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Old 13th August 2003, 06:56 PM   #21
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Quote:
Originally posted by SY
Guys, I'd really appreciate it if you'd stick to the technical issues and cut out the sniping. The technical portions are interesting and well-argued. The sniping back-and-forth is silly and childish.
Hi Sy,
I don't feel the technical issues are interesting and already well treated by John Curl.
The sniping about a non-excistant technical isssue is why I am out.

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Old 14th August 2003, 06:44 AM   #22
jcx is offline jcx  United States
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Default (mostly) peace Charles

I thought I was displaying my technical “point” about the bias openly and certainly didn’t expect anyone skewer themselves with it, I can’t fault anyone for missing the bias detail on a first pass look at the schematic since I clearly overlooked it in my first post even with the too large current source value central to my criticism

Suggesting tossing the fets is exactly the sort of witticism I would use or expect to receive in pointing out the bias error between engineers (not to mention a few lunch time asides like: “no he’s not Biased, <pause> …or at least not Reverse biased”)

I would expect other electronic engineers or technicians to quickly pick up the error and move on, however I can’t recommend the circuit to a hobbyist at Greylord’s level (apologies if I’m assuming to much from your question about current measurement)

The circuit clearly needs more engineering to work at all, much less at all well

I would genuinely like to hear an explanation by an experienced discrete jfet design practitioner on the technical reasons for the perceived audible superiority and how to handle the huge device variability and 2 dimensional matching requirements to get repeatably superior results. Borbley just scratches the surface of what I would like to know and doesn’t really go beyond the National and Siliconix app notes I read 25 yrs ago

So how about some really technical audio design contributions from “real experts”*


* least anyone think I’m being too generous (or am over medicated) – rest assured that the phrase “p*r*i*c*k*l*y Prima Donna” was muttered occasionally while composing this post and not just with Charles’ overreaction in mind
[no I don't mean to emphasize the four letter word, which is not the root meaning of this word; I just refuse to let stupid filter software spoil my alliteration]
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Old 14th August 2003, 07:58 AM   #23
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Cool Help

Hi JCX,
maybe this is helpfull (in French)
http://perso.club-internet.fr/drouvot/Plantefeve.htm

http://haute.fidelite.com.online.fr/...to-JFgros.html

and the source:
http://perso.wanadoo.fr/jm.plantefeve/sche.html

AS the circuit is symmetrical I would match the JFET's for IDSS and Gm. Best contact Jean-Marc Plantevève in French. Or maybe in English......
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Old 14th August 2003, 05:56 PM   #24
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Quote:
Originally posted by jcx
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
Hello JCX -

Thanks for the nice query. I'm actually quoting your previous post, as that has the most relevant questions. Here is my take on what the appropriate answers are:

1) If this were a mic preamp or a phono preamp, this might be a valid criticism. Since it is a unity-gain buffer, it is safe to assume that the noise contribution from a 650 ohm resistors is negligible.

2a) I don't think the idea of "wasting" 15 mA of current in a current mirror is pertinent, except in the case of battery-operated equipment.

2b) The current mirror won't "copy supply noise". The reference current for the mirror comes from ground. Any fluctuations on the rail are common to both "halves" of the current mirror and will tend to be rejected.

3a) Complementary FETs are, in fact, close enough complements to get good distortion reduction.

3b) You can't cascode the FETs, because they are followers. I'm not sure what you mean by "linearizing the outputs".

4) The biggest variable in the output stage bias will be the variability in Vgs in the FETs. Even this will not be too big, as the specified Toshiba FETs are pre-sorted by Idss. However, by using 100 ohm emitter resistors for the outputs, any variability is minimized. While there still will be some minor variation in output bias from sample to sample, I don't think this will be an issue in real life. If one were worried about it, you could either match FETs or change the 750 ohm resistors in the current sources to trimpots.

5) The output bias will be roughly 10 mA. This means that the output stage can drive a short circuit (through the 650 output resistor) and still stay in class A up to 24 V P-P. This almost to clipping, so I think there isn't really a problem here.

In your later post you asked "why FETs"? One advantage is it's about the only safe way to avoid the requirement for an input coupling cap. Another advantage is much greater linearity at RF frequencies. This helps avoid sonic nasties from the real world, where we are bathed in RF from scores of sources. Jocko has done some measurements in this regard, although he may not want to share them.

The bottom line is that they sound good. Most people don't use them simply because they don't understand them.

Best regards,
Charles Hansen
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Old 14th August 2003, 06:37 PM   #25
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Jocko, who just has to be different, uses BJTs in his designs.

And then has to take steps to reduce the minute overshoot problems.

Yeah, one of the few palces that I do not use JFETs in a low-level circuit. Sometimes I wonder why.

Then I remember how I can play games with the bias in the final stage easier without screwing up the offset.

Jocko
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Old 14th August 2003, 07:55 PM   #26
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Default Jfet is a four letter word

I must profess to be another "Mosfet and Jfets sound better" advocate. Biasing and matching really a non issue for one off and small quantity circuits. The Toshiba output stage BJTs are great but I will generally stick to fets in general. even for current sources and cascodes. Almost all the reported better sounding op amps have Jfet inputs. You will see a very definite trend to the use of Jfet and Mosfet front ends in lot of amplifier designs. The increased immunity to RF intermod is what attracted me to them from a design point as well as the better sound. RFs effects on audio circuits is one of the most important influences to how they sound and is one of the least understood factors by most audio designers. It is also the most lied about aspect of audio cable design.

I believe p*r*i*c*k*l*y is actually based on a five letter word. I have no idea what a Prima Donna is since I am known for my easy going conformity to the desires of the group and the term has never come up.......... Get off the floor and settle down before you hurt yourself Jocko!

Fred (another four letter word)
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Old 14th August 2003, 09:30 PM   #27
bocka is offline bocka  Germany
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Hi JCX,

first of all nice to see you back Fred although I do not confirm to a lot of your opinions. But maybe that becomes from the way we realise our audio equipment. Most often there is more than one way to get the desired result.

JCX you asked why FETs sounds superior to BJT. Well, look to nearly every implemenented amp design. You see BJTs most often, first of all they are cheap, second some BJT parameters vary only a very bit. This makes designs stable and rugged to parameter stray. FET design off chip is more difficult than on chip. Of course this has nothing to do with audible differences between BJT and JFET/MOSFET.

Ask yourself why do you need an open loop gain of about 100db. Most topologies do so. I can't see any reason for it in audio circuits. Try to lower open loog gain. Not only with local feedback, lowering topology gain is better. BJTs have more gain than FETs...

Intelligent use of BJTs AND FETs is the way to superior sound. Use as less as possible parts in the signal path. My designs only have a single amplification stage and only 3 (preamps) to 5 (power-amps) components in the signal path. Compare this to what you see in most commercial designs. And compare again to what tubes do. To my experience you can't do this, if you are only using BJTs or FETs. Combine them in the most intelligent way.

Yes, RF immunity is important, very important, use RF Filters, although they are inside the signal path. That's it.
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Old 15th August 2003, 12:18 AM   #28
jam is offline jam  United States
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How about this

1) Add a trimpot across R2 to adjust for DC offset and remove coupling capacitor.

2) Replace R10 and R11 with 4ma current diodes.

3) Attach fet drains to junction of output transistors and 100ohm resistors.

Would this give better performance and solve some of Greyhorse's problems?

Regards,
Jam
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Old 15th August 2003, 12:33 AM   #29
jam is offline jam  United States
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Default Oh! Oh! Steve is in trouble......

Here is one for you Steve.
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Old 15th August 2003, 02:00 AM   #30
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Quote:
Originally posted by jam
How about this

1) Add a trimpot across R2 to adjust for DC offset and remove coupling capacitor.

2) Replace R10 and R11 with 4ma current diodes.

3) Attach fet drains to junction of output transistors and 100ohm resistors.

Would this give better performance and solve some of Greyhorse's problems?

Regards,
Jam
Hello Jam -

It won't work as you described. Build it and see why not. (Here's a hint -- it will probably work OK if you simulate it.)

Charles Hansen
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