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Old 23rd January 2003, 02:50 PM   #11
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I'm a little bit allergic of designs which are impossible to build without selecting parts. Sometimes it's more or less impossible to get it right. I remember when I made my sub which had JFET input stage. It took me 100-150 fets to find two descent pairs! Not very good as a DIY project unless you want to buy lots of transistors and/or pay someone to match for for.
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Old 23rd January 2003, 08:33 PM   #12
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Hi ,
the Kaneda has incredible high cult status. Word is that it only sounds good with those oldfashioned hard to get FET's like 2SK30 etc.
I do prefer 25912 or 2SK389 dual monolitic.
And a simple emittor follower as ouputstage....?
The double diamond buffer LH0003 style is more to my liking.
Folded cascode sounds better than this circuit and....

The situation is similar to tubes in a way. Only those hard to get NOS extremely expensive ones will bring you to audio Nirvana.
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Old 24th January 2003, 12:07 AM   #13
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Kaneda was and is a prolific writer, and has published many, many designs in the pages of MJ. The one posted here is a very early design - he subsequently shifted to constant-current sinks feeding the input differential, Widlar current mirrors summing the second-stage differential, push-pull outputs, and somewhat later, constant-current output (only for preamps and phono stages, not for power amps). He was a stickler for components, and precisely specified what semiconductors, trim pots, resistors, capacitors, electrolytics and so on should be used. Kaneda also put much effort into power supplies, and experimented with battery operation, AC mains, switching regulators, series regulators, push-pull regulators, and so on.

Today, most of his designs consist of simpler tube-SS hybrid circuits on AC power - a concession, I believe, to the lowered design and engineering abilities of Japan's current DIY community.

I have not built any of Kaneda's later hybrid designs, but based on his 80's-vintage designs, I agree with Elso, that a folded-cascode has greater potential to sound good than a Kaneda-style dual-differential. But as always, careful, intelligent engineering is the key, and a well-executed dual-differential will almost certainly measure and sound better than an indifferently produced folded-cascode.

For a second opinion, I believe that Hartmut Quaschik converted his Kaneda to folded-cascode operation a few years back, although I don't have the particulars of the circuit that Hartmut ultimately used. From memory, Hartmut claimed that the results were a mixed bag - in some ways the folded cascode was preferable, in other ways he preferred the dual-differential.

regards, jonathan carr
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Old 24th January 2003, 12:36 AM   #14
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Jonathan.

I like Elso am a big proponent of the folded cascode, partly because it models as a single stage and correctly implemented sounds good. Could you give us an example of a dual-differential circuit that you like, a schematic perhaps?

Jam
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Old 24th January 2003, 01:45 AM   #15
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The circuit that I like is whatever happens to be most suited for the job at hand.

Folded-cascodes have certain topological advantages that translate into ease of stabilization, good bandwidth, and fast slew-rate. However, they have some clear disadvantages that you should keep in mind.

Discrete-device folded cascode topologies tend to top out at about 100dB open-loop gain max, while you can push a dual-differential to at least 150dB. If you want to make a single-stage active RIAA phono amp with a target closed-loop gain of 65dB, a folded-cascode is not the right choice.

Due to the biasing of the folded cascode (unless you are using a complementary scheme), the clipping behaviour is highly asymmetric. The side of the circuit with the folded cascode will clip long before the other side, meaning that utilization efficiency of the voltage rails ain't good. If you want a circuit that can swing lots of volts from within limited supply rail voltages, a folded-cascode is not a good choice.

It is far easier to minimize the voltages placed across the input devices with a dual-differential than it is with a folded cascode. BJTs are not much of a problem, but due to gate leakage currents, N-channel JFETs usually need to be used well inside their maximum Vds ratings unless you can deal with relatively high amounts of DC offset. The situation is somewhat better for P-channel JFETs, but in general, if you want any serious voltage swing at all from a folded cascode, cascoding the input devices is de rigor. Put another way, the reasons for cascoding the input devices of a folded cascode and a dual-differential are not the same.

In my experience, folded cascodes are more tolerant of power-supply quality than dual-differentials, but differences in grounding practices are more obvious with folded cascodes. And folded cascodes still retain an Achilles heel when it comes to PSRR - the summing current mirror. I believe that it is possible to minimize this PSRR issue by selecting a device with the appropriate parameters, but you are almost certain to require access to SPICE files - data sheets won't suffice.

Speaking of, does anyone know where there is a good collection of SPICE files for small and medium-signal BJTs made by Toshiba, Sanyo, Hitachi, NEC and other Japanese manufacturers?

regards, jonathan carr
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Old 24th January 2003, 06:34 AM   #16
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Default Re: Cult Status

Quote:
Originally posted by Elso Kwak
Hi ,
the Kaneda has incredible high cult status. Word is that it only sounds good with those oldfashioned hard to get FET's like 2SK30 etc.
I do prefer 25912 or 2SK389 dual monolitic.
And a simple emittor follower as ouputstage....?
The double diamond buffer LH0003 style is more to my liking.
Folded cascode sounds better than this circuit and....
Thank you Elso, you said it before me. I have always thought, even in the 80's, that Kaneda seemed to be too good to be true, not really bad but not very advanced if we talk circuit solution. Now I hear that Mr Kanega added some "candy" in his design.

Has the Kaneda amp really earned it's good reputation? Just wondering.
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Old 24th January 2003, 03:50 PM   #17
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Jonathan,

Thanks for your insight of the differences between the folded cascode and dual differential. You raise a several good points particularly about power supply quality.

My first experience with the dual differential was the Kenwood L-07 amplifiers and the Hitachi app. notes for front end design for use with their mosfets. These designs always seemed to be on the verge of instability and required a heoric amount of compensation. Maybe I took the easy way out and should have persued them further.
I did come to the couclusion that the second differential in the dual differential did not work as well with a current source and a current mirror worked better, probably due to current starvation of the next stage ( especially in the case of power amps.)
Any further thoughts?

Jam
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Old 24th January 2003, 04:39 PM   #18
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Jam:

>My first experience with the dual differential was the Kenwood L-07 amplifiers.<

The amps that allowed you to extend the NFB loop out to the speakers? Toshiba had amps with similar technology around the same time (last part of the 1970s, early 1980s), under the Aurex brand. Good idea if you were making a self-powered subwoofer, but not so hot for mid-range and treble duties. In general, this type of scheme requires heavier phase compensation to stabilize the amplifier than would otherwise be required. I don't think the Kenwoods are good or typical examples of dual differential circuits.

>Hitachi app. notes for front end design for use with their mosfets<

Which are pretty mundane circuits by today's standards.

>Maybe I took the easy way out and should have persued them further.<

It probably would have been worthwhile to pursue dual-differential designs further - but as for pursuing designs like the Hitachi application notes, or the Kenwood "feedback amok" monsters, it likely would have been a waste of your time.

>The second differential in the dual differential did not work as well with a current source and a current mirror worked better, probably due to current starvation of the next stage (especially in the case of power amps.)<

Is it possible to be more specific? Do you mean that your second differentials worked better with a current-mirror feeding the emittors instead of a normal current-source? Or are you referring to the signal-summing current-mirror after the second differential?

regards, jonathan carr
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Old 24th January 2003, 04:55 PM   #19
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Jonathan,

I ment a current source on the tail of the differential and a current mirror on the collectors (2nd. differential)

Jam
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Old 24th January 2003, 05:41 PM   #20
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Jam:

"Current starvation" could mean different things, depending on whether you are using a MOSFET or BJT output stage, and in case of the latter, the hfe of the devices, and whether you are using some sort of Darlington or compound transistor arrangement. But as long as you aren't trying to drive a low hfe BJT directly from the collectors of the second differential, and as long as you push enough current through the first and second differentials (you may have to tolerate some extra noise), you should not have any major problems.

I usually use current sources to feed both differentials. To some extent, the amount of current from a current source will change according to the voltage across it. Also, the output impedance of a current source isn't all that high. For these reasons, I cascode any current source intended for an application that I consider to be critical. The cascode bases are fixed, and ensure that the current from the current sources remains constant under all conditions.

I agree that a current mirror works well on the collectors of the second differential. I prefer a current mirror to resistors in this location, and I prefer a Wilson to a Widlar. But even a Wilson has its problems (manifested as poor PSRR). Because the underlying cause is the Early voltage of the devices, the problem remains the same whether the Wilson mirror is used with a dual differential or a folded-cascode. And this is why I was asking if anyone knew of a comprehensive collection of SPICE files.

regards, jonathan carr
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