Still some problems with Crescendo. Oscillation or what? - diyAudio
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Old 17th August 2003, 07:45 AM   #1
Eccu is offline Eccu  Finland
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Question Still some problems with Crescendo. Oscillation or what?

After summer holidays I fix that hum problem by isolating RCA connectors from chassis. Thanks Callmart.

But now there is other problem again. When I play test CD which includes "frequency sweep" from 0Hz to 20kHz some oscillations appears. Not all frequencies but allways on same frequencies.
Wierd? What should I do next?


But there is still some quiet hum when I connect CD-player to amplifier. When I disconnect it, hum disappears totally. If I disconnect amplifier RCA cable shield from RCA connector and CD-player is connected then there is very loud hum?




Previous thread:
Crescendo protection circuit?


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Old 17th August 2003, 09:02 AM   #2
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Default Re: Still some problems with Crescendo. Oscillation or what?

Quote:
Originally posted by Eccu
[snip]But now there is other problem again. When I play test CD which includes "frequency sweep" from 0Hz to 20kHz some oscillations appears. Not all frequencies but allways on same frequencies.
Wierd? What should I do next?
[snip]:[/
What do you mean: oscillations occur? What do you hear? How do you know there are oscillations?

Jan Didden
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Old 17th August 2003, 12:38 PM   #3
Rudy is offline Rudy  Belgium
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do you have hum when only one channel is connected to the CD player, lets say the left channel

Greetz Rudy
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Old 18th August 2003, 05:19 AM   #4
Eccu is offline Eccu  Finland
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Sorry my hasty description.

-Jan-
I mean that, when I start to play that "frequency sweep". When "sweep" is between (about) 30-70Hz, sound is…. hmmm how I say…. like square wave (clipping)… I'm not sure is that even oscillation. Then about between 70Hz-150Hz sound is quite clear. etc….
It's quote hard to explain. I can do some additional measurements to clarify this problem if you need.
I can use oscilloscope and signal generator.
Damn… Other (left) channel works nice but this another (right) makes me crazy.



-Rudy-
Yes, I have connect only other channel at time when I test that hum problem. Anyway, that hum problem occurs on both channel.
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Old 20th August 2003, 07:42 PM   #5
Eccu is offline Eccu  Finland
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Default I will jump to lake!


I'm unlucky or what..... I cannot believe that.....
I tested again both channel and I just noticed after more accurately listening that both channel have some kind clipping effect or so. Sound is not so natural what it supposed to be. Is that possible that green led can cause that because original scheme there is red leds on place D1 and D2?
Or can those T7 and T9 cause this because those are not BC560C and BC550C... but those are BC550B and BC560B?


Should I give up and throw that Crescendo project to dustbin.



This project has been trial for me.... but it is too much for me at the moment... I can't managed to fix it without you help, DIY guys....
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Old 21st August 2003, 12:43 PM   #6
yeti is offline yeti  Germany
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Default oscillations

Is it possible that these oscillations are coming from your speakers?
It seems that you've got cabinet resonances and your driver reaches his excursion limits.

Arne
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Old 21st August 2003, 12:58 PM   #7
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You don't have oscilloscope?

If you have clipping sound atmeduim level, you _can_ have broken driver transistors. I had one BF471 with secondary breakdown around 30 volts. The got nasty over 30 volts peak.

The Crescendo is much of a standrard solution and works very well.

Try to borrow a oscilloscope! If you can measure all currents and voltages it can be a help for us.
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Old 21st August 2003, 02:29 PM   #8
Eccu is offline Eccu  Finland
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Thanks again peranders.... I will borrow my firend o-scope and do some measurements. Can you give me some advice where should I measure? Driver stage or what?
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Old 21st August 2003, 07:06 PM   #9
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If you connect the oscilloscope both the input and the output and compare. Then test with no load and low frequency (1 kHz) and check the waveform. Then connect load, still looking good? Then increase the frequency to max 10 kHz. DON'T test at >100 kHz or above! Do you get oscillations over a certian postive voltage? If yes, increase the gate resistors up to 470 ohms in small steps. You may have oscillating N-channel devices.

Check how much output signal you get. Do you get as much you expect (5-10 volts less than supply voltage)?

If not start to measure currents and voltages in order to determine the working conditions.
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Old 21st August 2003, 07:33 PM   #10
joensd is offline joensd  Germany
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Hi Eccu,
as you know I had similar problems with my amp.
They started clipping at 25Vpeak cause the voltage over D3 and D4 wasn´t appropriate.
Check those as well being at it.
And yes, a scope, a sine generator and dummy resistors would be nice to test the amp.
Don´t blow your speakers.

I´ll press my thumbs
Cheers
Jens
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