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How to use pro line level rack effects in guitar rig (properly)?

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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby Hugh Robjohns » Sun Oct 04, 2020 1:33 am

DC-Choppah wrote:Or perhaps a way to convert the +4 dB signal to guitar amp without using transformers?

The MX200 is quite happy feeding an unbalanced destination, and the user manual illustrates a setup Where the outputs are plugged straight into a guitar amp... So you don't need the transformer for unbalancing the signal.

You may not need the reamp's attenuation either, depending on how well the guitar amps' input level controls work.

The only potential need for the transformers would be for galvanic isolation to prevent ground loops.

So, you may find you don't actually need the reamper at all -- try connecting the MX200 outputs directly to the amps with standard guitar cables -- but be careful with the amp input level controla. If that works, great. If you get hums, reinstate the reamper. Personally, I'd locate it near the amps, running long balanced cables from the MX200 and shorter nbalanced cables to the amps... And keep it well away from mains transformers and guitar pickups.
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby DC-Choppah » Wed Oct 07, 2020 1:42 am

Amp is back from the tech, he says he had to replace: 12AX7 Preamp Tube
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby DC-Choppah » Mon Oct 12, 2020 5:40 am

Setting the rig back up, I find I must use the reamp box, the hiss is fixed, but the loud hum remains.

The repaired Fender amp now has a normal amount of low level hiss with nothing plugged in.

The two amps I use on the right side (Polytone and Hiwatt combo) sound normal, and have no hum when I plug straight in.


Now adding in the effects, the only remaining problem is hum.

If I use unbalanced cables and go out of the Lexicon MX200 rack effects unit, and go right into the amps, the Fender amp breaks up badly even with the amp set at 1. The Hiwatt breaks up to.

It seems that the pro line level signal is just not compatible with the instrument inputs on the tube amps. There is no output control on the MX200, so you get a hot signal coming out. Sounds like tube breakup, but very harsh and snappy/cracky.

So I put the reamp box in by using balanced cables out of the MX200, and unbalanced cables coming out of the reamp box.

The left side of the reamp box feeds the Fender Deluxe Reverb, and with the repair this sounds clean, no hum and no extra hiss.

But on the right side the reamp box feeds an ABY switch which feeds both amps (Polytone and HiWatt). These amps sound good, but there is a loud hum as soon as the connection is made. No hiss, just hum. Quite loud hum.

Changing the gain setting in front of the amp has little effect on the hum. The hum is only affected by the gain knob on the amps. And if I unplug the cable from the amps, the hum goes away completely.
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby CS70 » Mon Oct 12, 2020 8:28 am

DC-Choppah wrote:If I use unbalanced cables and go out of the Lexicon MX200 rack effects unit, and go right into the amps, the Fender amp breaks up badly even with the amp set at 1. The Hiwatt breaks up to.

It seems that the pro line level signal is just not compatible with the instrument inputs on the tube amps. There is no output control on the MX200, so you get a hot signal coming out. Sounds like tube breakup, but very harsh and snappy/cracky.

That is normal and expected I believe? The amp input is made to handle the voltage produced by a guitar (instrument level) and line level is much higher. You're overloading the amp input.. If you have another effect which does have an output control, you can patch it after the lexicon and use that. Basically if you want to use the rack in front of the amp, you last box must have an attenuator (volume knob) at the end of the chain.

So I put the reamp box in by using balanced cables out of the MX200, and unbalanced cables coming out of the reamp box.

The left side of the reamp box feeds the Fender Deluxe Reverb, and with the repair this sounds clean, no hum and no extra hiss.

But on the right side the reamp box feeds an ABY switch which feeds both amps (Polytone and HiWatt). These amps sound good, but there is a loud hum as soon as the connection is made. No hiss, just hum. Quite loud hum.

Maybe it's nothing to do with it, but it seems to me you're basically splitting the same voltage in three... each destination will see a third of what they expect, so the s/n ratio will suffer. These things work in a pinch (for example, if I have forgotten my headphone amp I have used a Y cable to split the headphones out when recording... but since it's only temporary, for monitoring while playing, the extra noise is not that important), but I wouldn't do for anything more permanent.
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby Hugh Robjohns » Mon Oct 12, 2020 11:11 am

DC-Choppah wrote:But on the right side the reamp box feeds an ABY switch which feeds both amps (Polytone and HiWatt). These amps sound good, but there is a loud hum as soon as the connection is made. No hiss, just hum. Quite loud hum.

Sounds like you're back to finding and solving ground-loops....
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby Hugh Robjohns » Mon Oct 12, 2020 11:18 am

CS70 wrote:Maybe it's nothing to do with it, but it seems to me you're basically splitting the same voltage in three... each destination will see a third of what they expect, so the s/n ratio will suffer.

It has nothing to do with it, ;) ...and each destination does not receive a third of the signal!

The whole point of the 'constant-voltage interface', where the source has a low output impedance and the destination has a high input impedance -- which is exactly what DC has here between the reverb out and the amp inputs -- is that each device connected in parallel across the source receives near-identical input voltages, and that input voltage will be almost the same as the source output voltage (usually within a fraction of a decibel). I won't bore you with the maths, but it's a simple extension of the maths needed for calculating potential dividers if you want to explore further.

In the typical line level case, with an output impedance of 100 Ohms and input impedances of 10k Ohms, three receivers wired in parallel across the output will see 0.25dB loss of level compared to a single device. And even ten devices in parallel would only see 0.8dB level loss. Increase the input impedances to 47K Ohms and those figures change to 0.05dB and 0.1dB! And to 680K (a common input Z for a guitar amp) and they become 0.004dB and 0.01dB! So definitely not losing a third of the level!

Yes, guitar amp inputs are not designed to welcome high-level (up to +24dBu) pro signal levels. Typically the first valve input stage will be overloaded -- which is what appears to be happening for DC. That's one reason why re-ampers were invented!

The loud hum will be because the ABY box is sharing signal grounds between all sorts of equipment and thus creating ground loops which the unbalanced amp inputs are only too happy to amplify noisily!
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby CS70 » Mon Oct 12, 2020 12:10 pm

Thanks, good to know!
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby DC-Choppah » Mon Oct 12, 2020 6:31 pm

Hugh Robjohns wrote:
DC-Choppah wrote:But on the right side the reamp box feeds an ABY switch which feeds both amps (Polytone and HiWatt). These amps sound good, but there is a loud hum as soon as the connection is made. No hiss, just hum. Quite loud hum.

Sounds like you're back to finding and solving ground-loops....

I started out with three problems:
Hiss
Ghosts
Hum


First two are solved. Thank you for the help! Now since I am into this, I have to figure out this hum before going back to 100% play music. Hum really does spoil the fun.
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby Hugh Robjohns » Mon Oct 12, 2020 6:50 pm

I suspect you're going to need to install a transformer isolator box in the feed between the ABY box and one (or both) amps.

As an experiment, you could try disconnecting each amp, one at a time, from the ABY box (which should still be connected to the reamp box and Lexicon) to see if the ground loop affects one or both amps.

If one (or both) amp(s) is (are) clean when connected alone to the ABY box, which is likely, you'll only need one transformer isolator, and it could probably be used in either channel equally well.

If you don't mind hacking guitar cables up, you may be able to avoid the transformer isolator by cutting and isolating the cable screen to the sleeve connection in the TS plug at the amp input end of one amp, since that would also break the suspected ground-loop.
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby Hewesy » Mon Oct 12, 2020 7:09 pm

For perhaps Hugh or more knowledgeable to confirm but if you come out of the stereo unit through something like this, you should sort the ground loop: https://www.thomann.de/gb/ebtech_by_mor ... ator_2.htm

One side to the Fender, the other to the ABY and two amps.

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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby DC-Choppah » Mon Oct 12, 2020 7:37 pm

I have one of those two channel isolators. The Dual Ramp Box now sends to the two channel isolator. The isolator then send one output to the Fender, the other output to the ABY switch (in Y mode) that then goes to the Polytone and Hiwatt.

This is now hum free, hiss free, and ghost free. I can set levels to adjust the gain stages to keep the noise low.

It is sounding great now!! I basically only hear the normal amp noise from my playing position. Great!


I am going to try and pack these two transformer gadgets (The DRB and Hum Eliminator) back into the rack, mount them to the interior walls of the rack, and try to forget they exist!

I hope the ghosts won't return.
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby Hugh Robjohns » Mon Oct 12, 2020 9:45 pm

DC-Choppah wrote:It is sounding great now!! I basically only hear the normal amp noise from my playing position. Great!

I love it when the science works out! :D :ugeek:

I am going to try and pack these two transformer gadgets (The DRB and Hum Eliminator) back into the rack, mount them to the interior walls of the rack, and try to forget they exist!

I hope the ghosts won't return.

Make sure you keep them away from each other AND away from any mains transformers in the other rack units!
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Re: How to use pro line level rack effects in guitar rig (properly)?

Postby Hewesy » Tue Oct 13, 2020 6:58 pm

Great work, nice result.

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