Question about output stereo/mono output on Hughes & Kettner Rotosphere

[Sleipnir]

Howdy,

I read a few posts online about people who have pedal boards set up to run two amps in stereo but who also want to easily be able to use the same pedal board for one amp gigs. Some wanted to buy/make a pedal/box to use at the end of the pedal chain that would allow you to basically leave all pedals plugged in for stereo operation but then either connect one or two amps to this pedal/box where in the mono setting, the stereo signal from the pedals got summed to mono. One of the issues brought up was that summing to mono could cause phase cancellation issues depending on the type of stereo pedal(s) used as the left and right outputs might not be phase aligned, and there was also some concern about the effective impedance when summing the two outputs.

I only use one stereo pedal, an H&K Rotosphere, and I got curious about it's output design and found a schematic online. From what I can tell, when the Mono R output is used, the signal that normally goes to the L output jack is switched at the L jack to connect directly to the signal at the Mono R jack. When in stereo with plugs connected to both the L and R jacks, the switch on the L jack is open. Is it safe to assume that the L & R outputs must therefore be completely phase aligned since mono simply means adding the L signal to the R?

How about output impedance? It seems to me that it must be different for mono versus stereo operation with this design? I should point out that in the manual for the Rotosphere, there is a suggestion that when run in mono, the Mono R jack will produce a less intense effect than the L jack. I assume this has to do with the fact that in this scenario, you only output the circuit that connects to the L output jack rather than both the L and R circuits in parallel as when using the R Mono jack, but I don't necessarily understand why one is more or less intense.

Here is page 1 of the scematic


Here is a zoomed in view of the output jacks


I would be curious if anyone could take a look and let me know if my observations are indeed correct regarding phase alignment and the use of the switched L jack, and if so, perhaps even elaborate on why the signal is more or less intense using one or the other output in mono.

Thanks!

[PRR]

It's a complicated circuit.

> some concern about

Concern is cheap. Analysis is tedious and error-prone. Real World Tests tell you what you need to know.

If you split one signal, Roto it, and sum the outputs, does it sound strange? Does the sound level change badly?

I think there is a phase-shift or delay (not shown). If so, it WILL cancel some of the signal, that's the point. Weird out-of-space sounds (maybe only a Leslie effect, which would be stunningly strange except we are all used to it).

[Sleipnir]

PRR, as always, thanks for your input here and on other forums.

I finally got around to playing around with the Rotosphere pedal a bit today and I experienced something which I do not understand. I plugged my guitar into a Fulltone True-path ABY pedal which allows you to send the guitar signal to one or two outputs, with one of the outputs being transformer isolated. I plugged the outputs of the ABY pedal into the two (L & R) inputs of the Rotosphere and then the (L & R) outputs of the Rotosphere to my two amps. With the ABY pedal set to A & B, the sounds out of the Rotosphere was so attenuated I could hardly hear it. If I instead switched the ABY pedal to either A or B, the output of the Rotosphere was much better. If I then unplugged the cable from the L input of the Rotosphere (while making sure the ABY pedal was set to the one output going to the Rotosphere R input), the output got even slightly louder.

I tried the same setup using a passive ABY splitter with no transformer isolation, and the Rotosphere did not have the same super attenuated output when run in A & B mode.

If you're wondering why I would run the Rotosphere after an ABY pedal it is because I want to build a Stereo/Mono bypass pedal allowing me to bypass the Rotosphere or run it in either mono or stereo. I have a design, but now I am confused about this attenuation issue.

Can anyone attempt to explain why running the A & B mode of the Fulltone pedal into the Rotosphere L & R inputs would cause this attenuation?

[PRR]

Kinda sounds like the Fulltone True-path ABY pedal's true path isn't so true? 

[Sleipnir]

No, it appears that this was user error on my part. I normally have the phase switch on the True-Path ABY set to reverse as my two amps are of out of phase. I spent a lot of time thinking about this issue yesterday after experiencing the problem, and then still dumbfounded, tried again this afternoon. Well, it was simply caused by the phase switch being set to reverse. I could have sworn I flipped that switch back and forth when troubleshooting yesterday, but I must not have? With the phase switch set to reverse, essentially, I sent two opposing signals into the L&R inputs, thereby canceling each other out. Sorry for wasting your time!

My stereo bypass pedal should in theory work, but I will need to swap the + and - leads on the speakers of one of my amps to correct for the phase issues as I would not be able to use the ABY pedal for that purpose if placed in front of the Rotosphere.

[anotherjim]

Well, the design does simply merge L&R when only the R/Mono jack is used. This is done on a great many items of stereo gear. Yes it can create some side effects due to phasing - not necessarily bad, but will sound different compared to 2 separately amplified speakers mixing in the air.

Sometimes, I find it better to use only the non-mono output, the L in this case which won't give a merged output.  As the simplest way to convert a stereo mic setup to a mono mic setup is - remove one mic!

You have to try either merged or single & see which one you like better in performance.