A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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JR.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. »

If bias current is a concern maybe try to find some low noise JFET devices (I think Brad was messing around with some, but he was using an exotic topology too IIRC).

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by mediatechnology »

I would like to see a citation where someone has actually measured degradation.
The one post I did locate could not find, in a MM cartridge, any measurable performance change with bias currents.
Not sure I bookmarked it but I recall it being on DIY Audio.
I'd like to see some data where it does.

Having said that I don't think the Zetex transistors have to be run at such crazy high currents to still get decent performance.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans »

The BF862 FET has 0.8nV/rtHz.
but maybe others are even better.

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. »

Hans wrote:The BF862 FET has 0.8nV/rtHz.
but maybe others are even better.

Hans
yup, for MC you want lower than 0.8nV/rtHz

I am not up on the modern devices, but recall some exotic military JFETs down around 0.7nV back decades ago(?), but too expensive to interest me.

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by ricardo »

Hans wrote:All effort has been successfully invested in getting a very low noise input noise, but what about DC current flowing into the cart.
With 5 to 10mA collector current per ZTX851, Ibias and for that reason Ios must be in the many multi micro amps.
Isn´t that much too high and therefore over the top for an MC element?
No it is not.

I looked at this very carefully circa 1980. There are 2 possible problems.

- the current might deflect the 'central' position of the stylus
- current through the cartridge might skew the 'core' leading to 2nd HD.

With my Transducer Designer hat on ... in a MC cartridge, the Transduction Eqns are V = B l v where
V voltage
B flux
l length of conductor in flux
v velocity

& F = B l i
F force
i current through coil

B l is exactly the same in both eqns and these are the two links between the Mechanical & Electrical worlds. You get B l directly from the sensitivity of the cartridge. If the force due to the current is much less than the tracking weight, it is of no consequence.

I tried to find a difference in THD and/or 2nd HD on test discs with IIRC, up to 100uA through the cartridges, but saw nothing.

The real reason for the tests were to justify putting Ib of 5534/2 through a MM cartridge. MMs & MIs have the same relation as above though the 'explanation' is slightly more roundabout. I then tried some Ortofon MCs cos I expected their greater power efficiency would make this more serious but again saw nothing. My MC preamp had 0.28nV/rtHz performance and even the noise spectrum / level didn't change.

Wayne, I think I posted all this here a couple of years ago.
____________________
The other thing you mention is 1/f noise.
In 1980, I (and Great Guru Baxandall) also tested loadsa medium power devices to find stuff with low rbb'. The ZTX devices had good rbb' but required selection to weed out popcorn noise with poor yield. The truly EVIL & audible yuckiness of popcorn isn't reflected in any of the noise weightings.

Once I found Hitachi 2sa108x / 2sc254x, I gave up looking as they were better than the medium power stuff and didn't have popcorn.

I'm pleased that Wayne, Horowitz & Hill etc have found 21st century ZTX clean of popcorn. :mrgreen:
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans »

ricardo wrote:
I tried to find a difference in THD and/or 2nd HD on test discs with IIRC, up to 100uA through the cartridges, but saw nothing.

The real reason for the tests were to justify putting Ib of 5534/2 through a MM cartridge.
I then tried some Ortofon MCs cos I expected their greater power efficiency would make this more serious but again saw nothing.
My MC preamp had 0.28nV/rtHz performance and even the noise spectrum / level didn't change.

Wayne, I think I posted all this here a couple of years ago.
Hi Ricardo,

Your info is valuable, but I have a few comments:

1) A Cart coil is made from a very tiny wire, that like all wires can only have a max current.
That is said to be the reason why (some) manufacturers mention that you should never measure the internal resistance with a multimeter.

2) The second reason is that a DC current could magnetize the coil to some degree.
There is equipment on the market to demagnetize. Stereophile once did a test and found a positive effect on the sound reproduction.
I have no opinion on this because I never had the opportunity to test it.

3) Testing the effect on DC current for one element gives some insight, but it cannot guarantee that it is true for all elements with their different magnets and coils.

4) Measuring is not the same as listening, and that should be the final proof of the pudding. Not everything can be measured.
Einstein once said: Not everything that counts can be counted.

5) Last but not least, what should be really helpful is to have a guideline as to how much Cart current is allowed.
Such a guideline missing in your posting.
I'm sure that when allowing a DC current to be 1% of the current at max modulation , you are on the safe side.
But maybe it might be 10% or even more, but at least you have some ballpark figure related to the specific Cart.
And just allowing 100uA flowing into an expensive Cart, is not the kind of risk I would like to take, unless the manufacturer would give his O.K., what they won't do.

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by mediatechnology »

Richard - It's been awhile. I was hoping you would visit us and join in. Yes I do remember you posting that here. I recall it being when I was mulling FETs/vs op amp inputs for the MM preamp.

I still have to think that MC carts capacitively coupled with huge capacitors to elevated transistor bases are going to see peak DC currents in the tens if not hundreds of uA at turn-on.
After a good long warm-up and forming maybe a few uA.
But if capacitor leakage specifications are consistent with 0.01CV for the values required it could be far higher after stabilization in the tens of uA.
Lots of commercial MC stages are this way.

Regarding forces exerted on the cart I wanted to share the following observation.

Consider a conventional NE5532-based single-ended preamp directly-coupled to a MM or MC cart.
A better example is the PNP input LM833 where bias current flows out.
If +1 uA is flowing into cart Lead L+ and +1 uA into R+ the force exerted is lateral and toward the disc edge.
The "Ios" between input stages, i.e. the difference in bias currents, is a Vertical force.

Both are small WRT lateral skating forces and VTF as Richard points out.

I too have seen the warning regarding DCR measurement of both MC carts and step-up transformers.
The manufacturers have no control over Ohmeter tests currents and have a keen understanding that although their customers may be fanatics they also may not be the sharpest knife in the drawer either.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans »

Ever looked at these Fet's from Interfet ?

IF9030 0.5nV/rtHz
IF3601 0.3nV/rtHz

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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. »

Hans wrote:Ever looked at these Fet's from Interfet ?

IF9030 0.5nV/rtHz
IF3601 0.3nV/rtHz

Hans
Those are pretty impressive, including the cost.

I wouldn't expect the Ciss/rss to be a huge problem in context of MC carts that are low impedance. But the only way to tell is to build one up.

At $15 each I might be tempted to use my later topology (P-100) that only used one JFET per input but the Crss might have to be factored into my passive 75uSec pole at the drain of the input JFET.

I just noticed the link to my old P100 copy was broken so here it is again

Image
R2 would need to be scaled smaller for more voltage gain and C2 tweaked smaller to retune the 75uSec RIAA pole.

I make no assurances this would even work, and at $15 a JFET i don't care enough to try.

FWIW I vaguely recall that Brad used an unusual topology that had poor PSRR so had to roll his own low noise PS too.

JR

[edit] i apologize to Wayne for the veer and my general bad attitude about new phono preamps in this century. [/edit]
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by mediatechnology »

Was that your broken link JR or one here? If its mine I'll fix it.

I did find this P100 sch: viewtopic.php?f=12&t=151
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