Does this design require selected JFETs? - diyAudio
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Old 10th August 2003, 01:34 PM   #1
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Default Does this design require selected JFETs?

I found the attached schematic on a site that is unfortunately now closed. Since I am unable to contact the designer, I was hoping someone here could help me with building the design.

1. Could someone please tell me if the JFETs in this buffer, or in the power supply, require selecting for a specific IDSS? If they do, roughly what IDSSs should I be selecting for?

2. At this time, I do not own a multimeter that can measure current. Is there still a way for me to measure IDSS? The only circuit I have been able to find to measure IDSS is on the Borbely website.
http://www.borbelyaudio.com/ae599bor.pdf

Thank you in advance.
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Old 10th August 2003, 01:52 PM   #2
SY is offline SY  United States
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Interesting topology- and not that different than what I do with my tube-FET hybrid stuff. Yes, you'll want to match Idss and, if you can, Vp. One way to kill two birds with one stone is to just switch FETs in and out of this circuit and select for equal currents from both halves.

You can measure current by sticking a small value resistor in series with the drain and measuring the drop. Alternately, you can set the FET up as a current source on a breadboard for something near the current used in this circuit using a source resistor for adjustment, then switch FETs in and out, selecting for best current match (as determined by the voltage across the source resistor).
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Old 10th August 2003, 05:41 PM   #3
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Hello -

To my eyes, the audio circuit is pretty straightforward. Basically you just have complementary source followers driving complementary emitter followers. So it is just a unity-gain buffer. The extra bipolar transistors act as current sources for the source followers.

If unmatched JFETs are used, there will be a DC offset, but that will be blocked by the coupling cap. Ideally, they will be matched, but another way to approach this problem would be to change R10 and R11 to trimpots. Then you could dial in the desired operating current for each FET.

The power supply is kind of wacky, in my opinion. He has a current source feeding a shunt regulator, which is a good approach. I don't really like zeners as they add a lot of noise. The other oddity is that the source followers are supplied through a pretty high resistor value, which is not the ideal approach.

Good luck,
Charles Hansen
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Old 10th August 2003, 07:04 PM   #4
jcx is online now jcx  United States
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For all of the circuit symmetry the design is very “unbalanced” in a engineering design sense:

Low noise fets with hot bias current of 15 mA, noise potential is wasted by 650 Ohm output resistance thermal noise (and some fets will not even have Idss of 15 mA)

1:1 current mirrors waste current and just copy supply noise rather than reduce it in the fet bias current sources

Complemetary fets are not in fact symmetrical enough for any reasonable distortion cancellation - real improvements would be cascoding fets and linearizing output

Bipolar output bias is a function of the sum of 6 different voltages, the 2 fet Vgs being the most variable by device, the output bias has no trim provision

With little clue to intended output stage bias and load it is possible that the bipolar transistors will dominate the distortion (or at least add odd harmonics) which would again seem to not fit philosophically with the rest
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Old 10th August 2003, 09:55 PM   #5
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Lightbulb J-fets vs bipolar

JCX- that's right.Greyhorse:I mean that better is do this circuit with bipolar transistors on positions of J-fets.If you will see on datasheets of high speed buffers(Burr-Brown,Harris etc.),you will get inspiration. Certainly is better "buildt" this circuit to the feetback loop of any operational amplifier-distortion will be much lower.
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Old 10th August 2003, 11:28 PM   #6
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You should keep the jfets. The reason that you will not see them in IC's is that they can't make them complementary. Almost all discrete high end circuits use comp. jfets, rather than bipolar transistors. Matching is recommended, but it would probably work without close matching. The circuit is either idealized or compromised, depending on your position, by leaving out self leveling gate-source resistor in the current sources in the power supply.
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Old 13th August 2003, 06:08 AM   #7
jcx is online now jcx  United States
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actually regardless of the general desirability of jfets, you can safely toss them out in this circuit, J1,2 will always be forward biased (ie acting as diodes) for the power supply and bias resistor values shown, R18,19 will drop more than half the supply volts from the Q1,2 and Q3,4 current source current flowing through them with the given R1,2,10 and R3,4,11 values
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Old 13th August 2003, 06:18 AM   #8
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Default This is annoying...

Greyhorse originally asked about whether the FETs needed to be matched in this circuit. Somehow the thread devolved instead into a critique of the circuit and JFETs. Many of the opinions expressed here are unfounded and without merit. But when jcx posts that "you can safely toss [the JFETs] out" because "they will be ... acting as diodes", that is just plain wrong and needs to be corrected.

There is nothing so annoying as an instant "internet expert" that spouts off meaningless nonsense. The original poster may or may not be able to distinguish the helpful advice from the useless BS. I've got better things to do than to try and monitor the dispensation of useless information. So my request is to keep from posting unless you *really* know what you're talking about.

Charles Hansen
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Old 13th August 2003, 07:16 AM   #9
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Default Re: This is annoying...

Quote:
Originally posted by Charles Hansen
There is nothing so annoying as an instant "internet expert" that spouts off meaningless nonsense. The original poster may or may not be able to distinguish the helpful advice from the useless BS. I've got better things to do than to try and monitor the dispensation of useless information. So my request is to keep from posting unless you *really* know what you're talking about.
You wouldn't mean meaningless nonsense like... those open-loop emitter followers you'd mentioned before would you?

se
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Old 13th August 2003, 09:06 AM   #10
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Default I agree with Charlie........

Add SE's nonsense to the list.

Hey Steve......you might try building something for a change instead of running down everyone else. Especially people who know a lot more than you do.

Jocko
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