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Grounding issue on my CT624M w/piezo

Posted: Tue Feb 06, 2018 2:47 pm
by mayor_of_pc
I've been struggling with a 60Hz hum grounding issue on my CT624 with acoustic saddles. I sent an email to customer service requesting a wiring diagram, but I didn't get a response. Maybe somebody here can help.

I'm getting obnoxious hum when using the magnetic pickups. I'm using a noise gate, but I need to set the threshold way too high to reject the hum.

I’ve discovered that the ground side of the magnetic pickup/volume/tone circuit is isolated from the cavity ground and has a DC bias of about 3.6V. The bare shield wires of the pickups are connected to this “ground” and I can fix the hum by connecting the bare leads to the cavity ground. Unfortunately, this introduces unwanted feedthrough on the magnetic circuit when using the piezo circuit.

It looks like the pickups are intentionally isolated from cavity ground, but it would be nice to get confirmation. If I could get my hands on the wiring diagram for the interface between the magnetic pickups and the piezo preamp, I could probably figure out a way to fix the hum problem.

If I can’t, I'm considering a retrofit with a Graph-Tech Acousti-Phonic preamp. Anybody have any experience with these? The Graph-Tech preamp senses between a mono or a stereo jack which avoids the existing cumbersome two jack arrangement. There are also some nice wiring diagrams on their website including options to directly connect the magnetic pickups to a jack without interfacing to the piezo preamp.

Side note: During troubleshooting, I noticed that some strings weren’t connected to ground. I was using Di-Addario XL strings with color coated ballends. Since the bridge is electrically isolated by the acoustic saddles, the ferrules in the body are used to ground the strings. But, the paint on the ballend was insulating the string from the grounded ferrule. I fixed it by scratching the paint off, but I don’t use these strings anymore.

Re: Grounding issue on my CT624M w/piezo

Posted: Sat Feb 10, 2018 12:32 pm
by mayor_of_pc
I'm getting no love from Jeff and his guys, but I managed to come up with a custom modification that's acceptable. The hum was pretty bad, so I'm surprised that no one else with the AS option has this problem.

First, I separated the humbucker bare wire connections from the elevated volume ground and connected them to the cavity ground. This extends the cavity ground out to the coil pole pieces and solves the 60Hz hum issue:

But, as I mentioned in the first post, a very strange blow-by issue is created. When using one cable for the magnetic pickups and another for the piezo pickups, the magnetic signal is present when the mini toggle is in the piezo position. It's even more strange that the magnetic signal is the highest when the magnetic volume is at the minimum position. I don't understand this at all, but I can surmise that the magnetic signal is somehow feeding into the elevated volume ground. Since the output signal at the cable is referenced to the cavity ground, any signal between the two grounds will be heard.

So, I solved the blow-by issue by placing a capacitor between the two grounds. By experimentation, I found that 0.1uF was not enough, 100uF was too much (this created volume pot noise, and I have no idea why), and 0.47uF does a good job of reducing the blow-by to acceptable levels:

So, I'm happy with the new arrangement and won't need to retrofit to another piezo preamp.

Re: Grounding issue on my CT624M w/piezo

Posted: Tue Feb 13, 2018 7:29 am
by Alienadin
That sucks, mate.
I want my next build with the AS as well, but this has me worried. I know nothing about soldering, and my first Kiesel exhibits terrible single coil hum, which annoys me greatly too.
I find the lack of after purchase customer service very disappointing.

Glad to see you have found a solution, but this shouldn't be necessary in the first place.

I would really like to hear a response from Kiesel here.