Author Topic: ABY-box with buffer/phase flip issue  (Read 330 times)

Halkbi

ABY-box with buffer/phase flip issue
« on: February 27, 2021, 09:26:18 AM »
I've been looking to help a friend getting more out of his '65 DRRI. I thought I'd throw together a simple ABY-box for channel switching, but with an added buffer to prevent tone suck + a phase flip switch for one of the outputs so that the channels can be jumpered and still be in phase (the reverb gain stage of the vibrato channel puts them out of phase stock).

https://www.parasitstudio.se/stripboard-layouts/simple-buffer-and-phase-inverter

I used this simple buffer which utilizes a dual opamp for a buffer and phase flip but I modded it so that the phase flip stage has it's own input, added output caps and power filtering and bumped up the value of the input cap to .22 just to be safe. The signal hits the buffer stage before the ABY-wiring, which was put together using this diagram from Musikding:

https://www.musikding.de/A-B-Y-Switch

I then wired it so that the signal going to one of the outputs can be run through the phase flipping stage using a DPDT switch. So far so good. Trying it out fresh off the bench everything works as expected, except that the non-phase flipped channel does a pretty big volume drop when hitting the Y-switch with the phase flip active on the other output. I'll say it again; the non-phase inverted output exhibits a volume drop when the Y switch is engaged along with the phase flip on the other output. The phase flip seems to to it's thing just fine (since there isn't any audible difference when it's engaged :icon_wink: )  when the Y-switch isn't active, and the Y-switch works as expected as long as the phase flip isn't active (which ultimately defeats the purpose of both of them in this particular application). It does resemble a phase cancellation issue, but I can't for my life figure out how both the buffered and the phase flipped signal could be summed with the non-phase flipped ditto since it's an active stage that only goes to one of the output jack. Any help appreciated.
« Last Edit: February 27, 2021, 09:42:03 AM by Halkbi »

iainpunk

Re: ABY-box with buffer/phase flip issue
« Reply #1 on: February 27, 2021, 11:31:32 AM »
3 things we need to help you:
- an accurate, legible schematic, a picture says 1000 words
- sharp, well lit, pictures of your boards, both solder side and component side.
- IC voltages, DC on all pins.

i have a strong feeling that there is a problem in the transiation of the schematic form the way it was to the split stages.

cheers, Iain
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

Halkbi

Re: ABY-box with buffer/phase flip issue
« Reply #2 on: March 01, 2021, 06:17:27 AM »
I have a hard time thinking something's wrong with the actual buffer/phase flip circuit since it behaves as expected on it's own, but upon further evaluation I've realized that the Musikding ABY-wiring isn't very good to begin with. Since the non-active output jack isn't shorted to ground it gives a lot of hum on the output not in use. I'm honestly a bit shocked that such a subpar solution is being sold as a kit, as it's easily resolved by using 3PDT-switches. This seems like a much better way to do it and I'll be rewiring mine accordingly to see if it helps to resolve the issue stated above:


Bigshredder

Re: ABY-box with buffer/phase flip issue
« Reply #3 on: March 01, 2021, 03:10:24 PM »
I am about to build a phase flip switch into one of the output of the splitter kit from musikding. I am using isolated jacks on that build because tha jacks that came with the kit didnít even hold the ts-plus in place. The tip-jacks were to short. But my thought is to isolate the switch from the enclosure to ensure that the 2 output never connects. Is your an aby box? Could it be a grounding issue?

Halkbi

Re: ABY-box with buffer/phase flip issue
« Reply #4 on: March 05, 2021, 11:01:54 AM »
I am about to build a phase flip switch into one of the output of the splitter kit from musikding. I am using isolated jacks on that build because tha jacks that came with the kit didnít even hold the ts-plus in place. The tip-jacks were to short. But my thought is to isolate the switch from the enclosure to ensure that the 2 output never connects. Is your an aby box? Could it be a grounding issue?

Did you proceed with this yet? As stated above, the Musikding wiring will give you that loose cable hum on the output that's not in use since it never shorts anything to ground. You will be much better off using the 3PDT solution that I posted.

I have now rewired mine using said method and it doesn't exhibit any hum. However it still gives the same issue when using the Y-splitting in conjunction with the phase shift (the non-flipped output drops in volume). I'll try and do a more proper explanation of my circuit.

First off the signal hits the first opamp stage of this circuit (the buffer). Pin 1 and R3 is not connected.



The signal from pin 1 and 2 goes through a 220n decoupling cap to the input of this ABY-wiring:



The output going to jack B is routed via a DPDT switch which sends the signal either straight to the jack or to the jack via the phase flip stage (into R3/out via another 220n output cap coming from pin 7/R4).

This works fine until you use the Y-functionality along with the phase flipping on output B, which results in a drastic volume drop on output A. In A/B mode it works just fine with the phase flip engaged, and without the phase flip the Y mode works fine.

I still think that there's no issue on how either circuits operate but rather a problem with the combination of the Y mode and the phase flip on output B. I just cant understand why there shouln't work. Nothing seems wrong with my board and wiring and I've cross referenced it with the original schematic multiple times without finding anything that doesn't match. Do you still want voltages and pictures?

PRR

Re: ABY-box with buffer/phase flip issue
« Reply #5 on: March 05, 2021, 07:55:16 PM »
I don't see the DC conditions. There are no coupling caps out of these opamps?

I sure can't follow the audio through the switch from a word-sketch. If the two outputs meet they sure will cancel. If DC is not isolate the opamp may be working at the edge of its ability.


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Halkbi

Re: ABY-box with buffer/phase flip issue
« Reply #6 on: March 06, 2021, 06:46:11 AM »
I don't see the DC conditions. There are no coupling caps out of these opamps?

I sure can't follow the audio through the switch from a word-sketch. If the two outputs meet they sure will cancel. If DC is not isolate the opamp may be working at the edge of its ability.



Thanks thanks for making the effort to try understand my explanation despite the lack of a proper schematic. I don't know what I was thinking trying to word it out (especially since my english is so-so). Your interpretation wasn't too far off, I've added a schematic below. Not sure what you mean by DC conditions as the power supply is shown in the right of the original schematic (9v going to pin 8, 4.5v to R2 and pin 5, ground to pin 4). Originally the circuit had no coupling caps (as stated in the blog post that I linked to in my original post, it's designed to be used as a tool within another circuit and hence the lack of coupling caps/power filtering/polarity protection) so I added a 220n on each output.

Finally, here's my circuit. I can't for my life figure out how the added phase reverse stage on output B can cause the aforementioned volume drop on output A when it's engaged in conjunction with the Y-mode. Especially since output B remains unaffected.


iainpunk

Re: ABY-box with buffer/phase flip issue
« Reply #7 on: March 06, 2021, 03:02:50 PM »
put a capacitor between the bypass switch and the R3 10k resistor! you are putting DC on the normal output the way you did it, tube amps don't like it, especially since its DC voltage.

cheers
half man - half snail - 6 feet to scale - Snail man's - not frail - He's been - to jail - snail man is fuccing real
『snailpilled』

Halkbi

Re: ABY-box with buffer/phase flip issue
« Reply #8 on: March 08, 2021, 08:31:26 AM »
put a capacitor between the bypass switch and the R3 10k resistor! you are putting DC on the normal output the way you did it, tube amps don't like it, especially since its DC voltage.

cheers

Added another 220n at the phase reverse input and the issue has been resolved! Lesson learned. Thanks a lot :)