Flat Phono Preamp Based on John's P10 and 2SK389

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JR.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Mon Sep 05, 2011 11:40 am

More great citations thanks again Wayne...

I am not sure if it has been mentioned over here but the 2sk389 appears to be obsolete. perhaps not an issue for DIY (yet), and making phono preamps is probably not a consumer audio pursuit these days, but any thinking that way may need to source other JFETS.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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mediatechnology
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Sep 05, 2011 2:18 pm

Thanks John.
I am not sure if it has been mentioned over here but the 2sk389 appears to be obsolete.
Which is also why I have a pile of Toshiba 2SK389s. They're still using 12AX7's "over there" aren't they? Unobtanium normally doesn't phase the :ugeek:

Linear Systems still make 2SK389s in the form of the LSK389. The original Toshiba is still fairly available though pricey.

As you did 30 years earlier, discrete JFETs are still vailable including the LSK170. I had no problem getting the LSK170s from distribution in bulk and re-sold a few. A few people grumbled about Idss variations being out-of-grade for the LSK170 vs. the Toshiba 2SK170 but they also seemed to be whiners. I'd say select 'em and stop complaining. They're not that expensive. Be glad someone still makes these things.

Linear systems also make the PF5301: http://www.linearsystems.com/datasheets/PF5301.pdf

Fairchild has the PF5301 EOL but they have >10,000 units.

There's also Interfet still making FETS: http://www.interfet.com/Home/

I think it may be awhile before the world supply of junction FETs dries up. I'm more worried about water in Texas.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Mon Sep 05, 2011 3:19 pm

Yup, it's a shame Lee is on a bee line for Vermont and the already saturated NE instead of TX who could actually use some rain. Jet stream and rain patterns can get weird (as in chaotic). I'm sure somebody somewhere is blaming the TX drought on global warming.

The storm is moving so slow (2 MPH) I'd be surprised is there's much rain left by the time it makes it's way up north... It's been raining solid for three days here, while my rain ditches can easily handle the projected 10" spread out over days... I don't even expect the full 10" from what I've seen so far. While there was enough run off for my back to front overflow ditch to get a good workout this morning.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Mon Sep 05, 2011 5:03 pm

Found the discussion on optimizing input current density in low-noise preamps:

viewtopic.php?f=12&t=434
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by ricardo » Tue Sep 06, 2011 5:31 am

> AN104 that you provided may be what ricardo was talking about.

This is the sort of thing I remember but I think the AD note didn't apply the A wt directly and actually plotted the 8ve or 1/3 8ve noise. I could compare this with a 1/3 8ve noise plot which I've just managed to dig up. 30 yr old memories :o
Here's the 30 yr old plot. The B&K 2307 chart recorder has slipped by 1 tooth so 50Hz (hum) appears at 40Hz. 1dB/division

The MM is an Ortofon VMS20E. The Ortofon MC, either MC20-2 or MC10. The MC preamp is set for very high gain 28.8dB so if adjusted for equal loudness from the 2 cartridges, the noise advantage of my little circuit is even better. With RIAA, MM noise is whitish while MC is redder than pink.

At the time, every other MC step up device was at least 6 - 10 dB worse (usually much worse) depending on how you measured the noise. So you normally don't see this cos the noise of the step up device.

If you can match a MC properly, you'll get better S/N than MM. MCs have quite high power output even if voltage is low. MM is inherently inefficient in comparison. Also the inductance doesn't let you get close to true efficiency except for 1 frequency.

Does ultra low noise on vinyl playback matter? I think so. From analysing my listening test results and ignoring any obvious Golden Pinnae raving, there is a sense of less grittiness and also the system seems to disassociate clicks, pops & record noise from the music. Maybe a noise modulation effect. But I'm probably pontificating from the wrong orifice and it was 30 yrs ago. :lol:

from the Golden Pinnae AudioXpress design
Above 7kHz, the typical 0.5mV MC cartridge has slightly better SNR (lower noise with reference to signal) than the MM cartridge ..
I'm not sure he sees that with low output MC cartridges. His example, the Dynavector has MM size voltage output. His use of AD797 is of course inappropriate for MM but good for the Dynavector. NE5532 would give several dB improvement in noise for MM.

The loading capacitor also affects the noise, improving it for the usual MMs.
Attachments
MCnoise2.jpg
MC vs MM noise
MCnoise2.jpg (224.87 KiB) Viewed 16647 times

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Tue Sep 06, 2011 11:38 am

This is ancient history for me and while I am a little curious, I am not curious enough to assemble my old circuit.

FWIW on my spec sheet I claimed a 3dB better S/N ("A" wtd) for the MC version than my MM version, while I don't recall what I used for dummy cartridge loads to make those bench measurements. That was more than 30 years ago.

My MC input noise would be dominated by 2x 2sb737s @ around .4nV/rt Hz, and 15 ohm resistor in gain leg. Cart was terminated into 100 ohm.

In hindsight a SE input could be quieter, but 15 ohm gain R would need a big ass cap in series, etc. The feedback I got from pretty tweaky audio types was good and they did not say anything about noise, but i never even owned a MC cart myself... they were expensive :lol: ... I actually worked out a trade with Shure for a few MM carts, since Shure wanted a couple delay line kits from me when they were making a run at surround sound market. They probably got the better of me in the trade since their cart costs were squat, but i was still getting a better deal than buying carts through distribution.

FWIW the 2sb737 and 2sd786 were actually developed by a tiny japanese company for MC head amps. That small company was later bought by ROHM who thanked me for using them in my kit article because it gave them a lot of exposure. Popular Electronics had something like a half million subscribers at their peak.


JR
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by ricardo » Tue Sep 06, 2011 5:18 pm

JR. wrote:FWIW the 2sb737 and 2sd786 were actually developed by a tiny japanese company for MC head amps. That small company was later bought by ROHM who thanked me for using them in my kit article because it gave them a lot of exposure. Popular Electronics had something like a half million subscribers at their peak.
Hitachi 2sa1084 / 2sc2546 etc were the best I found in da early 80's. I measured rbb about 5R7. Before that, some medium power devices were best. GG Baxandall did a lot of work on these for QUAD.

Rohm 2sb737 / 2sd786 and Toshiba 2sc3329 have published rbb about 2R but I've never tried them. Today, all these devices are Unobtainium. A few secret hoards of 2sc25476 & 2sc3329 are still extant.

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Tue Sep 06, 2011 7:27 pm

I have a stash of about 100 2sb737s (s= beta of 600-800).. We had 2sd786 in the system at Peavey but they went obsolete 10 years ago or maybe more.

JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by mediatechnology » Wed Sep 07, 2011 5:24 am

ricardo - Thanks for sending that plot. IIRC he mentions in the TAA article that the DynaVector had enough output that it could be used with MM inputs.

John's got a stash of 2SB737s.
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Re: Flat Phono Preamp Based on John's P10 and 2SK389

Post by JR. » Wed Sep 07, 2011 2:05 pm

Yup, how many you need?

I was thinking of someday doing my own version of DC coupled mic preamp with them, bot the real deal design needs me to roll my own A/D convertor too, and float it up to +48V. I could use some off the shelf standard A/D but that isn't as cool. The world doesn't need yet another specialty mic preamp (from me),


JR
Don't only half-ass tune your drums. Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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