(Below I am talking about putting a 100 ohm resistor between the preamp output jack and ground, "lifting the ground" of the signal to prevent ground loop hum. I am not referring to taking the ground off the power cable.)
Hi all thanks for looking. I was building a preamp, and I was considering putting an optional ground switch on my output. In my study of schematics, I looked at different guitar amplifier schematics. For example, I was reading the Marshall JCM 900 schematic to see what they did.
Rather than putting a ground lift switch on the effects send, the direct out, and the "recording compensated, (ie speaker simulator/voiced/filtered), they simply placed a 100 ohm resistor on all grounds preemptively.
My question is, should I put no ground lifts, ground lift switch, or preemptively place a ground lift resistor?
Can having a 100 ohm ground lift resistor that is not needed cause problems?
Thanks anyone for your experience and expertise.
Hi all thanks for looking. I was building a preamp, and I was considering putting an optional ground switch on my output. In my study of schematics, I looked at different guitar amplifier schematics. For example, I was reading the Marshall JCM 900 schematic to see what they did.
Rather than putting a ground lift switch on the effects send, the direct out, and the "recording compensated, (ie speaker simulator/voiced/filtered), they simply placed a 100 ohm resistor on all grounds preemptively.
My question is, should I put no ground lifts, ground lift switch, or preemptively place a ground lift resistor?
Can having a 100 ohm ground lift resistor that is not needed cause problems?
Thanks anyone for your experience and expertise.
I don´t think this will help to prevent loop hum.
It is possible to prevent loop hum by using a transformer.
It is possible to prevent loop hum by using a transformer.
Sometimes a resistor is put in the signal return (0V) is that what you mean? It's usually more around 5 to 10 ohms
100 ohm resistor on fx send and preamp outputs
One time i had hum between preamp and amp and it was suggested i add a small resistor between the grounds and it worked like a charm. Yhis was suggested by a saavy member of this forum. Here is a link to an article what Im talking about. Avoiding Ground Loop Hum
I have attached a small portion of a schematic to illustrate what im talking about. It looks to me like they are placing 100ohm resistors on jack output grounds. I have assumed this is to prevent hum. Maybe there is some other purpose.
One time i had hum between preamp and amp and it was suggested i add a small resistor between the grounds and it worked like a charm. Yhis was suggested by a saavy member of this forum. Here is a link to an article what Im talking about. Avoiding Ground Loop Hum
I have attached a small portion of a schematic to illustrate what im talking about. It looks to me like they are placing 100ohm resistors on jack output grounds. I have assumed this is to prevent hum. Maybe there is some other purpose.
Attachments
Yes, it can help, in that article it says 10 to 100 ohms, I'd use the lowest resistance that does the job, in a well designed and laid out system it shouldn't be necessary
So what did you do in the end with the 100R to lift the circuit ground from earth?
I have this implemented on my power amplifier as it was built, and not on my preamplifier.
I am intrigued to see if I can reduce my small background hum if I remove this and put a low impedance link to earth (as I have on my pre amp)
Thanks if anyone is still watching this thread 🙂
I have this implemented on my power amplifier as it was built, and not on my preamplifier.
I am intrigued to see if I can reduce my small background hum if I remove this and put a low impedance link to earth (as I have on my pre amp)
Thanks if anyone is still watching this thread 🙂