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Old 8th August 2008, 03:43 PM   #1
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Default John Curl - Parasound amp Issue

I have a really nice John Curl Parasound amp that i bought last year from a client that reported a problem with it. He stated that sometimes the amp wont turn on. Meaning, the power comes out but it wont come out of protect. This same client also said that he had the amp plugged directly into his TV so he could use the remote as the volume control!

Assuming that the output of the TV was more at fault then anything. I brought the amp home, set it up in the shop and the amp ran great. no issues what so ever. I took it home and ran with with a passive pre and my Sony SCD-1 for months, no issues at all.

Assuming the amp was in perfect working condition. i sold the amp and shipped it off to its new home. The new owner reported the same problem as the person i bought the amp from. Sometimes it wont come out of protect?

So he shipped the amp back, i refunded his money and once again i put the amp on the bench and it tested great. ran it at home for 4 months now, no issues.

SO, with the interest of selling it again. I brought it back to the shop and this time i connected it to a different oscillator, one that has an adjustable DC offset control. I can literally dial in over 1 volt of DC offset. Thinking that maybe that a pre-amp with some DC offset was triggering the protection in the amp.

I proceeded slowly, and worked my way up to .5v+ and down to .5V- and the amp every time kicked in within a second or two. it does get slightly slower the higher you go. This amp does not have DC blocking caps at the input as i understand it. to it has to servo out the DC. every time the amp kicked on and ran perfectly.

I didn't dare go much higher then +/- .5 volt and was cautious to even go that high.

I want to sell the amp again but i want to make 110% sure it is in perfect working condition. the amp works great for me, sounds great, looks great I mean its great! But i do NOT want to have to ship this bugger and have someone be unhappy with it.

OH, also, if it makes any difference, i tested it with both 8 and 4 ohm loads and bridged mono with .5v of offset into the RCA jacks. I have not done any tests with the XLR jacks. I should test those but some discussion about how i should test those is in order. Injecting a balanced signal is fine and dandy but what about DC offset. the nature of the balanced input should null any effects of dc right?

Any suggestions on ways i can test this to assure its good to go? I have done everything i can think of.



Zc
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Old 8th August 2008, 05:06 PM   #2
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This is one of the problems with direct coupling, you can't make it foolproof. I would suspect that either you are oscillating for some reason or you have significant DC input to trigger the protection circuits. I did not design either so I have little control over the parameters. It would be best to contact Tony@Parasound.com and get his take on the problem. By the way, what model is this amp?
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Old 8th August 2008, 06:33 PM   #3
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Its the HCA-2200, can't remember if its the series II or not, I think so.

The last customer was using it bridged mono with another 2200 and reported that the other 2200 ran fine.

The fact that i can't duplicate the problem under any circumstances is what is driving me mad. IF, i could get it to not turn on even once then that would be something. But for it to run for months on end without failure...

I had thought about building up a small box with a couple of high grade caps inside as an external DC blocking device to send along with the amp in case the new owner experienced the same problem. BUT after bench testing with up to a half a volt of DC offset feeding the amp i don't think thats the issue? I'm not sure what else i can do???

I talked with Tony on the phone just after i got the amp and he didn't have much to say about it.
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Old 13th August 2008, 04:43 AM   #4
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Ok FINALLY i was able to observe the issue with this amp. Of course it happened as a customer was here to check out a pair of speakers i had for sale! when i first powered the amp on. the left side didn't work, right side worked and sounded great. Powered the amp off, powered it back on...no go. wiggled RCA jacks, checked speaker cables. nada.

I switched over to another amp for a bit then decided to try the Parasound again. connected everything back up, hit the switch, worked sounded great...

Now i just need to duplicate the issue on the bench so i can see what the hell is going on inside. At least i know WHICH channel has the problem.

Zc
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Old 14th August 2008, 07:38 PM   #5
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I have since been able to duplicate the issue and carefully watch and listen on power up. When the problem happens. The amp powers on and you can hear the relays clicks out of protect and the power light turns green as normal. The right channel plays and the left doesn’t. While in this condition I flipped the RCA/XLR switch several times and No change. I flipped the bridged/stereo switch and Again no change. I gave it the old TV service mans trick and thumped it on its side, lifted up the left side slightly and let it drop. Thinking maybe it was just a bad solder joint or something. (I know not very anaylytical but I didn’t have tools at the moment and i was in a hurry to diagnose.)

I let the amp sit feeling the heatsinks. Both seemed normal. I raised the volume up quite a bit, no change in the left side, right side fine. I lowered the volume and sat there for a moment. Then a blip of sound out of the left side. A few seconds later came another. Then after a bit the left side come on and played and stays playing. There are no relay clicks at this time so I believe the issue is maybe post input stage but pre output stage. No DC offset from the output of the amp or anything out of the ordinary. It is as if it is muted. when the sound blips or comes on. there are no abrupt pops or anything.


I have schematics And I am anxious now to get it back on the bench and do some investigating. I will open it again and inspect for cold solder joints, oxidize IC sockets/pins the usual.


Any suggestions on what else to look for??
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Old 14th August 2008, 08:40 PM   #6
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Just a suggestion:

I had an amp in the past showing similar behaviour.
A friend borrowed my old Philips DFA 888. As far as I understood
it was played loud eventually overloading and damaging one
B&W monitor speaker.

After a while the amp was returned to me, showing the behaviour
you explain. One day everthing fine, few days later the left
channel did not work. Few days later everything was fine again.

To make a long story short: In this case I had to replace the
output relay. Problem solved.
After replacement I opened (cracked) the old relay and found that
the contacts were severly burnt.

I do not know if this is similar in your case but maybe it is worth
a try to make sure that the relay contacts are in good order.
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Old 19th August 2008, 02:00 AM   #7
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Well I believe I have found the problem! 1- cracked solder joints on the XLR jacks and two dirty speaker output relays!

I checked over the preamp board with a stereo microscope and made sure every solder point was clean. then pulled off the left channel module and check it over as well.

I put it all back together and checked it out. I connected one channel of my scope directly to the output of the amp module before the speaker relay and one after. I could watch the amp channel come on instantly when i applied power and then watch the output relay kick on. thats when i noticed that the output after the relay had some noise where as the output from the amp module was clean. I used a plastic tool and pressed through the rear vent on the relay PCB and as i pressed on the board i could watch the signal after the relay change level!

So i pulled the amp back apart and pulled the relay PCB off. what a bunch a work THAT was! sheesh. But once i got it off i was able to carefully remove the covers off the relays. I connected a 24V power supply to the relays and measured the resistance through the contacts. the left channel relay measured anywhere from 24K to 20-30 ohms. I noticed that the armature was slightly bent and that the contacts didn't hit each other square. I was able to bend the armature slightly so that the contacts hit each other a bit better. I then used a piece of linen card stock to clean the contacts with 24V applied. I then cleaned the contacts with alcohol just to make sure. I don't have any deoxit on hand.

The right channel relay was in a bit better shape. resistance measured about 1-2 ohms before cleaning. and the contact points sat square with each other. None of the contacts look charred or pitted so just a cleaning should and they should be ok.

Tomorrow i will get it all put back together and see if it acts up anymore.

I suspect that when i was pushing on the relay PCB it was flexing the plastic relay bodies slightly and that was changing the contact resistance a bit. All the solder joints look fine and the board was fastened to the binding post's tightly.

However. I also noted that the blue stuff used on screws and nuts to hold them in place, doesn't seem to work so well. and in fact on the ground connections. it seems to have seeped under the terminals a bit. I think i will clean these those areas a bit. Just FYI for anyone that reads this later trying to fix there amp.


Stay tuned!
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Old 19th August 2008, 10:23 PM   #8
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After letting the relays sit over night after cleaning. I ran the same test today and i am happy to report that I now show .15 ohms through the relays and that is typically the resistance of my meter leads. I have the amp reassembled and retested and it appears much more stable now when the relays kick in. Previously the signal would waver for a half second. now that relay clicks and its rock solid! Now i need to run it for a few days and retest but it looks like that solved the problem!

The previous owner did report that he never used it much. so maybe the contacts just oxidized from lack of use or something. I wish i has some type of cramolin product on hand to apply, but I cant seem to find anyplace that sells it locally and i keep forgetting to order some.

haven't decided if i am keeping this one or not. It has been a struggle to let go of it thats for sure.
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Old 20th August 2008, 04:42 AM   #9
BrianL is offline BrianL  United States
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Interesting commentary on "high end" equipment. All this fanciness of circuitry, components and "man jewelry" chassis. Then, foiled by a cheap relay in the high-power signal path.
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Old 20th August 2008, 03:26 PM   #10
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Relays are a PITA no matter what you use. Low voltage High current devices that work reliably for power is one thing. but audio is another!

Switching the amp on or off with the volume turned way up can arc the contacts and cause the resistance to go up.

There is room for improvement in this amp for sure. Parasound used Omaron 25amp relays soldered directly to a PCB that is mounted to the backs of the 8 terminal post's on the back of the amp.

Logically thinking I would add two mounted to the bottom of the amp (there is plenty of room) relays that have multiple sections all paralleled so that way you have 2,3,4 contacts points working together to help keep resistance low through the relay rather then the one contact point thats in the relay's used.

Only concern would be how much current can the control circuitry handle and will the new relays exceed that??
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