need help with power supply hum, why there is hum?

[Olegarich]

Hey Diy Family!

I build power supply for my guitar pedals. It's based on LM317T voltage regulators which can handle 1.5A. the circuit is simple. Diode rectifire, couple 1000uf cap, 0.1uf cap, then it's going to input of regulator and after output there is 1uf tantalum and 470uf electrolytics. There is resistor, cap and trimpot going in to the regulator to regulate voltage. Simple circuit. So what i did is I took 14V 3.5A toroidal transformer and pluged circuit to it. Everything works quite and well. Then I soldered 10 same circuits and plugged in to the same transformer. So it looks like that from transformer is going in  to the 10 same circuits and after to ten gt pedals.

When I plug 6 pedals  which need small mA it works well but when i plug  pedal which needs more, like 250 mA there is a  big hum coming out of amplifier. To my counting transformer can handle much more current and each circuit can handle more then 500mA easily, why is there hum coming out? Because when I unplug other pedals and plug in one that needs more current it works well, quite with no hum. When i plug other without this one, there is no hum but if plug together there is a big hum. why?

Thanks

 

[Olegarich]

by the way, i\ve plugged pedals and measured amperage in transofrmer secondaries and it's gives me only 220mA

 

[ruffrecords]

by the way, i\ve plugged pedals and measured amperage in transofrmer secondaries and it's gives me only 220mA

What value is the output voltage to the pedals?

Cheers

Ian

 

[Olegarich]

it's 9V,

 

[Olegarich]

is it possible that waht cause problem is that i use one tranformer with one secondary and that going in to the 10 circuits? do i have to have transformer with 10 secondaries?

 

[audiomixer]

food for thought:

you feed all pedals 9V? ten independant regs seems an awful lot of regulators... now if you would require independent voltages that would be something different. how about only three circuits, that should be enough to match a very large peadl board....

how about testing one single regulator up to specs  - with the required cooling on the TO-220 package and lets say 1.5A load. beware, unlike LM78xx series the tab is not ground but output and must be isolated from each other on LM317. did you inadvertently connect all ten regulator outputs together? might upset them, certainly as soon as you draw some current. an other thing to look at is your 14V transformer. you will get around 20V after rectification.  so 11V to drop @ 250mA - 2.75W to dissipate. you might run into thermal problems with your setup if not cooled properly.
I don't think you will readily find a transformer with ten secondaries. and while it's not obvious for beginners, you should not require large-ish capacitors after regulation. your 470uF  is probably redundant if not actually bad on the regulator board. If memory serves me the datasheet has something like 1 to 10uF there....

how about a quick picture... ;-) might help solve your mistery.

- cheers, Michael

 

[Olegarich]

Ten regulators is to make isolated power supply in future i plan some more transformers with 4-5 secondaries. So that all the power supply would be isolated.

 

[audiomixer]

you're sure you're not over engineering this one? I probably know best about over engineering my self, but do you have a valid technical reason trying to do independant supplies besides 'I'am sure it's going to boost audio quality, perhaps'. effect pedals are single ended, common ground trough the jack connection anyway - so why bother?

ok, not meaning to be a bad ass.... - smile

hope we can help you on the actual prob.

- Michael

 

[keefaz]

What are the DC voltages at input and output of the regulator when you have hum?

Guitar pedal aren't really low noise, I would have tried a cheap wall mart 9V switching PSU first

 

[Olegarich]

Hey guys, everything seems to work now. Actually don't what was it. Maybe because all using one secondaries of transformer but what i did is i bought small transformers with little amperage and made that it's fully isolated, plugged same pedals and there is no hum and all working as it should. Still don't what could it be.

 

[ruffrecords]

I am looking for a 14VAC toroidal transformer at about 3.5 amps. where did you get yours?

Cheers

Ian

 

[Matador]

I would guess the dropout requirement of the regulator was being violated as the current draw increased.

These regulators need a minimum input to output differential in order to maintain their regulated output voltage.  If the transformer secondary R was high, as the current increases the voltage sags, and if it approaches 9V then the regulator begins to "let go" of the 9V output and will float along with the incoming AC signal.  This essentially will inject PSU ripple directly on to the output and PSRR drops rapidly.

What people don't realize is that the dropout voltage is a function of the output current and temperature.  According to Farichild's datasheet, at 25C and 20mA output current, the minimum input-to-output differential is about 1.6V.  At 1A, it's close to 2.2V!

You didn't say what the AC secondary rating was for the transformer, but if it was 9VAC this could easily cause this.  9VAC peak voltage is perhaps 12.5V, and after diode rectification becomes more like 11V(ish).  This leaves only 2V of headroom across the regulator, and at low currents this may be ok, but as current increases, you may be violating this spec.

You'll notice that all of the datasheet specs are given at a minimum of 5V input to output differential.  This implies that for a 9V supply, you need a peak AC secondary rating (before rectifiers) of about 15V, which implies at least a transformer rated at least 11VAC (12VAC is more common and available).

 

[Olegarich]

probably that was it. Ruff records this was custom made transformer by guy in my place. he makes good quality transformers and it allways about just to give him a call and say what you need in couple le daya you have tranaformer but as i know he makes only power transformers. by the way this one is 14v 3.5A and im not  lot small current trafos so if you interesting in getting this one give me pm.

one more time thank you all alot.

 

[PRR]

One transformer, *TEN* rectifiers, ten loads, all on the same audio ground, WILL act badly.

The circuit is common at the transformer, and at the audio grounds, but the *ten* rectifiers will NOT match exactly, and throw tenth-Volt of buzz difference in the ten outputs, which is about as big as your guitar signal.

Using ten transformers fixes this, as you planned, as you confirmed.

Next time you want many loads on *one* transformer, use *one* rectifier and main cap. You may use multiple regulators, though in many cases a single regulator can do the job, keep it simple.

 

[CJ]

nothing beats battery power for effects,

maybe a 12 V motorcycle gel cell with dropping resistor,