Hum from Russian (Green) Big Muff Pi DC Jack

[bryantabuteau]

HI,

I just put a dc jack (boss barrel style)  into my Green Big Muff Pi.  It works fine, but using the jack over the battery comes with a heck of a lot of hum.   I use the adapter in all my other pedals, and i get negligable hum.  I did a quick search and found this....

There will be room inside the BMP for your buddy to park another smoothing cap between the tip and shaft connections of the jack. Try using a 220uf/16v cap for a bit more hum rejection.

can anyone confirm that this may be a solution to my problem?  
suggestions on type of cap etc?

the BMP isn't modded at all except for the 8-10k resistors to Q2 and Q3? being changed out to 4k7, which I did while installing the dc jack.

[niftydog]

I would perhaps try something closer to a 2200µF cap just to be sure of proper filtering.

The only capacitor you'll get that is that big is an electrolytic.

the BMP isn't modded at all except for the 8-10k resistors to Q2 and Q3? being changed out to 4k7, which I did while installing the dc jack.

so... it IS modded then!

Not at all familiar with the circuit so I can't say if that has changed things, but a word of advice; modify things in stages so that you don't end up with a nightmare troubleshooting procedure at the end of it. (You should have moded the resistors then checked it before adding the DC jack.)

Does the pedal work properly from batterys still?

Stuff to check;

- the adapter voltage under load - too high perhaps.
- make sure the battery is disconnected when the adapter is plugged in.
- use your multimeter on AC volts setting to check for ripple on the adapters output.

[bryantabuteau]

thanks, yeah the unit works sweet on a battery, so i figured my mods weren't to blame.  I'll definately check the AC ripple and put a cap in there.  thanks.

[bryantabuteau]

hmmm.....  I measured 18v AC coming out of my adapter.  That doesn't seem right does it?  around 10v DC without a load.   With a 1000uf cap and 2x400uf caps across the dc jack pins ( i assume thats how it works - didn't have a 2200uf handy) the hum is reduced slightly but not near enough to use.  My onespot adapter died a sad death a few months ago, so all i got is unregulated ones hanging around.  Maybe i need to invest in a regulated adapter or make a little power box myself.

[niftydog]

I measured 18v AC coming out of my adapter. That doesn't seem right does it?

WOOF! Um, no that's not correct. Are you sure it's meant to be a DC adapter and not an AC adapter?!?!
The best way to be sure would be to have a look at it on an oscilloscope. Chances are that it's pure AC, which would mean that a cap filter isn't going to do the job. It would require recification as well as filtering.

I would maybe expect a few hundred millivolts of ripple on top of the DC. Problem is, when there's that much AC, you can't distinguish between DC that is supposed to be there and DC that "appears" to be there because of the interaction between the supply and your multimeter.

[bryantabuteau]

well this is definately marked as a DC adapter, and i've been using it for 6 months powering a boss pedal tuner, daisy chained to a sansamp GT2 and a jim dunlop wah, and a rotovibe and a phase90, and had no noticable hum.  Maybe something went wrong with it recently, or all these other pedals have some sort of filtration installed in them.  but as you said, its an enormous amount of AC coming through, so who knows.  I also noticed on a GGG big muff schematics, there was an extra cap on the board itself that was mentioned for dc jack hum reduction.  

Can't really afford a new adapter at the mo, so might just leave it for a while, and just use a battery with it for that one.  do you think the AC coming through could be damaging my other pedals?

I'd love to look at it on a scope, but don't have one handy.

[niftydog]

with that much AC (if that is actually what's happening) damage is a definate possibility. However, as you say, if there's sufficient filtering it might be ok... but if it's pure AC there's still the chance of damage due to the negative swing of the AC.