Regulated powersupply doesn't like long wirelenghts

[alfafalfa]

Hi,

In my rack I have a regulated powersupply with 12 volt and 9 volt  that I  use with a 15 foot wire to feed my stompboxes. All goes well with my distortions, a TS 9/ 808 , a BSIAB 1 & 2 , a Red Fuzz , Ross comp and others but my newly built Tremulous Lune doesn't like being fed  with such a long wire. It just cuts out , the effect starts a crackling noise and no sound , I think it may be oscillating.javascript:void(0);
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Has anyone ever encountered something similar ??
I already fixed it by the way by putting another 7809 Voltage Regulator inside the TL box no more problems so far.

Alfafalfa 

[bancika]

yeah, cable could pick up some noise...did you try using shielded wire? It might help.

[alfafalfa]

Thanks , I'll try that tomorrow. Will report .

Alfalfa

[ward]

If you feed a 7809 regulator with 9volt it can't be regulating.   
The voltage drop is about 3 volts I think.

[bancika]

yeah, you need at least 12V to feed 7809

[alfafalfa]

Ward and Banicka ,

You're right about the 12v but the Tremulous moon worked when I plugged it in.
With exactly 8.95 volts I got 7.50 volt from the regulator , no problem for the TM, maybe a bit less volume ( headroom ? ).
When I plugged in my 12 volt ( actually 11.89 V) I got 8.95 volt at the output of the 7809.

Now for the shielded cable, it worked but the weirdest thing was  the standard wire I used also worked
for a couple of minutes until it started acting up again. I 'll keep on using the shielded cable just to be sure.

Thanks for your help,  Alfalfa

[bancika]

maybe something changed in that few minutes that changed electro-magnetic field around cable.
About regulator, it's not wise to use it that way...you won't get flat current with less voltage on regulator. Better use 12V DC before pedal...or if shielded cable works without regulator don't use it at all.
Cheers

[R.G.]

Your regulator or the TL was probably oscillating at radio frequencies.

Three terminal regulators are special cases of operational amplifiers set up to feed only one signal. They have high gains and negative feedback to keep things at the same level all the time. Such amplifiers can be sensitive to loading under certain conditions, and can oscillate if things aren't right. capacitive/inductive loading like with long cables can under some circumstances start such oscillation.

Large signal RF running through audio amps often sounds like an angry hissing or crackling and completely elbows out the audio.

Why did it quit? You changed some little thing and it got pushed back below the edge of oscillation.

Try putting a 0.001 to 0.01uF CERAMIC  cap, either disk or monolithic, out in the TL, along with a 100uF power supply cap. If it still happens, put a 100 ohm resistor in series with the +9V before it hits the caps in the TL.

The commentary is correct - if you feed a 7809 less than about 11V, it is not regulating.

[alfafalfa]

Thanks R.G. for your clear explanation , I have one more question !
"Try putting a 0.001 to 0.01uF CERAMIC  cap, either disk or monolithic, out in the TL, along with a 100uF power supply cap. If it still happens, put a 100 ohm resistor in series with the +9V before it hits the caps in the TL.
"
Do you mean a 100 uF and the 1nF ceramic paralel from positive 9 V to ground ?

Alfalfa

[R.G.]

Yes - 100uF and a ceramic cap in parallel from +9V to ground, out in the TL. These caps lower the impedance of the power supply as seen locally by the TL. Otherwise, the TL sees a power supply with a long inductor/capacitor feeding it power - that cable. The series resistor before the local caps acts as damping on the tendency of the long cable to oscillate by eating up any ringing energy, very much like putting your hand on a bell eats up the ringing energy. This would make things worse for the TL if you did not also have the local capacitors to lower the impedance as seen by the TL again.

[alfafalfa]

Even with 30 feet of cable it works like a dream .
Much obliged R.G.

Alfalfa