4CM, multi-channel amps, channel volumes and S/R levels - can this be done scientifically?

PolarityMan

So Im debating trying 4CM again with my helix in the loop of my VH4. The first test I always do i get my main riffing tone setup so that the helix is in the signal path but has no effects engaged so I can get the system as transparent as possible.

I spent half an hour fiddling a while back and wasn't convinced I got there but I figure that there should be a scientific way to do this just by looking at the specs of the amp and helix.

So was wondering if anyone could follow my working and point out anything that doesn't make sense etc.

So to start with:

Goals
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1.) Main tone comes from the amp preamps
  1b) The ability to create special patches that bypass the amp preamp and go straight to the FX return is a nice to have 
2.) Main tone is not compromised by using 4CM 
3.) Relative patch volumes can be adjusted using the pre-amp volume controls (within some level of tolerance) ie/ if I balance against channel 1 at noon I prob don't need to accommodate channel 4 dimed. 
4.) The signal must never be clipped inside the helix 

Assumptions
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Although loops 2 is switchable it is parallel only, so although I could avoid the helix being in the loop when not required using loop 2, this would likely cause phase problems as in my experience there is usually cross talk even at 100% wet on the amps mix control. Therefore I will use loop 1 and have to live with a double AD/DA conversion even on patches where there is no processing in the loop.

From the VH4 specifications:
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The individual channel volume controls determine the signal strength at the send jacks.The range is - ... to +10dB. The output impedance is 4.7 kΩ

from the Helix Manual
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Relevent global settings:

Guitar In Pad: If your guitar or bass has active or really loud pickups, you may want to turn this on. There's really no rule; use what sounds best.

Send/Return: Choose "Instrument" when using a Send/Return pair as an FX loop for stompboxes; choose "Line" when using a Send/Return pair as an FX loop for line-level rack processors, or as additional inputs and outputs for connecting keyboards, drum machines, mixers, and other gear.

1/4" Outputs Choose "Instrument" when connecting the Helix 1/4" outputs to stompboxes or the front of guitar amps; choose "Line" when connecting to mixers, studio monitors, or standalone recorders. When using a single amp or speaker, connect only the LEFT/MONO 1/4” jack

Forum Knowledge
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https://line6.com/support/topic/16279-unity-gain-setting-signal-to-noise/
Helix would be the same. If the VOLUME knob is maxed and your Merge > Mixer and Output blocks are set to unity, the signal should be unity all the way through (assuming you've set your Global Settings > Ins/Outs > Levels appropriately). (from @Digital_Igloo )

Send/Return Block Settings
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Send: Adjusts the level sent to your external devices.
Dry Thru:  Adjusts the level of the signal passed through the Send block, independent of the Knob 1 (Send) level. Normally, this should be set to 0.0dB.
Return: Adjusts the level received at the Return jack.
Mix Blends the Return signal vs. the dry signal passed through the Return block. When set to 0%, the path bypasses the Return completely. When set to 100%, the entire signal is fed from the Return, and no dry thru signal is heard.

Question 1: Given the info above I think that the VH4 is a line level loop. Is this correct?

My assumption is that my starting point should be:

Amp Channel Volume: maxed
Amp channel Gain: maxed
Guitar in pad: off
Send/return: Instrument for send1
Send/Return: Line for return 2 (I think this might be the bit I missed before)
1/4" out: Line
Helix Volume: Full
Dry Thru: 0
Mix: 100%

I believe that should give me unity all the way through for the maximum input signal the unit will ever be presented with.

If this clips then I should then back off the amp channel volume until clipping no longer occurs and re-balance my other channel volumes.

Question 2: How can I detect clipping in this scenario (other than audibly). The helix has no meters.

one approach might be to sue the digital out to feed reaper and I can then look for digital overs at any point in the chain by moving the split feeding the digital out?

If I diagnose clipping then I find the first point in the chain it was introduced and dial back the volume. 

Question 3: I'm starting out with the loudest channel with gain maxed and volume maxed on the assumption that if I produce no clipping in this scenario then all scenarios with less volume will also be fine but if the helix processing path does not support the full dynamic range of the amplifier would I be better starting out more conservatively.

Question 4: How do I determine where "unity" is for channel volumes? Does such a concept even exist given the purpose of the preamp is to amplify the signal?

WiresDreamDisasters

Aren't you over complicating this a bit dude?

Take the master volume knob on the Helix out of the equation by assigning it to digital only.

Guitar > Helix Input
Send 1 > Front of amp (possibly through an Orchid Isolator box if need be)
Send of amp > Return 1 on Helix
Main output on Helix > Return of amp
Midi cable connecting the two.

In your patch, put FX Loop 1 somewhere. With zero effects in the chain and everything else set to 0dB, this should be the closest you can possibly get to unity gain - IE: The guitar at the input of the Helix should match the guitar at the send of the FX Loop 1, the amp then does it's thing, and you use the amps volume controls and not the Helix.

Basically....

 Therefore I will use loop 1 and have to live with a double AD/DA conversion even on patches where there is no processing in the loop.

This is the easiest and simplest thing to do. I wouldn't bother mucking around with the parallel loop on the amp. Just ignore it.

This is how I run my Helix with my amps, and the double AD/DA conversion is perfectly fine. 

The signal will not clip inside the Helix unless you're EXTREMELY loud on the channel volumes, but then your amp wouldn't be balanced at all. FWIW I run the channel volumes at noon usually and don't go any higher. You can run at 11 o'clock and not clip the Helix, it will be fine.

PolarityMan

That's the first thing I did and I lost some tone. This isnt a post about how to wire it up and get it working functionally so much as getting it more transparent.

When writing that post I think the thing I was missing was the send / return instrument versus line level. I suspect I was using instrument level to feed the FX return and probably returning the preamp signal at instrument level to the helix then trying to counter this with the send / return gains.

WiresDreamDisasters

PolarityMan said:

That's the first thing I did and I lost some tone. This isnt a post about how to wire it up and get it working functionally so much as getting it more transparent.

When writing that post I think the thing I was missing was the send / return instrument versus line level. I suspect I was using instrument level to feed the FX return and probably returning the preamp signal at instrument level to the helix then trying to counter this with the send / return gains.

Sorry if I missed it, what was the nature of the tone loss? Loss of high-end ?

PolarityMan

Felt like restricted dynamics and blanket over speaker kind of thing..didnt put an analyzer on it but potentially could have been high end loss. Ive had that before with 4CM where it just doesnt sound quite right. with the GT-10 it had metering at every block so was a bit easier to see what was going on and even then i never felt like i totally got rid of it.

WiresDreamDisasters

Hmmmm. Blanket over speaker kind of thing certainly sounds like high-end loss to me. Make sure you're not slamming the front of the amp with a line level signal - don't have FX Loop 1 set to "line" set it to "instrument". The main 1/4" outputs can be set to line I believe.

PolarityMan

Presumably you would need the return to be line as well, im fairly certain one of those settings would have been incorrect as I dont beleive i realised you could change them independently at the time. 

WiresDreamDisasters

Let me check mine here and see what settings seem to work best.

WiresDreamDisasters

Okay. So here it's:

Send/Return 1 = Instrument
Send/Return 2 = Instrument
Guitar In Hi-Z (input block) = 1M Ohm
Output block = Send 3/4

Cables:
Guitar > Helix Guitar Input
Helix Send 1 > Front of Amp
Amp Send 1 > Helix Return 1
Helix Send 3 > Amp Serial Return

Make sure you've got FX Loop 1 in the patch. No changes to the send and return values. Job done. Sounds 99.9% the same as plugging straight into the amp to me.

PolarityMan

I was definitely using the main out last time....what lead you to use send 3?

WiresDreamDisasters

PolarityMan said:

I was definitely using the main out last time....what lead you to use send 3?

Just allows me to keep the main outs connected to my monitors if I need to use it as a soundcard at the same time.

PolarityMan

ahh cool, hadnt thought of that use case..I started going through my patches to add a send that has an amp sim so I could have a backup if I eneded no amp so Ill prob switch to use the mains for that.

PolarityMan

WiresDreamDisasters said:

Okay. So here it's:

Send/Return 1 = Instrument
Send/Return 2 = Instrument
Guitar In Hi-Z (input block) = 1M Ohm
Output block = Send 3/4

Cables:
Guitar > Helix Guitar Input
Helix Send 1 > Front of Amp
Amp Send 1 > Helix Return 1
Helix Send 3 > Amp Serial Return

Make sure you've got FX Loop 1 in the patch. No changes to the send and return values. Job done. Sounds 99.9% the same as plugging straight into the amp to me.

So finally tried this out. I noticed that there was a huge volume boost running return 2 in instrument so I have that set to line. I did notice there is a definite loss of range on the channel volumes though, wondering if my first attempt i was just fooled by a volume difference. I also notice a lot of noise which is eliminated when I bypass send 1 and plug straight from guitar to amp leaving the rest of the helix in place...havent eliminted dodgy cable on that one though as got interrupted by kids