Problem with K&K Pure Mini volume wheel hum

[HeyMikey]

I just installed a Pure Mini with the (passive) volume control wheel into my Martin. It sounds fantastic as long as the volume knob is at 100%. However, when I back it off, even the slightest bit, it hums. If I stick my finger near the volume control the hum gets louder. If I touch the volume control underside it gets very loud. I'm playing direct into a new Fishman Artist.

Again it is perfect as long as the volume control is at 100%. Has anyone experienced this problem? Any thoughts?

[llew]

A while back I had a K&K mini in a Kevin Kopp K-35. Hated it. Like you said...if the volume wan't all the way up it sounded terrible. I was told by folks "in the know" that K&K had since remedied that problem but I see no need to try again? I use the K&K mini now almost exclusively (w/o the volume wheel) and love it. But it's the standard version.

[AHill]

You'll get more responses posting this in the Amplification forum. I suspect your amp isn't impedance matched. A K&K ideally wants an impedance of 1 Megaohm vs. the more standard 10 Megaohms. You may get better performance going through a K&K preamp.

[HeyMikey]

Ahill - Thanks.

1) Will any pre-amp/di solve this impedance issue or does the device specifically need to do that? Had I known I would have just bought the regular Pure without the volume control.

2) Could this be some kind of grounding or shielding problem that can be simply fixed? Like I said when I reach into the sound hole toward the volume wheel the hum gets louder, and really loud when I touch it underneath where the wires connect. I've also reached out to K&K but haven't heard back yet.

[jonfields45]

Sounds like a simple problem K&K's tech support can walk you through. My guess is the volume pot ground is floating.

[HeyMikey]

Originally Posted by jonfields45

Sounds like a simple problem K&K's tech support can walk you through. My guess is the volume pot ground is floating.

One would think. So far this is what I've gotten back from K&K tech support. My snarky comments in paren's.

• Yes, its a common phenomenon (i.e., problem) with this unit (though they continue to sell it)
• A lot of people just use it as an on/off switch (instead of what we advertise it to do and charge a premium for)
• We don't offer a shielded version, or have a retrofit option at this time. It is complicated (see update below)
• Buy a K&K pre-amp which may help (spend more $ )
• Remove it, and buy a replacement end-jack unit without the volume knob (spend more $ )

I think at this point I will leave it always full on/off, and just suck up the fact that I paid extra $ for this; and then wait until I buy some other piece of gear like a TC Acoustic Play or a standalone pre-amp to see if that helps. Ideally, having on-board volume control would be terrific going from strumming to fingers, so I will wait. On the plus side the Pure Mini pickups do sound terrific going straight into the Fishman's EQ (gain to 9, bass to 10, very faint reverb 2)

Update: Tech support suggests it might be possible to shield the pcb, connecting to the grounds / wire shields, but insulated from the rest of the circuit points by perhaps using a copper sheath or by constructing a metal housing around it (which they should do IMHO). I think I'll try some copper sheath/tape and see it that helps.

They also say that their "active" volume unit with the phase control does not have this problem. Had I known I would have gone that route...

[Rudy4]

Originally Posted by HeyMikey

One would think. So far this is what I've gotten back from K&K tech support. My snarky comments in paren's.

• Yes, its a common phenomenon (i.e., problem) with this unit (though they continue to sell it)
• A lot of people just use it as an on/off switch (instead of what we advertise it to do and charge a premium for)
• We don't offer a shielded version, or have a retrofit option at this time. It is complicated (see update below)
• Buy a K&K pre-amp which may help (spend more $ )
• Remove it, and buy a replacement end-jack unit without the volume knob (spend more $ )

I think at this point I will leave it always full on/off, and just suck up the fact that I paid extra $ for this; and then wait until I buy some other piece of gear like a TC Acoustic Play or a standalone pre-amp to see if that helps. Ideally, having on-board volume control would be terrific going from strumming to fingers, so I will wait. On the plus side the Pure Mini pickups do sound terrific going straight into the Fishman's EQ (gain to 9, bass to 10, very faint reverb 2)

Update: Tech support suggests it might be possible to shield the pcb, connecting to the grounds / wire shields, but insulated from the rest of the circuit points by perhaps using a copper sheath or by constructing a metal housing around it (which they should do IMHO). I think I'll try some copper sheath/tape and see it that helps.

They also say that their "active" volume unit with the phase control does not have this problem. Had I known I would have gone that route...

That's unfortunate. I've worked with electronics for many years and I've always understood that piezos won't work with a passive volume due to the ultra-high impedance of the transducer. When I saw them advertising the Mini with passive volume control I wondered how they were circumventing basic physics. Evidently they weren't.

I love the K&K mini and I've installed a half dozen in my guitars and a few for others, but always with the understanding that if volume control was needed it would be done at a pre-amp sitting on the floor. K&K also swears that the Mini can be plugged directly into many amps without degradation of the sound. That's another claim that skirts the laws of physics and I've found not to be true.

I wish that they simply represented it for the truly great transducer that it is.

[jonfields45]

OK, you've presented some new info that the full off is not humming. That means that all the terminals on the POT are properly connected but the extra impedance of that POT not full on or off, in the signal path is making the guitar susceptible to hum. The picture I see on-line shows a connector between the heads and the volume control. If that is the system you've got, try taking that connector apart and cleaning both sides with contact cleaner (the Home Depot stuff will be fine) and reseating. Next try cleaning that POT with DeOxit fader cleaner (not contact cleaner). The wiper on the POT might not be making good contact with the carbon resistor between the (metal?) end points. A 1M ohm Pot should make an acceptable passive volume control for a K&K Mini given a very high impedance input like any Fishman Loudbox (they spec 10M ohm).

[HeyMikey]

Thanks Jon. I'll look into those products. I don't know who carries them, or at least the Fader, locally but I can order online if needed.

[jonfields45]

Originally Posted by HeyMikey

Thanks Jon. I'll look into those products. I don't know who carries them, or at least the Fader, locally but I can order online if needed.

The more I think about it, I think it is a bad pot with the wiper not making good contact with the carbon resistor between the end terminals. For it to work well it needs to be a high resistance pot (>1M) which means touching the center terminal should hum when it is not full off, but a bad contact would explain a lot. How about a simpler approach and just send the volume control back for replacement and see if you do better with a new one. I'm a retired EE and many low cost parts are only sample tested in production and testing a high resistance resistor (not quite as tough as testing a fuse :~) is not easy. Also the home base for K&K is high humidity country and not a great place for storing parts that are not baked and dry packed (hermetic plastic bags). K&K is not the kind of high volume manufacturer where that kind of precaution would be routine. Age and humidity are going to accelerate the failure of a marginal pot.

There is no way K&K would sell a product that routinely malfunctioned and I would not take the tech's poorly chosen words as gospel just because when someone calls him about that product, no surprise, it is because it doesn't work. He has no experience with users where it does work...they don't call.

[bho]

Hello,

Can anyone tell me if this problem with the K&K volume control (without preamp) has been remedied?

Or, any other experiences to share?

Thanks.