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Any issue with stereo outs from FX Pedal to two different DI boxes?

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Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby cldswm » Thu Jul 12, 2018 2:05 pm

Hey there,

I would like to send synths output to a preamp channel strip and the line outs to a re-amp box to bring the signal down to instrument level, then into some fx pedals. The fx pedals stereo outputs will go into two DI boxes then into my recording interface.

Before this I was sending synth -> preamp -> fx pedals -> audio interface. The problem was this introduced lots of hums and hiss that degraded the audio signal.

I was troubleshooting to see where the noise is coming from and it came down to the line level signal output from the preamp into the fx pedals. I was able to bring down the line level into instrument levels into the fx pedals then out to a JDI DI box which I have already. I don't know why but in theory the output from the fx pedals should be okay to go directly into my audio interface, but adding the JDI DI box before the audio interface eliminated the 60's cycle hum.

Since many pedals have stereo outputs I would like add another JDI DI box for the stereo outputs.

My question is

1. Am I making this too complicated trying to correct the noise issue? Is there a better way.

2. If I use 2 seperate DI boxes to receive signal from the stereo out of a fx pedal would I have problems with phasing issues that I should worry about? As these 2 units might be slightly different.

Many Thanks!
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby James Perrett » Thu Jul 12, 2018 3:25 pm

Personally I would take the channel strip out of the system, feed the synth straight into the effects pedals and then feed the output of the effects pedals into the line inputs of your audio interface. Anything involving foot pedals is going to be less than optimum when it comes to sound quality so I wouldn't worry too much about using DI and Reamp boxes to match levels (unless your interface doesn't have line inputs for some reason).

It would help if you could include the make and model number of the channel strip and audio interface as there may be other options.
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby cldswm » Thu Jul 12, 2018 4:36 pm

James Perrett wrote:Personally I would take the channel strip out of the system, feed the synth straight into the effects pedals and then feed the output of the effects pedals into the line inputs of your audio interface. Anything involving foot pedals is going to be less than optimum when it comes to sound quality so I wouldn't worry too much about using DI and Reamp boxes to match levels (unless your interface doesn't have line inputs for some reason).

It would help if you could include the make and model number of the channel strip and audio interface as there may be other options.

I’m using the channel strip to add color. Is there any reason you think I should take it out of the system? As mentioned above the DI and revamp box helps tame the noises.
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby Hugh Robjohns » Thu Jul 12, 2018 6:04 pm

cldswm wrote:I would like to send synths output to a preamp channel strip and the line outs to a re-amp box to bring the signal down to instrument level, then into some fx pedals. The fx pedals stereo outputs will go into two DI boxes then into my recording interface.

Messy... but doable.

It might make more sense to drive the FX pedals directly from the synth outputs, and then feed the DX pedals into the preamps (if needed) and on to your recording rig.

Before this I was sending synth -> preamp -> fx pedals -> audio interface. The problem was this introduced lots of hums and hiss that degraded the audio signal.

I suspect a typical ground loop problem created the hums. The hisses would be down to the complicated gain structure.

...adding the JDI DI box before the audio interface eliminated the 60's cycle hum.

Yep. The transformers in the DI and re-amp boxes are designed to break ground loops...

Am I making this too complicated trying to correct the noise issue? Is there a better way.

The whole thing is complicated! I'd suggest ditching the preamp, and connect synth into FX pedals into DI box into recording interface. But if you really want the channel strip for colour, and want that colour before the FX pedals, then you have to do it the way you are: synth > preamp > re-amp > FX > DI > interface.

If I use 2 seperate DI boxes to receive signal from the stereo out of a fx pedal would I have problems with phasing issues that I should worry about? As these 2 units might be slightly different.

A stereo DI box would be best (I generally use a Radial ProD2 for my stereo keyboards), or two identical mono units would be next favourite. You can use different mono boxes, but they will have slightly different characteristics in terms of frequency and phase responses, and may also have different gains. If you're really unlucky, they might even have opposite polarities! In all probability you'll get away with it without noticing anything untoward, but it's not ideal!

H
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby James Perrett » Thu Jul 12, 2018 6:35 pm

If you really want to use the preamp I'd put it after the pedals and feed its output into the line input of your interface. A good mixing desk can handle a wide range of input levels so I'd expect anything calling itself a channel strip to be able to do the same so there would be no need for a DI box (unless you need one to remove a ground loop).
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby cldswm » Fri Jul 13, 2018 12:25 am

Hey Thanks for everyone's replies, really appreciate it.

The preamp I'm using is the Shelford channel. I agree it would be more ideal to feed the fx into the preamp.

BUT,

1. I couldn't afford another one for stereo
2. In my situation I feel I get better sound by going through the preamp first with some light eq'ing & comp + some distortion then into the fx pedals. (I've tried feeding fx into the preamp and by the time the signal reaches the preamp it will drive the signal too hard to sound pleasing. To my ears anyways).

NOW,

I think my problem isn't really to match the impedance, but to break the hum. Are there any other ways fix this?

The instrument section and the pedals are drawing power from one socket, my interface, rack gear & speaker are on another, they are aligned on the same wall so I suspect they are drawing power from the same line. The computer is drawing power from an opposite wall.

I've also connected everything to two Samson S-Patch plus. I think these are introducing hisses but bare-able ard upper 8k +.
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby ef37a » Fri Jul 13, 2018 6:36 am

http://orchid-electronics.co.uk/dual_isolator.htm#

I would suggest one of the above to earth isolate the synth outputs? After that I think you could hook up anyway you liked.

"Impedance" is almost never an issue with line'ish level signal exchanges. Almost always level/gain staging. You might find the use of simple 10k Ohm log pots in tins allows proper levels to be obtained (I think you see that the synth might be too hot for pedals which generally clip at or under +10dBu.)

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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby Hugh Robjohns » Fri Jul 13, 2018 8:55 am

cldswm wrote:1. I couldn't afford another one for stereo
2. In my situation I feel I get better sound by going through the preamp first with some light eq'ing & comp + some distortion then into the fx pedals.

Fair enough -- I was expecting you to say something along those lines.

I think my problem isn't really to match the impedance, but to break the hum.

With a (very) few notable exceptions, we don't worry about 'impedance matching' in the world of analogue audio. The rule is very simple: the destination's input impedance must be at least 5 (and ideally 10) times greater than the source's output impedance. And with electronic devices (synths, preamps, FX pedals etc) you'd be hard pressed to find anything with an output impedance more than a few hundred Ohms (1K at the most), and all line inputs are at least 10k Ohms, and most rather higher. Most mic pres are 2K or more, too.

So, forget 'impedance matching' in this situation completely. All you need to worry about are balanced/unbalanced, signal levels, and especially those ground loops...

Are there any other ways fix this?

Audio isolation transformers are the easiest and most versatile solution, but you need to be methodical in connecting your system together, identifying exactly where the ground loops are, and not making the system worse in the 'process' of fixing it! I often find that drawing out a schematic of the whole system is the easiest way to figure it all out -- taking special note of which devices are class1 (grounded) and which are class 2 (double-insulated or ground-free).

The instrument section and the pedals are drawing power from one socket, my interface, rack gear & speaker are on another, they are aligned on the same wall so I suspect they are drawing power from the same line. The computer is drawing power from an opposite wall.

Never assume that sockets on the same wall are closely connected, and it doesn't take much cable to generate a hum voltage if there's a lot of leakage current in the system...

Ideally, your first step would be to re-arrange the mains wiring so everything is fed in a star-fashion from a single double-wall outlet. That step alone might well resolve much of the problem.

If you can't do that, then you are going to have to get busy with the isolating transformers... and making sure that you don't have circumventing ground loops via the metalwork of rack frames or keyboard stands etc!

I've also connected everything to two Samson S-Patch plus. I think these are introducing hisses but bare-able ard upper 8k +.

A patchbay is a passive thing. It can't generate noise... but it can very easily create additional ground loops.

H
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby ef37a » Fri Jul 13, 2018 10:08 am

"A patchbay is a passive thing. It can't generate noise... but it can very easily create additional ground loops.

H"

And being 'passive' it cannot of itself CREATE earth loops..But I know what you mean Hugh! A patch bay, by definition gets all sorts plugged into it and linked up so you have to be very methodical as to what is connected.

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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby Hugh Robjohns » Fri Jul 13, 2018 11:28 am

Anything that joins the grounds of separate devices can create a ground loop, and since many patchbays tie all the socket screen connections together they can sometimes create multiple ground loops between the equipment plugged into the patchbay. With balanced connections this isn't normally a problem because the ground plays no part in the signal referencing, but with unbalanced sources or destinations, where the ground does provide the signal reference, any ground loop currents flowing around the patchbay will result in audible noise...
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby James Perrett » Fri Jul 13, 2018 1:09 pm

cldswm wrote:The preamp I'm using is the Shelford channel. I agree it would be more ideal to feed the fx into the preamp.

I've just had a look at the manual for the preamp and see that it is already transformer coupled on both input and output. So there should be no need to add any more transformers into your signal chain - simply wire in the existing ones in the channel strip correctly.

This means using mono jack to XLR leads that use XLR pin 2 as the signal and pin 3 as ground while leaving pin 1 disconnected. As Hugh says, keeping everything on the same mains socket will also help.
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby cldswm » Mon Jul 16, 2018 3:27 am

Hugh Robjohns wrote:
cldswm wrote:1. I couldn't afford another one for stereo
2. In my situation I feel I get better sound by going through the preamp first with some light eq'ing & comp + some distortion then into the fx pedals.

Fair enough -- I was expecting you to say something along those lines.

I think my problem isn't really to match the impedance, but to break the hum.

With a (very) few notable exceptions, we don't worry about 'impedance matching' in the world of analogue audio. The rule is very simple: the destination's input impedance must be at least 5 (and ideally 10) times greater than the source's output impedance. And with electronic devices (synths, preamps, FX pedals etc) you'd be hard pressed to find anything with an output impedance more than a few hundred Ohms (1K at the most), and all line inputs are at least 10k Ohms, and most rather higher. Most mic pres are 2K or more, too.

So, forget 'impedance matching' in this situation completely. All you need to worry about are balanced/unbalanced, signal levels, and especially those ground loops...

Are there any other ways fix this?

Audio isolation transformers are the easiest and most versatile solution, but you need to be methodical in connecting your system together, identifying exactly where the ground loops are, and not making the system worse in the 'process' of fixing it! I often find that drawing out a schematic of the whole system is the easiest way to figure it all out -- taking special note of which devices are class1 (grounded) and which are class 2 (double-insulated or ground-free).

The instrument section and the pedals are drawing power from one socket, my interface, rack gear & speaker are on another, they are aligned on the same wall so I suspect they are drawing power from the same line. The computer is drawing power from an opposite wall.

Never assume that sockets on the same wall are closely connected, and it doesn't take much cable to generate a hum voltage if there's a lot of leakage current in the system...

Ideally, your first step would be to re-arrange the mains wiring so everything is fed in a star-fashion from a single double-wall outlet. That step alone might well resolve much of the problem.

If you can't do that, then you are going to have to get busy with the isolating transformers... and making sure that you don't have circumventing ground loops via the metalwork of rack frames or keyboard stands etc!

I've also connected everything to two Samson S-Patch plus. I think these are introducing hisses but bare-able ard upper 8k +.

A patchbay is a passive thing. It can't generate noise... but it can very easily create additional ground loops.

H

Hey thanks again for everyone who took the time for the INPUT.

So I've been trying to isolate the 60hz hum issue. First thing I did was to connect everything to the same power outlet as suggested. I think it did improve the problem but did not solve the issue completely. I find that I'm still getting noises if the setup were interconnected via the patchbay. I'm assuming there is an issue with connecting unbalanced output to balanced
input, thus when unbalanced synth output goes into the balanced patchbay hum noises arises. I was able to eliminate the hum to a minimum when the patchbays were not involved. With the Shelford DI input Ground Lift engaged. The LINE output of the Shelford is then fed into the JDI in reverse.

In my setup I have all my unbalanced synth output connected to the back of the Samson S-Patch plus using TRS cables. The preamp i/o s are connected to the back on the same patchbay, along with the inputs of the audio interface. On a second patchbay I have all my fx pedals connected to the back.

This is typically what I do when I'm recording : synth -> DI Input of preamp -> JDI to various fx pedals -> audio interface. This is all connected via the patchbay for flexibility. The idea is to be able to interconnect different fx pedals on the fly in different orders. This may seem unprofessional to recording engineers and is probably not the right way of doing things. But to me its more inspirational making music this way.

Anyways, I'm wondering if this is where the hum is coming from, connecting unbalanced gear to the balanced patchbay then out to balanced preamp then back out to unbalanced fx pedals. On the back I'm using TRS cables for the synth / preamp & audio interface. With the fx I'm using TS cables.

I have been doing some research regarding this issue and found these articles.

http://www.rane.com/note110.html
https://www.presonus.com/learn/technica ... Unbalanced

Should I try and modify my cables?
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby cldswm » Mon Jul 16, 2018 6:47 am

So I've been trying to isolate a 60hz hum issue in my system. First thing I did was to connect everything to the same power outlet. I think it did improve the problem but did not solve the issue completely.

This is typically what I do when I'm recording : synth (unbalanced) -> DI Input of preamp channel strip -> JDI to various fx pedals -> audio interface. These are all connected via two Samson S-Patch patchbays (balanced) for flexibility. The idea is to be able to interconnect different fx pedals (unbalanced) on the fly in different orders.

In my setup I have all my unbalanced synth output connected to the rear of the patchy bay using TRS cables. The preamp i/o s are connected to the rear on the same patchbay, along with the inputs of the audio interface using TRS cables. On a second patchbay I have all my fx pedals connected to the back using TS cables. I patch these together in the front with TS cables.

I find that I'm still getting noises if the setup were interconnected via the patchbay. I'm assuming there is an issue with connecting unbalanced sorces to balanced input, thus in my situation, when unbalanced synth output goes into the balanced patchbay hum noises arises. With the preamp channel strip DI input Ground Lift engaged & the LINE output of the channel strip is then fed into a Radial JDI in reverse, I was able to eliminate the hum to a minimum when the patchbays were not involved.

Anyways, I'm wondering if this is where the hum is coming from? mixing unbalanced and balanced sources.

I have been doing some research regarding this issue and found these articles.

Sound]404 Not Found System Interconnection
Balanced and Unbalanced Connections | PreSonus

Should I try and modify my cables? Pls point out where I might be doing something wrong where balanced or unbalanced cables should be used instead.

Many Thanks!
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby cldswm » Mon Jul 16, 2018 6:52 am

https://www.soundonsound.com/sound-advi ... g-solution

And this one by yourself :headbang:

Any tips on how would I go about modifying my existing cables?

Thanks!
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Re: Any issue with stereo outs from FX Pedal to two different DI boxes?

Postby ef37a » Mon Jul 16, 2018 8:12 am

Whenever you try to mix balanced and unbalanced inputs and outputs you are likely to fall foul of hum issues. This is especially the case with a patch bay setup because you might get one routing layout hum free but patch in another device and you get hum again.

The best solution is to have ALL ins and outs balanced and of course this is the way 'pro' studio do things but this can be expensive or impractical. There are however measures you can take.

1) Balance EVERY output. Best done with transformers, 1:1 "600 Ohm" output transformers* but yes, expensive. The other option is to "impedance" balance unbalanced outs. You have done considerable research so the technique should be familiar?
You can of course incorporate the required resistors inside the TRS plugs but this is fiddly and leaves you with non-standard cables. A better approach IMHO it to build "breakout boxes" which contain the jacks and resistors and possibly earth lift switches and the RF 'keeper' caps. Some pieces of equipment can have Z balanced jacks fitted to them, e.g. I have 2 cassette machines and a Minidisc recorder so equipped.
2) Provide earth lifts, either by special cables (mark them up!) or as in 1^.

*Inputs are not so easily balanced. You need rather expensive "10k-10k line bridging" transformers or electronic balancing front ends. Mind you, if you did go for some active electronics you could also use that to "normalize" your operating levels? Something else the pros do!

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