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Old 25th March 2003, 10:06 AM   #1
Eccu is offline Eccu  Finland
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Unhappy Problems with Crescendo ME - Need help!

At the first startup my amplifier speaker output measurement give 0,2VDC (between speaker + and -), which slowly decrease to zero and then few millivolts to minus side (-0,03V).
Question(1) Is this normal manner? Elektor magazine instructions says that normal offset is 0,1V or less.


I measure also quiescent current by measuring voltage between test points tp1 and tp3 and I get about 0,1VDC when trimpot was set to fully left (counter clockwise). Elektor magazine instructions says that recommed voltage is between 0,044V and 0,055V, so I have double. When I turn little bit trimpot to right then voltage rise little bit too, so I turn it back to left.
When amplifier was switched on about one minute then heatsink get quite warm but not hot.
In RCA input I have one 220ohm resistor to prevent any oscillation in input, but I don't know do that give any effect.


I have little differencies in my schematic too. In amplifier part those T7 and T9 should be BC550C and BC560C, but I have BC550B and BC560B because local shop don't have those C -models.
Question(2) Have I misunderstand that letter after numbers, is that current amplification factor (hfe)?
Question(3) Will this cause this strange behaviour?

Also those D1 and D2 (leds) should be red, but I have green.
Quoestion(4) Can this cause this strangeness, because I remember that different color leds have different threshold voltage?

Also in protection circuit that offset compensation circuit should have opamp OP77, but I have OP07. In OP77 datasheet have been written that OP77 is replacement for OP07. I use that OP07, because local shop don't have those OP77 models.

Quoestion(xxx) What should I do next??? Is there some page or have some one Cresendo owner measured any currect and voltage values from different points in circuit, so that I can compare those values mine ones.
Any "operation smoke" have not seen, but is that possible that I have already burnt some components, because I get so much quiescent current?

(No protection circuit schematic included yet)
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Old 25th March 2003, 10:27 AM   #2
joensd is offline joensd  Germany
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Hi Eccu,
as I assembled the crescendo myself and had problems likewise I wrote to Elektor to get some measurements from them.
Have a look here : New adventures in hifi

On the second page is the mail from Elektor.
I´m no expert but it really might be the choice of components as your quiescent current is definitely too high at the lowest setting.
Did you thermally couple input differential pair and LED/transistor?
I´d definitely put red LED´s in and BC5X0C transistors as it isn´t a cost factor.
Hope you didn´t buy the wrong Z-diodes, just like me ;-)

Regards+Good luck
Jens
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Old 25th March 2003, 12:40 PM   #3
Eccu is offline Eccu  Finland
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Hi joensd...
How is going?


Yes, I have thermally coupled those LEDs and transistors.


Local dealer does not have rectancular red LEDs, only green, so I must order those from different place.

I must test other channel today and if that act same way, then I must buy those BC5*0C transistors.

Do any substitute transistors exist (BC550C and BC560C)?
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Old 25th March 2003, 01:53 PM   #4
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Quote:
Originally posted by Eccu

Do any substitute transistors exist (BC550C and BC560C)?
BC549C and BC559C should do fine. Even B types, which maybe easier to find, would do the job well.


Carlos
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Old 25th March 2003, 04:10 PM   #5
Eccu is offline Eccu  Finland
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Thanks carlmart
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Old 25th March 2003, 04:34 PM   #6
Rambi is offline Rambi  Netherlands
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Default Re: Problems with Crescendo ME - Need help!

Quote:
Originally posted by Eccu
At the first startup my amplifier speaker output measurement give 0,2VDC (between speaker + and -), which slowly decrease to zero and then few millivolts to minus side (-0,03V).
Question(1) Is this normal manner? Elektor magazine instructions says that normal offset is 0,1V or less.
That's fine, the slow decrease to -0.03V is caused by the offset compensation circuit. (So at least that's working properly

Quote:

Also those D1 and D2 (leds) should be red, but I have green.
Quoestion(4) Can this cause this strangeness, because I remember that different color leds have different threshold voltage?
The threshold voltage of the LED determines (with R17/R20) the current setting for the current source around T5. Usually (IIRC) a green LED has a higher threshold voltage than a red LED -> higher current. This could cause the other problem you describe: not being able to reduce the bias enough (only down to 0.1V between tp1 & tp3, which is about 450mA).
Instead of getting a red LED you could also adjust the value of R17/R20 to the green LED's threshold voltage. The current should be about 3.9mA.
Measure threshold voltage and choose R17/R20 to appr. (Vth-0.6)V/3.9mA (if Vth is 2.4V, R17/R20 should be about 460 for example).


Quote:
Also in protection circuit that offset compensation circuit should have opamp OP77, but I have OP07. In OP77 datasheet have been written that OP77 is replacement for OP07. I use that OP07, because local shop don't have those OP77 models.
Since the offset reduces to -0.03V this should be OK.

Good luck!

Remco
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Old 25th March 2003, 04:54 PM   #7
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Notice that this type of DC-servo which is used here can get a small "hang-up". This is caused by the base currents of the input transistors. Right now I don't remember if it was positive (NPN lower Hfe) or negative (PNP lower Hfe) input bias currents.

This phenomina is easy to observe and not possible to overcome if the DC-servo is located where it is. In my vinyl amp I have an input bias servo AND a DC-servo (with chopper stabilized opamps), overkill yes, but that's me.
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Old 25th March 2003, 05:38 PM   #8
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I must correct myself. I suspect that you actually can cure this hang-up, just connect a current source and inject the same current as the bias current. A minor disadvantage of this is that you have a servo plus and manual adjustment. I prefer no manual adjustments at all.
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Old 25th March 2003, 05:45 PM   #9
joensd is offline joensd  Germany
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I don´t like to threadjack but whilst talking about it...
Is it normal that my crescendo-ME amp make a slight turn-on thump??
The protection switches the output on after a few seconds so what could be the reason?
Think I´ll have to post the protection circuitry as well.

Regards
Jens
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Old 25th March 2003, 06:29 PM   #10
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Quote:
Originally posted by carlmart


BC549C and BC559C should do fine. Even B types, which maybe easier to find, would do the job well.


Carlos

Sure? They are only 30V parts.
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