Types of current sources for audio amplifier applications

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danny92

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Joined 2012
#1
Hi everyone,

I'm a newbie at audio amplifiers and I'm studing in Bob Cordell's book "Designing Audio Power Amplifiers", I've only made two designs but never tested it yet.
I've a question to do:
Which is the best current source, in terms of linearity, thermal stability and output resistance?

In both books I've got from Bob Cordell and Randy Slone the best current source is allways the feedback current source, in which both transistors act together to mantain their collector currents. Another form of making a current source is using a current mirror with a known voltage and a resistor (I think that this could also be made with a wilson current mirror or enhanced current mirror), this current mirror should be degenerated with 10:1 degeneration or a little more. The other type of current source which is also an acceptable design is the current source based on diode anode-cathode (Vak) voltage, you could use normal silicon diodes such as 1N4*** series, a led or a zerner diode.

Thank you very much for your atention,
Best regards,
Daniel Almeida
 
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#2
Some images of the current sources.
 

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#4
Hello Bonsai and thank you very much for your help,

I can use two feedback current sources one for the LTP and another one for the TIS, instead of zener current source for ltp and feedback current source for TIS?

Best regards,
Daniel Almeida
 
#6
Thank you very much for posting, this project with depletion-mode MOSFET seems very interesting, those CCS have very good output impedance characteristics even at higher frequencies, but I've a few questions to make:

The output current in those current sources are adjusted by the current set resistor, right?

What's the minimum current that can be set?

Whats's the minimum operational voltage?

The MOSFET draws current with the gate shorted to ground?

Thank you very much for your help,
Best regards,
Daniel Almeida
 
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#7
Thanks for the review of these topologies. I had seen some of these in schematics here, wondered what they were for. My transistor books are very old. Wikipedia constant current source wasn't very useful, had one schematic, a transistor with a zener on the base leg. To keep my parts count down I ended up using JFET's with a gate tied to the source, a part which went obsolete last month, 2000 minimum.I have one J174 left. !@#$$%$^ Yes, I can buy them somewhere else, for $8-10 freight.
 
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#8
danny92 said:
Hello Bonsai and thank you very much for your help,

I can use two feedback current sources one for the LTP and another one for the TIS, instead of zener current source for ltp and feedback current source for TIS?

Best regards,
Daniel Almeida
Click to expand...

Of course.

The reason I like. Zener reference is tat it is extremely wide band. You do need to a. filter it and b. decouple it so that means two extra caps though.

My second favorite CCS is the two transistor active type.
 
#10
Bonsai said:
...
The reason I like. Zener reference is tat it is extremely wide band. You do need to a. filter it and b. decouple it so that means two extra caps though.

My second favorite CCS is the two transistor active type.
Click to expand...

Someone did a series of simulations of different current sources to choose for an amp. That was you IIRC?
Did you ever compare any of the commercial CCSs? Seems like a suitable use for an IC.
Also did you miss my question on your transformer thread?
And best source of power supply capacitors -about 10000Uf 100V - to use with it?

Best wishes
David
 
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#11
danny92 said:
The output current in those current sources are adjusted by the current set resistor, right?
Click to expand...

Yes.

danny92 said:
What's the minimum current that can be set?
Click to expand...

1 uA to 10mA with LM334. Look at the datasheet. For higher current see LT3092 or use a trimmer.

danny92 said:
Whats's the minimum operational voltage?
Click to expand...

Just a few volts (1.8~2.2 DN2540, 2.9~3.5 10M45S).

danny92 said:
The MOSFET draws current with the gate shorted to ground?
Click to expand...

Yes, depletion mode (normally on). Make drain positive vs. gate to turn off.

Very, very good CCS. Ixys IXCP 10M90S is good for 900V.
 
#12
Of course.

The reason I like. Zener reference is tat it is extremely wide band. You do need to a. filter it and b. decouple it so that means two extra caps though.

My second favorite CCS is the two transistor active type.
Click to expand...

Zener references need lots of filtering because they are noisy. This is why I prefer a low Goss J-fet bias at Idss, cascode with a high Goss J-fet in series with a resistor||small COG cap. This is how Borbely does it. Although the output Z may be higher, this makes a very quiet voltage reference. If you like J-fets as much as I do then this is not a problem, using another J-fet as a buffer. I honestly cannot see how anyone who designs audio circuits could not like J-fets, they are so wonderful.:hug: I use them for whatever, whenever I can.😎

BTW, N-ch J-fets typically are less noisy. Keeping a constant Vds for the low Goss device helps a lot, it is worth the cascode. J177 would make a good P-ch source, cascode that with the J174.
 
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#18
danny92 said:
Hi everyone,
Which is the best current source, in terms of linearity, thermal stability and output resistance?
Click to expand...


Firstly, all current sources are negative feedback current sources. What Bob refers to as the negative feedback current source is, in fact, an amplified negative feedback current source.

The amplified negative feedback current source would be my first choice for both the TAS and the TIS, as its output impedance is very high, approaching that of a cascode source.
 
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