7070 popping when relay closes

[goldswimmerb]

Hey all, so I have what seems to be a simple issue, or so I thought it was... I have a 7070 here that makes a fairly loud pop when the relay closes, what I noticed was that if I disconnect the normally closed pins which have 12 or so V from the muting circuit the pop goes away, the muting circuit does seem to sort of work as I do see the transistors in it switching when I change inputs or push the phono 1/2 switch but there is a faint pop when I do that with no input. Any ideas? I'd rather not remove that 12V from the outer relay pins as I don't want to mess anything else up.

DC offset at the terminals starts at 4.5v when the relay is open and then drops to less than 20mv when the relay closes (and the outer pins rise up to 12v)

 

[chazix]

DC offset at the terminals starts at 4.5v

At the speaker output terminals? I don't see how that could happen if there were speakers connected. Is it speakers that you are hearing the pop from? If so, is there anything relatively exotic about the speakers, like electrostatic drivers?

 

[goldswimmerb]

At the speaker output terminals? I don't see how that could happen if there were speakers connected. Is it speakers that you are hearing the pop from? If so, is there anything relatively exotic about the speakers, like electrostatic drivers?

From what I can tell is the unit has a nominal voltage at the terminals on the outer terminal of the relay that is used in part with the Preamplifier muting circuit. But it's bizarre

 

[kwayde]

I just worked on a 7070 for a gal who said hers had a slight pop when the relay engaged. I never heard the pop when I had it, so I had her bring it back. We connected it up and it seemed fine. She then said it happened with a tape deck, so I connected one and still no pop. I asked her to take it home to see if she could eliminate the things connected to it as being the trouble. Turns out she had new speaker cables made up to connect the Ohm speakers to the receiver, which are 4 ohms, and had just been worked on by somebody else. She then eliminated the cables as being the trouble. Turns out it's the speakers giving her the pop. A different set of 8 ohm speakers worked fine. I don't know what was done to the Ohm speakers, but she brought me some other gear to work on this morning and I asked her to bring the 7070 to have a new relay put in. Won't know if the problem will still exist until she picks it back up.

 

[chazix]

From what I can tell is the unit has a nominal voltage at the terminals on the outer terminal of the relay that is used in part with the Preamplifier muting circuit. But it's bizarre

I agree it's a bit bizarre - why does the preamp need to be muted when the relay is disengaging the speakers anyway?

But what puzzles me is how that setup could lead to enough energy being delivered to the speakers to produce a pop when they do engage. The preamp muting circuit will be trying to push about 1.2 mA into the NC terminals of the relay. If there are speakers connected and their DC resistance was, say, 8 ohms, then that current would develop about 10 mV across the speakers, which I wouldn't expect to be enough to make much of a pop when it is disconnected by the relay energizing. If there are no speakers connected, then you would measure a larger voltage - or if the speakers have a very high DC resistance, which is why I wondered about electrostatic drivers.

 

[goldswimmerb]

I agree it's a bit bizarre - why does the preamp need to be muted when the relay is disengaging the speakers anyway?

But what puzzles me is how that setup could lead to enough energy being delivered to the speakers to produce a pop when they do engage. The preamp muting circuit will be trying to push about 1.2 mA into the NC terminals of the relay. If there are speakers connected and their DC resistance was, say, 8 ohms, then that current would develop about 10 mV across the speakers, which I wouldn't expect to be enough to make much of a pop when it is disconnected by the relay energizing. If there are no speakers connected, then you would measure a larger voltage - or if the speakers have a very high DC resistance, which is why I wondered about electrostatic drivers.

So I figured it out, turns out it has to do with the setup, my test bench was only a single speaker on one channel, the moment I hooked up an 8ohm load up to the other channel the pop went away entirely. Bizarre, but I guess it's good to document quirks... Seems odd because that means that with low impedance headphones and no main load (like the ones I was testing with) you'll get a pop as well.

Was running it on my ELAC UB5 after I got it closed up and off the bench, which are 4 ohm speakers and got no pop with both L and R connected. Something weird I guess.