They are slightly better (I verified it a while ago) but the issue is impact ionization and at 10V there are 100's of pA of input current as SY noted. There are not even model parameters for this. BTW there is no impact ionization on P devices so balanced circuits run at 10V are really messed up. Again for MC this does not matter since the source is 2-30 Ohms or so.
Without the source impedance it is hard to compare. -145dBV is about .4nV unweighted so the source R better be < 10 Ohms.
Hi John,
I disagree. There is more than one good way to skin a cat. I'm guessing that you are doing that so that you can use BJTs for the subsequent amplifier, but that is unnecessary.
Cheers,
Bob
Hi,
This was measured with Rs = 0 Ohm. And yes, MC cartridges with more than around 10 Ohm DCR will have more noise than the input Stage.
Most MC's are below 10 Ohm, so Ideally one would get another 6dB or so SNR, but it was hard enough work to get the noise that low, while having wide gain adjustment and the ability to use the same stage for MM Inputs without excessive input C and so on.
And as a good Marxist creed is "Work is the curse of the drinking Class", I declined the invitation to make the input stage even quieter and instead raised a glass of Cabernet...
Ciao T
This was measured with Rs = 0 Ohm. And yes, MC cartridges with more than around 10 Ohm DCR will have more noise than the input Stage.
Most MC's are below 10 Ohm, so Ideally one would get another 6dB or so SNR, but it was hard enough work to get the noise that low, while having wide gain adjustment and the ability to use the same stage for MM Inputs without excessive input C and so on.
And as a good Marxist creed is "Work is the curse of the drinking Class", I declined the invitation to make the input stage even quieter and instead raised a glass of Cabernet...
Ciao T
John,
the Vendetta is very often mentioned as one heck of a MC amplifier with extremly low noise, but it is also said to be impossible to build today due to the lack of the ultra low noise JFET:s (2SJ72 and 2SK147).
Now, the schematic, or at least a try for the schematic, has been placed on this forum in 2006.
If one does not want or need the extremly low noise of the Vendetta, the Vendetta should be possible to make even today as long as one has the 2SJ72/SK170 JFET:s. One could call it The Vendetta Lite (as in not super low noise).
The MOSFET:s has to be changed aswell.
My question is if you would allow me/us to present the schematic as drawn in 2006 (not drawn by me)?
the Vendetta is very often mentioned as one heck of a MC amplifier with extremly low noise, but it is also said to be impossible to build today due to the lack of the ultra low noise JFET:s (2SJ72 and 2SK147).
Now, the schematic, or at least a try for the schematic, has been placed on this forum in 2006.
If one does not want or need the extremly low noise of the Vendetta, the Vendetta should be possible to make even today as long as one has the 2SJ72/SK170 JFET:s. One could call it The Vendetta Lite (as in not super low noise).
The MOSFET:s has to be changed aswell.
My question is if you would allow me/us to present the schematic as drawn in 2006 (not drawn by me)?
A step in the direction of a "universal" phono preamp could be achieved by making the number of input JFET pairs switchable. Going from four pairs to one would give a 12dB reduction in gain and a four-fold reduction of non-linear input capacitance.
If the gain of at least one input pair is reduced (by means of higher source resistors and/or different JFETs), a bigger ratio can be achieved, with sensibly sized steps in between.
A nice feature of the (simplified) schematic below is that the switch settings don't affect the quiescent operating point, so gain can be changed "on the fly" without fear of overly loud ticks or pops through the speakers.
If the gain of at least one input pair is reduced (by means of higher source resistors and/or different JFETs), a bigger ratio can be achieved, with sensibly sized steps in between.
A nice feature of the (simplified) schematic below is that the switch settings don't affect the quiescent operating point, so gain can be changed "on the fly" without fear of overly loud ticks or pops through the speakers.
Attachments
That will certainly be MC only. 10 - 1.1nV op-amps in parallel, marketing certainly hopes that catches on.
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Unless someone provides compelling evidence to the contrary those old top hat FET's where bigger die from essentially the same process. I don't have my data sheets here but I got some quick data off the web.
2SJ72 gives 40mS (gm) @ 185pF Ciss
2Sj74 gives 22mS @ 105pF Ciss
2 - 2SJ74 gives 44mS @ 210pF Ciss ratiometrically darn close.
Complementary total you need about 100mS to get .4nV giving yourself a little breathing room and depending on topology. Remember JFET's are majority carrier devices, holes in one case and electrons in the other. That is why complementary ones are virtually impossible to make.
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Hi,
For most intents and purposes you can sub each 2SK147 in the Vendetta with two 2SK170 in parallel and each 2SJ72 with a two 2SJ74 in parallel. I think this would only be needed in the input stage, the other stages could likely sub in 2SK170/2SJ74 with minimal if any change.
I find the Vendetta quite interesting, it was revolutionary for it's time...
Ciao T
For most intents and purposes you can sub each 2SK147 in the Vendetta with two 2SK170 in parallel and each 2SJ72 with a two 2SJ74 in parallel. I think this would only be needed in the input stage, the other stages could likely sub in 2SK170/2SJ74 with minimal if any change.
I find the Vendetta quite interesting, it was revolutionary for it's time...
Ciao T
It is true my recent 2SK170's show excess noise at the low end (picture in PASS forum), but this is a day of the week/phase of moon thing.
Hi,
The noise may be marginally higher, but I doubt it is material.
I actually achieved lower noise than the Vendetta measured by Stereophile had using only 4pcs of 2SK170 (and a bunch of tubes, all of it zero feedback as well).
Ciao T
The noise may be marginally higher, but I doubt it is material.
I actually achieved lower noise than the Vendetta measured by Stereophile had using only 4pcs of 2SK170 (and a bunch of tubes, all of it zero feedback as well).
Ciao T
Hi Godfrey,
This is an interesting idea, although one would have to balance the cost and hassle of the switches against the cost of just having dedicated input stages for the MC and MM functions. Also, you might want to make sure those switches are make-before-break so that no gates are ever allowed to float, even briefly.
My philosophy is that if we are really going for highest performance, we use different input amplifiers for the MC and MM functions, each finely tuned and optimized for its individual application. Indeed, if necessary to achieve the highest performance, even some or all of the RIAA equalization might be duplicated. Of course, the ultimate end of all this would be to have separate MC and MM preamps, where selection between the two applications is done at line level.
Cheers,
Bob
I have the front end of the JC-80 (schematic) at work if no one can find it. John already did not mind me posting the line stage during a servo discussion. It's 30yr. old anyway
. A head amp with SK170/SJ74 substituted as Thorsten suggested would be easy to pull off. I figure 2-SJ74's are 110% of an SJ72, this could easily be taken into account. The VFET's for the line stage are unobtainium today.Thorsten - Isn't the 400pF or so input C getting a little dicey for some MM ?
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I still find the Vendetta (according to the schematic that someone here made 2006) very interesting! Noise wise I am not interested in 80 dB SNR as most LPs only are around 60-65 dB, but the simplicity of the design is elegant indeed!
Yes, the 2nd stage can be using 2Sk170/SJ74 as I see it. The folded cascode could be made out of BJTs instead of SJ72/SK147. I have such a design working well in the simulator.
I think that the MOSFETs are the hardest to replace. Hmmmm...maybe some Zetex Zxx3310:s could do the job.
Yes, the 2nd stage can be using 2Sk170/SJ74 as I see it. The folded cascode could be made out of BJTs instead of SJ72/SK147. I have such a design working well in the simulator.
I think that the MOSFETs are the hardest to replace. Hmmmm...maybe some Zetex Zxx3310:s could do the job.
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