Odd output from relay and PGA2311 volume control!

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abza

Member
2014-05-28 4:26 pm
Right, I've got a PGA2311 volume controller set up and being controlled from a simple Arduino sketch.


The output from the PGA2311 is being fed to a relay, which toggles between:



1. Sending the audio to a headphone output (for further amplification in another part of the chassis).


2. Sending the audio to a pair of dual op-amps, set in a unity buffer configuration, and then on to a pair of dual RCA jacks to act as preouts on the back of the chassis.


Schematic:


8J2Yztk.jpg



Problem:


When the relay is in position 1 - the headphone output - the output is great, the audio is great, and it looks as I would expect:


IUGzxlI.jpg



However, when the relay to position 2 - opamps and RCA jacks - I get the following on the scope for both of the RCA jacks, both left and right channels:


dJsJUPo.jpg




  • I tried swapping the opamps, no change. I even tried removing the opamps from their sockets, and the result is the same!
  • When scoping the output right at the relay common pins, the result is the same - when the relay is in position 1, the output is clean, and when in position 2 is strangely distorted.
Any ideas what is happening here? How can the state of the relay be affecting the output of the PGA2311?
 
Do you power the PGA and the relays from the same supply voltage? The switching relays may cause the voltage to drop, or produce a spike of some sort that could upset the PGA.
I have had some trouble with a PGA where I used the input selector to switch not only the input signals, but the input grounds, too. Due to the different potentials of the connected sources (PC, CD Player, etc.) the PGA would eventually stop working after several times of switching between the sources.
 

abza

Member
2014-05-28 4:26 pm
Okay, problem solved! It's a weird one, still not entirely sure why I got the behavior I experienced.


It seems (for the first time ever) I had a "bad" resistor - namely, the resistor pulling the MUTE pin low on the PGA. When I use an ohmmeter to test the resistor, it seems to flutter between being 10Kohm and a short circuit. I replaced the resistor and all works fine.


Now of course, what I can't explain is why energizing the relay would have made the circuit work at all, if the MUTE pin was being randomly shorted to ground... Or why the non-energised relay wouldn't work at all! Truly bizarre.
 
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