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Old 24th August 2005, 09:40 AM   #21
tlf9999 is offline tlf9999  United States
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I am looking at your last version. a few comments:

a) Vce on T2/T3 appear to be too small. Ic going through T1/T5 is about 1.6ma. so voltage drop over R4/R8 is about 750mv. Voltage over R8 is about 700mv (voltage over D1/D2). As such, Vce on T2/T3 is about 650mv. I tend to run them a little bit higher (in the 1.5v range). part of the reason is high degenerate resistors R4/R8: i use 22ohm in general.
b) this is less of an issue with CCS but I usually set the current going through the Vbe resistors at about 1/10 of the current going through the whole Vbe setup. In this case, I would dial them up about 10x. Using a small capacitor here (0.1-1u) will help as well.
c) R16/R19 seem to be too big.
d) I usually make R21 identical to R22, for reasons I elaborated in my post earlier. I also used the same value for R20 and R22. IRFP140 class mosfets drop about 4ma (peak) gate current at 20khz, and I usually run my driver stage at about 15 - 20ma.
e) game resistors for the mosfets are very small. I use 110ohm to just slow down the mosfets.

Hope it helps.
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Old 24th August 2005, 01:15 PM   #22
quasi is offline quasi  Australia
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Quote:
Originally posted by tlf9999
I am looking at your last version. a few comments:

a) Vce on T2/T3 appear to be too small. Ic going through T1/T5 is about 1.6ma. so voltage drop over R4/R8 is about 750mv. Voltage over R8 is about 700mv (voltage over D1/D2). As such, Vce on T2/T3 is about 650mv. I tend to run them a little bit higher (in the 1.5v range). part of the reason is high degenerate resistors R4/R8: i use 22ohm in general.
b) this is less of an issue with CCS but I usually set the current going through the Vbe resistors at about 1/10 of the current going through the whole Vbe setup. In this case, I would dial them up about 10x. Using a small capacitor here (0.1-1u) will help as well.
c) R16/R19 seem to be too big.
d) I usually make R21 identical to R22, for reasons I elaborated in my post earlier. I also used the same value for R20 and R22. IRFP140 class mosfets drop about 4ma (peak) gate current at 20khz, and I usually run my driver stage at about 15 - 20ma.
e) game resistors for the mosfets are very small. I use 110ohm to just slow down the mosfets.

Hope it helps.

a) In practice the current through R4 & R8 is about 1.3mA. I prefer to have larger value resistors here for better linearity. No reason why the amp would not work with a variety of values here.
b) I am not sure which part of the amp you are talking about, i.e. which Vbe.
c)Yes, these in fact can be removed and replace with links but T10 and T9 actually need very little drive, because there is less load on them than if they were driving a transistor output stage. The base resistors afford some protection to previous stages. But hey each to his own.
d)The value of R21 was selected during bench tweaking. I found the 100 ohm value gave the best performance in terms of symmetry, clipping and linearity.
e) I use lower value gate resistors in order to keep the drive impedance to these FETs low. The FETs used have a fairly high input capacitance. If FETs with a lower input capacitance are used then higher value resistors could be used. Again each to his own.

Thank you for your observations.

Cheers
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Old 24th August 2005, 01:44 PM   #23
cd-i is offline cd-i  Romania
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hello guys


Again, i like this schematic but the power level is too high for me. I only need a power around 100 - 150w maximum.

My question is how can i reduce the power but keep the performace?

A solution is to reduce the supply voltage and the number of power transistors (to use only two pairs or even one pair of MOS-FETS)

Another solution is to reduce the gain of the amplifier (and perhaps the number of output transistors?)

I like the idea of reducing the gain but i don't know if that is the best solution because i have no experience in designing power amplifiers.
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Old 24th August 2005, 01:59 PM   #24
quasi is offline quasi  Australia
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Quote:
Originally posted by cd-i
hello guys


Again, i like this schematic but the power level is too high for me. I only need a power around 100 - 150w maximum.

My question is how can i reduce the power but keep the performace?

A solution is to reduce the supply voltage and the number of power transistors (to use only two pairs or even one pair of MOS-FETS)

Another solution is to reduce the gain of the amplifier (and perhaps the number of output transistors?)

I like the idea of reducing the gain but i don't know if that is the best solution because i have no experience in designing power amplifiers.

Sorry cd-i

Forgot to answer your question.

If you use +/- 50 volt rails this module will deliver just over 110 watts into 8 ohms.

You will need a transformer with 38 volt windings plus a 25 amp bridge rectifier and a minimum of 10,000 uF capacitors per rail per module.

You can use the existing board layout and just use 4 FETs.

You could reduce the gain of the amplifier by changing R17. The value of this resistor divided by 1000 (R18) gives you the gain. I would not go below 22k.

Cheers
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Old 24th August 2005, 11:03 PM   #25
cd-i is offline cd-i  Romania
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hello, quasi


I made up my mind. I will build the amplifier developed by you and presented here

http://www.diyaudio.com/forums/show...8232#post498232


Obviously I will build the schematic posted by you at the end of that thread after you worked your magic on it

I will also use the printed circuit board you designed for this schematic.

At first i will build the amplifier "as is" and after i see it working i will try to reduce the power.

Anyway this will take some weeks due to my school exams session that begins next week.



Thanks for your help.
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Old 24th August 2005, 11:25 PM   #26
tlf9999 is offline tlf9999  United States
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Quote:
Originally posted by quasi
a) No reason why the amp would not work with a variety of values here.

You want to have enough Vce to maintain linearity. at 650mv, it is dangerously close to its saturation voltage.

Quote:
b) I am not sure which part of the amp you are talking about, i.e. which Vbe.

Sorry. I was talking about the Vbe multiplier.
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Old 24th August 2005, 11:27 PM   #27
tlf9999 is offline tlf9999  United States
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Quote:
Originally posted by cd-i
At first i will build the amplifier "as is" and after i see it working i will try to reduce the power.

you can just build it, with maybe a pair of output devices. you can always parallel a resistor with the feedback resistor to dial back the gain.
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Old 25th August 2005, 12:23 AM   #28
quasi is offline quasi  Australia
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Hey cd-i

The link you posted did not work, I presume you mean this one.

http://www.diyaudio.com/forums/attac...amp=1111237713
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Old 25th August 2005, 01:22 AM   #29
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funberry-

"Ok here's another:
citation amp"

man that is a Pass Classic!, these articles are awesome!
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Old 25th August 2005, 08:09 AM   #30
cd-i is offline cd-i  Romania
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hello quasi,

yes that's right i posted the wrong link. I'm sorry but i'm a new member of this forum and i don't ktow wery well how to use all it's facilities. But don't worry i'm young and i'm lerning


Thanks again for the schematic. I can't wait to see it running !!!

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