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Old 1st February 2003, 09:21 AM   #1
joensd is offline joensd  Germany
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Default New adventures in hifi

Hi all,
just am about to finish my actual project: a crescendo ME.
The sound is astoundingly good->Iīm happy

BUT : Turning on the oscilloscope for some final checks made me .
At +-47 supply voltage the output swings at +-30V (25RMS).
Input signal was a sine wave of 1VRMS and it doesnīt matter if the output is loaded or not. It starts clipping at 25RMS in both channels BTW ...

Iīm using a 10kOhm pot as passive preamp at the moment.
Input impedance of the amp is 45kOhms.
Could this cause any problems? Should I maybe match the pot more to the input impedance ? (like 50k)

Another point probably causing problems is that I took 1.3W zeners (D3,4) instead of 0.5W so they might be not biased enough.

Power supply is 2*35VAC/600VA , 2*33000uF for both channels.
Bias is set at 300mA.

Iīd appreciate any hints cause I love this amplifier and want to have the last watts as well!

Thanks in anticipation
Jens
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Old 1st February 2003, 09:36 AM   #2
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it seems to start clipping at 78W , if im right ... the guys published for a 90W @ 8 Ų sounds non sense.. have u tried contacting them?? ??
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Old 1st February 2003, 09:42 AM   #3
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Lightbulb maybeeeeeeeeeeeeeeeeeee

the suggested bias is from 200 to 250 mA....I say maaaaaaaybe this causes your fault....thats 44-55 mV on R34....not 300 mA u r using.. now ....is this amp really good? a friend of mine is building one .....hope u get this fixed... Tschus!!!!
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Old 1st February 2003, 10:00 AM   #4
halojoy is offline halojoy  Sweden
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Default Re: New adventures in hifi

Quote:
Originally posted by joensd
Hi all,
just am about to finish my actual project: a crescendo ME.
The sound is astoundingly good->Iīm happy

BUT : Turning on the oscilloscope for some final checks made me .
At +-47 supply voltage the output swings at +-30V (25RMS).
Input signal was a sine wave of 1VRMS and it doesnīt matter if the output is loaded or not. It starts clipping at 25RMS in both channels BTW ...

Iīm using a 10kOhm pot as passive preamp at the moment.
Input impedance of the amp is 45kOhms.
Could this cause any problems? Should I maybe match the pot more to the input impedance ? (like 50k)

Another point probably causing problems is that I took 1.3W zeners (D3,4) instead of 0.5W so they might be not biased enough.

Power supply is 2*35VAC/600VA , 2*33000uF for both channels.
Bias is set at 300mA.

Iīd appreciate any hints cause I love this amplifier and want to have the last watts as well!

Thanks in anticipation
Jens
Good choice. I wouldn't mind building one myself.
I would rather build this one than Pass, Gainclone or almost any other project.

I don't think it is the power supply. It is more than strong enough.
I am thinking of the load when you are testing.
are you ure it is 8 ohm. Is it a resistor?
Speakers can have lower impedance in real life.
---------------------------------

peranders
should know about Crescendo ME.
He has some little about it on his homepage.
/PA Crescendo

Anyway, A Great Amplifier!
You will soon get more than 25 Watt RMS in 8 ohm.
somebody will find the problem.

Picture is from peranders site

/halo - loves a good crescendo in music
Click the image to open in full size.
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Old 1st February 2003, 10:57 AM   #5
joensd is offline joensd  Germany
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Quote:
it seems to start clipping at 78W , if im right ... the guys published for a 90W @ 8 Ų sounds non sense.. have u tried contacting them?? ??
Sorry for the confusion. Of course 25VRMS are 78W.
But Iīve got +-30V on the scope so peak voltage is 30V.
This comes to 21.21VRMS and 56W into 8Ohms.
For making the confusion complete my Fluke (TrueRMS) says something like 25VRMS (thatīs where the value came from).
But regardless which value I take and calculate with:
At +-47V I think I should get a swing of +-40-42??

Contacting Elektor might be a good idea anyway.

Bias can be set up just like you want it.
Recommended is not to go below 200mA.
You can crank it up to 1A. Ouch!
I think William (Wuffwaff) explained it somewhere here.
This way you get 16W of pure class A.

To the sound of the amp:
I can only compare to a shitty Marantz amplifier (PM-52) and a LM3886 amp.
The LM3886 outperforms the Marantz and has a powerful but analytic and sometimes cold sound.
The Crescendo sounds even more powerful and though much more relaxed. It seems it doesnīt have the treble of the LM3886 but when you listen to it you donīt miss anything.
Itīs warm, gets never boring and is like...cream&chocolate.

Thanks a lot so far
Jens
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Old 1st February 2003, 11:04 AM   #6
joensd is offline joensd  Germany
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Thatīs my baby. Pretty much like in the article.
The case is not done by myself and gets so nicely warm....
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Old 1st February 2003, 12:00 PM   #7
halojoy is offline halojoy  Sweden
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Arrow 25 volt rms says nothing of the Watt

I asked you this before:
Quote:
I am thinking of the load when you are testing.
Are you sure it is 8 ohm. Is it a resistor?
Speakers can have lower impedance in real life.
25 volt rms = 35,3 volt peak
gives
If LOAD is
7 ohm = 89 Watt
6 ohm = 104 Watt
5 ohm = 125 Watt
4 ohm = 156 Watt

No speaker is exactly 8 ohm.
Can be 5, 6, 7 or 8ohm.
----------------------------------------
If you measure BOTH Current and Voltage
you can get Watt, and also see Speaker Impedance at
your test frequency.
Current can be measured over
the small emitter resistors in Output transistors.
-------------------------------------------------------

If you have a Power Resistor on Exactly 8 ohm
that can take 100-200 Watt,
than you only have to measure VoltageRMS
to get Power.

/halo - did not get any answer to his LOAD-question
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Old 1st February 2003, 12:02 PM   #8
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Jens,

It is normal to give up 10-12V of headroom with this kind of
circuit, so you should be getting about 35V peak (25RMS) from
the amp. Could your 'scope be wrong?
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Old 1st February 2003, 12:57 PM   #9
joensd is offline joensd  Germany
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Quote:
I asked you this before:
Sorry I havenīt answered to this but if you read my first post carefully youīll see that it doensīt matter if there is a load or not.
Itīs clipping anyway and so it canīt be the overload-protection of the crescendo.

Regarding the measurements :
Yes, there must be something wrong and I donīt think itīs the Fluke. The scope is old and should use one of my friends to finally check again.


Quote:
It is normal to give up 10-12V of headroom with this kind of circuit
Really? I actually canīt tell cause Iīm not that experienced but it just seems a little too low for me.
Somebody on the forums who build the crescendo was stating as well that the output swing should be more than +-40 (80Vpeakpeak with a sinewave).

Iīll sleep over this one, go for another scope, wait for Elektor to answer and savour the amplifier till then.

Thanks to all of you
Regards
Jens
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Old 1st February 2003, 01:32 PM   #10
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Hi Jens,

yes it is normal to loose 10..12Volts with this circuit, roughly speaking you loose the gate voltage of a FET and the voltage of the zener in the second stage (I canīt figure out the number on my monitor...)

So you loose 5V more than expected. I think you should check
1. if the zener has the wrong voltage
2. if the second stage is biased wrong. Current should be somewhere around 40mA (if I didnīt misread values or made other mistakes), and if it is significantly lower (around 13mA my guess...) than you have the problem. You can take out R30/R31 to see if the clipping gets away (may get unstable though), because at 30 V they will take up all the current (if itīs 13mA) and the voltage cannot go higher. So my guess - wrong bias (check LEDīs).

Hope this helps
Andy.
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