Parallel Preamp Selector Using MEGX + GFI Duophony

[scratch17]

I have a lot of tube preamps that I want to use with a MEGX. I want to confirm that this will work before I try it out. My idea is to route two mono series loop paths in parallel and use a GFI Duophony to mix one preamp from each path. I will use my Radial JDV MK3 to split my guitar's output. Each side of the split will feed each of the parallel loop paths.

I plug one output of the JDV MK3 into the input of the MEGX. Then I convert all of the loops to split mono, providing me with 12 mono loops. Now I populate loops 1, 7, 2, 8, 3, and 9 with one of my preamps. I now have 6 preamps that are in series. But I want two series paths in parallel. So I sarifice loops 4 to create a parallel routing path.

To do that, I physically connect the loop 4 send to the left input of the Duophony. I still use a TRS to 2 x TS cable. I set up the loop order so that loop 4 is after loops 1, 2, 3, 7, 8, and 9.

To route the second series path, I plug the second guitar output from the JDV MK3 into the return of loop 4. Now I populate loops 5, 6, 10, 11, and 12 with five other preamps.

Finally, I connect a TS cable from the MEGX output to the right input of the Duophony. Duophony will allow me to mix one preamp from the six preamp path with one from the 5 preamp path. The balance between the two paths can be saved as a Duophony preset.

BTW, I want to credit Mark Johnson of the Secret Weapons YouTube channel. He demonstrated mixing a UA Dream and a UA Ruby in parallel using a Duophony.

But there's more...

Since I can rearrange loop order on the MEGX, I can move preamps from one path to the other. Why would I want to do that? Well, I might want to mix two of the preamps that are physically connected to loops in the same path. In that case, I'd move one of the preamps to the other path. Note that I can also move loop 4 to a different position in order to change the break point between the two paths.

And then there's the possibility of using drive pedals in series with a preamp. I will also try gain staging two preamps in series to see what kinds of tones I can get.

So, @rjmmusic. Is this theoretically a sound routing structure? Do you see any issues I might have to overcome?

 

[Mario]

Running preamps in the loops of audio switchers has some potential issues. Oscillation (internal feedback) is a serious concern especially with high(er) gain preamps. Ground loop hum and noise is the other issue, which would require a significant investment in audio isolation transformers.

To minimize or prevent oscillations, you may need to use two MEGXs. One to serves as a switchable signal splitter with its SENDs feeding the input of the preamps. The second MEGX would have the outputs of the preamps feeding the RETURNs.

There will still be the potential of ground loop hum and noise, but generally just isolating either the inputs of the preamps has worked.

 

[scratch17]

Thanks for the reply Mario. This was the info I was looking for.

As far as ground loop prevention, I have an 8 channel Ebtech Hum Eliminator. I think that should work. Of course it adds the cost of 16 added mono cables to the bill. But I have plenty, so that won't be an issue.

WRT oscillations, I have some questions.

1. You infer that moderate and lower gain preamps might not be at risk for this issue. Am I correct about this?

2. In most cases I only plan on having one preamp on at any time in the two parallel paths. Both will stay separately routed until the input of the Duophony where the two signals will be mixed. Until mixing, neither preamp's signal will be connected to the other. So where does the feedback get started? What could cause oscillation? Are you thinking that oscillation could begin by internal leakage between the input and output stages in the MEGX?

3. Here is a list of my tube preamps. Some are in heads or combos with effects loops so I can access the preamps and power amps individually. Which preamp(s) would you consider to be high enough gain to be a potential problem?

'68 Fender AB-768. This is in my Silver Face Bandmaster. I had the amp modified by adding an effects loop. The loop has true bypass. Note also that input and output level can be set individually.

Tone King Imperial II preamp.

Tone King Royalist preamp.

Mesa Boogie Rectifier Recording preamp.

Synergy Syn20-IR preamp. Modules will be Marshall JCM 800, Dr. Z Z Wreck, Synergy IICP+

Humble Amps Bumble Bee.

Marshall DSL40-CR (Sweetwater mod version).

4. Say I use two switchers. Do they both have to be MEGX? I want the MEGX to be my preamp selector. But could I use my Voodoo Lab GCX as the output switcher? I don't need to be able to reorder outputs because I'll only ever use two dirrectly connected to the Duophony's inputs.

5. You say that "generally just isolating either the inputs of the preamps has worked." Did you mean isolating the inputs from the outputs?

6. If I used two MEGXs to isolate the inputs from the outputs of the preamps, how would I have parallel preamps, which is what I am trying to accomplish. Unless I use loop 4 in the manner I suggest above to break apart a 6 preamp path from a 5 preamp path in the two loopers.

If I am incorrect about this, could you please do a routing diagram of how the two loopers would be connected to provide parallel inputs and outputs?

7. Would gain staging a pedal in front of a mild preamp like my Fender or TKIP potentially cause oscillation? I was thinking of adding some mild gain stages from my pedals in front of the preamp I select in a chain. Of course, I am aware that doing so can always create feedback if driven hard enough. I would not normally intentionally do that; but I might want to end a phrase with a few seconds of feedback. Could doing so electrically harm any of the devices, or is it just my ears I'm putting at risk?

 

[Mario]

Thanks for the reply Mario. This was the info I was looking for.

You're welcome!

As far as ground loop prevention, I have an 8 channel Ebtech Hum Eliminator. I think that should work. Of course it adds the cost of 16 added mono cables to the bill. But I have plenty, so that won't be an issue.

Cool!

WRT oscillations, I have some questions.

1. You infer that moderate and lower gain preamps might not be at risk for this issue. Am I correct about this?

Generally yes.

2. In most cases I only plan on having one preamp on at any time in the two parallel paths. Both will stay separately routed until the input of the Duophony where the two signals will be mixed. Until mixing, neither preamp's signal will be connected to the other. So where does the feedback get started? What could cause oscillation? Are you thinking that oscillation could begin by internal leakage between the input and output stages in the MEGX?

The oscillation is not limited to just the MEGX, it can also occur in any loop switcher, whether ours or other brands. And regardless of the switching element used in a loop switcher, whether it's a relay or matrix IC. The oscillation potential occurs when you connect both the input and output of a preamp to the send and returns of a loop, and those signals get routed to the switching element, where they can get into close proximity to one another.

3. Here is a list of my tube preamps. Some are in heads or combos with effects loops so I can access the preamps and power amps individually. Which preamp(s) would you consider to be high enough gain to be a potential problem?

'68 Fender AB-768. This is in my Silver Face Bandmaster. I had the amp modified by adding an effects loop. The loop has true bypass. Note also that input and output level can be set individually.

Tone King Imperial II preamp.

Tone King Royalist preamp.

Mesa Boogie Rectifier Recording preamp.

Synergy Syn20-IR preamp. Modules will be Marshall JCM 800, Dr. Z Z Wreck, Synergy IICP+

Humble Amps Bumble Bee.

Marshall DSL40-CR (Sweetwater mod version).

Impossible to say. It depends on a variety of factors, signal impedances, gain, signal levels.

4. Say I use two switchers. Do they both have to be MEGX? I want the MEGX to be my preamp selector. But could I use my Voodoo Lab GCX as the output switcher? I don't need to be able to reorder outputs because I'll only ever use two dirrectly connected to the Duophony's inputs.

They do not need to be the same.

5. You say that "generally just isolating either the inputs of the preamps has worked." Did you mean isolating the inputs from the outputs?

I meant using an audio isolation transformer between the SEND of a loop switcher and the input of a preamp. Generally there should be only one earth ground connection for a rig. Any other earth ground paths (connections) could result in ground loop hum and noise.

6. If I used two MEGXs to isolate the inputs from the outputs of the preamps, how would I have parallel preamps, which is what I am trying to accomplish. Unless I use loop 4 in the manner I suggest above to break apart a 6 preamp path from a 5 preamp path in the two loopers. If I am incorrect about this, could you please do a routing diagram of how the two loopers would be connected to provide parallel inputs and outputs?

Because of the potential complexity and issues involved here, I am moving in baby steps and haven't even started to consider parallel routing.

7. Would gain staging a pedal in front of a mild preamp like my Fender or TKIP potentially cause oscillation? I was thinking of adding some mild gain stages from my pedals in front of the preamp I select in a chain. Of course, I am aware that doing so can always create feedback if driven hard enough. I would not normally intentionally do that; but I might want to end a phrase with a few seconds of feedback. Could doing so electrically harm any of the devices, or is it just my ears I'm putting at risk?

Pushing a preamp with a pedal could potentially result in oscillation, but that's if we're still talking about just using one switcher with the preamps connected to the send and return of each loop. Using two switchers shouldn't result in oscillation. Traditional feedback is cool, hard to believe a generation of players will never know what that's like, etc. There are a lot of Mesa Boogie's that continue to function after oscillation, lol. I used to work for M/B so I am allowed to make that joke, lol.

 

[Mario]

WRT the parallel path, off the top of my head, it would be best to use the MEGX and connect it's loop RETURN jacks to the outputs of the preamps. The OUTPUT on the MEGX will be used for one (output) path. You will need to use the SEND of one other MEGX loop as your second (output) path, this loop would always be on. For the two preamps you want to be feeding the two paths, you will need to turn on their loops in the MEGX, and you will rearrange the loop order so one loop is before the MEGX OUTPUT and the other loop is before the loop that you're using the SEND of as your other (output) path.

 

[scratch17]

Thank you so much Mario for your detailed answers. I think I am starting to get a handle on the potential for oscillation.

You said above that, "oscillation potential occurs when you connect both the input and output of a preamp to the send and returns of a loop, and those signals get routed to the switching element, where they can get into close proximity to one another."

That's why you are suggesting two loop switchers; one for preamp inputs and one for preamp outputs. In effect, you are saying that I need to keep the input signal route isolated from the output signal route to stop the two signals from leaking into one another across the switching element. It also occurs to me that even if there isn't enough gain to cause oscillation, it doesn't mean that there is no leakage across the loop's switch at all and that could potentially degrade sound quality.

So I think I have to use two loop switchers.

 

[Mario]

You're welcome!

 

[scratch17]

Actually, I think I should do some testing first. I already own a GCX loop switcher. Regardless of whether I set up stereo preamp paths, I will be getting a MEGX.

I could set up a multi stage test with each preamp. I would start with a preamp's I/O just connected to one loop to see if there was an oscillation issue. If so I could set up a test with that preamp using two loops on the MEGX. One loop would be for input; the other for output. Assuming that eliminated the oscillation issue, I could test my other preamps. Some might require isolation (two loops); others might only require one loop.

Worst case, with one MEGX I can get 12 mono loops. So I can isolate six preamps' inputs and outputs. I'd just not get the parallel processing I'd like. But I'd have a zero issue way to select six preamps on the fly in my guitar rig.

 

[Mario]

Different switchers will yield different results in regards to oscillation. Testing with the GCX will only tell you how things will work using the GCX.