Adjust DCP timing?

BlueCrab

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#21
I agree with you Don, but it's the lessor of two evils. Wasn't the relay in the PL4000 made to switch the PL400? Or the PL700? I could put up a placard stating: "Turn on: Turn on PL4000 first. Turn on PL400 second. Turn off: Turn off PL400 first. Turn off PL4000 second."

As I said before, it all sucks.

A time delay relay would work, but it's yet another piece of equipment.
 
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#22
That's the way I learned to do it when I started using separate components, Amp on last, off first. So far so good.
 

jbeckva

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#24
The outlet on the preamp is 2 wire. Outlet strips are generally 3 wire and I would not recommend a ground buster adaptor. The inrush current to the amp could damage the relay. I don't know if they are readily available. An alternative would be to use the preamp outlet to start a time delay relay for the amp. There are some cheap ones on ebay. There may be one that would fit inside the 4000.
I'd go this route i.e. delaying the power up on the amp in it's entirety. Those meter "slams" can't be healthy. But yeah, power down would still be another event to get 'em.. maybe not as much?
 

Gepetto

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#25
I think the intern at PL designed that delay circuit in the PL4000. Looks like possibly a misguided muting attempt on power up.
 

Gepetto

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#26
I should have mentioned that as built C10 was a 100uF capacitor, not 470uF as shown on the schematic. This gave a timing of about 9 seconds. I changed it to 220uF to extend the time.



You're right, but just. Tests that I conducted this morning, the DC transient was essentially over in a little more than 2 seconds and the DCP did not trigger. To do this test I disabled the timing circuit by shorting the emitter and collector of Q5 which causes Q6 to saturate pretty much when B1 comes on.

The transient causes the needles on the meters to slam full max for a moment and then relax just before I hear the DCP relay click on. They slam again at turn off, but after the DCP has disconnected the speakers.

Don & jbeckva - Thanks for the timing info. I don't see the need to go beyond 10 seconds or so.

Here's my current plan:
  1. I've ordered the Brown Dog adapter to replace the RC4136 with two OPA2134s. The hope is this will reduce that initial transient, but even if it doesn't, the OPA2134 is a better op amp and better is the enemy of good enough.
  2. Either shorten or disable the PL4000 timing circuit. I don't see the need for it. Transient exists either way. I just want the transient over before the DCP energizes.
  3. Extend the DCP to about 6 to 10 seconds. That's more than sufficient.
  4. Plug the PL400 into a power strip and plug that power strip into the switched outlet of the PL4000. Normal operation would be to have the power switch open (PL400 off), turn on the PL4000, count to 5 potatoes, and then turn on the power strip (turning on the PL400). This would prevent all the needle banging, but if I forgot to turn off the power strip, everything would come up alright in the end. This kind of sucks, but the alternative would be to leave the PL4000 on all the time - just like Dynaco did with the PAT5 to prevent the same issue.
Feed your meters after the DCP if the meter banging is concerning to you, others do that
 

BlueCrab

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#27
Excellent point, I'll do that.

I think the intern at PL designed that delay circuit in the PL4000. Looks like possibly a misguided muting attempt on power up.
Ha, ha. Don't sugar coat it, tell us how you really feel. It was indeed an attempt to mute the output on power up. There are notes in the service manual concerning "Excessive turn-off thump", where they recommend reducing the power supply capacitance (from 470uF to 220uF) among other things. Yuk.

Whereas I love the look of the PL4000 and like the attempt at compression/expansion & correlator functions, it leaves a lot to be desired. I just thought it was the coolest looking preamp back in the 70s.
 

Gepetto

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#28
Looks cool but a single supply preamp is just a bad idea all around. Saves $$$ though.

And the RC4136 used in it was like the first quad op amp that appeared on the commercial market in the early 1980s produced by Raytheon. I used them in motor control circuits in that same time frame, things that had a bandwidth of about 500Hz. They were fine for that purpose. The ones that TI now makes probably are a little better process wise but that is about it. It is basically a quad version of the lowly LM741 which is a good utility op amp but not an audio op amp by any measure.

BTW, I have a PL4000 so I have the complete right to complain :) It sits on my shelf and looks cool...
 

nakdoc

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#29
C2 and C4 ought to be changed. Now way the opamp need 15 seconds to 'settle". Note that the 470 ohm output termination resistor should bleed off any DC. I wonder too if your power amp's input has any DC on it? You may be discharging that voltage through the 470 ohm resistor when the relay closes?
 

BlueCrab

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#30
C2 & C4 were changed as were all electrolytics in the preamp. With the timing circuit engaged, once it times out and applies power to the op amp, the transient settles out in about 2 seconds. That is, the op amp settles in about 2 seconds. There is no dc on the preamp output once the transient is over. There is no dc on the power amp's input with the preamp disconnected.

The positive side of C2 & C4 have to charge to 15vdc when power is applied. This pulls up the negative side of C2 & C4 until the transient is over. When the power to the preamp is turned off, the opposite occurs and the preamp output goes negative as C2 & C4 discharge - no instantaneous voltage changes across a capacitor.
 
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#32
Looks cool but a single supply preamp is just a bad idea all around. Saves $$$ though.

And the RC4136 used in it was like the first quad op amp that appeared on the commercial market in the early 1980s produced by Raytheon. I used them in motor control circuits in that same time frame, things that had a bandwidth of about 500Hz. They were fine for that purpose. The ones that TI now makes probably are a little better process wise but that is about it. It is basically a quad version of the lowly LM741 which is a good utility op amp but not an audio op amp by any measure.

BTW, I have a PL4000 so I have the complete right to complain :) It sits on my shelf and looks cool...
I have a 4000 and it sits on a shelf where I can't see it because I've had night mares working on 4000's.
 

BlueCrab

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#33
I have changed the timing of the DCP - installed a 100uF cap, which measured about 90uF - delays the relay click to about 5 seconds. A little short of where I wanted it, but sufficient. After a couple of beers & a single malt, I accidentally tested the DCP when I switched on the amp first and then the preamp second - fool. Failed to read the placard. Guilty of LUI - Listening While Intoxicated. Got a loud pop out of the speakers, but the DCP killed anything worse coming down the line. Speakers seem none the worse. I was ready for another single malt.
 

BlueCrab

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#34
Oh, and wired the meters to be after the DCP - no meter slamming. I have a delayed turn-on relay on order.
 
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#36
...Guilty of LUI - Listening While Intoxicated. Got a loud pop out of the speakers, but the DCP killed anything worse coming down the line. Speakers seem none the worse. I was ready for another single malt.
Sometimes we have to learn our lessons the hard way. LUI is more likely to do that. When I tested the design I used a knife switch to short across an output with a sacrificial 8" speaker and no crossover. Each time it yelped, but never died. I must have done it 20 times.
 
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#39
Just to finish up on what actions I took. Initially I bought one of the Delay-On Relays I mentioned in an earlier post. Cost about $30, which I thought was reasonable. And it does the job. It was easy to mount inside the PL4000, timing is easy to set, you can use either AC or DC for control, and the switch contacts are rated to 16A. But then I noticed a high-pitched whine (to my ears anything over 1kHz is high-pitched), which seemed to be coming from the relay. I had my 28 year old son verify this whine. And somehow it ended up being fed to the speakers, but not volume sensitive. I tried changing the control voltage from AC to DC - same thing. So I gave up on this approach.

I went to my junk box and pulled out my array of old chips and built a simple timing circuit based on the LM555, which were used like salt and pepper for all sorts of circuits way back when. Mounted this on some perf-board (which I hate because soldering to it sucks). All the active components for this circuit are probably as old as the PL4000 itself. This circuit switches the original relay in the PL4000 and has a delay of about 9 seconds. This works great - no high-pitched whine. The PL4000 settles to steady state by 9 seconds, then the PL400 is switched on followed by another 5 seconds until its relay switches the speakers in.

While I was in the PL4000 and in the spirit of "better is the enemy of good enough", I changed the RC4136 op amps to OPA4134s. Well, at least 2 out of 3. I was able to remove the RC4136 op amps without destroying them and added sockets. I did change the one on the Peak Unlimiter, but performance definitely was worse, so in the end, left the original RC4136 in place.

IMG_0691.JPEG IMG_0692.JPEG
 
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#40
The time delay relay likely has a switching power supply in it for control circuits. Since it was mounted next tot he volume control I could believe it radiated noise into the signal. Perfboard is a PITA. The 555 route occurred to me earlier in this thread but desigining and building the circuit is beyond the capabilities of a lot of people. It looks like you did a pretty good job.
 
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