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Novel Topology MC pre-preamp

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5th March 2010, 04:37 AM

#1

analog_guy

diyAudio Member

Join Date: Apr 2007

Novel Topology MC pre-preamp

I have been simulating an MC pre-preamp which
has a slightly different topology from the usual. It is a no GFB design,
where the V- inputs are shorted to gnd. The gain of 24 dB is set by a
combination of gm and the drain resistors. Upper and lower halves are
shorted together at AC frequencies. This configuration offers two
advantages. The first is a substantial (30 dB or better) reduction in
distortion. The second is a approx a 3 dB increase in SNR due to the
effective paralleling of two correlated signal sources whose noise is
uncorrelated. Attached is the active circuit schematic.

Simulated
distortion is lower than the noise floor for inputs up to 10 mVPP. The
simulated input noise for 3x paralleled 2sj74/2sk170 is 0.28nV/sqrt(Hz)

Attached Thumbnails

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Last edited by analog_guy; 5th March 2010 at 04:50
AM. Reason: Error on Schematic

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10th March 2010, 03:06 PM

#2

ji4m

diyAudio Member

Join Date: Mar 2005

Location: Texas

Looks interesting, although I don't have
enough 2sj74 to build it. Do you simulate with slightly unmatched devices,
or assume perfect pairings? I hope this isn't bad form, but would you be
willing to share the spice cir file?

Last edited by ji4m; 10th March 2010 at 03:16
PM.

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10th March 2010, 06:19 PM

#3

analog_guy

diyAudio Member

Join Date: Apr 2007

Fet Matching

Quote:

Originally Posted by ji4m

Do you simulate with slightly
unmatched devices, or assume perfect
pairings

For the initial simulations
I assumed matched devices. The noise floor is not substantially changed
due to device mismatch, but the distortion is. Devices of a given polarity
need to be matched, but devices of opposite polarity do not. That is why
the pos and neg sections are tied together at audio frequencies by the two
0.1 uf caps.

Matching devices for distortion entails matching three
parameters: Rs, beta, and Vto. For the devices in question lambda is small
enough to not require matching. The generalized matching problem can be
addressed by using external passive components to adjust the effective Vto
and Rs, leaving only beta, which can be matched by device screening. It
may be sufficient to match only Idss. Once I buy a number of devices and
do a spice parameter extraction I'll know.

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10th March 2010, 08:34 PM

#4

Joachim Gerhard

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Join Date: Mar 2009

I see 1uF not 0.1uF.

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12th March 2010, 02:04 AM

#5

analog_guy

diyAudio Member

Join Date: Apr 2007

Capacitor Value

Quote:

Originally Posted by Joachim Gerhard

I see 1uF not
0.1uF.

Good catch. However, the two
capacitors form a voltage divider with the gate impedance of the follower
jfets. So the corner frequency for either 0.1 or 1.0 uF will still be
below the audio range.

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12th March 2010, 03:28 AM

#6

Joachim Gerhard

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Join Date: Mar 2009

Then i whould prefer a 0.1uF because they are
awailable in very good quality.

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12th March 2010, 06:40 AM

#7

EC8010

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Join Date: Jan 2003

Location: Near London. UK

FETs and MCs don't really mix - take a look
at the 1/f noise frequency. On the other hand, distortion really shouldn't
be a problem at such low levels.

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12th March 2010, 07:02 AM

#8

analog_sa

diyAudio Member

Join Date: Aug 2002

Location: Sofia

Quote:

Originally Posted by EC8010

FETs and MCs don't really
mix

This statement probably
has a solid ground in some textbook but is in direct contradiction with
many of the best existing designs. Should owners scrap their Vendettas or
just use them with MMs?

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12th March 2010, 10:12 AM

#9

Joachim Gerhard

diyAudio Member

Join Date: Mar 2009

In the 2SK170 and 2SK74 1/f corner frequency
is quite high at 1 to 4kHz but the rise is very slow.
In praxis the
noise can be very low.
I have build several stages with 4 or 8 units in
parallel that had excelent noise performance.

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12th March 2010, 12:18 PM

#10

Rod Coleman

работник

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Join Date: Jul 2004

Location: Warwickshire UK

Quote:

Originally Posted by EC8010

FETs and MCs don't really mix - take
a look at the 1/f noise frequency. On the other hand, distortion
really shouldn't be a problem at such low
levels.

The flicker noise is
certainly not a problem for practical designs. The moving coil presents
near to a dead short to the gate-source at low frequencies, which doesn't
hurt. I use only one 2SK369 (7V, 7mA, shunt cascode) with 11 Ohm in the
source for degeneration, and this works perfectly for a Lyra Lydian-Beta.
If you get an ear close enough to the tweeter, only some higher frequency
noise is perceptible, contrary to the 1/f suspicion.

Just watch
out for Igsx (excess gate current) when applying the Toshiba JFETs: it
starts at about 8V for practical drain currents, I suspect 12V is too much
- even small Igsx current is likely to be noisy.

I would be
interested to see any device that is claimed to be better for
interfacing a moving coil cartridge than a TOSHIBA large geometry JFET, as
shown in this design.

I am assuming that BJTs are out due to the
need for coupling capacitors, and although I'd enjoy using a valve, the
grid leakage current keeps me away from them - turn-ON pulses of microamp
level are hard to avoid (and the Lyra's designer, Jonathan Carr is on
record in these pages as declaring these to be a factor in needing to
fluxbust). That's before you tackle the noise problem with valves, or
their high loop area at the input (electromagnetic 'moment') degrading the
external field susceptibility. Enough said about Microphony! MOSFETs are
out for certain due to noise level.

What's left? Transformers? I'm
not fee-paying enough to buy one that's possibly good enough to try it
out. Mr van den Hul has given reason enough why he wouldn't use one, and
as a designer of magnetic systems generating this very signal, I'll be
content to respect his opinion!

The TOSHIBA 2SK369 and his
relatives were designed explicitly for use as "first stage in MC and head
amplifier", and I find their low noise, low voltage operation, dc-coupled
input, pA input current, high gain AND low output impedance circuit
possibilities are all excellent properties when solving the MC input
problem. And so long as you can buy thousands of these excellent parts for
the cost of a trafo that might begin to compete, I see only one
choice!!!

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