Using one buffered pedal to drive multiple amps....

[olddawg]

I don't do this much anymore. In the old days I just parallel up the inputs and delt with ground loops with ground lifts and ignored any loading problems. I'm playing a high profile situation where I will be using multiple amps outdoors. I'm not switching between amps. The last pedal in my chain is a buffered pedal (Ibanez DE 7). Will that be enough to avoid the usual issues if I just use a simple Y splitter after the buffered pedal into 2 amplifiers? Or do I need to use a dual buffered splitter like the JHS Buffered Splitter?

[pdf64]

It would be best to have a buffer feeding a direct output and one via an isolation transformers, see http://www.geofex.com/FX_images/splitter.gif
This will avoid a connection between the signal 0Vs of the 2 amps, which is what creates a ground loop (assuming that the amps are properly safety linked via mains ground).

[Leo_Gnardo]

I don't do this much anymore. In the old days I just parallel up the inputs and delt with ground loops with ground lifts and ignored any loading problems. I'm playing a high profile situation where I will be using multiple amps outdoors. I'm not switching between amps. The last pedal in my chain is a buffered pedal (Ibanez DE 7). Will that be enough to avoid the usual issues if I just use a simple Y splitter after the buffered pedal into 2 amplifiers? Or do I need to use a dual buffered splitter like the JHS Buffered Splitter?

Your last pedal being buffered, you'll have no problem losing bandwidth splitting into two amps. BUT you still have the ground loopage to contend with. pdf64 suggests the geofex diy, and that's good but a bit more complex than necessary. There's a commercial item, Radial Bigshot for $90, has no internal buffering just a single transformer & a couple switches. If you buy one, put some extra glue on the transformer as it likes to tear loose from its circus board. Or build yourself a Bigshot copy, choosing which switches you may or may not need. It's very handy to have a polarity flip switch if you use two amps simultaneously. The Mouser TL018 transformer suggested in the geofex schematic works fine; I've used it to replace Bigshot transformers that broke loose & left no wire to solder to.

[tedmich]

In theory this should work well but I haven't made mine work yet

[olddawg]

Your last pedal being buffered, you'll have no problem losing bandwidth splitting into two amps. BUT you still have the ground loopage to contend with. pdf64 suggests the geofex diy, and that's good but a bit more complex than necessary. There's a commercial item, Radial Bigshot for $90, has no internal buffering just a single transformer & a couple switches. If you buy one, put some extra glue on the transformer as it likes to tear loose from its circus board. Or build yourself a Bigshot copy, choosing which switches you may or may not need. It's very handy to have a polarity flip switch if you use two amps simultaneously. The Mouser TL018 transformer suggested in the geofex schematic works fine; I've used it to replace Bigshot transformers that broke loose & left no wire to solder to.

Most of these passive switcher/splitters have one direct out and one isolated out. Is there any signal loss with the isolation transformer and does it have to be treble compensated on that output? I'm very surprised that I can find a simple inexpensive passive splitter with isolation. Everything is an A/B/Y switch.

[Leo_Gnardo]

Most of these passive switcher/splitters have one direct out and one isolated out. Is there any signal loss with the isolation transformer and does it have to be treble compensated on that output? I'm very surprised that I can find a simple inexpensive passive splitter with isolation. Everything is an A/B/Y switch.

If your source was not buffered, you'd have to expect some level loss & frequency bandwidth loss. Even with buffering, if you measure carefully I'm sure you'll find the Bigshot or Mouser TL018 aren't perfect. OTOH these little Mouser tran's are downright cheap, and those I know who use the Bigshot are perfectly satisfied. If they had to tweak a volume or treble control some fraction of a turn to compensate, they did it & didn't complain. You can theorize until the cows come home, or you can buy yourself a transformer that costs less than a Big Mac & experiment, find out if your ears will be satisfied, then add switches as you need, leave out those you don't need.

There's no reason not to use 2 transformers if you're concerned that the direct & transformed signals will be different in level & eq. Still cheap.

One hazard with iso transformers: they will pick up hum from stray magnetic fields so place your split box somewhere it won't pick up hum. Away from wall warts & power transformers of all sorts. One of my crustomers bought a stereo line isolation box, then parked it right atop the power transformer in his Mesa 90+90W amp. Woooooops! Moving it to the other side of the rack, quiet as could be.

[olddawg]

Good point. Guess I'm ordering a couple of transformers.

[Dave H]

I used to use something like this.

[Chuck H]

Forgive me if I'm stepping on any safety toes, and I'm not out to uncomplicate a problem that needs solving if complicated is the goal, but...

If all the amps have the circuit ground at the chassis and there is a ground loop then there is also redundant chassis grounds. Why not just make an AC plug box with ground lift switches.

Please ignore if any of the amps aren't using the chassis for circuit ground.

EDIT: I know that there are arguments for not doing this because "What if a cable is kicked loose?" or "What if the grounded amp gets unplugged?" I'm the only guy in control of my gear when I'm on stage and I damn well know if either of those things happens. These aren't more than potential and momentary rare possibilities that need to be solved for before continuing anyhow. More convincing is that with AC ground lifting the chassis grounds for the AC ground lifted amps is actually using the instrument cable shields. I'm honestly not too worried about this either, but I can understand if someone else is.

[pdf64]

If all the amps have the circuit ground at the chassis and there is a ground loop then there is also redundant chassis grounds. Why not just make an AC plug box with ground lift switches.

Because if the amp without secure connection between it's chassis and mains ground develops a fault that sends mains current to its 0V / chassis, the path that current will take to ground is via instrument cable screens / phone jack connectors, which are unlikely to accommodate sustained fault current before fusing open circuit, which may happen more quickly than the mains fuse blowing.
That will allow various jack sleeves, amp chassis, fx pedal metalwork, possibility even the instrument (and so that player) to be pulled up the mains voltage, thereby presenting a fatal hazard.
Not just only to the player, but to anyone that they come into contact with whilst energised.
Safety systems have been developed, implemented and maintained to mitigate risk; there's a cost associated with that but it's a safe bet to assume that cost is lower than the cost to human life, however that might be extrapolated.
So it may be seen to be a reckless risk to human life to disable / bypass such safety systems, with criminal and civil liabilities probably falling on someone taking such action.

[Leo_Gnardo]

Forgive me if I'm stepping on any safety toes,

Well, safety, yes & pdf64 has sounded off on that to my satisfaction. I've also tried the "AC cable ground lift" solution, and it helps reduce hum but doesn't do as complete hum/buzz solution as transformer-isolated inputs.

FWIW I made a 5-way iso splitter for a customer in 1998, much along the lines of what Dave H shows in post #8, with mute switches and transformers following each output op amp. Nothing too special, I snitched the circuit from Bob Bradshaw, just used affordable Mouser transformers rather than rock star priced Dean Jensens. Worked a charm!

[Chuck H]

Well sure, there's THAT

I agree that maintaining a fixed ground for each system is the only guaranteed safe thing to do. But I also know that the lifting AC grounds when connecting multiple amps was common on big stages at a time when the players amplifiers were their only source of volume. And I just haven't heard of anyone being shocked or electrocuted by doing this. Other reasons, but not specifically this. I've been shocked because of inverse wired AC plugs though. Which is also a good enough reason to isolate the instrument.

I only mentioned it as a possibility because if I were up against a time limit and didn't have time to order something I would probably handle it this way. I know that's not the case here, so safety first.

[Dave H]

I agree that maintaining a fixed ground for each system is the only guaranteed safe thing to do. But I also know that the lifting AC grounds when connecting multiple amps was common on big stages at a time when the players amplifiers were their only source of volume.

As you make your own amplifiers you could incorporate something like this for ground lift. It's much safer to lift the circuit 0V from ground and have the chassis always grounded.

[Leo_Gnardo]

But I also know that the lifting AC grounds when connecting multiple amps was common on big stages at a time when the players amplifiers were their only source of volume. And I just haven't heard of anyone being shocked or electrocuted by doing this.

Now you have heard of it: I used to get shocked all the time. From the first jam sessions in hi school up to the first day I encountered the Bradshaw output switcher in 1992. That was the day I thought to myself "why hasn't anybody done this before?" Expense, mostly, with a box full of Jensen nickel/iron. All amps safety grounded & no more shocks, what a relief!

[Dave H]

Expense, mostly, with a box full of Jensen nickel/iron. All amps safety grounded & no more shocks, what a relief!

My circuit above in post #8 doesn't use any transformers. It's cheap, just a few op-amps and resistors.