Amp Block input select - Sum L+R vs Left. Is there a difference?

[pitmonkey]

When I was revisiting old presets a couple of nights ago I realised that some of them have the amp block input select set to Left and some have it set to Sum L+R. I know the sum is supposed to be the default and I never knowingly changed that setting so I'm not sure why some of my presets are one way and some are the other but hey...

When I switched it to Left only I swear the sound became slightly clearer and more focused, as if there was a tiny bit of phasing or a couple of miliseconds difference between the input signal that was being summed into the amp block. I know that it shouldn't make a difference, my signal path before the amp block is always mono and usually only has a compressor and a drive block, but somehow it seemed like it did. I'm also totally willing to accept that it's psychological on my part!

Has anybody else noticed this?

Just interested to see if it's actually all in my head/ears...

 

[Matteodem]

Interesting. Same question for cab block

 

[Rock Chalk]

I would think it all depends on what blocks you're running in front of these bocks...right?

 

[GlennO]

Yes, there is a 6dB difference.

 

[Matteodem]

Yes, there is a 6dB difference.

Do you mean that switching to L+R boosts the input of the amp of 6db?

 

[Bakerman]

If the left & right signals arriving at the amp block are identical, the input select setting doesn't matter.

 

[IronSean]

Yes, there is a 6dB difference.

I see in the wiki that if you are using L+R as a global input mode while only plugged into the L channel then your signal will be attenuated by 6 dB and you might pick up noise from the unused R input. But this applies to the global input. It also implies that the Sum L+R must compensate for the 6dB boost of summing the same signal together because it results in a 6dB loss when you sum one signal with silence.

 

[Boltneck Music]

I stay with left the entire time unless the block preceding is stereo. I normally use two cabs panned hard and left so I set the input for the cab block to Stereo. Never do I use sum LR. Hope this helps

 

[pitmonkey]

I would think it all depends on what blocks you're running in front of these bocks...right?

Yeah, I get that. If I was running a stereo effect ahead of the am block it would definitely make a difference. I'm not, though.

 

[pitmonkey]

If the left & right signals arriving at the amp block are identical, the input select setting doesn't matter.

That's what I thought. The perceived difference could totally just be me.

 

[pitmonkey]

I s'pose on the basis that there is no difference it doesn't matter if I use left only...

 

[IronSean]

If you want a second opinion you could reamp a DI twice, once with each setting, and then compare the final results.

 

[GlennO]

If the left & right signals arriving at the amp block are identical

That's the catch. For the important use case of digital input, for example when reamping a mono DI, they're not the same, and that's when you'll see a difference between "left only" and "sum l+r". I'm pretty sure the exact difference between the modes has changed over firmware versions, and that might explain the incorrect instruction in the manual about how to manually compensate for the difference.

 

[pitmonkey]

If you want a second opinion you could reamp a DI twice, once with each setting, and then compare the final results.

Yep, might give that a try.