hum noise increase - turning down volume on a strat.

markblack

Any idea why when I roll the volume off and get to 8/9 there is an increase in hum on my Strat. When it hits 10 it quietens down, same when it get's below 8.

Cheers

Mark

p90fool

Could be a grounding issue, or maybe a cheapo unscreened patch lead?

dogload

I came across this on a strat at work that we'd been given, and it had the output wires the wrong way round! Earth to tip and hot to barrel lugs on the jack. 

Probably not the problem but easy to check.

markblack

i'll have a look.

MattFGBI

Is it in all positions or just 2 and 4? 

musteatbrain

Interested in this thread. My single pickup CS Cabronita does this

ICBM

Because the maximum resistance from the signal output to ground occurs when the pot is at about 8. That means that noise being picked up by the cable is least well damped at that point.

It may mean you're using a cable that is prone to noise, or possibly that the amp is not very well earthed - but *some* effect like this is normal, so it may not be a fault.

musteatbrain

Thanks. I'll check my cables

markblack

@ICBM that seems to be it. Cheers.

M

knuckleberryfinn

ICBM said:

Because the maximum resistance from the signal output to ground occurs when the pot is at about 8. That means that noise being picked up by the cable is least well damped at that point.

I'm not doubting the truth of this at all - in fact I've noticed this noise issue myself.  But why?!  It makes no sense.  Surely the resistance should be at its maximum when the pot is turned all the way?!

ICBM

knuckleberryfinn said:

I'm not doubting the truth of this at all - in fact I've noticed this noise issue myself.  But why?!  It makes no sense.  Surely the resistance should be at its maximum when the pot is turned all the way?!

No, because then the resistance of the pickup is in parallel with the pot, which reduces the effective value to ground, to that of the pickup DC resistance.

The maximum resistance from the pot centre terminal to ground occurs when the resistance directly to ground via the lower part of the pot track is equal to the resistance indirectly to ground via the upper part of the pot track and back via the pickup.

knuckleberryfinn

ICBM said:

knuckleberryfinn said:

I'm not doubting the truth of this at all - in fact I've noticed this noise issue myself.  But why?!  It makes no sense.  Surely the resistance should be at its maximum when the pot is turned all the way?!

No, because then the resistance of the pickup is in parallel with the pot, which reduces the effective value to ground, to that of the pickup DC resistance.

The maximum resistance from the pot centre terminal to ground occurs when the resistance directly to ground via the lower part of the pot track is equal to the resistance indirectly to ground via the upper part of the pot track and back via the pickup.

Ah, now I get it.  Thanks!

p90fool

That's why I suggested cables. A reduction in noise as you roll it up to ten is a classic symptom.

Ruptor

I have the same or probably worse on a Strat kit I made but I tried different cables none of which give problems on other guitars. The noise is about 5 mV at full volume & 800mV at half volume so you can't use the guitar at less than full volume. I would have started a new thread but can't figure out how to do it.

ICBM

Ruptor said:

I have the same or probably worse on a Strat kit I made but I tried different cables none of which give problems on other guitars. The noise is about 5 mV at full volume & 800mV at half volume so you can't use the guitar at less than full volume.

That’s caused by poor shielding in the guitar, if it’s not the cables.

Ruptor

ICBM said:

That’s caused by poor shielding in the guitar, if it’s not the cables.

If you looked at this guitar you would say what shielding? The pots are all tied together and there is a piece of aluminium foil stuck to the plastic panel that holds the pots but all the wires are not screened. I have never seen such a large amount of noise it is as if the pickups tune with the volume control to pick it up. I put some aluminium shielding in the wood and disconnected the tone controls to get rid of most of the wiring but it made no difference. Could I wire the volume so the wiper shorts the signal to ground to reduce the signal? I guess the experts won't like that idea since the signal is loaded more as the volume is turned down but at least the signal won't be swamped by noise that would be cut by the same amount.

ICBM

Is the bridge grounded?

Ruptor

ICBM said:

Is the bridge grounded?

Yes at the start I checked all the other ground connections and wiring. I found a fault on the prewired assembly where they had the wires round the wrong way on the centre pickup that meant it cancelled out the signal when switched in circuit and stopped any output. Because you suggested the leads were the most likely cause and I thought the jack socket was a bit weak I cleaned the contacts and bent the tip contact for a tighter connection. This has improved things a bit so now you have to move the volume further to the middle to get the noise but it is still a lot compared to my Humbucker guitar. On a normal input to the amp it is usable off of centre volume but through a high gain unit like fuzz the noise is amplified when no signal is present. One thing I did try was to increase the pickup output by putting a neodymium magnet on the back of the ferrite ones and the output doubled. A pretty easy upgrade but I only have one magnet that wasn’t long enough to cover all the strings. Thanks for the help ICBM it is a lot better now. In hind site perhaps there was manufacturing grease or other contaminate on the jack inside of the metal tube causing a bad connection.

Ruptor

I was still not happy that the noise was so bad & output level of signal seemed a bit low so I took another look. What a dipstick I am. When I fixed the wrongly wired panel I had to remove it that meant unsoldering the jack socket. When I soldered the wires back on the jack I put them the wrong way around so the tip was earthed. I am amazed that it worked so well. Now I don't have any noise and it is quieter than an original Telecaster probably because of the extra shielding I put behind the pots. The main thing to take from this is if you have buzz off of full volume on a Strat check the polarity of the signal through the wire. Using a multimeter set to measure resistance you should get practically zero between any metal of the guitar and the shield of the jack plug. Between the tip of the jack plug and the metal you should see the resistance of the volume pot when you turn it. I have stated this obvious situation in case anybody else has built one of these Strat copies and if anybody has had a cable that is wired the wrong way around with the tip as the shield that you can't tell by looking at it. I will remember this for sure because the guitar seems to work.