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Old 22nd August 2003, 12:11 AM   #11
Rudy is offline Rudy  Belgium
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Eccu,

This can take a some time, but edit the file in ... paint or wathever and add the DC values to the schematic.

With this information it is a LOT more easy to help you, and if you could be so kind, change the value on the schematic to the ones you used, if you changed something that is offcourse.
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Old 22nd August 2003, 03:04 AM   #12
sam9 is offline sam9  United States
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The two ranges you cite center on 50Hz (mains AC) and 100Hz (2 x mains freq i.e.; ripple). Any chance thishas something to doi with your to the power supply?
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Old 22nd August 2003, 04:41 AM   #13
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Replace the Green LEDs that you have used with the correct Red LEDs, as these have different voltage drops. This is critical since they hold the base voltage of the current source transistors. Proceed with testing after taking this first step.
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Old 22nd August 2003, 07:51 AM   #14
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I wouldn't worry too much over the LED'. You wíll get slightly higher currents but the main thing is the T7 and T9 get sufficient Vce (not get saturated), becuase this get less with higher currents (unless you reduce R25 and R27).

The red arrows above are good measurement points. If you can give us those we can get a picture of the problem. Using B-models instead of C for BC transistors _may_ need adjustments.

Note also if you have turned a BJT wrong (interchanged collector and emitter) , it works with gain Hfe of 3-10 and max voltage 5-7 volts. Make sure once again that every part is turned right. Can you take a picture maybe?
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Old 24th August 2003, 04:07 PM   #15
Eccu is offline Eccu  Finland
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Hello all.....

It's sad but true that I do not have chance to use just right components what those supposed to be. This because local dealer does not have exactly right values what I need.

1st thing.... Those D3 and D4... I have used 1N4730A zeners which are (3.9V, 1W Zener Diode) so should I change those to 0,5W models?



I start now make some measurements.... be ready
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Old 24th August 2003, 04:23 PM   #16
joensd is offline joensd  Germany
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Hi Eccu,
about D3,D4...
I used 1.3W zeners and due to too less bias current I had about 2V over them.
If I remember correctly zeners should be "biased" with about 1/10th of Imax so go for 0.5W types or raise current through them.
If the voltage is <3.9V your amp definitely clips before it should.

See here

Good luck
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Old 24th August 2003, 05:59 PM   #17
Eccu is offline Eccu  Finland
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Thanks Joensd... I have been read that earlier today.

Here are measurements from left channel. Nothing connected to input or output.
So.... over zeners is only 3V.....

Next step is to buy couple zeners

Someday on the next week I will get oscilloscope and I will make some additional measurements of both channel.


Any way... is those other measurements right?
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Old 24th August 2003, 09:59 PM   #18
Rudy is offline Rudy  Belgium
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Eccu,

here we go,

FIRST OFF ALL, BEFORE DOING ANYTHING MAKE SURE THE BIAS CURRENT OFF THE MOSFETS IS SET TO 0 Amps.
You don't want these babies to be destroyed, there mutch too expensive for this.

1. becouse u use green led's the current through the VAS stage is to high, actually about 50% to mutch, better solve this by changing the led's to red one's ( 3mm ) or by replacing R17 & R20 with 390ohm resistors.
2. Replace the zeners D3 & D4 with 0.5W ( 0.4W ) types, ore if these are hard to get try changing the resistor with some lower value's, like 8K2 or 6K8, still the silution with the 0.5W zeners is the best way.

Well, after all this crank up the bias till 200mA and measure all this again, so we are able to see iff all of this had any effect.

The "rare" soundings in the 50...100Hz region can be the result off the bad current settings off the amp, therefor it was not able to deliver any power to the speakers, this is the region where your speakers demand the most power ( actually < 50Hz ask's more power, but we are not able the hear distortion in this region ).

Good luck and let us know about the results

Greetz Rudy
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Old 25th August 2003, 04:17 PM   #19
Eccu is offline Eccu  Finland
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Hello again...
Very helpful peoples here.... thanks!


I have earlier changed those R17 and R20 from 270ohm to 460ohm because bias was too high.
More about that in this thread below:
Problems with Crescendo ME - Need help!

Yes.... next step is that I will change those zeners.....
Lets hope that is final cure.....
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Old 30th August 2003, 08:46 AM   #20
Eccu is offline Eccu  Finland
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Now I have changed those D3 and D4 zeners from 1W to 0,4W models.... but before I assemble all stuff again together for testing I have few question still open.
Can I replace coax -cable (RCA) which goes from backplate to amplifier board with twisted pair cable. Is there any disadvantages in this kind cable which should carry low level signal?

And should I connect 10ohm resistor and 100nF capacitor in parallel and connect it between to RCA input frame and backplate (ground) ? And will this prevent some ground loop (hum)?


I ask these because every time I have some problem in my amplifier I must disassemble so must stuff before I can do any modifications to boards. At the moment I have been disassemble total four times and its not so funny thing....
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