2 power supplies 1 transformer

[PurpleTheory]

Hey guys,
I built a practice amp that runs on an 18v bipolar PS, i also built a preamp to use with it that runs on 9v. the transformer that i have has several different taps for different voltages so i built both PS using the same transformer. when i connect the preamp it makes a really loud hum and no guitar signal gets though, but when i connect the preamp to a 9v battery it works fine. i checked both supplies with a DMM and the voltages read fine. what am i doing wrong?

thanks,
matt

[R.G.]

I'd have to see the schematic to tell for sure, but your comment that "the transformer that i have has several different taps for different voltages so i built both PS using the same transformer" makes me think that you may have used some of those other taps and not have a common ground reference, so the power supply is at least partly shorted.  That would fit the idea that it works fine with an isolated battery supply.

But I can't really tell without seeing what you did in schematic form.

[PurpleTheory]

I drew this real quick, just to show how i have it set up, it might be too simple to be of any help, i can draw up another if this one doesn't make sense.



Thanks for taking a look.
Matt

[grapefruit]

We'd really need to see the schematic of the 18V supply and 9V supply.
Is the 18V supply running half wave, with the blue wire going to GND and the pink wire going to the diodes?
And the blue wire on the 9V supply going to GND?


Stew.

[R.G.]

There is a way that could work, but it's probably not what you have wired up. Yes, we need the detailed power supply schematic to tell completely.

However, unless you have that blue wire soldered hard to signal ground and are only half wave rectifying +/-18 and +9, the signal ground will short out one or the other of the transformer windings through the rectifiers when the rectifiers conduct.

You could use a 7809 and make 9V from your +18V supply and everything would be fine.

[PurpleTheory]

This is how i have them set up:



Would it be best to get rid the bridge recitifier and wire 9v ps onto the end of the 18v ps?

Thanks,
Matt

[grapefruit]

Your 18V +/- supply is not really suitable for the LM3886. It will work to a degree, but you'll get nowhere near the available power from the LM3886.
You don't need a regulated supply for the LM3886. It won't be able to supply enough current. You really need a full wave rectified supply with big filter caps. I'd use a 24V centre tapped transformer. (24-0-24). You'll have to check the power rating of the transformer, current rating of the bridge, and voltage rating of the filter caps.

Check the LM3886 datasheet.
I just found a circuit here that includes power supply
http://hepso.dna.fi/misc/LM3886_amplifier.html  it doesn't show the transformer though.

[R.G.]

This is how i have them set up:

And that tells the tale. The blue wire is connected to ground at the +-18V end, and to your bridge rectifier at the 9V end. When the yellow and red wires go negative, the transformer secondary is shorted through the diode bridge because the green wire for +/-18V and the black wire for 9V are tied together.

You could make this work somewhat by removing the bridge rectifier and substituting in one diode from the yellow wire to where the (+) is on the bridge rectifier now and tying the blue wire to where the (-) is on the bridge rectifier. That at least would not short out the transformer.

However, it gives you half wave rectification on your power supplies, which is not good practice for any amount of significant power. The 3886 will not be happy, and you'll likely be plagued by power supply ripple.

Would it be best to get rid the bridge recitifier and wire 9v ps onto the end of the 18v ps?

Probably. However there's a more pertinent question: Is the transformer really suited for the application?

It might be. What's good for a 3886 is about +/- 15 to +/-30Vdc. Does that transformer have a centertapped winding? If it does, make your 3886 voltages out of that and then derive the +9V from the positive supply with a 7809.

What voltage taps does the transformer have?

[PurpleTheory]

thanks for the help guys.....now i'm not quite sure what i want to do; build a new ps, or try to fix what i've got? as it is it's pretty loud, but not nearly as loud as i hoped.

What voltage taps does the transformer have?

it has taps for 6v,9v,12v, and 24v, it's also rated 1A. is this a suitable transformer then?

thanks,
matt

[R.G.]

it has taps for 6v,9v,12v, and 24v, it's also rated 1A. is this a suitable transformer then?

Maybe. It won't be all the power you could get out of the 3886 chip.

Is the "12" tap a centertap? That is, if you measure from the 12V tap to the 24V tap, it it equal to the 0V tap to 12V tap voltage? If so, you can get about +/- 16Vdc out of it with a bridge rectifier across the 0-24 taps while using the 12V tap as ground, ignoring all the other taps. This will give you much better results than the half-wave doubler power you're getting now, and you won't need to regulate the power amp power. You'd get +9 from a 7809 between ground and +16.

If the voltages are not the same between 0 and 12 as 12 and 24, then although it may be possible to use it, it's not really a good choice.

As another espistle in the "Mother Nature" series, Mother is trying to teach you that guitar amplifiers are in all actuality just big power supplies that let some of the power through in a carefully controlled way to the speakers, with some other small junk thrown in to help control the power. Properly, the design of an amp starts with the amount of power you need, then a power supply design, then the rest of the amplifier.

The optimum transformer for a 3886 driving an 8 ohm speaker in a musical amplifier is about 20-0-20Vrms to 22-0-22Vrms at 1.5A to 2A. That gives you about +/-30V, and you'll get 50-60Wrms out of it. With a 24Vct (12-0-12Vrms) transformer, you  get about +/-16V, and about 12-15Wrms. Note that this is only slightly quieter than the 50-60W version because of the peculiarity of the human ear.

On the other hand, whatever parts you actually do have and don't have to order from Timbuktu are much more useful than the perfect part that's still *in* Timbuktu.

[PurpleTheory]

Thanks, i really appreciate all the help. Well, today i went and picked up a more powerful transformer, it's 36VCT and 1.5A (i think). So before i get started i thought it would a good idea to show you how i thought to wire it up before actually trying it ( since i already burnt one up a while back, learnt it the hard way). Is this how it should be set up?;



Also, how big and how many caps should i use, i have a few 1000uF and 3300uF, would those work? what would be ideal?

Thanks again,
matt 

[RDV]

Here's what I used:


RDV

[PurpleTheory]

Cool, thanks for the schemo. One question though, where it says "Earth (ground)" is that the middle prong on the AC connector? And what's the difference between the downward arrow made of lines and the hollow downward arrows? also, do i use rectifier diodes on the gnd? sorry if these are dumb questions.

thanks,
matt

[R.G.]

That's a much better power supply for an LM3886. Assuming that you have the + end of the caps in the square pads, yep, looks good.

Use all the capacitance you can get there, up to about 10KuF. Put in four 3300's or four 4700's if they'll fit.

what's the difference between the downward arrow made of lines and the hollow downward arrows?

Down arrow of lines is safety ground from the AC wall socket. Hollow down arrow is signal ground. How you interconnect them can make a difference in hum in some situations.

also, do i use rectifier diodes on the gnd?

Maybe, but I'd recommend not. Have you ever gotten shocked by touching a guitar then a microphone? That was caused by AC leakage not being properly grounded by the safety ground. Possibly (probably!) one of the pieces of equipment the guitar/mike/etc was attached to had only two wires in its AC cord, no third wire safety ground. The safety ground is a direct copper wire through all the power wiring to a rod or cold water pipes actually buried in the dirt around the building. It's only function is to provide a dead short to AC leakage and to keep that current out of your personal body.

To do that, it has to be connected to every piece of metal you can touch, particularly signal ground. The problem with*that* is that two pieces of equipment, both with third wire grounds may have slightly different potentials on the safety ground wire where they contact it, so their signal wires are at different potentials, and when you connect the two signal grounds with a guitar or mike cord, you get hum.

The rectifier/resistor/cap arrangement between earth ground and signal ground in Rick's picture is a setup recommended sometimes for lifting the grounds a bit so you don't get the hum. The idea is that the rectifier diodes will conduct if the hum voltage ever gets over a diode drop big, and then it will save your own personal body from shocks. The problem with that is what if the diodes are not perfect? I suspect Rick may have picked this up at ESP's web page. There they note that you should use a 25A rectifier bridge for those diodes in case there is a major AC fault that they have to carry. If you use 1N4148's - or 1N400x's - and the fault current burns them out, then the electricity can have its way with your own personal body.

I'd rather cope with hum in my equipment on some occasions than be dead even once, however infrequently. I'm just funny that way, I guess. You may make different choices; if you do, I hope it's not Darwinian.

[RDV]

http://sound.westhost.com/

I picked up that schem and lots of other ideas there. It's a good site.

RDV