Running two amps.

[Keith Hilton]

I have not had good luck running two amps, with one being a tube amp, and one being solid state. Seems like I always get a difference in ground potential which creates hum.
I played a dance last Saturday, and noticed another problem when trying to run two different solid state amps. The amps were from two different manufacturers. One amp seemed to suck all the signal away from the other amp. For the volume from both amps to be about equal, this meant I had to turn one amp's volume real low, and turn the other amp's volume up real loud.
Both of these amps were 200 watt amps. The only thing I can figure is one amp has a much higher input impedance. To suck so much signal away, seems to me it has to be something else going on besides input impedance. Because I am thinking all solid state amps have a pretty high input impedance. One of the amps was a bass amp, and the other amp was a regular guitar amp. Maybe a bass amp has a much lower input impedance compared to a regular guitar amp. Maybe someone can explain why one amp was sucking all the signal?

[b0b]

What are you using to split the signal between the two amps?

[Ian Rae]

The only thing I can figure is one amp has a much higher input impedance. To suck so much signal away, seems to me it has to be something else going on besides input impedance.

Not necessarily. If they are totally different amps, then input impedance would be enough to explain it unless they're buffered in some way as b0b hints at.

[Cartwright Thompson]

The speakers might be out of phase.

[Michael Butler]

try the humX by ebtech. it has worked for me between my 59 princeton and 66 deluxe reverb.

it may depend also on how you are running into the amps.

play music!

[Keith Hilton]

bob, the signal was not split into two separate signals. The signal was sent to the two different amps from a single OP Amp buffer running at unity gain. The output of the OP Amp was "Yed" to each amp. I thought the low impedance of the OP Amp would be adequate to drive both amp inputs. I also thought the input impedance of both amps would not be that far apart. The amps I had hooked up was a newer Evans guitar amp, and new Trace Elliott ELF bass amp. The ELF bass amp was eating all of the signal from the OP Amp output "Y" connection. I had to turn the ELF bass amp down real low and the Evans amp up really high. Out front friends said I had the best sound they had ever heard from me. On stage it did not sound that great. Plus, I could never seem to get the two amp's volumes equal.
We are lucky to have Georg Sortun on the FORUM. My opinion is George Sortum has a great electronic mind. George, good audio transformers are hard to find, and expensive. I do understand that a transformer is probably the best way to eliminate hum caused by a difference in two amp's ground potential. In this case, noise was not an issue, it was the difference in amp volumes. I suppose the solution using OP Amps is to send the signal to two separate high impedance OP Amps. Then have separate gains for the output of each separate OP Amp. As I see it, the Trace Elliott bass amp has a much lower input impedance than the newer Evans guitar amp. With a lower input impedance, voltage goes to the least resistance. Thus sucking all the signal.

[John McClung]

I used a ground lift on one of my twin amps, that eliminated hum. Speakers out of phase is another strong possibility. I did that for one set, sounded from where I sat like the steel was in every corner of the room, love it, but the club owner said I was ungodly loud in the room, a problem I'd never had. So I put them back in phase, all was well.

[George Kimery]

When I was in Jeff Newman's school in Nashville, he was running two amps. He had a chorus on one amp only. What I will never forget, to connect the two amps together, he just had a guitar cord going out of one of the inputs on one amp to one of the inputs on the other amp. I couldn't understand, and still don't, how you can use an input as an output. They were both Peavey amps, but don't remember which model.

[Greg Lambert]

[Lee Baucum]

Many years ago, when Jeff was running two amps, I started playing my Emmons U-12 through a silverface Fender Vibrosonic and a Peavey Nashville 400. I just used the two outputs of my Goodrich volume pedal to split the signal. As Jeff recommended, I lifted the ground on one of the amps.

It was a huge sound.

[Keith Hilton]

Greg, look in the direct box you posted, and see if it has coils like Georg was describing?

[Nic Neufeld]

I'm no expert but I've used my console steels in a biamp rig using the "tuner out" of the first amp (small bass amp) into the input of a small tube guitar amp. Have had no troubles with it set up that way...

[Dave Mudgett]

From the ELF owner's manual:

Input Impedance:
>10 meg Ω

That is way higher than the typical guitar amp, usually more like 500 KΩ - 1 MΩ. Depending on exactly how everything is connected, this type of disparity in input impedances could cause imbalance in how much signal is coupled to each amp.

Not sure exactly what the Evans amp input impedance is. This Danny Hullihen link http://danny.hullihen.tripod.com/id26.htm says 1MΩ at Input 1 and 80KΩ at Input 2 for an Evans FET-500. I didn't find any info on the Evans website for newer amps.

[Godfrey Arthur]

Try this. About $170.

This allows you to use either amp or both with full clarity and no hum.


The Twin-Cityâ„¢ is an active amp selector that lets you connect any two guitar amps and switch between them or drive them both simultaneously without noise, loss of gain, or any degradation to your natural guitar tone.

[Keith Hilton]

Now I am really confused Georg. Yes, amps were in parallel from the OP Amp source. Yes, the combined input impedance of the two amps is a lot more than 10 times the output impedance of the OP Amp. But Georg--both amps were not affected equally. The ELF sucked all the signal. I had to turn the ELF volume way down, and the newer Evans amp volume real high.

[Randy Schneider]

The ELF sucked all the signal. I had to turn the ELF volume way down, and the newer Evans amp volume real high.

Does the signal on the Evans come back up to 'normal' if the input to the ELF is removed?

[Greg Lambert]

Greg, look in the direct box you posted, and see if it has coils like Georg was describing?

Im not sure what kind of coils it has. I had the same hum problem years ago when trying to run two amps. Adding the direct box eliminated the hum problem.

As far as volume , I really didnt notice any difference in volume of the amps that I can remember. It may have been there but it wasnt much or I would have noticed it.

[Keith Hilton]

Randy, the Evans volume level comes back to normal once the ELF is taken out.

[Blake Hawkins]

A lot of good advice and solid theory here.
I think that you need to completely isolate one amp from the other.
Georg suggested that and I know his line drivers would do the job if they were available.
Perhaps, you could do that with a small stereo mixer.
Feed the left channel to one amp and the right channel to the other. That way you could optimize the level to each amp. Feed the guitar into a mike channel and select mono.
As long as the amp does the mixing before the output stage
it will work. Of course you have to be mindful of the levels and be sure that they do not overload any of the circuits.

[Garry Vanderlinde]

The way Jeff Newman taught me to do it was that you had to have two outputs on your volume pedal FIRST! or a splitter so you had two dry outputs.
Then each output went to a separate amp with it's own cord.
One amp is set normal with a little amp reverb and the other is also set normal but with a little chorus (the Electro-Harmonics Small Clone was very popular at the time) set very lightly.
STEEL GUITAR MAGIC
Jeff said you are not playing in the band...You ARE the band
...and of course lift the ground on one of the amps...

[Marc Jenkins]

This device from Radial has everything covered - SAFE ground lift, phase switching, also tuner out:

http://www.radialeng.com/product/bigshot-aby