Output impedance of discrete phaser

[Gabriel Santos]

I'm about to order a PCB from one of those chinese manufacturers, the ones who sell 5 dual layer 10x10cm boards for 2 dollars + shipping. But this article from ETI UK was posted on the american radio history website: https://worldradiohistory.com/UK/Electronics-Today-UK/70s/Electronics-Today-1974-06.pdf and it shows 4 resistors I wasn't counting on, R71 to R74, those not being showed on SQ Quadraphonic - A Dead Surround Audio Format.... or is it?, my 1st source for such circuit. Upon seeing the original circuit, I did some simulations with and without the omitted resistors and came to the following conclusions: on Adrian Bruce's Art&Technology version, Rf and Lf give out about 822mV with an output impedance of about 6Kohms and Rb output gives out [email protected] and Lf [email protected] and, as it supposed to be, the voltage on the four outputs was the same upon adding the 2 resistors at Rf and Lf, but the output impedance was increased by 9.4Kohms at Rf and Lf. During the simulation I fed both inputs with a 500mV@1KHz. So, Is it expected for the rear channels to have a lower voltage level and also a lower output impedance? If so, how much lower should the rear impedance be compared to the front impedance? About 5 times, thus making R71 to R74 necessary?

 

[Soundfield]

I'm about to order a PCB from one of those chinese manufacturers, the ones who sell 5 dual layer 10x10cm boards for 2 dollars + shipping. But this article from ETI UK was posted on the american radio history website: https://worldradiohistory.com/UK/Electronics-Today-UK/70s/Electronics-Today-1974-06.pdf and it shows 4 resistors I wasn't counting on, R71 to R74, those not being showed on SQ Quadraphonic - A Dead Surround Audio Format.... or is it?, my 1st source for such circuit. Upon seeing the original circuit, I did some simulations with and with the omitted resistors and came to the following conclusions: on Adrian Bruce's Art&Technology version, Rf and Lf give out about 822mV with an output impedance of about 6Kohms and Rb output gives out [email protected] and Lf [email protected] and, as it supposed to be, the voltage on the four outputs was the same upon adding the 2 resistors at Rf and Lf, but the output impedance was increased by 9.4Kohms at Rf and Lf. During the simulation I fed both inputs with a 500mV@1KHz. So, Is it expected for the rear channels to have a lower voltage level and also a lower output impedance? If so, how much lower should the rear impedance be compared to the front impedance? About 5 times, thus making R71 to R74 necessary?

They are of no consequence. They only provide a signal path for the mono switch and to equalise the working of the volume and balance controls. The output impedance is not a issue worth worrying about with or without them. The real issue is that it is a very poor decoder as the phase shift networks are so crude and the channel separation is minimal.

 

[Gabriel Santos]

They are of no consequence. They only provide a signal path for the mono switch and to equalise the working of the volume and balance controls. The output impedance is not a issue worth worrying about with or without them. The real issue is that it is a very poor decoder as the phase shift networks are so crude and the channel separation is minimal.

Right, what about the voltage discrepancy between front and rear channels, is it something I should worry about? I also designing a switch bank for the outputs of the decoder so they can go straight to the tone controls and then power amp or they can go to a logic circuitry, based on the motorola IC's and then to the tone control and power amp.

 

[Soundfield]

Right, what about the voltage discrepancy between front and rear channels, is it something I should worry about? I also designing a switch bank for the outputs of the decoder so they can go straight to the tone controls and then power amp or they can go to a logic circuitry, based on the motorola IC's and then to the tone control and power amp.

I cannot comment on the accuracy of your simulations or any assumptions you might be making. Break it down into smaller circuit elements and see if you can find a discrepancy. Any gain errors can of course be easily corrected in the pre-amp stage.

 

[Gabriel Santos]

I cannot comment on the accuracy of your simulations or any assumptions you might be making. Break it down into smaller circuit elements and see if you can find a discrepancy. Any gain errors can of course be easily corrected in the pre-amp stage.

Thank you again, but what I am concerned about is if should the rear channels have a lower voltage level than the front ones say in the MC1312 or on the phaser part of a Sony SQD2020? If no, I'll ceranly have to review my simulation and check for possible mistakes.

 

[Soundfield]

Thank you again, but what I am concerned about is if should the rear channels have a lower voltage level than the front ones say in the MC1312 or on the phaser part of a Sony SQD2020? If no, I'll ceranly have to review my simulation and check for possible mistakes.

I'd expect them to be within a couple of dB of each other. You need to validate your simulations against the performance of the actual circuit. They can be very different, often because you are not simulating what you think you are simulating! Have you not built any part of this on a 'breadboard' you can physically measure?

 

[Gabriel Santos]

I'd expect them to be within a couple of dB of each other. You need to validate your simulations against the performance of the actual circuit. They can be very different, often because you are not simulating what you think you are simulating! Have you not built any part of this on a 'breadboard' you can physically measure?

I have, but I wasn't concerned about such aspects before this ETI UK issue showed up. All I was concerned was in listening to the Dark Side of the Moon in 4 channels, but now I am indeed worried about those aspects duly noted on the sugestion on do some physical tests. Also, should the 4 channels have flat or almost flat frequency response?

 

[Soundfield]

I have, but I wasn't concerned about such aspects before this ETI UK issue showed up. All I was concerned was in listening to the Dark Side of the Moon in 4 channels, but now I am indeed worried about those aspects duly noted on the sugestion on do some physical tests. Also, should the 4 channels have flat or almost flat frequency response?

You'd be lucky to get a really flat frequency response through those phase shift networks, but it shouldn't be too bad. Good luck with your tests.