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Helix alternate tube preamps switching 4CM, Lehle, Hum reduction


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Just got the Helix rack and control board one week ago, so far the unit is marvelous in terms of usability, flexibility, and sound preservation.




Yes, a cannot emphasize this enough how important the preservation of an original sound for me is.


I use 2 amps, Mesa Boogie MKV and a Marshall Jubilee 2555, both wired with the 4 Cable Method.




So far the Helix maintains the sonic integrity of the MKV and Marshall.


When using just one amp, hum is no issue ether with either MKV or Marshall 2555.






When cabling the preamps into effects Loop 1 MKV and effects Loop2 for the M 2555, I can alternate between both preamps. In this case the hum involved (ground loop - DC coupling) makes the whole setup unusable.






When cabling  the preamps  into effects Loop 1/2 (Setup as Stereo) for MKV and the M 2555, I can alternate between both preamps using a Lehle SGoes Amps switcher (the red one) which uses a High Z transformer for  avoiding DC coupling.




In this case the hum involved (DC coupling -ground loop) is reduced significantly




Here is the Routing:




I use the send from Helix effects loop 1 wired into the Lehle Input.




The 2 Lehle Outputs are wired to


     MKV Input, and Marshall Input, 


     the MKV effects send is wired to Helix effects loop 1 return,


     the Marshall 2555 effects send is wired to Helix effects loop 2 return 




In both A and B, the Helix Outputs are wired to the effect Returns of MKV and 2555




The sound preservation is amazing, but the hum remaining still needs to be addressed.




Any ideas how to get even lower hum levels with this setup?




Thanks for any helpful advise  Regards


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Thx for info. 


Yes, of course  i tried the ground lift.

Expectedly there was no change, as the ground lift  option is tied to working with the XLR outputs (balanced).

The 1/4 TS connectors are not affected by this.


I could think of working with the XLR, but in this case I would have to modify the effect  returns of the MB MKV and Marshall to balanced operations, via transformer or electronic  balancing. The chance of sonic signal degradation is existent due to further signal processing-transformation.


  • Some progresss I achieved, by maintaining max. audio levels across the signal chain, in order to optimize the signal to noise ratio.
  • 2 more approaches I have in the pipeline, 
    • a. using a Lehle p-split device for the preamps out of MKV and 2555 to further improve the dc-coupling  reduction.
    • b. transform the Effect returns of MKV and 2555 to XLR Operation in order to use the ground lift option

As soon as I got the P Split I'll post again.



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Thx for the  info, as we work with 240 V AC  in Germany, its not exactly suitable. Considering the user responses , the chance it might not work is pretty high either.


So far I found a simple, but in terms of safety to be considered, solution.


I removed the Lehle 2 SGOes  switcher  and rerouted the Helix for two separate, alternately activated  mono  effect loops ( 1 MB, 2 Marshall).


Now I taped off the  ground (earth) contact of the AC plug of the Marshall.


As the Marshall is still connected to earth via the  Audio cables (Input, send and return) to  the Helix (earthed) there is a remaining  earth connection for the Marshall.


The result is just spectacular, no hum at all, and no sound Degradation by additional equipment.   


I keep working on the hum issue, maybe I  find one  that maintains sonic integrity/hum elimination + safetey ensurance alltogether.

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Maybe You need to isolate one of the preamps completely which requires 2 iso boxes.

E.g. signal flow ... helix send. Iso.. Preamp1. Iso.. Helix return.

Not sure exactly for your set up but u will need more than one isolation box.


The best solution i know is made by John at orchid electronics uk and sold at a non brand name price... they are the highest quality and audio quality and cheaper than band name solutions.

Email him tell him your exact set up, what u need to be able to achieve and he will tell what you need.

He can custom make units for your needs.


Soundonsound website have two glowing reviews of his products... and I can confirm they are outstanding solutions..

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I would be scared of lifting the ground on the Marshall. Considering the 400 to 500 volts inside. Imagine a short and sending that back to you through your system.


Leave the Marshall grounded try to HumX the Helix. Or like above, isobox etc.


Edit: never used a HumX, maybe try one or two on the Marshalls?

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P.s. I gave the wrong link... the dual is for a stereo set up like I used...

Tho I no longer use that set up as too big to move around 2 stereo 2x12 guitar combos and a helix.

I now use a Yamaha thr100hd which has dual amp set up and a stereo power amp and stereo send return into the 2x12 thrc mismatched speaker cab and no ground issues with the helix as the two signals are all within the same electrical path. Small portable set up... sounds incredible with the mismatched speakers and options to choose the power amp tube and class of operation for each side of the power amp.

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Thx for all the ideas,


1. concerning the earth  Isolation:


 Yes, I certainly do NOT recommend this solution to anyone else.


 As I myself work with tube amp for quite a long time now (repair and mod), I can  classify the risks involved.

The earth Connection via the connected  equipment remains, and besides the DC  B+ voltage form Tube amps,  the 240 VAC from any power supply remains enough to fry anything, unless you have  fast fuse protection - and or Fi guards. 


2. concerning the hum isolatores , yes  I Isolated  one preamp completely, the reduction in hum was evident but certainly not 0.

In terms of sound quality I can only speak in the highest regards from Lehle.  


So far, they are the only ones where Idid not perceive any sound degradation.  Even the famous AMP switcher by Mesa Boogie with top notch  

Jensen transformers  did not live up to the Lehle.


So far I consider my issue solved (for the moment). Stiil I'll pursue the quest for solving the taped earth issue.   



Thx again.

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I just received an ebtech hum eliminator yesterday. It worked wonders.


Got a PRS Custom 50 last week and was not going to be able to take it out live. This thing worked wonders.


The rest is RJM Mini Amp Gizmo for channel switching and everything is through an old power conditioner. I know not doing much, but everything in room is on same circuit.


That hum eliminator plopped into the effects loop really worked wonders, I was blown away.

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