Linear Systems LSK489 Application Note by Cordell

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ricardo
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Re: Linear Systems LSK489 Application Note by Cordell

Post by ricardo »

JR. wrote: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.
The problem isn't getting below 1nV/rtHz on a mike preamp but doing it with full P48V and protection.

I've doodled a Cohen circuit with 0.9nV/rtHz, ie slightly better than Millenia Media but can't see how Earthworks do 0.6nV/rtHz with full P48V bla bla

I want this for my vintage STC 4038 ribbon. I'm building a cold room as a recording booth but its difficult to get the liquid Helium in Cooktown. And I have to teach the musicians to sing and play while breathing from SCUBA :D
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JR.
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Re: Linear Systems LSK489 Application Note by Cordell

Post by JR. »

ricardo wrote:
JR. wrote: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.
The problem isn't getting below 1nV/rtHz on a mike preamp but doing it with full P48V and protection.
I am not very sure what exactly are these hurdles. Are you talking about current leakage in the 48V blocking caps as the P48V problem?

And is protection another source of noise? Perhaps some series R to limit current ?
I've doodled a Cohen circuit with 0.9nV/rtHz, ie slightly better than Millenia Media but can't see how Earthworks do 0.6nV/rtHz with full P48V bla bla
I can't confirm final numbers but the same preamp I posted the schematic for (arguably a "Cohen", although I never heard that nomenclature used until years later) has a MC version using 2sb737 well less than 1nV bipolar parts. I made many mic preamps using the 737s before my time at Peavey, and once there I used the 2sd786 (the npn complement) that was in their system and also < 1 nV rt/Hz. Note: both parts are now obsolete but I have a few 737s stashed.
I want this for my vintage STC 4038 ribbon. I'm building a cold room as a recording booth but its difficult to get the liquid Helium in Cooktown. And I have to teach the musicians to sing and play while breathing from SCUBA :D
I think the government has stockpiled helium, not sure where it comes from and too lazy to google it.

JR
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ricardo
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Re: Linear Systems LSK489 Application Note by Cordell

Post by ricardo »

JR. wrote:
ricardo wrote:
JR. wrote:I am not very sure what exactly are these hurdles. Are you talking about current leakage in the 48V blocking caps as the P48V problem?
I think current leakage can be solved with the right Electrolyics and perhaps leaving P48V on all the time.
And is protection another source of noise? Perhaps some series R to limit current ?
Yes. 0.6nV/rtHz (Earthworks spec) is Rnv 22R36 which is nearly taken up by the 2x10R series protection THAT recommend.

Of course my STC 4038 example is a red herring cos it doesn't need P48V. It's then straightforward to get well under 1nV/rtHz short circuit noise with a dedicated ribbon mike amp. using Cohen without any P48V and protection.

But the 4038 ribbon is a passive nominal 200R source. The important noise spec isn't Short Circuit Noise (spec'd by Rnv) but Noise Factor .. how much the preamp increases noise above the theoretical noise of a 200R resistor (1.82nV/rtHz). THAT 1510/12 can get within 1.3dB of perfection for this.

My guess is Earthworks use the Cohen circuit. What I can't figure out is if it is immune to shorted cables on one side with P48V connected. Their stuff is encapsulated.

One could argue the 0.6nV/rtHz Earthworks is overkill. I can't think of any P48V mikes that would benefit from 0.6nV/rtHz over the 1.6nV/rtHz or so of a competent THAT 1510/12 P48V design. It's also not quiet enough for the, once common, 30-50R nominal dynamic/ribbon mikes .. for which I've got my own variant of a Cloudlifter type device.

But 0.6nV/rtHz isn't harmful PROVIDED it doesn't compromise protection against The Phantom Menace etc.

BTW, for the wannabe LN gurus out there, 'straightforward' & 'competent' doesn't mean 'easy'. Your PCB layout, earthing, decoupling & other skills need to be first class to get the above numbers.
think the government has stockpiled helium, not sure where it comes from and too lazy to google it.
I believe the USA has more than 90% of the World's known sources of Helium.
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JR.
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Re: Linear Systems LSK489 Application Note by Cordell

Post by JR. »

ricardo wrote: Of course my STC 4038 example is a red herring cos it doesn't need P48V. It's then straightforward to get well under 1nV/rtHz short circuit noise with a dedicated ribbon mike amp. using Cohen without any P48V and protection.

But the 4038 ribbon is a passive nominal 200R source. The important noise spec isn't Short Circuit Noise (spec'd by Rnv) but Noise Factor .. how much the preamp increases noise above the theoretical noise of a 200R resistor (1.82nV/rtHz). THAT 1510/12 can get within 1.3dB of perfection for this.
I have worked in NF terms for decades because absolute noise voltage depends on bandwidth and other qualifications. NF is absolute and meaningful for comparison. IMO

If we realize a 1.3dB NF, think about how much cost and effort it takes to improve that? How much would you spend to lower a noise floor a fraction of a dB? Can you then hear that fraction of dB with the mic in any room you will likely encounter.

As a designer I understand the pursuit of perfection, but once you get NF <2dB I'd be tempted to stick a fork in it, and have.
My guess is Earthworks use the Cohen circuit. What I can't figure out is if it is immune to shorted cables on one side with P48V connected. Their stuff is encapsulated.

One could argue the 0.6nV/rtHz Earthworks is overkill. I can't think of any P48V mikes that would benefit from 0.6nV/rtHz over the 1.6nV/rtHz or so of a competent THAT 1510/12 P48V design. It's also not quiet enough for the, once common, 30-50R nominal dynamic/ribbon mikes .. for which I've got my own variant of a Cloudlifter type device.

But 0.6nV/rtHz isn't harmful PROVIDED it doesn't compromise protection against The Phantom Menace etc.
The solutions for "phantom menace" have the unwritten specification of being cost effective since consoles may have several tens of these inputs. I could imagine a heroic input clamp approach perhaps involving power mosfets that could absorb the amps of discharge current without the large series R. Not practical for consoles, but perhaps the occasional money channel.
BTW, for the wannabe LN gurus out there, 'straightforward' & 'competent' doesn't mean 'easy'. Your PCB layout, earthing, decoupling & other skills need to be first class to get the above numbers.
think the government has stockpiled helium, not sure where it comes from and too lazy to google it.
I believe the USA has more than 90% of the World's known sources of Helium.
Probably planning to corner the dirigible market. :lol: I vaguely recall some claims that it was strategic while I never bothered to understand why. Perhaps for the superconducting electro-magnets in the rail guns.

JR
Cancel the "cancel culture", do not support mob hatred.
ricardo
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Re: Linear Systems LSK489 Application Note by Cordell

Post by ricardo »

JR. wrote:If we realize a 1.3dB NF, think about how much cost and effort it takes to improve that? How much would you spend to lower a noise floor a fraction of a dB? Can you then hear that fraction of dB with the mic in any room you will likely encounter.
With you all the way. :D

Only worthwhile if you can do it even cheeper :mrgreen:
My guess is Earthworks use the Cohen circuit. What I can't figure out is if it is immune to shorted cables on one side with P48V connected. Their stuff is encapsulated.

One could argue the 0.6nV/rtHz Earthworks is overkill. I can't think of any P48V mikes that would benefit from 0.6nV/rtHz over the 1.6nV/rtHz or so of a competent THAT 1510/12 P48V design.
The solutions for "phantom menace" have the unwritten specification of being cost effective since consoles may have several tens of these inputs. I could imagine a heroic input clamp approach perhaps involving power mosfets that could absorb the amps of discharge current without the large series R. Not practical for consoles, but perhaps the occasional money channel.
Somehow I doubt Earthworks do anything as clever as that. My guess is its not protected .. but I'm not keen to zap a borrowed Earthworks. :o

I'd still like to know though ;)
I believe the USA has more than 90% of the World's known sources of Helium.
Probably planning to corner the dirigible market. :lol: I vaguely recall some claims that it was strategic while I never bothered to understand why
It WAS for dirigibles or rather Zeppelins and considered strategic. USS Shenandoah to USS Macon. Foreigners had to make do with Hydrogen which crashed & burnt while American airships only crashed.
carlmart
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Re: Linear Systems LSK489 Application Note by Cordell

Post by carlmart »

I was about to ask of suggestions for RIAA MM designs using still available parts, and searching I found this Bob Cordell suggestion.

It looks interesting, even if there are two things I want to avoid: electrolytic capacitors C3 and C5.

Is there any other design you might suggest that has no capacitor in the signal path (except those belonging in the RIAA filter), particularly electrolytic?

This is not for any commercial design, only my private leisure. I want co convert all my LPs to CD or high sampling/bitrate files.
Last edited by carlmart on Sun Dec 21, 2014 12:44 pm, edited 1 time in total.
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JR.
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Re: Linear Systems LSK489 Application Note by Cordell

Post by JR. »

I haven't used an electrolytic in the direct audio path of a RIAA preamp since 1978... my later P10 in the early '80s had an electrolytic inside the gain stage open loop transfer function but not in the closed loop path.

My later P-100 published mid '80s is far tweakier with not only no electrolytic caps even inside the open loop path, but the first gain stage is run completely without NF and followed by a passive 75uSec RIAA pole.

check this thread for more discussion about phono preamps than you can possibly use. viewtopic.php?f=12&t=151&hilit=p100

Perhaps post follow up RIAA discussions there? or here, or ...

JR

PS: Vinyl recordings are obsolete IMO, but don't tell the hipsters... I wonder if hula hoops are coming back too.
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carlmart
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Re: Linear Systems LSK489 Application Note by Cordell

Post by carlmart »

Thanks for the tip. I will go there.

Where's that P100 to have a look?

Some people that deal with pro recordings today, and use high quality gear, may differ on your opinion that LPs are obsolete, if played with a quality player and through a high quality preamp.

IMHO you have to spend a lot more of money to make things work well with LPs, and they die a little every day you play them.

But normal CDs can be proven inferior sounding to a good quality LP setup, and by inferior I mean not as close to the actual musical event. Digital is more practical, not necessarily better sounding.

In any case we could spend hours discussing over this and get to no agreement. I come from the analog recording days, Nagra and else, so I got to have (when my ears were younger too) some relationship between the actual audio situation and the recording chain. And it was movies I worked with, so even the sprocketed magnetic tape in those days did sound pretty good. Optical ruined things later on.
Craig Buckingham
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Re: Linear Systems LSK489 Application Note by Cordell

Post by Craig Buckingham »

mediatechnology wrote: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.
Hi Guys, new here, first post.

My guess it was chosen for low dissipation factor. Generally higher voltage electrolytics have lower dissipation factors for the same capacitance value.

Also, calmart, you mentioned in a later post about avoiding C3 and C5 electrolytics. I like to avoid electrolytics also. Although C3 is in a benign DC location and not going to cause any noticeable harm.

C5 however is likely to harm things. It's late here and I am a bit sleepy, so I'll try and get this right. The LM4562 has worst case I/P offset voltage of +/-700mV and I/P offset current of 65nA. Looking at the inverting terminal only, I make it a DC Av of (61.67) with R13 connected to ground. The 65nA of I/P offset current works out to be 186.5uV offset voltage across R13. Added to a worst case of 7mV I/P offset voltage = 886.5uV I/P offset voltage. Multiplied by the DC Av 61.67 gives +/-54.67mV output offset voltage. Mind you these are worst case and the typicals are a factor of 7 better than this. Which would be around +/- 7.8mV O/P offset for the last stage.

How much DC can the next stage take is the big question. The offset could be trimmed out without too much effort with a bias current compensation scheme.
carlmart
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Re: Linear Systems LSK489 Application Note by Cordell

Post by carlmart »

Nice, Craig.

Now what about using some of TI's first schematic here and see if there's a way to add Cordell's FET input?

http://www.ti.com/lit/ds/symlink/lm4562.pdf

Problem is Cordell put part of the RIAA filter on the last stage, didn't he?
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