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Old 2nd August 2003, 05:56 PM   #1
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Default Shunt regulator for a phono pre

Having lived for a while with a gainclone i decided (maybe naively) that a full solid state system will be cool. Not that i don't like the sound of my valve phono and DHT (PX25) power amp, it's just that the success of the GC gave me hope that a simple but good sounding system is possible with premium quality opamps. So far the results have been encouraging, rather than exilarating, but of course i've always known that a good phono stage is a lot more difficult than a a good power amp.
I started the prototype with the intention to build several different power supply regulators and compare the sonics. Initially it was built with the 'standard' 317/337 combo and the sound was admittedly not too bad, but the usual 'greyness' and 'thickness' of the regulators was unmistakeable. My first 'improvement' was a simple tl431 based shunt fed from a ballast resistor. The midrange cleaned up remarkably but in general the sound became less balanced, with very weak bass. Next the resistor was replaced with a 317 hooked as a current source at 70mA. Each rail of the phono draws around 50mA, so i thought 20mA for the shunt transistor should be enough. The bass came back, but interestingly a lot of the 'greyness' i've learnt to associate with the 317 came back as well. So, maybe a 317 is not a perfect CS after all .
Finally i built the simple CS shown on the diagram: it's based upon a forward diode drop. As anticipated the sound was now really better. If anything, the bass was maybe a tad too soft compared to the 317/337 regulators but everything else more than made up for it. As you can see i use very substantial transistors for both the current source and the shunt element. Initially tried BD139/BD140 but they really didn't sound as good.
So, happy as i am with the result, there is certainly room for improvement. I'll appreciate all comments and suggestions and some time next week will try to compare this to a Jung type regulator.


cheers
peter
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Old 2nd August 2003, 07:02 PM   #2
PGW is offline PGW  United States
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You are some way ahead of me in your phono amp regulator study.

I had thought of trying the Jung super-reg, probably using ALW's circuit+boards, and then following that with a shunt regulator to see what difference it makes. Interestingly, ALW uses the 317/337 as a pre-regulator for the super-reg, so I am guessing that the "greyness" you hear is cleaned up by the super-reg.

You might clean up your own current source by using a LED or buried-channel zener for the voltage reference at the input.

I downloaded an article on shunt regs a few weeks ago - I don't seem to hav the URL but a google on wenzel and finesse should find the source.

I will be interested to hear how your invetsiagtion progresses.

--John
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Old 2nd August 2003, 07:19 PM   #3
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John

I started this journey 'backwards' with the simple shunt-reg as i wanted something small, cheap and not prone to oscillations and problems. Mostly because i intend having a set of the regs next to each op-amp/buffer. The Jung solution is not so well suited to the idea of having 2 or 3 per channel. Also i have serious doubts about the preregulators. Having never built a Jung/ALW reg i still have some experience with what Audio Research used to use when the SP10 was the best preamp ever. It is similar to Jung in using an opamp with a series-pass transistor, but it's output is floating and not referenced to a particular voltage, just used to buffer each stage and fed from a preregulator. I never got to like what it did to the sound.
Interestingly Elso Kwak seems to also dislike preregulators, i'll certainy try them, but against clear prejudice.

cheers
peter
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Old 4th August 2003, 05:33 AM   #4
Rob F is offline Rob F  New Zealand
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Hi,

I find that bypassing the "upper" voltage divider R on the
TL 431 with a capacitor improves the sound markedly. Try using
a good quality 0.47uf. Works for me.

Rob.
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Old 7th August 2003, 03:35 PM   #5
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Hi Rob

After reading your post i was annoyed for not having thought of this myself A .47 seems a bit low for a 5k1 resistor so i tried a 3.3uF Wima MKP (admittedly not my favourite). To my surpise the sound did not get any better; to the contrary - it lost dynamics. Weird?

cheers
peter
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Old 7th August 2003, 06:46 PM   #6
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An interesting and thoughtful post Andy. I used tl431 primarily for the low noise and simplicity. If low noise is not a priority i'll gladly get rid of it although most of the discrete circuits i've seen typically consist of differential amp with/without cs/c mirrors, current amp,shunt element - easily 7-8 transistors per rail, not counting the CCS. Not ideal if you want 4 of these.

cheers
peter
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Old 7th August 2003, 06:48 PM   #7
andy_c is offline andy_c  United States
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Default Re: A few thoughts

Quote:
Originally posted by ALW
As has been pointed out to me recently, the SPICE model is DC only and hence useless for AC simulation
If you're using LTSpice, a good TL431 model is available in the files section of the Yahoo LTSpice user's group at http://groups.yahoo.com/group/LTspice. This model includes AC effects. It also solved the convergence problems I was having with the first TL431 model I downloaded from the net, which I believe was provided by TI. I think you need a Yahoo email account to access these groups though. But that's no problem. They are easy to set up.
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Old 7th August 2003, 07:32 PM   #8
A 8 is offline A 8  Sweden
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If you want to use the tl431 why dont you just switch to the ts431 by taiwan semiconductor.
It has lower noise then the tl431 by TI.
I think 48 nV/square hz 10Hz-10mHz compared to 130nV/square hz for the TI TL431.
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Old 7th August 2003, 10:50 PM   #9
ALW is offline ALW  United Kingdom
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Quote:
If you're using LTSpice, a good TL431 model is available in the files section of the Yahoo LTSpice user's group at http://groups.yahoo.com/group/LTspice. This model includes AC effects

I'll look at that - thanks.

It may be easier to emulate it with discrete's though, using the data sheet topology as a guide.

BTW, if anyone wants a remarkably accurate model of an LM317, take a peek here: -

http://www.aeng.com/pdf/regulator.pdf

http://www.aeng.com/pdf/regulator2.pdf

Andy
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Old 7th August 2003, 11:42 PM   #10
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Hi,

Congratulations on post #6, Andy.

While my own experience is mostly valve related, the same principles apply.


Good insight,
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