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Old 22nd December 2011, 10:28 PM   #11
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Richard Murdey
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DMC-10 simplified schematic, for reference.
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Old 23rd December 2011, 05:48 AM   #12
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Hi,

Quote:
Originally Posted by rjm View Post
DMC-10 simplified schematic, for reference.
As you can see, it is essentially the same kind of circuit as the Pioneer, but with J-Fet inputs (NPD5566 like so many US Pre's of the time, matched 2SK246 can be substituted with minor changes, according to Kaneta and some of his followers 2SK30 is best for input pair) and lateral Fet outputs (VN.. and VP.. parts long obsolete - nowadays I'd use 2SK214/2SJ77 or related).

The J-Fet inputs improve input level handling and linearity, as well as RFI resistance. The Fet outputs largely remove the loading on the VAS (some nonlinear capacitance remains, but very little).

With the Hitatchi TO220 laterals I recommend above you can easily run a few 100mA quiescent current to get pure class for even low impedance headphones.

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Old 24th December 2011, 10:27 AM   #13
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A couple of questions.

The DMC-10 schematic is missing a few details. What's a good (best?) way to generate the bias for the output pair?

If I'm getting this right, the Pioneer and the Spectral share the basic topology: differential input pair, second differential pair as the voltage amp, bias, and output pair. The difference is one of substituting the bipolars for fets.

Is this basically true of all discrete solid state preamplifiers or is there any completely different way to approach the matter?

[that's a little bit disingenuous of me: I just got the 47 labs 0247 headphone amp kit and was quite thunderstruck by the circuit used. Think diamond buffer with voltage gain and feedback, direct coupled to a Szilkai pair running open loop as a buffer. So I'm naturally curious now to know what else is lurking out there.]
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Old 24th December 2011, 12:35 PM   #14
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Hi,

Quote:
Originally Posted by rjm View Post
The DMC-10 schematic is missing a few details. What's a good (best?) way to generate the bias for the output pair?
If using laterals a resistor, if using vertical fets a VBE multiplier, if using J-Fets at Idss, nothing will do fine.

Quote:
Originally Posted by rjm View Post
If I'm getting this right, the Pioneer and the Spectral share the basic topology: differential input pair, second differential pair as the voltage amp, bias, and output pair. The difference is one of substituting the bipolars for fets.
Yes, that is so. You can add many more Preamp's including Sony TA-88ES, Goldmund and many others.

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Originally Posted by rjm View Post
Is this basically true of all discrete solid state preamplifiers or is there any completely different way to approach the matter?
There are probably more different approaches than you can shake a stick at.

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Originally Posted by rjm View Post
[that's a little bit disingenuous of me: I just got the 47 labs 0247 headphone amp kit and was quite thunderstruck by the circuit used. Think diamond buffer with voltage gain and feedback, direct coupled to a Szilkai pair running open loop as a buffer. So I'm naturally curious now to know what else is lurking out there.]
The "diamond buffer with voltage gain and feedback" is known as "current feedback" It is quite common, as is a Sziklai Output.

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Old 25th December 2011, 11:40 AM   #15
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"The "diamond buffer with voltage gain and feedback" is known as "current feedback" It is quite common, as is a Sziklai Output."

Are you sure about this? A diamond buffer has nothing to do with current feedback.
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Old 25th December 2011, 12:01 PM   #16
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Hi,

Quote:
Originally Posted by Bonsai View Post
"The "diamond buffer with voltage gain and feedback" is known as "current feedback" It is quite common, as is a Sziklai Output."

Are you sure about this? A diamond buffer has nothing to do with current feedback.
Yes, I am quite sure about that.

The description in parenthesis was taken from the OP's post and his description of the circuit. The actual circuit is the one I commented on, a current feedback system with diamond transistor front-end (e.g. AD844 in IC's).

Of course, to understand this would actually require reading posts and knowing a few things about basic electronics, prior to commenting.

This is something that long gave up from certain people, these days I only expect from those are educated, well mannered, reasonable and knowledgeable...

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Old 25th December 2011, 06:01 PM   #17
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My question was asked in a perfectly civilized manner and without any other intentions.

However, since you chose to interpret my question as a hostile challenge, let me restate it it in terms you may better understand:-

You don't need a diamond output stage to create a CFA.

Do you understand how a CFA works, and why?
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Old 28th December 2011, 01:22 AM   #18
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Thorsten is correct, it's a current feedback circuit element. (see here) There are symmetrical current mirrors on the collectors of the diamond buffer output transistors, with the output load of the mirror tapped to return a proportion of the signal back to the diamond buffer output.

When I was parsing the circuit earlier I saw the feedback resistors but my brain did the I-V conversion automatically and I saw it as voltage feedback rather than current feedback. Sorry if I was unclear.
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Old 28th December 2011, 02:46 AM   #19
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rjm,

Not sure why you want to build a C-21 when you can find a used Pioneer at reasonable price. I got one and it's a very decent preamp. I can put it up against Threshold all FET (FET1 MKII) preamp which I also have from that era. Sometimes, simpleton circuit is all it needs to sound GREAT! I checked out mine to see if anything needs to be done since it's over 30 yrs old. I found the Parts Quality is really good and Step Attenuator adds to the simple BJT circuit. Even the old lytics are a OK on my LCR meter.
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Old 28th December 2011, 04:42 AM   #20
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James, your post reads like a classified add, but I agree it would be simpler and cheaper (in the long run) to buy one rather than build a copy.

The electrolytics in a 40 year old component are of questionable status, despite your checks. Let's face it they must be on their way out by now. That's not what's holding me back though; it's just a matter of purpose - I'm more interested in the circuit than the preamplifier per se.
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