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Old 28th January 2003, 08:50 PM   #31
jcarr is offline jcarr  United States
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I frequently see folded-cascode circuits that use resistors as current sources, but in my opinion and experience, this is not a wise idea. The better the current source is, the better the folded cascode circuit will work and sound. I use cascoded current sources for my folded cascode circuits, and I accomplish all of the biasing via voltage regulators. If you want good bass, current sources that absolutely don't waver and ultra-stiff biasing schemes will both pay handsome dividends.

Although a folded cascode may require additional support circuitry, and may therefore _look_ more complex than dual differentials, the signal path is actually simpler and cleaner than dual differentials, and a folded cascode is easier and more straightforward to stabilize.

To quickly describe some of the advantages of folded cascodes over dual differentials, let me quote from the Analog Devices AD-797 data sheet, page 9. "The AD797, due to its single-stage design, has the property that its noise is flat over frequencies from less than 10Hz to beyond 1MHz. This is not true of most DC precision amplifiers where second-stage noise contributes to input-referred noise beyond the audio frequency range. In sample data systems, where aliasing of out-of-band noise into the signal band is a problem, the AD797 will out-perform all previously available IC opamps."

Now, as to whether we should rank a particular device or class of devices according to personal preference, my advice is rather than painting yourself into a corner, to use whatever devices are best suited for the application. And the better you are at analyzing and comprehending the circuit that you have, the better your chances will be to locate the optimal devices.

I do not deny that subjective aspects like musicality, tonal complexity and "magic" are nice to have, but it is surprising (perhaps shocking is a better word) how much these qualities are tied to mundane, unromantic issues like how the wiring networks are designed and routed, or how the circuit is constructed (including the circuit-board layout). If the circuit topology and circuit construction issues are all up to snuff, it is entirely possible to make a magical-sounding component using completely normal components that many audiophiles would treat with disdain.

hth, jonathan carr
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Old 28th January 2003, 09:17 PM   #32
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Question Current Sources?

Hi Jonathan,
Thanks for elaborating but were are the current sources implemented ?(for the long tailed pair, the curent mirror load of the folded cascode or....?)
It might be helpfull to include a simplified schematic of your ideas. I find this thread hard to follow.
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Old 28th January 2003, 11:10 PM   #33
jcarr is offline jcarr  United States
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Dear Elso:

For this discussion, let us assume that we are talking about an N-input differential combined with a P-folded-cascode.

IMO, regardless of whether the overall topology is dual-differential, complementary differential or folded cascode, the long-tailed input pair of _any_ differential must sit on a constant current sink for proper differential operation. Fixed-cascode current sinks are a good method for this task, because they insure that the current value remains truly constant regardless of operating conditions, and they can have very high output impedance. Another valid option would be a current-source feeding the input of a current-mirror, with the output side of the current mirror acting as the current sink. Because the input side of the current mirror wouldn't be connected to anything relating to the signal, this would prevent the current from being modulated as the input pair swung according to the signal. But OTOH, the output impedance from a current mirror isn't as high as you can get from a good fixed cascode. So on the whole, my vote would be for my original suggestion of a current-sink feeding a fixed cascode.

Moving on to a folded cascode, one way of analyzing such a circuit is to consider it as a master current source (let's call this Itotal) feeding two current sinks - the input differential (let's call this Ia), and the folded-cascode section (let's call this Ib). From the resulting equation, Itotal=Ia+Ib, we can see that as long as we insure that any two of the current elements are fixed and constant, the third current element will be fixed and constant, too.

Although on paper it is possible to make all three current elements constant-current, in practice there will be some difficulty in matching the current values so that Ia+Ib actually do equal Itotal. I tried this once, using individual current elements, and the result was a bunch of split transistors and a burnt circuit. _If_ I were to attempt this again (for example, to test out the bootstrapped summing current mirror used in the Analog Devices AD797), I would arrange everything as a single master current source supplying an array of current mirrors.

If you would like different, in-depth analysis of the folded cascode, please look at this patent by one of my design heros, Masao Noro:

http://patft.uspto.gov/netacgi/nph-P...S=PN/4,406,990

hth, jonathan carr
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Old 29th January 2003, 12:55 AM   #34
jam is offline jam  United States
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Jonathan,

What is your reason for using regulators for cascode biasing in place of zeners. Lower noise? And what type of regulators do you use?

Jam
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Old 29th January 2003, 11:57 AM   #35
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Default KANEDA PSU.

Hi,

No idea whether this Kaneda PSU is of any interest to you all,here goes:

Cheers,
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Old 29th January 2003, 01:42 PM   #36
jam is offline jam  United States
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Frank,

I think there is an error in the drawing.

I believe the base of the 2SC1777 and 2SA872 should be tied to the positive and negative rails through a resistor respectively for the regulator to work. This forms a voltage divider for the above transistors to work. The schematic shows the bases tied directly to the output rails.

Jam
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Old 29th January 2003, 01:58 PM   #37
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Default KANEDA MOD.

Hi Jam,

Quote:
I think there is an error in the drawing.
That's quite possible,I really can't help you here.
I'm rather ignorant as far as solid state designs go.

If I can trace back the circuit source I'll be glad to inform you.
Can't remember where I got this from.

Cheers,
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Old 29th January 2003, 06:45 PM   #38
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Quote:
Originally posted by jam

I think there is an error in the drawing.
Jam,

the gate of the negative rail current source JFET is tied to the drain and not the source.

Fdegrove,

Thanks for reposting my schematic of Kaneda power amp regulated power supply.

http://www.diyaudio.com/forums/showt...pply#post55559

regards,
Hartmut
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Old 29th January 2003, 07:05 PM   #39
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Default kaneda 1995 schematic

On demand of some fellows, I hereby show you the schematic of the Kaneda preamp as published in MJ, June 1995. BTW, it is numbered as realisation no. 128 by Mr.Kaneda.

regards,
Hartmut
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Old 29th January 2003, 07:07 PM   #40
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Default Kaneda 1995, matching power supply schematic, positive rail

subject says it all

BTW, again, there is an error in the schematic. Who's first to see it ?

regards,
Hartmut
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