs. Any chance the ballast resistors for the 18v section have gone missing? Have you tried current sourcing?

 

[Black Heart]

Hi Black Heart

Many thanks for publishing your designs. Any chance the ballast resistors for the 18v section have gone missing? Have you tried current sourcing?

Happy to post the designs. Not completely sure what you mean about the ballast resistors, but assuming you're talking about the series resistors normally used with a shunt regulator to drop the voltage, in this case the input chokes are used as the series element for the 18V section. I did talk a little about the topology in the first post, but now it has probably gotten a bit spread out The 18V section powers my DAC analog output stage and is dropped to 5V with the next regulator section across the 300 ohm resistors for the D/A ICs and to power the regulator opamps. The last image I posted would be the more typical shunt reg topology with the series (ballast?) resistors if you weren't using a choke input supply like mine.

Not sure about the current sourcing question. Could you explain more? Thanks for the comments.

 

[analog_sa]

Ok, so you're using the chokes. Are the 40 uF following the chokes not interfering with the operation?

A common improvement (at least technically) of a shunt reg is to feed it from a constant current source instead of a resistor. My experience with this is ambivalent; i guess the CCS design makes or brakes this idea.

 

[jackinnj]

I would suggest deocupling the lithium battery from the opamp input with a 470R or 1K resistor, perhaps even filtering the battery as reference. I know that NiCads are noisier than alkalines -- I haven't done any testing of Lithium batteries, however.

 

[Black Heart]

Ok, so you're using the chokes. Are the 40 uF following the chokes not interfering with the operation?

Well, 40 uf is a pretty small amount considering many series designs use thousands of uf following the control element. But fast transient response of the regulator wasn't one of my primary design goals since all the circuitry it feeds is very stable.

A common improvement (at least technically) of a shunt reg is to feed it from a constant current source instead of a resistor. My experience with this is ambivalent; i guess the CCS design makes or brakes this idea.

Yeah, the key word there is "technically". In high end audio we only care about best sound, and that usually is a result of tampering with the signal as little as possible. There's a lot of potential merit to a design with only copper between the rectifier bridge and the load

I would suggest deocupling the lithium battery from the opamp input with a 470R or 1K resistor, perhaps even filtering the battery as reference. I know that NiCads are noisier than alkalines -- I haven't done any testing of Lithium batteries, however.

I haven't really looked into the chemistry of all the different batteries, but lithium cells have a very low self discharge rate, with shelf life specified in decades as opposed to the weeks and months of some types. So from that standpoint, they couldn't really have any noise as that would imply energy dissipation, and in this circuit they are used almost as if they're still on the shelf since the input curent for the AD711 is in the low picoamp range, worst case. I am curious though about your noise measurements since I haven't seen that info before. Are you talking about batteries under load? I agree that batteries aren't ideal low noise power sources and usually benefit from some decoupling and filtering when used to supply current to a low noise circuit. But not in this case when used as a low impedance reference. At least not IME in this circuit, or in the discrete versions I made with mosfets.

Interesting comments. Thanks.

 

[jackinnj]

There are a series of articles on ADI's website relating Reference Noise and the measurement acuracy of ADC's -- which got me started -- you can specify the noise over a 100mHz to 10Hz bandwidth for a period of time (10 seconds) to get an idea how "acurate" the system is going to be -- better viewed on a scope than measured with anything (except perhaps a thermal averaging meter.)

this all started back in a Quantitative Analytical Chemistry grad course which I took decades ago -- back in the days of Nixies and Dewar containers.

Here's my noise measurement amplifier (it's a work in progress, this is iteration #3), cobbled from an HP465a, incorporating measurement from 100mHz to 10Hz and 10Hz to 100kHz:
http://www.tech-diy.com/hp465a_amplifier_modernization.htm

 

[agent.5]

I have 2 questions:

[1] How does lithium battery voltage reference compare to some of the late model, low noise reference, such as MAX6126?

[2] If I am building an amp with say +/- 30VDC output, what is the best way to get clean +/-15VDC to power the op amp?

Batteries? Or do i need to build another power supply just to power the op amp?

 

[jackinnj]

here's an article by Ron Mancini from Texas Instruments which appeared in EDN last winter -- how to make a good reference into a great reference:

http://www.edn.com/contents/images/480491.pdf