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Old 12th March 2010, 05:00 PM   #11
EC8010 is offline EC8010  United Kingdom
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As has been pointed out, transformers are a much more expensive solution (although they do allow you to treat the signal as balanced, and therefore reject hum etc very nicely).

Valves are noisier than FETs because although it is possible to make the gm comparable, their grid current is noisier than a FET's gate current.

Despite the fact that I have some LSK170, I will probably use one of the superbeta bipolar transistors designed for low noise (Analog Devices SSM2210).

In the end, the implementation matters at least as much as the device...
The loudspeaker: The only commercial Hi-Fi item where a disproportionate part of the budget isn't spent on the box. And the one where it would make a difference...
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Old 12th March 2010, 06:00 PM   #12
jcx is offline jcx  United States
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I don't see the reason to think bjt i_b flowing in mc is a worry - the coil is setting in a air gap and the current can't have much effect on the near saturated PM biased pole pieces - the situation is completely unlike mm
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Old 12th March 2010, 06:25 PM   #13
wahab is offline wahab  Algeria
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I have designed and built a bjts input MC preamp, but i hesitate to use it.
Noise contribution is about 0.35nV/sqrt hz, but i have trouble
reducing the input DC voltage under 2uV.
In the waiting, i m using an AKAI preamp (that i did save from being
scrapped at the local scrapyard !!) that use 2SK150 dual fets at the input.
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File Type: pdf 2SK150.pdf (100.1 KB, 95 views)

Last edited by wahab; 12th March 2010 at 06:25 PM. Reason: typo
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Old 12th March 2010, 07:28 PM   #14
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Wahab, could you post the circuit ?
2uV is not much. In a 5 Ohm cartridge it generates only 0.4uA or 8E.-13 W.
I do not think that is a problem.
I did a lot of transimpedance amps that generated much more and it did not matter how i abused the cartridge (Lyra Titan i). I always worked perfectly well afterwards.
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Old 12th March 2010, 11:11 PM   #15
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Default Simulated Noise Floor for JFET MC Amp

Here is the simulated noise PDF for the MC amp with a 20 - 20KHz BPF applied. With a gain of approx 14, the equivalent noise referenced to the input stage is 0.27 nV/sqrt(hz). This was simulated with the input shorted
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Old 12th March 2010, 11:30 PM   #16
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Default Simulated Distortion for the MC Amp

Here is the simulated distortion for a 20 mVPP input into the MC amp. 20 mVPP is, of course, considerably in excess of what an MC cartridge can deliver. At lower input levels the distortion measurement is limited by the amp's noise floor. Remember also, that this is a zero GFB design.
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Old 2nd April 2015, 01:14 PM   #17
tvi is offline tvi  Australia
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i m using an AKAI preamp (that i did save from being scrapped at the local crapyard !!) that use 2SK150 dual fets at the input.
This one PR-A04?

included a Sharp for fun
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File Type: png PR-A04.png (155.5 KB, 319 views)
File Type: png sharp.png (383.7 KB, 323 views)
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Old 2nd April 2015, 11:58 PM   #18
Ejam is offline Ejam  Australia
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Default Rod Coleman - Shunt Cascode Questions


Some clarification on your 2SK389 shunt cascode: is that 7mA per fet or for the pair and similarly 11 ohms per source or for the pair?
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Old 3rd April 2015, 05:05 AM   #19
Calvin is offline Calvin  Germany
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I wonder why You use a differential input stage structure in first place?
Why not use the simpler complementary grounded source instead?
It'd require 6 JFETs and 2 resistors less for the same outcome.
At first thought the only difference could be lower pssr and a phase inversion of the output.
I'd say the differential Input comes handy if we have to deal with (and/or combine) two signals at the same.
If we only have to deal with a single signal there's no advantage over a SE structure, but added complexity and hassle.
The lower THD is imho only due to complementary action and not because of the differential Input structure.


Last edited by Calvin; 3rd April 2015 at 05:12 AM.
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Old 3rd April 2015, 05:33 AM   #20
jcarr is offline jcarr  United States
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+1 to Calvin's comments.
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