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Old 30th Jul 2020, 9:23 pm   #1
bikerhifinut
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Default Device to simulate a dual rail power supply from a single DC source

Ok this isn't a world stopper, but it occurred to me and would be handy from my point of view if I could use a DCwallwart or battery via a 2 pin DC power socket and thence convert it to a dual supply for use with such a circuit.
I know its dead easy to just use a couple of batteries and dead easy to make, for instance, a +12 0 -12V dc supply with an AC transformer either twin secondary or single. So lets leave that one alone.
So I have had a quick look online and ended up going round in circles. I believe there are Chips out there that facilitate this by creating a "virtual earth" which sits in the middle of the DC provided.
This could be very useful from my point of view, but if its too fiddly or expensive no problem.
And yes I have some 24V DC sources to play with....

If anyones wondering whats going on in my thick head it's because I am playing with dual rail opamp circuits for phono stages and very probably in due course some other preamp applications including a sensible and effective baxandall tone control.
And my bench PSU is a single rail job. That's got me on the case to build an adjustable dual rail Job, that'll be interesting.

Daft Andy.
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Old 30th Jul 2020, 10:16 pm   #2
SiriusHardware
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Default Re: Device to simulate a dual rail power supply from a single DC source

Have a look at the ICL7660 IC, which generates a negative version of whatever positive supply it is running from, up to +10V (for -10V out). Information about the Maxim version (MAX7660) is here.

https://www.maximintegrated.com/en/p...s/ICL7660.html

That particular IC doesn't have very high current output capability, about 10mA which may well be enough to power a phono preamp.

If not, try the MAX660 - runs on (and generates) a maximum of +5V / -5V but gives up to 100mA output.

https://datasheets.maximintegrated.com/en/ds/MAX660.pdf

Both of these ICs typically require just the IC and one or two capacitors to work.
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Old 30th Jul 2020, 10:23 pm   #3
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Default Re: Device to simulate a dual rail power supply from a single DC source

thanks Sirius.
They don't quite meet the specification as I'd like to get at least 12 0 12V and 50mA as a bare minimum. Eventually a board will have to run a full suite of circuits from RIAA, and thence into the usual buffers and active tone control circuits. Plus a feed for a remote motorised Volume control although that could come straight off the Raw DC input.
Thanks anyway.
I suspect its going to be a non starter and I'll settle for a 24V AC feed and rectify and split/regulate internally.
Andy.
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Old 30th Jul 2020, 10:33 pm   #4
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Default Re: Device to simulate a dual rail power supply from a single DC source

Regarding your mention of splitting a single supply equally, I remember I built this function generator back in the 80s which uses a circuit with a buffered potential divider.

See page 51 bottom right hand corner of the page, circuit using 1458 dual op-amp and 4 transistors, fed from single 18V supply giving regulated +/- 8V2 outputs
Change the single 8V2 zener if you need more volts on the outputs

https://worldradiohistory.com/UK/Pra...2-05-S-OCR.pdf

Best Regards
Chris

Last edited by unitelex; 30th Jul 2020 at 10:36 pm. Reason: corrected IC type
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Old 30th Jul 2020, 10:46 pm   #5
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Default Re: Device to simulate a dual rail power supply from a single DC source

Or you could use this simple dedicated rail splitter device from TI

Up to 40V input, divide by 2 output, up to 20ma source sink
For more current you could possibly use 2 in parallel with a sharing resistor on each of the outputs...

https://www.ti.com/lit/ds/symlink/tl...ct%252FTLE2426
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Old 30th Jul 2020, 11:00 pm   #6
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Default Re: Device to simulate a dual rail power supply from a single DC source

How about a DC/DC converter. This one takes 9-18v DC in and has +/-12v DC out at 60mA per rail.

https://uk.farnell.com/tdk-lambda/cc...61970?st=DC/DC

There are probably many variants available from the same supplier.
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Old 30th Jul 2020, 11:06 pm   #7
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Default Re: Device to simulate a dual rail power supply from a single DC source

Quote:
Originally Posted by unitelex View Post
Or you could use this simple dedicated rail splitter device from TI

Up to 40V input, divide by 2 output, up to 20ma source sink
For more current you could possibly use 2 in parallel with a sharing resistor on each of the outputs...

https://www.ti.com/lit/ds/symlink/tl...ct%252FTLE2426
That looks the business!
and probably the 8 pin variant.
Is it really that simple? just one external capacitor?

A.
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Old 30th Jul 2020, 11:23 pm   #8
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Default Re: Device to simulate a dual rail power supply from a single DC source

on eBay blimey! pricy.
RS online..... 1.75 each + VAT if you buy 5 at a time. But not bad and I figure a useful chip for future low current op amp jobs. Especially in the Camper van where I only have a single 12V supply and if the circuit is ok on + - 5V then its good.
Reminds me I need to check the rating of my inverter, My missus wants me to fit our old but good Brennan JB7 in the van, and the blessed thing runs on 24V DC so its the inverter into mains PS, shame but I cant see any other way round it. OT sorry.

A.
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Old 30th Jul 2020, 11:43 pm   #9
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Default Re: Device to simulate a dual rail power supply from a single DC source

I've used dc-dc converters for op amps like that. No problems with them. They're used for instrumentation amplifiers in industrial controls too.

Bear in mind some are regulated and some are not, and some need a minimum load and some don't. Might as well get one with everything in the one box.
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Old 31st Jul 2020, 1:07 am   #10
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Default Re: Device to simulate a dual rail power supply from a single DC source

I've seen common audio amp ICs (e.g. LM386) used as supply splitters. They bias their output to about half supply...
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Old 31st Jul 2020, 8:11 am   #11
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Default Re: Device to simulate a dual rail power supply from a single DC source

Bikerhifinut,

I've got a few salvaged dual rail modules at home if that's any use to you?
I know I've got one that will give +/- 15V @ at least 100mA per rail.
I doubt I will use it/them, I'll have a proper look over the weekend & get back to you.
FOC by the way, just a donation to a charity of your choice if I'm successful!
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Old 31st Jul 2020, 12:18 pm   #12
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Default Re: Device to simulate a dual rail power supply from a single DC source

Quote:
Originally Posted by Karen O View Post
I've seen common audio amp ICs (e.g. LM386) used as supply splitters. They bias their output to about half supply...
Yes, that's all you need. It tends to be cheaper than buying a dedicated IC, especially as an audio company is already buying audio op-amps by the bucketload, so will be getting excellent prices... I'd prefer an op-amp to the LM386 because you're relying on the internal biasing of that to keep it at the mid-point rather than external circuitry - but obviously the LM386 can supply a bit more current.

It's literally 2 equal value resistors followed by a unity gain follower. Just about any op-amp will do, but check the current rating of the output stage. Having said that, there really won't be much in the way of DC currents in the earth circuit, so it's not important. Many years ago, Quad used just 2 resistors for some of their power amplifiers!

When doing this, remember that DC precision is not a big deal - audio circuits don't mind if the rails aren't perfectly symmetrical, especially if you are nowhere near the clipping point.

Some years back I built a super-simple version of this using just 4 or 5 transistors - details are at work, but I can look them up next time I go in to the office. An op-amp circuit would obviously work better, especially in terms of DC precision, but it was an idea I wanted to experiment with one lunchtime. Plus I wanted the circuitry to take almost no operating current, so the reading on the bench power supply was still fairly accurate, and we didn't have any low-current op-amps in stock at the time...

I mounted it in a small diecast box, with leads from the single bench PSU coming in one end, and three 4mm binding posts on the top. The box obviously acting as a heat sink, should there be significant earth currents. I arranged the circuit so that the two collectors of the power transistors (TIP31/32 IIRC) were connected to 0V, so no need to insulate them from the box. Nice and simple.

At the other end of the scale, Calrec did this with an op-amp and some beefy Darlington transistors - this circuit took 36V DC from a separate regulator and turns it into +/-18V. This would be a good one to copy if you wanted better DC precision.

Also, once the experimenting is done, and you're ready to implement a final circuit, it's usually easier to just modify your circuit if you wanted single-rail operation - you almost certainly won't need extra active devices to do that.

Cheers,

Mark
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Old 31st Jul 2020, 12:25 pm   #13
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Default Re: Device to simulate a dual rail power supply from a single DC source

Oh, and remember that the LM386 is limited to 12 or 18V operation, depending on which version you buy, so that's only +/-6 to 9V
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Old 31st Jul 2020, 2:55 pm   #14
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Default Re: Device to simulate a dual rail power supply from a single DC source

Bikerhifinut,

Having had an earlier than expected finish I've had time to root out the modules that might be suitable, one is perhaps a bit heavy duty & needs a minimum of 36VDC input, the part number is PM30-48D12 from Lambda. I've tried it on the bench & it works but it MUST have 36VDC. The other is a module made by TRACO part number is TMR 3-2412, these are a single output at 12 VDC but the output is floating so by using two (I've found 5, so far!) you could get your +/- 12VDC easily. If you are still interested then drop me a PM. Have a good weekend.
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Old 31st Jul 2020, 5:10 pm   #15
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Default Re: Device to simulate a dual rail power supply from a single DC source

This is from a commercial product that I repair, TR5 is a 2N2905, R44 and R45 1K and the op-amp is part of an LM324 though any op-amp would do
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Old 31st Jul 2020, 5:23 pm   #16
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Default Re: Device to simulate a dual rail power supply from a single DC source

Just be aware that that is designed for a particular application, and probably won't be suitable for general purpose use.

The problem with it is simple: it can only sink current.

So if the positive half of the circuit being supplied takes more current than the negative half, no problem. That will be the case in the product that it lives in. TR5 conducts and sinks current to the -ve rail as necessary to bring the "0V" back to the centre.

But if the negative half of the circuit takes more, there is nothing to source current to that half. So the mid-point "0V" will move down towards the negative rail, and there's nothing the poor op-amp and PNP transistor can do to change it...

The fix is simple - add an NPN transistor. Join the bases and emitters together, and tie the collector to the positive rail. In other works, follow the op-amp with an unbiased output stage. The unbiased arrangement might cause a "dead spot" that needs attention, but that's easy enough to fix if necessary.

Hope that makes sense - not trying to be negative (no pun intended! )

Mark
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Old 31st Jul 2020, 9:58 pm   #17
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Default Re: Device to simulate a dual rail power supply from a single DC source

Re post #12, an op amp has three outputs, I must remember that. Could come in useful for current limiting.
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Old 1st Aug 2020, 7:00 am   #18
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Default Re: Device to simulate a dual rail power supply from a single DC source

There's a whole family of audio amplifiers founded on the trick of having an opamp try to drive ground directly (or through a small load resistor) then using its supply currents to drive power transistors which drive the real output.

It might just be photocopying smear, but the Calrec circuit seems to have NPN and PNP darlingtons with the same type number!

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Old 1st Aug 2020, 10:14 am   #19
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Default Re: Device to simulate a dual rail power supply from a single DC source

If you zoom right in, you can see that the print quality is the problem - it's not a draughting mistake.

Of the various Calrec schematics I found at the time, this was the best quality print, believe it or not. The audio schematics were plotted on huge fold-out sheets in very high quality, so I've never been sure why the PSU schematics came to us as poor photocopies.

Well-spotted though
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Old 1st Aug 2020, 11:20 am   #20
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Default Re: Device to simulate a dual rail power supply from a single DC source

Thanks for the input guys, I've been offered someone elses spare split rail PSU board and as I have a couple of suitable transformers in the parts bin I think the sensible way forward is to build it up as an adjustable bench supply for my prototypes. Then if I Like stuff enough to make them permanent I can put a permanent split rail in the finished article.
But thanks because you never know when you need to do something along these lines.
At a couple of quid each including the VAT and shipping the TLE2426 from RS aren't a bad deal at my end of the hobby and I think that's how I'd go. I even found some online info using the 2426 with a power transistor for a high current solution.

A.
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