extra hum when guitar volume not maxxed

[zjokka]

In a few builds, I noticed that the hum of the effect (fuzz face, booster) was a lot louder when I put the volume down. When I dialed back from 10 to 8 the hum becomes clearer. Somebody already suggested that it's the guitar's wiring, which is very likely.

Now tried to tackle this phenomenon by using my stargrounded and shielded strat with the fat booster I'm doing now, and it has this problem too, but a lot less (like 30%), so I could live with that. It is the guitar.

But is there a way of eliminating this completely?
Any experience on this?

thanks
zj

[jimmy54]

I'd like to see an answer to this as it happens to me too.

[zachomega]

I don't know if this is your problem, but I used to have a Harmony Rocket II from the 60's and it had a floating ground system.  I'm not sure if this was to prevent the user from getting shocked or because it was just a more convenient way to "ground" the guitar.  Anyway, when the volume was at 10, the thing was more or less as quiet as any other guitar, but when the volume was brought down at all, the guitar would hum quite a bit...Enough anyway to make sure I always kept the volume at 10. 

So perhaps it is a grounding problem? 

-Zach Omega

[zjokka]

zach, thanks

that grounding is somehow the problem is clear. with guitar volume on 10 there is 0 Ohm resistance between the guitar and the booster, dialing down the volume it builds up to say 80-100k at 7, where the hum is the loudest. So with some resistnance on the ouput, some ground signal is bleeding into the signal path, probably through minimal conductivity of your pot, between taper and casing, because it is grounded like that...

I'm just freewheeling here.

Still, if you got into the right situation with that Rocket you might really fly because if it does that, it's sure as hell not stargrounded properly. Could never have avoided electrocution with wrong polarity amp by stargrounding though. You need to put a 400v 0.33uF cap between shield and starground point for that.

Never heard of floating grounding... you mean grounded on the pot casings in different locations? At least that's what I think the problem is.

like I stated before, the problem is clearly in the guitar as my properly shielded and stargrounded strat hardly has this problem. Though I can still hear it if I put amp at serious volume. at normal volume it is indistinguishable from the amp noise.

Probably it only goes as far as your stargrounding skills go. Was thinking it could have been elimated completely


zj

[zachomega]

I don't have the rocket anymore...It was worth many times what I paid for it so I sold it. 

But from what I remember...The strings were not connected to ground at all.  It had a trapeze style tailpiece and a wooden saddle.  Additionally, all of the electronics were shielded with some armor style cable.  I never took it apart to see the details of it but I know the strings themselves were not grounded hence why I referred to it as a floating ground.  Not sure if that is the proper terminology for that. 

Anyway, in that situation, it is impossible to be shocked while touching the strings.  Touching the pickups or the jack on the other hand could be a different story. 

-Zach Omega

[Blues Lyne]

If it only happens with certain pedals, it may not be the guitar.  I had the same problem with my Bartolini Tube-It.  The hum was the worst with the volume at 7 or 8 depending on the guitar.  It went away as I turned the guitar up, but also got somewhat better if I turned the volume down lower. 

I asked an engineer friend who has worked installing radio studios and building studio components and is also a musician and he said that it had to do with impedance.  I don't remember the details, but he said that at 7 or 8 the pot was probably at the point where there was equal resistance on either side of the wiper and that was why it was at it's worst at that setting.  It boiled down to the input impedance of the pedal and the output impedance of the guitar not working right together.  I put a buffer or a buffered pedal in front of the Tube-It and no more problem.  The problem is, you don't want a buffer in front of your Fuzz Face, and I've never had that problem with my fuzz face clone.  But it might be worth a try to see if it takes care of the problem.

[Izzy]

I am having the same problem. When I put my guitar volume at 0 there ia more  hiss than when the volume is at max in my ds-1. But it had output buffer too. So I didnt understand why it was making hiss. One of my friend told me that putting a FET transistor before the buffer as in actual achematic helps, but I am not sure,
Any help?

[DDD]

Extra HUM and extra HISS are two different things astually.
With the guitar volume pot NOT at max or zero position the output impedance of the guitar is higher (the worst case is the pot resistance divided by two). The higher the source impedance - the higher the possibility of crosstalk, RF interference, hum, etc.
Good shielding, good cable and good connectors eliminate the problem.
Extra hiss with the volume pot at zero position can be caused:
1. with the bad contact inside the pot just in the zero point (it happens sometimes),
2. RF interference detected by the input stages of pre-amp, especially if there is not enough shielding or bad contact in the jack/plug (it happens more oftenly than one can expect),
3. Output/input impedances' mismatch, which can add some extra noise to the input stage (actually, too "theoretical" situation).

So - shielding and shielding again, no ground loops, good jacks and plugs.

[mac]

I like pedals that can be controlled with the guitar knobs, so this was an issue for me too when I connected the them to a sound card. Lowering the guitar volume introduced a lot of bassy hum, and as said before it was louder when it was at medium resistance. But with my amp I did not had this problem, basically because it was grounded to the plumbing. Adding a wire from the effect case to the water pipe solved the problem a 90%.
The plumbing probed to be better than the usual buried copper speer because it is longer and has more surface in contact with gnd. Well at least for me.

mac

[Meanderthal]

 WOW! I haven't heard of grounding to plumbing since I built a crystal radio when I was about 9 years old or so... We didn't have any ground in the household wiring at all(old house with ancient cloth insulated wiring), so that's what I had to do. It does work though.

[zjokka]

With the guitar volume pot NOT at max or zero position the output impedance of the guitar is higher (the worst case is the pot resistance divided by two). The higher the source impedance - the higher the possibility of crosstalk, RF interference, hum, etc.
Good shielding, good cable and good connectors eliminate the problem.
So - shielding and shielding again, no ground loops, good jacks and plugs.

OK! did try with my shielded guitar and the hammond fx box properly closed and its gone from the realm of audibility. Have to keep into mind that this is something which cannot be fixed wih a trick or mod. Reflects the quality of our build, Mother Nature at least is fair 

Adding a wire from the effect case to the water pipe solved the problem a 90%.The plumbing probed to be better than the usual buried copper speer because it is longer and has more surface in contact with gnd. Well at least for me.

WOW! I haven't heard of grounding to plumbing since I built a crystal radio when I was about 9 years old or so... We didn't have any ground in the household wiring at all(old house with ancient cloth insulated wiring), so that's what I had to do. It does work though.

double    -- guess that's how you learn about grounding and never forget

[mac]

WOW! I haven't heard of grounding to plumbing since I built a crystal radio when I was about 9 years old or so... We didn't have any ground in the household wiring at all(old house with ancient cloth insulated wiring), so that's what I had to do. It does work though.

I live in an 60yr old building. No gnd wiring, that´s why I use the plumbing.

BTW, in some houses I built I had trouble with the copper stick buried deep in the ground, because copper surface gets rusty with time. I had some customers complaints about this. It is a good idea to locate the copper stick and replace it from time to time. And plumbing is not an option since the introduction of thermofusion pipes long ago.
And please do no use old metal gas pipes!!!


mac