JM, thanks for your reply.
Here is the link to the professor tweed schematic. http://www.runoffgroove.com/professor.html
The schem has it running off of +9v, but for use as a pre most folks (I checked at DIY audio forum) use +12v for more headroom (with no ill effects).
I am glad to hear that it won't be an issue to just derive the lower voltage from the +24v rail in terms of current draw.
Some questions, however:
Why does one need to use a Zener diode? to keep current from flowing "backward"? or is it for voltage drop? Is it a big no no to just use a resistive divider to get the required lower voltage?

Just another quick thought. would an isolation transformer between the preamp output and the poweramp input solve this potential problem? Would I need another at the preamp input?

Hi all, I am 50% finished with my lm3886 amp build. I am designing the power supply for the preamp portion, which will be a "proffessor tweed" from ROG, and I have a question about the way I am proposing to derive the lower voltage rails for the preamp. I have done the calcualtions to get +/- 6v supplies from my +/-23.4v poweramp rails. The design uses a basic resitive divider from the +23.4v to ground to get the +6v rail, and the same thing from the -23.4v to ground. The idea is to use the -6v rail as the ground point for the preamp, effectively making it a +12v design.
   My question is this: will running the premap with a ground point that is offset -6v relative to the ground of the poweramp create an awful groundloop/hum problem? Would it be better to derive a non symetrical single +12v supply rail from the +23.4v rail instead? I thought of this, but did not want to create an uneven current draw from one rail that might cause some sort of oscillation issues... Any advice would be helpful and greatly appreciated!

You should be able to connect them directly together. Since the "volume pot" of the ruby comes before the IC, you may need to add a "master volume" pot (100k log would be okay) between the ouput of the ruby and the input of the Sonic T.
  You should also check to see what the gain is set to on the Sonic-T. If it is set too high, you may get some distortion when driving it with a pre. A gain of, say 20ish, would be okay, where as gain higher than 30 may begin to produce some distortion when running the pre at full volume... Remember that with solid state amps, if you want distortion, you want it in the preamp stage, NOT in the power amp... The ruby should give you this, or build you could build a noisy cricket for the pre if you want more control over the distortion than is offered by the ruby...

J M,
   Thanks for leaving me a reply on this old thread... This was my first posting here, and in the months since then, I have learned A LOT more about this kind of stuff. That preamp was my first successful build, and I think I was more blown away that I had builtit and it actually worked! Since then I've built more things,a nd heard more circuits,a nd read a lot more. Back then I was still under the illusion that it was "tube sound" I was looking for. Now I know that waht I'm looking for is amplification with tone that sounds good with the Harmonica. Getting that tone has a lot to do with harmonicas and microphones, but also amplification for the frequencies of sound that harmonicas generate. These are very different from guitar frequencies. In fact, harmonica amplifaction shares more with bass guitar  and for regular human voice (not singing, just talking) amplification than it does with guitar amplification.
   I now think that I was just lucky that this particular circuit happend to also sound good for harmonica. It probably has to do with it originally being designed as a microphone preamp rather than any "magic" properties of the design (ie. all the tube emulation theory that gabe writes about at his site)... I'm now experimenting more with FET preamp/overdrive circuits and trying to figure out ways to tweak them for harmonica amplification. If you (or anyone else) has any ideas about this, let me know! I'd be glad to hear them!

This is a pretty sweet resource... Great to see that we can actually get something usefull for our hobby out of our tax dollars!

Hi all, I just came across this page:
http://www.fairradio.com/catalog.php?mode=view&categoryid=1546

  As you will see they stock MANY very affordable transformers, of varying voltages and amps. Many of them are right for building chipamp dual-rail powersupplies with (right VA rating, and also center tapped). None are toroidal, however... I just thought I might pass on the news!

Hey there, I went over to the description of that tranny at partsexpress. It actually has THREE secondary windings, one of which is 35VAC at 4 amps (VA = 140). It does not specify that that winding is center-tapped, however. In fact it is very vague about the center-tapped-ness of the whole thing, except for the winding you mentioned (16-0-16 at .5 amp), but, looking at the picture, there are A LOT of black wires hanging out there, and only a few colored wires (in the wire harness). So IF some of those black wires were the center-tap for the 4 amp secondary, THEN this sucker would work for an LM3886. Of course you would not be getting 60 watts out of it, but maybe something like 40 watts (i did not bother to do the real math)...

By the way, does anyone know of good place to get deals on speakers? I know bout antique elctronics supply and partsexpress, but is there anywhere better (besides the local pawn shop  )

I recently shopped at futurelect.com
They have very good prices, and also stock lot's of audio IC's that you can't get at the goldmine or smallbear, especially the lm3886. They also have the BEST deal on bundles of resistors, caps, diodes, etc... The one problem is that, even though you pay in USD, and their accounting office seems to be in the US, they ship from thailand, and it took weeks for my order to get to me. Everything I got was of great quality though...

Just a quick question. Why do you have the 10uf caps bypassing each diode on your bridge rectifier? I'm only asking becasue I'm about to build the exact same type of power supply (2 x non center tapped transformers in the same configuration you have there) for my lm3886 amp, and I haven't come across a PS schematic that indicates you should do that. I guess I mainly wnat to know what are the advantages of having those bypass caps?

Hi Guys, This is the original schematic from the Olive Audio site... Cheers!

n9voc, I have read probably close to a hundred different explainations about power supply designing and "how to pick the right transformer" for a power amp, but your short post has made clear to me what these other explainations couldn't! Thanks for this!

Teemu,
    I now understand that you meant that it is advantagous to have a preamp that iscapable of overdriving the amp, but not that the preamp should be overdriving the poweramp. It makes sense to have the extra output from the preamp stage, since, as you say, the volume potentiometer will be shunting significant voltage.

Quote from: teemuk on April 19, 2008, 05:43:57 AM
I don't understand how the series volume potentiometer you describe could ever work. A shunt configuration works because in it the potentiometer forms an adjustable voltage divider. I don't see that happening in the series configuration. You'd be just increasing the impedance of the circuit but this alone has no effect on signal levels. If you need to control the attenuation then you need the resistive divider somewhere. At this point you're pretty much forced to use the shunting configuration anyway. The only way I see the series configuration could work is when it's a part of an inverting amp (its input resistor to be exact) and as such defining the gain of the stage. I don't recommend trying this idea in a power amp, as it does not sound like a very stabile solution.

  I think I did not fully explain what I meant by this. If you go to the following link http://www.lwharpamps.com/techmicpot.html you will see three ways of wiring up a logarithmic potentiometer to act as a volume control. The first way is the standard that, as you say, introduces a variable voltage divider, and shunts varying amounts of the signal to ground. This produces a taper that has very fine control at the begining of the pot's travel, but increasingly coarser control as the pot travels (ie less control at higher volumes).
  In the second image the pot is wired "backwards", which still introduces a voltage divider, but offers the "finer" control at the loud end of the pot's travel (which is counter clockwise now)
  The final image shows the situation I was alluding to. It seems that this will, as you say, create a variable output impedance. But won't it also offer finer variation in voltage attenuation? You will never get "total 0" volume with this type of control since there will always be some voltage across the pot, but if you have a significantly high value pot, you should be able to get fine control from very low volumes (pot at max resistance) up to very high volumes (pot at min resistance). I would think that as long as the input impedance of a non-inverting amp is set to be higher than the max output impedance that will be passed from this control, there shouldn't be any problems? Correct?

  I'm not sure that the fine versus coarse issue holds true for linear taper pots. Would it be less advantageous to use a linear pot in this case? (I mean will the "linearness" of the pot's taper make volume adjustments wierd?)

Anyway, thanks for the very informative posts! I just wanted to clarify what I actually meant!

Hi Teemu,

  In your discussion about impedance matching between Pre and Power amps, it seems like you are suggesting that it is good/advantagous to have a preamp that can overdrive the LM3886. Am I correct in this assumption or did I totally miss the point?
  Anyway, I had thought that the reason to use an ROG design is that it is the Preamp that should be creating the desired distortion, and the poweramp should just be cleanly amplifying the signal (regardless of whether it is distorted or clean) that is provided from the preamp. In this case, it seems to me that the output voltage range (3-4 volts) from a stock ROG (or other FET distortion pedal) design should be what you want. ie, they are high enough to be amplified into nicely audible volumes by the poweramp, but not so high as to actually overdrive the power amp itself.
  As for the effect of the "vol" pot: would not it make more sense to wire it as a variable resistor (in series with the output of the pre) rather than as a volume control (shunting variable amounts of signal to ground)? Would you not then have finer control over the output voltages over the entire travel of the pot? i wouldd think a pot value of 500k should give a good amount of contol when used like this...
  What do you think about these issues? BTW, I'm awaiting my lm3886 and j201's mpf102's etc. from futurelec, and I'm planning to build a multi preamp lm3886 amp...