Linear Systems LSK489 Application Note by Cordell

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mediatechnology
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Linear Systems LSK489 Application Note by Cordell

Post by mediatechnology »

Bob Cordell has published a very interesting application note for the Linear Systems LSK489.

http://www.linearsystems.com/assets/med ... n_Note.pdf

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LSK489 Phono Preamp, Cordell, Linear Systems Application Note

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LSK489 Discrete Op Amp, Cordell, Linear Systems Application Note
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Re: Linear Systems LSK489 Application Note by Cordell

Post by emrr »

Thanks, a quick skim looks very promising in many apps.
Best,

Doug Williams
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Re: Linear Systems LSK489 Application Note by Cordell

Post by JR. »

Thanks for this Wayne. I have always enjoyed Cordell's work.

That LSK489 looks like a sweet part, while I have to keep reminding myself that the world doesn't need yet another phono preamp.

At 1.5nV/rtHz it is a little noisier than the 2SK117 (1nV/rtHz) I used in my last phono preamp (to end all phono preamps) back decades ago, but is significantly lower capacitance by a factor of 1/5 or 1/6th..

I like the way Cordell made the 75uSec pole in his RIAA playback EQ a passive real pole (kind of like I did). This way the top pole in the RIAA rolls off forever (even though I determined that the record cutting lathe did not boost forever), less ultrasonic response is always a good thing, IMO..

Nice to know that good parts are still out there. I think the second generation low Rbb transistors for making discrete mic preamps, may have gone obsolete just like the first generation (2sb737,2sd786).

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Re: Linear Systems LSK489 Application Note by Cordell

Post by JR. »

Brad Wood is using a http://www.nxp.com/documents/data_sheet/BF862.pdf in his new moving coil phono preamp.

At 0.8nV/rtHz pretty quiet part... Not as low input C as the 489 but a few pF is not that big of a deal.

Might make an interesting mic preamps... if they weren't already decent.

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Re: Linear Systems LSK489 Application Note by Cordell

Post by mediatechnology »

The "Diamond" output stage in the discrete op amp looked kinda cool.
I ran across that recently in an old Burr Brown catalog.

That phono preamp might be fun to build.
JR the attention you've drawn to (early) passive poles makes me a believer in them.
Your P10 schematics and the images to the flat phono preamp get a lot of hits.
There still seems to be a lot of interest in phono preamps.

I hope Brad can continue to get those LN FETs from Philips.
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Re: Linear Systems LSK489 Application Note by Cordell

Post by mediatechnology »

John - Do you think it would be possible or advisable to replace C5 (in the figure 7 LSK-489 preamp) with a servo? That would seem to allow C5 - a 100 uF/100V bipolar electrolytic - to be replaced with a lower-value film.
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Re: Linear Systems LSK489 Application Note by Cordell

Post by JR. »

mediatechnology wrote:John - Do you think it would be possible or advisable to replace C5 (in the figure 7 LSK-489 preamp) with a servo? That would seem to allow C5 - a 100 uF/100V bipolar electrolytic - to be replaced with a lower-value film.
Possible sure, advisable (?), but some people are still interested. If I was going to kill more brain cells over reinventing phono pre's I might apply one of the new lower noise JFETs to my last design. I was scribbling on a way to further linearize the first open loop gain stage, despite the fact the as it was the distortion was already very low. 8-)

For fig 7. Just short across the 100uF and attach a servo detecting DC at the final output and sending correction through a large resistor to R8 or R9 depending on inverting or non-inverting servo topology. I would not expect any fast edges at the output of that stage, unless it is feeding the outside world and if so the N.I. servo could reject RF better. Hang a same value resistor as the servo from the other drain resistor to ground to maintain PSRR. I do not have a feel for how much correction it will take to zero that front end.

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Re: Linear Systems LSK489 Application Note by Cordell

Post by mediatechnology »

Thanks John.

After looking at it I realized that the input has a 1.6 Hz cutoff, the C5/R13 cutoff 0.6 Hz.
With the input rolled off early I doubt C5 wouldn't see any significant terminal voltage.
Probably best to just series-connect two 220 uF.
Wonder why he specified a 100V rating?
Easily done with 2X 220uF/50V but a lot of real estate.
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Re: Linear Systems LSK489 Application Note by Cordell

Post by JR. »

That front end will be +/- xV without a trim (I think). The circuit designer may have been too lazy to suggest measuring what the DC voltage actually was after the first stage and then use a polar electrolytic inserted based on that DC offset polarity. It seems like the final output of that stock figure 7. preamp would have this DC error, so maybe it is decent untrimmed. There are not many DC coupled power amps in use today.

The EQ topology I used in P-10 (a similar approach using flat front end gain stage) formed the IEC (7950 uSec) LF pole with a .16 uF film, so bumping that up to .33 or .47uF would probably be an acceptable HPF roll-off for records using decent film cap dielectric.

JR

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Linear Systems LSK189 Single JFET

Post by mediatechnology »

I see that Linear Systems now have a single LSK189: http://www.linearsystems.com/assets/med ... LSK189.pdf
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