I am trying to adjust the bias on my solid-state amp. The instructions says to measure the voltage across the emitter resistors.
I interpreted the emitter resistor to be any resistor connected to emitter like R6, R13 or R17 in the schematic. However the voltage drop across these is about 700mV and the bias pot has no effect on them. So I guess I have it wrong.
I would greatly appreciate you pointing out across which points I should measure. The manual states the following for bias adjustment -
Bias adjustment:
Note: Bias adjustment should be made without any load connected to the amplifier’s output, and no signal connected to the amplifier’s input.
1. Connect a multimeter to one of the emitter resistor leads and set the multimeter to mV scale.
2. Turn trimpot R16 counterclockwise until it reaches 0. A click should be heard.
3. Turn on amplifier, and let it warm up for 15 minutes on idle.
4. Adjust trimpot R16 to get a 14mV reading.
5. Let the amplifier warm up for a 24 hours period with signal and load.
6. Remove signal and load, and reconnect the multimeter, setted to mV scale, to one of the emitter
resistor leads.
7. Readjust R16 to 18mV
I interpreted the emitter resistor to be any resistor connected to emitter like R6, R13 or R17 in the schematic. However the voltage drop across these is about 700mV and the bias pot has no effect on them. So I guess I have it wrong.
I would greatly appreciate you pointing out across which points I should measure. The manual states the following for bias adjustment -
Bias adjustment:
Note: Bias adjustment should be made without any load connected to the amplifier’s output, and no signal connected to the amplifier’s input.
1. Connect a multimeter to one of the emitter resistor leads and set the multimeter to mV scale.
2. Turn trimpot R16 counterclockwise until it reaches 0. A click should be heard.
3. Turn on amplifier, and let it warm up for 15 minutes on idle.
4. Adjust trimpot R16 to get a 14mV reading.
5. Let the amplifier warm up for a 24 hours period with signal and load.
6. Remove signal and load, and reconnect the multimeter, setted to mV scale, to one of the emitter
resistor leads.
7. Readjust R16 to 18mV
The emitter resistors you are looking for are those of the output transistors which do not seem to be included
on this drawing. They may even be located at another point and detailed elsewhere. They will be the largest
transistors and mounted to a heat sink
Instruction (1) is incomplete but you must take a reading across both terminals of a resistor in order to get any
reading at all. Note that the polarity of your reading depends on choice of probe connection but is not important here.
on this drawing. They may even be located at another point and detailed elsewhere. They will be the largest
transistors and mounted to a heat sink
Instruction (1) is incomplete but you must take a reading across both terminals of a resistor in order to get any
reading at all. Note that the polarity of your reading depends on choice of probe connection but is not important here.
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Ian, thank you so much.
So in that case the resistor marked with "18 1W" would be the one right ? It is surprising that all the other resistors have a name but not this resistor or the lower transistor.
Azi
So in that case the resistor marked with "18 1W" would be the one right ? It is surprising that all the other resistors have a name but not this resistor or the lower transistor.
Azi
No, that's not correct... You see only the first transistors or perhaps buffer stage on this diagram. What you are looking for is not on this drawing at all but follows on from the connectors marked "positive" and "negative".
The resistors will be 2-5 watt types of a fair size. The value will be below 1R or just 1.0 etc - even big square chunky ceramic block
types are likely. Remember, you are physically looking for big parts - heatsink etc. and not on this drawing.
The resistors will be 2-5 watt types of a fair size. The value will be below 1R or just 1.0 etc - even big square chunky ceramic block
types are likely. Remember, you are physically looking for big parts - heatsink etc. and not on this drawing.
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I don't understand that schematic.
There are what appear to me to be cascodes fitted to the input LTP and to the CCS for the VAS.
But the voltage feed to the bases of these cascodes comes directly from the -ve supply rail.
What type of circuit is that?
Now back to the bias.
They are stating 18mVre.
I suspect that Re = 0r1 have been fitted to each output emitter.
There are what appear to me to be cascodes fitted to the input LTP and to the CCS for the VAS.
But the voltage feed to the bases of these cascodes comes directly from the -ve supply rail.
What type of circuit is that?
Now back to the bias.
They are stating 18mVre.
I suspect that Re = 0r1 have been fitted to each output emitter.
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Perhaps Classe aren't too sure now either. It may just be attempted
cascode elaboration that performed well enough at the time. ...Gaetan?
cascode elaboration that performed well enough at the time. ...Gaetan?
One that skimps on components, it will appear. The voltage offset is provided by base current times base resistor, something that seems a little risky in the light of transistor beta tolerance and aging.
> One that skimps on components, it will appear.
My knowledge of circuits is far elementary from yours, so I will not argue with you there. However, judging from external appearances alone, it seems unlikely that they skipped on components. This thing is majorly overbuilt with really large heatsinks, custom made 140, 0000 uF capacitors, the output section is 2400Watts/360 Amps per channel- it weighs 60 lb.
I think the DR-9 sold for about 3500USD when they debuted and evn in todays used market they fetch about 1200 USD. Decent reviews too.
Once again, I cant judge them on their design merit, I am citing the other indicators.
BTW, I did manage to get the other pages of teh schematic. So, for anyone else looking for the DR-9 manuals, check here http://bwgroup-support.com/classeservicemanuals.html If you look at any of teh other amps- DR-15 etc they have a similar output stage and you can use those to understand the DR-9 layout.
I threw in the towel and gave it for servicing- this was before I found the site. I will atttach a ciopy of the scehmatuic in my next post.
My knowledge of circuits is far elementary from yours, so I will not argue with you there. However, judging from external appearances alone, it seems unlikely that they skipped on components. This thing is majorly overbuilt with really large heatsinks, custom made 140, 0000 uF capacitors, the output section is 2400Watts/360 Amps per channel- it weighs 60 lb.
I think the DR-9 sold for about 3500USD when they debuted and evn in todays used market they fetch about 1200 USD. Decent reviews too.
Once again, I cant judge them on their design merit, I am citing the other indicators.
BTW, I did manage to get the other pages of teh schematic. So, for anyone else looking for the DR-9 manuals, check here http://bwgroup-support.com/classeservicemanuals.html If you look at any of teh other amps- DR-15 etc they have a similar output stage and you can use those to understand the DR-9 layout.
I threw in the towel and gave it for servicing- this was before I found the site. I will atttach a ciopy of the scehmatuic in my next post.
Here's the schematic for teh output stage. So the resistor in question is it acroos the two .27ohms, or is it one of them- which one then ?
Thanks again for helping out.
The measurement is between the positive speaker terminal and any of the 0.27 ohm resistors/emitter junctions. There is a discrepancy between circuit diagram and explanation as they talk about 21.5mV here.
The test points I suspect is P4 to P7and bias should be set to 21.5mV at any of the test points P4-P7 if you have a DR-15 power amplifier. This recommended setting here of 21.5mV provides about 0.8 watt class A operation and almost conforms to Nelson's first watt theory. I would set it to 24mV then the first watt will be the proverbial class A.
The test points I suspect is P4 to P7and bias should be set to 21.5mV at any of the test points P4-P7 if you have a DR-15 power amplifier. This recommended setting here of 21.5mV provides about 0.8 watt class A operation and almost conforms to Nelson's first watt theory. I would set it to 24mV then the first watt will be the proverbial class A.
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These are very good sounding amps, especially the model with lateral mosfet driving the outputs. Well worth the second hand price.
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