| joensd |
Hi all,
just am about to finish my actual project: a crescendo ME.
The sound is astoundingly good->Iīm happy :nod:
BUT : Turning on the oscilloscope for some final checks made me :bawling: .
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 ...:scratch:
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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| costiss |
| 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?? :confused:??:confused: |
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| costiss |
| 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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| halojoy |
| 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 :nod:
BUT : Turning on the oscilloscope for some final checks made me :bawling: .
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 ...:scratch:
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
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| joensd |
| 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 ****** 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. :nod:
Thanks a lot so far
Jens |
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| joensd |
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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| halojoy |
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 :mad: |
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| Joe Berry |
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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| joensd |
| 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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| AndyM |
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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| wuffwaff |
Hi Jens,
30 or even 35 volts is a little low. I get 24.5 volt RMS with a 42 volt supply so Iīm loosing not as much as you do (itīs the older design with 2sj50/2sk135 though).
One thing you can do is to make a seperate supply for the driver and input stage about 4 to 5 volts higher (so 50-52 volts). This will give you a bit more efficiency and better sound quality.
william |
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| joensd |
| quote: | 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. |
That sounds so good that it might be the failure. :bulb:
Over D3 and D4 I measured 2V if I remember correctly.
(I canīt stop listening to the amplifier so it goes from the workbench directly back into the rack ;-))
These diodes are rated 3.9V/1.3W but Iīll exchange them as soon as possible to 0.5W for proper bias.
The LEDīs are properly thermally coupled as given in the schematic but Iīll check them for correct bias.
Many thanks
Jens |
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| joensd |
| quote: | Originally posted by wuffwaff
Hi Jens,
30 or even 35 volts is a little low. I get 24.5 volt RMS with a 42 volt supply so Iīm loosing not as much as you do (itīs the older design with 2sj50/2sk135 though).
One thing you can do is to make a seperate supply for the driver and input stage about 4 to 5 volts higher (so 50-52 volts). This will give you a bit more efficiency and better sound quality.
william |
Hi William,
as another crescendo fan I hoped you might jump in for some advice.
A separate supply is a good idea but if thereīs any mistake I wanna have that repaired before I begin to tweak anything.
As i donīt really need the full power anyway I think I can live with it at the moment.
Regards
Jens |
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| joensd |
| quote: | | As i donīt really need the full power anyway I think I can live with it at the moment. |
Uups, how confusing of myself! :confused:
I was relating to Williams tweak of course and still want my amp to work properly.
Measured the voltage over the zeners D3 & D4 (3.9V/1.3W) again and got 2.8V.
Shouldnīt it be like 3.9V to have constant value or is it just a limiter to not exceed the 3.9V?
Jens |
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| wuffwaff |
Hello Jens,
yes it should be 3.9V all of the time. Are you shure youīve got enough current flowing through them? Try changing the resistor feeding them to get a constant value (lower itīs value by a factor 2 or so)
william
R24 and R25 5k6 should do the job. |
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| joensd |
| quote: | Originally posted by wuffwaff
Hello Jens,
yes it should be 3.9V all of the time. Are you shure youīve got enough current flowing through them? Try changing the resistor feeding them to get a constant value (lower itīs value by a factor 2 or so)
william
R24 and R25 5k6 should do the job. |
Changed the resistors to 6k and voltage over the zeners is 3.0V.
Getting an old book about electronics out it says a zener has to be biased with 0,1*Imax. In case of my 3,9V/1,3W zener diode this comes up to Ptot/3.9V*0,1~30mA which is a lot for the resistor to cope with at 42V and nonsense anyway so Iīll definitely go for a 0,5W diode which should be happy with about 12mA current.
Thanks
Jens |
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| joensd |
Hi,
back here to update the amplifier lacking output power.
Changed the D3,4 Zeners to 0.4W types and now the bias current of those is high enough so they give a proper 3.9V.
Output voltage of the amp rised to about 27VRMS which gives a good 90W. Yippie!
The treble response of the amp got much better with that change so it had to do with the failure as well.
For those who are interested and need some measurements of the crescendo ME design here the quote of Elektorīs email:| quote: | Hello,
Some measurements:
Related to ground or over resistor:
(0 V =< 10 mV, all measurements +/- 10 %)
R3: 0 V; R17,R20: 1,05 V; R8,R9,R13,R14: 1,9 V; between R8 - R9 (R13 - R14):
< 100 mV; R25,R27: 1V17; Gate T12 - Gate T13: 4 V; R34,R35 : 50 mV
(quiescent current somewhat higher); Tp3: 0 V, Bias : < +/- 10 V
The peak output voltage should be around 38 V for 90 W in an 8 ohm load. So
there might be some thing wrong. The gate-source voltage in the output stage
and the setting of T7/T9 and T8/T10 is the cause for a 10 V drive loss.
This is inherent to the design.
Ton Giesberts
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Thank you so much for all the help and ideas.
Jens - listening to his amp until he has to eat something |
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