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regulator booster

Discussion in 'd.i.y.' started by teddy_pardo, May 21, 2006.

  1. teddy_pardo

    teddy_pardo Trade: Teddy Pardo

    Here is a nice a simple mod that can be applied wherever a 317,337,1086 regulator is used.

    The idea here is to insert a VBE before the 3-leg regulator. The VBE filters high frequencies noise (in fact almost all relevant frequencies) before they get to the regulator, and isolates the regulator from the power rail in a way that noise from one regulated circuit doesn't get back to the power rail (let's say less noise...). Doing that significantly boosts the performance of these simple regulators.

    It looks like this (Carl, do you recognise your capacitors...):

    [​IMG]

    [​IMG]

    Note that you have to cut the strip-board between the collector and the emitter of the transistor. Otherwise it will still work, but no benefit... ;)

    [​IMG]

    Isn't it lovely?

    it's a pin-pin replacement with the original regulator, you can even reuse the original regulator. The original regulator circuit (resistors/capacitors) remains as is, no need to change anything. The VBE requires some dropout (about 1V), if you don't have it than simply replace the original 317 by a 1086 which is low dropout to compensate.

    The circuit is based on the same VBE circuit used on the Flea (BC547c, 3.3uF film, 10K, and 10/22uF tant), but other combinations can be used. As is, it can only be used for low current circuits (few 10s of mA), for higher currents smaller resistors and larger capacitors at the transistor base should be used. I achieved excellent results with 22uF tantalums, and 10-56uF oscons.

    I used this circuit to replace all the regulators in my CDX (there are about 20). I replaced each time 2-3 regulators, and each time the improvement was noticeable.

    Highly recommended!

    Ooops. I forgot to mention, a ground wire has to be connected from the circuit ground to the main PCB ground. I used a short wire connected to the ground leg of one of the main PCB tantalum (the Vout tantalum of the original regulator circuit).

    Enjoy, Teddy
     
  2. martin clark

    martin clark pinko bodger

    Very tidy! I'd agree the improvment can be worth the effort, too.

    Just one little note - the transistor (if it's still BC547) has a collector current limit of 100mA, so if you plan one of these for a rail drawing significantly more than say 70-80mA it would be a good idea to swap to some other transistor with higher current handling. Something like BD139 would be ideal - or BD140 (PNP) for negative regs. - though the lower Hfe will slightly increase voltage drop across the gyrator. A simple rule of thumb would be this: if the existing reg has a heatsink, use a small power transistor instead.

    The current limit of the BC547 (and the extra volt or two drop with small-power alternatives) is unlikely to be a problem on most analogue rails in, say, preamps and cd players though.
     
  3. teddy_pardo

    teddy_pardo Trade: Teddy Pardo

    Yes indeed, for higher currents I've used BC639/BC640. But since their hfe is too low I added a bc547 to build a darlington configuration (requires one more row on the strip-board). BC639 and BC640 can hold few hundreds of mA.

    On the CDX 15V rail there is a regulator that caries 1A (the one with the heatsink), for this one I used a power darlington.

    I would add to your rule of thumb, to measure the dropout on the transistor, if it's more than 1V then the resistor value needs to be reduced and the capacitor augmented accordingly. If that doesn't help or the transistor gets hot then it has to be replaced by something with more power handling capacity.

    Teddy
     
  4. hacker

    hacker Delicious and nutritious

    How much current can a 321 circuit draw when running flat-out? I'm thinking of trying this mod inside my pregregulating psu, but don't know if it's going to perform when playing music very, very loud... in essence, I want to know the vbe will work under the maximum current that a 321 will draw.

    The plan is to replace the 4 LM1086s in the psu with Teddy's new circuit.

    Cheers,
    Carl
     
  5. martin clark

    martin clark pinko bodger

    The line stages are class-A, and so run at very nearly constant current (c. 40mA off teh top of my head). No problem with the VBE trick.
     
  6. hacker

    hacker Delicious and nutritious

    Excellent. I take it the buffers use even less power. Even when I add in the miniscule current from the LED and relay, I'll still be withing tolerance. Cheers Martin.

    Carl
     
  7. PigletsDad

    PigletsDad pfm Member

    An idea which may not fly.

    The noise floor of a VBE is quite a bit lower than even the best 317/337 etc, although the output impedance is rather worse.

    There may be some applications where it is worth putting a VBE after the regulator - for cases where noise level is overriding. One possibility would be an MC phono stage.

    This will give some of the noise benefits of a SR from a simple circuit, without the very low output impedance.
     
  8. teddy_pardo

    teddy_pardo Trade: Teddy Pardo

    Yes, I agree, and indeed I'm using VBEs in places where I don't care about output impedence. That's what I did when I used a VBE for each resistor on my 102 and results where very good.

    Teddy
     
  9. martin clark

    martin clark pinko bodger

    Good point - just a point of reference for those try ing it, output Z for a transistor operated this way is approx. [26/(collector current in mA)]. So, draw 26mA gives an o/put Z of 1ohm. Pretty grim compared with any 3-pin reg...
     
  10. mikesnowdon

    mikesnowdon resU deretsigeR

    Hi Teddy.

    I need your help with something please. In this thread I am looking at how to implement your reg booster to a negative regulator. Ive used google and the forum search facility and tried to understand as much as I can from this thread. Im stuck though. Can you help?
     
  11. mikesnowdon

    mikesnowdon resU deretsigeR

    Planning to do this to the 7805 regs on the DAC's in my CA 640 V2. Can I sub the 3.3uF for ayting else as Im on a tight budget? Maybe a 2uf Film would be cheaper, of a Tant with a film bypass?
     
  12. hacker

    hacker Delicious and nutritious

    Tants should be fine, and bigger is better within sensible limits. 10uF, 22uF, 33uF - all are fine. As long as the voltage rating is sufficient you should have no worries.
     
  13. mikesnowdon

    mikesnowdon resU deretsigeR

    Cheers Hacker. Am I OK using the same value for both caps in the VBE or is there an ideal value for this implementation? I'm planning on using them on the +VDD and +VAA (Digital and analogue suplies) on he DAC IC's. Later the DAC's will be given their own dedicated supply from one of you first PSU boards.

    I got one of those 'lucky bags' of caps from Maplin and it came with a bunch of 3.3uF/25 Oscons. Could these be usefull? (bag also came with some Panny FC's and Elna Stargates :eek: )
     
  14. martin clark

    martin clark pinko bodger

    Use whatever caps you like. Same value will be fine.
     
  15. mikesnowdon

    mikesnowdon resU deretsigeR

    Are you sure I can use any old cap? Isn't there an ideal type for this application, like low ESR to reduce the overall impedance of the circuit?
     
  16. midcm1

    midcm1 pfm Member

    The vbe preceeds the reg therefore (I guess) the cap is not critical. Use anything but watch the start up, too large an RC combination will cause the supply to start too slow so the DAC may not lock...try???

    Should be a nice improvement though. The VBE just filters noise that the reg simply can't.
     
  17. mikesnowdon

    mikesnowdon resU deretsigeR

    Thanks Dominic, that's cleared it up a bit (pun intended) :D :D

    PS: You should receive a little package this week :)
     
  18. fran

    fran pfm Member

    I'm interested to hear how you get on mikesnowdon.... I also have a DAC awaiting some replacements for 7805s. I need to draw out teddys circuit on paper though as the 7805 has a different pinout compared to the 317 etc. Kind of a measure twice cut once approach!


    Fran
     
  19. Puffin

    Puffin pfm Member

    Mike. I assume you abandoned the PCB project for these?
     
  20. mikesnowdon

    mikesnowdon resU deretsigeR

    Hi Fran.

    Yes, Its nice simple little mod which is cheap and hopefully effective. You could build these up on stripboard like Teddy has done or do it dead-bug stylee.

    Hi Puffin,

    Afraid so. Trying to get the PCB in some way 'generic' so that virtually any regulator could be used - and for both positive and negative - got me a bit baffled. I had the idea on wire links to 'configure' the circuit but didn't fancy it. I might come back to it soon though. Now that I know any old cap will do I might draw it up using 10-33uF SMT Tants to make it more compact. Oh, another off putting aspect was the cost of getting the boards made. You'd be surprised how much it is for such a tiny pcb.

    EDIT: Another reason I gave it up was because many many questions I posted went unanswered, with no-reply for forum members who have the knowledge to help out. This seems to happen to me alot. My threads seem more like blogs......why is that? (end of mini rant) :D
     

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