OT: powering a 9vdc pedal with dc adapter

[xtian]

Sorry for the off-topic post. Irksome, but hopefully simple problem. I built this booster pedal kit: http://generalguitargadgets.com/pdf/ggg ... a841b3e85a

Works great when powered by 9v battery. But I want to power with "wall wart." I have tried several, and when connected, they cause a loud buzzing, with the guitar's signal barely audible. I eliminated variables by measuring the DC voltage and polarity, and using a model with a 9v plug adapter, so it looks exactly like a battery to the pedal. No help.

Why am I getting such nasty buzzing? Is this booster SO sensitive to a little rf noise or ripple or whatever that I can only use a battery? Or maybe I can throw an appropriate cap in there?

[ToneMerc]

I find that most of the "wall warts" have a fair amount of AC component riding on the DC. Also, the input filtering of many of these pedal projects tends to be conservative outside of using a battery as the source. It could be either of these or even a lead dress/layout issue.

TM

[M Fowler]

Did you add a DC power jack to your pedal or using a snap 9v lead?

Did you reverse the polarity?

I haven't had any problems with the booster pedal I built from their kit using Godlyke power-all.

[xtian]

Did you add a DC power jack to your pedal or using a snap 9v lead?

Did you reverse the polarity?

I haven't had any problems with the booster pedal I built from their kit using Godlyke power-all.

A DC jack AND 9v snap lead were both included in the layout. According to continuity testing, the DC jack's normally closed switch is working properly when I insert the DC adapter plug. Never mind, though--I'm using a DC adapter exactly like a 9vdc battery, connecting it to the pedal's 9v battery clip, and getting that nasty buzz.

I have a One Spot power adapter on order; I'll try that when it arrives.

[xtian]

Here is a photogram.

[M Fowler]

I'm wondering if you are then have switched the voltage + / - using the snap on 9v adapter?

[xtian]

I was careful about that. And I tried it both ways!

[Super_Reverb]

I suggest measuring the AC voltage at the output of the 9V DC wall wart. It should only be a few mV. Most of the modern adapters are based on more compact switching regulator designs; switchers operate at 10s or 100s of kHz.

You may need to find a quieter p/s or use some filter caps and a series choke to tame it. If this is the case, required inductance to filter at switching frequency will be much lower than typical 120Hz filter requirements.


cheers,

rob

[xtian]

I suggest measuring the AC voltage at the output of the 9V DC wall wart. It should only be a few mV. Most of the modern adapters are based on more compact switching regulator designs; switchers operate at 10s or 100s of kHz.

You may need to find a quieter p/s or use some filter caps and a series choke to tame it. If this is the case, required inductance to filter at switching frequency will be much lower than typical 120Hz filter requirements.

Reads about 30mV AC. Can you suggest how to filter with caps?

[tubeswell]

. Can you suggest how to filter with caps?

Same as a Pii filter supply in a tube amp (but you don't need such hi-voltage caps). If you have a +ve ground, then e-caps go in with the "+" side to ground.

[jjman]

Reads about 30mV AC. Can you suggest how to filter with caps?

Just get a One-Spot or other regulated unit. Will show 0vac.

[Masco]

Or add a 9v regulator.

[vibratoking]

This could be several things. Polarity is one possibility, but you said you verified that it is correct. An external power supply can introduce hum via a ground loop. Have you verified there is not a ground loop? A simple three prong to two prong adapter should fix it if one exists.

[M Fowler]

I think it's a polarity issue but you verified it isn't so I am at a loss here.

This pedal requires DC power supply or AC power supply?

Mark

[xtian]

Polarity is not the issue, nor is ground loop.

I have a One Spot arriving this evening, and I'll let you know if that solves the problem!