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Old 28th July 2009, 10:21 PM   #11
WimVL is offline WimVL  Belgium
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Question a Crescendo problem

I builded my crescendo, the 1982 one from elektuur already two times, but always have the same problem. The crescendo works but after a couple minutes there exist a noisy sharp sound that rises up. and then after about 5 minutes collaps C14 or C15(the big Elco's on the PCB's) only one will explode.I checked the polarity for mor than 100 times, but still have this problem, can anyone help me. I think is't something stupid,

Is there maybe a fault in the chematic that i don't have? I think sombody told me ones there was a fault in the chematic, is it true am I the problem.

I hope somebody can axplain my problem, because I builded it complete and it looks very ok, only the sound don't works.

I'm from Belgium.
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Old 28th July 2009, 10:29 PM   #12
llwhtt is offline llwhtt  United States
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Can you post a schematic so we can see what we are dealing with?

Craig
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Old 29th July 2009, 01:14 PM   #13
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Default Re: a Crescendo problem

Quote:
Originally posted by WimVL
I builded my crescendo, the 1982 one from elektuur already two times, but always have the same problem. The crescendo works but after a couple minutes there exist a noisy sharp sound that rises up. and then after about 5 minutes collaps C14 or C15(the big Elco's on the PCB's) only one will explode.
Sounds like oscillation. Maybe some of the information here is useful. The Crescendo seems to easily become unstable, when the bias setting is not correct.

Or could be thermal runaway, if not all relevant components have thermal contact. Transistors or diodes for the bias setting often need to be thermally connected to the same heatsink as the power stage or even directly to the power transistors.
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Old 30th July 2009, 12:32 AM   #14
WimVL is offline WimVL  Belgium
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Default schematic

here i have the chematic from my Crescendo!!!
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File Type: pdf crescendo1.pdf (94.9 KB, 147 views)
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Old 30th July 2009, 01:21 AM   #15
valo is offline valo  Norway
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Default Re: schematic

Quote:
Originally posted by WimVL
here i have the chematic from my Crescendo!!!

Yes this is exactly the chmatic that i did use 10 yeares ago, and did use the PBC layout that was in the article
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Old 30th July 2009, 06:25 AM   #16
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So do you have T8 and T10 on the same heatsink as T11, 12, 13, 14?

Did nothing from the link in post #13 solve your problem? There is a list of changes and an updated schematic in there with tips on the best location of certain components.
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Old 30th July 2009, 07:19 AM   #17
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The gate resistors could be increased to 330 ohms or 470 ohms. I had problems with this.
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Old 30th July 2009, 07:39 PM   #18
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Default Re: a Crescendo problem

Quote:
Originally posted by WimVL
I builded my crescendo, the 1982 one from elektuur already two times, but always have the same problem. The crescendo works but after a couple minutes there exist a noisy sharp sound that rises up. and then after about 5 minutes collaps C14 or C15(the big Elco's on the PCB's) only one will explode.I checked the polarity for mor than 100 times, but still have this problem, can anyone help me. I think is't something stupid,

Is there maybe a fault in the chematic that i don't have? I think sombody told me ones there was a fault in the chematic, is it true am I the problem.

I hope somebody can axplain my problem, because I builded it complete and it looks very ok, only the sound don't works.

I'm from Belgium.
Hi,

no regarding the original numbering of components , attached is one version that I had simulated. It has a few modifications ( ones from Elektor magazine and others from me ) but works well on simulator.

eD
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File Type: pdf crescend _ crescend.pdf (32.6 KB, 102 views)
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Old 31st July 2009, 04:01 AM   #19
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eD, the schematic you have posted is not that of the Crescendo but modified version of the Hexfet Amp. Your simulator version has a cascoded VAS and runs off higher rails.
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Old 31st July 2009, 05:35 AM   #20
Bonsai is offline Bonsai  Taiwan
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I've commented on this amplifer before, but here goes again.

This is a 1980's design and back then the philosohy was in general the higher the loop gain the better. Nothing wrong with that point of view, but then you have take some design precautions to ensure the amplifer behaves.

There is no dominant pole compensation so the amp perfomance is overly sensitive to layout, component tolerances, EMI ingress etc. On top of that, Mosfet suppliers are notorious for reducing chip size on a regular basis (to remain competitive) and this often affects key parameters like input capacitance etc. On this design, that will impact the stability.

This amp could do with miller caps between the VAS Cascode collectors and the bases of the VAS transistor (sorry, cannot read the references on the posted copy - they are a bit blurry).

Maybe Bob or Glen want to comment.

Anyway, good luck with your amp - if you can get it stable no doubt it will sound good.

(My first Elektor amp was the 'Equa' - I think it was about 1978 or 1979 - if anyone has a copy of the schematic, I'd really appreciate it if you would post it).
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