I'm taking another look at one of my own units that has been at the back of the line for ages.
It was occasionally blowing rail fuses and sending a big pop to the speaker when it did that. We fried a tweeter with it once. One of those issues that was infrequent enough that it is very hard to track down.
But it still puts a decent pop through to the speakers on every powerup unless I wait 10 or so seconds to turn on the speaker. I had another one of these units a few years before I got this one that did the same thing. But I had another client unit in recently that didn't pop at all. I could just leave the speaker switch on. I know many units that don't have a speaker relay still have some sort of circuit to prevent this pop, but I can't find one here. Is there one I have missed?
I've added a premade protection circuit but it clicks in a bit too fast. I have to figure out how to lengthen the delay a bit, but after seeing the other 730 that didn't pop at all, I'm wondering if there is something that is supposed to prevent this pop completely.
I've recapped it and done as many other things as I could think of, but never cured the issue. Power amp transistors are original, except I changed the input transistors with a new matched pair.
https://www.hifiengine.com/manual_library/harman-kardon/730.shtml
It was occasionally blowing rail fuses and sending a big pop to the speaker when it did that. We fried a tweeter with it once. One of those issues that was infrequent enough that it is very hard to track down.
But it still puts a decent pop through to the speakers on every powerup unless I wait 10 or so seconds to turn on the speaker. I had another one of these units a few years before I got this one that did the same thing. But I had another client unit in recently that didn't pop at all. I could just leave the speaker switch on. I know many units that don't have a speaker relay still have some sort of circuit to prevent this pop, but I can't find one here. Is there one I have missed?
I've added a premade protection circuit but it clicks in a bit too fast. I have to figure out how to lengthen the delay a bit, but after seeing the other 730 that didn't pop at all, I'm wondering if there is something that is supposed to prevent this pop completely.
I've recapped it and done as many other things as I could think of, but never cured the issue. Power amp transistors are original, except I changed the input transistors with a new matched pair.
https://www.hifiengine.com/manual_library/harman-kardon/730.shtml
If I understand correctly, you've experienced two different Model 730 receivers--- one did not thump, but yours does? Both ostensibly have the same design, no production revisions? Any way to confirm confirm this?
I've seen power amps that were claimed to avoid thump at power-on. I recall that at power-on, the amps had no active bias and the operating bias activated gradually (mostly symmetrically) so there was minimal thump.
Your amp lacks the complimentary input differential stages I've seem in some designs, but if I'm not mistaken, it does start initially with zero bias. You can experiment if you're feeling adventurous!
Be sure to experiment without speakers, or at least heavily current limited to avoid damage. To begin, I'd load the speaker terminals with 10K. Improvise a short across C406 that you can conveniently release (eg. with a small switch). With C406 shorted, apply power and check for DC at the speaker terminals. I think there should be only leakage currents present; voltage across the 10K load will tell you what's going on. Then open the switch and observe the power amp's turn-on behavior, with a scope if available. Voltage across R406 is about 40V, so charging current into C406 will be about 40uA. Voltage across C406 will rise at 40uA/220uF = 0.18V/s. C406 voltage will clamp when D402 and R408 begin to conduct. I think the voltage across C406 will need to rise to more than 1 V before Q406 and D404 begin to conduct. All this seems to suggest about 5 second turn-on delay. See how this correlates with the turn-on transient/thump behavior.
Have fun!
I've seen power amps that were claimed to avoid thump at power-on. I recall that at power-on, the amps had no active bias and the operating bias activated gradually (mostly symmetrically) so there was minimal thump.
Your amp lacks the complimentary input differential stages I've seem in some designs, but if I'm not mistaken, it does start initially with zero bias. You can experiment if you're feeling adventurous!
Be sure to experiment without speakers, or at least heavily current limited to avoid damage. To begin, I'd load the speaker terminals with 10K. Improvise a short across C406 that you can conveniently release (eg. with a small switch). With C406 shorted, apply power and check for DC at the speaker terminals. I think there should be only leakage currents present; voltage across the 10K load will tell you what's going on. Then open the switch and observe the power amp's turn-on behavior, with a scope if available. Voltage across R406 is about 40V, so charging current into C406 will be about 40uA. Voltage across C406 will rise at 40uA/220uF = 0.18V/s. C406 voltage will clamp when D402 and R408 begin to conduct. I think the voltage across C406 will need to rise to more than 1 V before Q406 and D404 begin to conduct. All this seems to suggest about 5 second turn-on delay. See how this correlates with the turn-on transient/thump behavior.
Have fun!
I've experienced 3 of these that I can remember. Two had the heavy thump/pop. One didn't.
I didn't notice any production differences but only the one unit is here now.
You might try to confirm R406 = 1M and C406 = 220uF. That's what leads to my estimate of 5 second delay of amp turn-on. Does that mesh at all with speaker thump?
I might have this one sort of figured out. It is the hardest kind of problem to diagnose. The kind that doesn't really exist.
Another 730 came in and isn't having the pop issue. But it seems to make the difference if the speaker buttons are not engaged in the first few seconds after poweron. My unit is more prone to that than this unit, but both seem okay if powered up with speaker engaged. I've found with cap coupled receivers I need to leave the speaker engaged when they are on, but haven't had other DC units like that.
The muting circuit seems to hold the signal back for a few seconds as it should but won't prevent a pop if speaker is engaged at the wrong time.
I also noticed the rail fuses are supposed to be slow blow. That's probably the cause of the original issue I had where it was blowing fuses and damaging speakers. Slow blow fuses would have withstood the input surge.
So I think all is good if I just leave the speakers on. Which also means taking out the relay circuit I added.
The guy who introduced me to this gear always insisted on having volume down and speaker off when powering off and on a unit. Having the speaker off doesn't seem necessary most of the time. And a very bad idea for some units like cap coupled units, and apparently this model. I do think it makes sense to keep the volume down though. Especially if there is a speaker relay. So there isn't a signal on the relay when it closes.
Another 730 came in and isn't having the pop issue. But it seems to make the difference if the speaker buttons are not engaged in the first few seconds after poweron. My unit is more prone to that than this unit, but both seem okay if powered up with speaker engaged. I've found with cap coupled receivers I need to leave the speaker engaged when they are on, but haven't had other DC units like that.
The muting circuit seems to hold the signal back for a few seconds as it should but won't prevent a pop if speaker is engaged at the wrong time.
I also noticed the rail fuses are supposed to be slow blow. That's probably the cause of the original issue I had where it was blowing fuses and damaging speakers. Slow blow fuses would have withstood the input surge.
So I think all is good if I just leave the speakers on. Which also means taking out the relay circuit I added.
The guy who introduced me to this gear always insisted on having volume down and speaker off when powering off and on a unit. Having the speaker off doesn't seem necessary most of the time. And a very bad idea for some units like cap coupled units, and apparently this model. I do think it makes sense to keep the volume down though. Especially if there is a speaker relay. So there isn't a signal on the relay when it closes.
Why is it a bad idea to turn speakers off before power down? I have a Yamaha that pops when turning off even with vol at 0. For now I turn off speakers first until I look at recapping it.
I doubt recapping will have any effect with power on/off pops. If that's the primary motivation for recap, I fear you'll be disappointed.
Not the reason and I should do a bias adjust first anyways. I picked up a 2nd backup CR-1020 and plan to do a full restore on it. In part for fun/learning and also to compare the two afterwards to see if there is any audible difference to me. Love the receiver.
Earlier, I avoided giving a direct answer because I doubt there's a universal best practice--- I think it's a function how the amp is designed and how much expense was devoted to ensuring graceful behavior with power on/off.
But since you mentioned your receivers were Yamaha CR-1020, I tracked down schematics out of curiosity. The output amps are direct-coupled to the speakers without DC blocking caps, but the amps must undergo some bias gyrations during power on/off. I would surmise most graceful power cycle would turn off speakers prior to AC off; turn on AC and wait a few seconds for bias transients to settle before enabling speakers. That should minimize thumps and clicks, I think. To automate this process, you could add speaker relay featuring delayed-close/instant-open control.
There's a receiver schematic in 2nd post of link below:
https://www.diyaudio.com/community/threads/yamaha-cr-1020-yama-humming.357549/
Best,
Steve
In general, not a bad idea if the unit is not cap coupled. This HK appears to be an exception. But turning the speaker off before powerdown would be okay. Engaging the speaker within the first few seconds after turning the unit on is where the trouble starts.
If a receiver has a relay circuit (The Yamaha CR-1020) and you are getting a poweroff pop, this actually can be caused by caps. A power supply cap can be discharging too quickly. Or something in the relay circuit can be causing the relay to not click off as fast as it should be. If the relay is slow to respond when you power down the unit, some pop will sneak through.
Also check DC offset. If it is too high you can get some pop sneaking through as well.
