A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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Gold
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by Gold » Sun Feb 10, 2019 12:42 pm

Most of the time I’ve found that flattest frequency response often isn’t subjectively the best sound. I’ve found this with both MM and MC carts. I usually find that the flattest frequency response often sounds “overdamped” to my ears.

In the case of the Denon 103 it leaves a resonance peak that sounds like it’s around 10K. I think there is a dip just a little bit higher maybe 11 or 12k. Sometimes this makes things sound brighter or darker depending on the program.

billshurv
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Sun Feb 10, 2019 3:55 pm

JR. wrote:
Sun Feb 10, 2019 11:36 am


Back in the bad old days such transducer design was done in the context of a closed system. Like old school microphones (transmitters), engineered with companion preamps.

I find it hard to get excited about vinyl, but I am not the target market for the recent revival (dead cat bounce).

JR
Nor am I (the target). I've had vinyl since the beginning and not got rid of it but the newest pressing I have is probably about 1993. I'm not paying £30 for a vinyl mastering of a digital source, even if it seems the vinyl is a little less squashed in mastering. I do however enjoy the ritual and exploring dead ends from the past that might improve replay. But I freely acknowledge that, at the push of a button my server delivers far more accurate reproduction.

(aside: its also fun to see the indignation of the true believer when you say you are doing RIAA digitially. I'm trying to build at least one setup that has the highest heresy possible).

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JR.
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by JR. » Sun Feb 10, 2019 4:11 pm

billshurv wrote:
Sun Feb 10, 2019 3:55 pm
JR. wrote:
Sun Feb 10, 2019 11:36 am


Back in the bad old days such transducer design was done in the context of a closed system. Like old school microphones (transmitters), engineered with companion preamps.

I find it hard to get excited about vinyl, but I am not the target market for the recent revival (dead cat bounce).

JR
Nor am I (the target). I've had vinyl since the beginning and not got rid of it but the newest pressing I have is probably about 1993.
The beginning? My dad was a recording engineer for RCA in NYC back in the 50's (Perry Como, Harry Belafonte, et al). Over a decade before that he worked at Western Electric and even Vitaphone (first sound movies used sound tracks cut to disc(s)). I have a stack of 78s and even some one-off lacquers my dad cut at work in his studio.

I have lots of old vinyl that got trashed by being abused at parties back in the 60s.

The good news is I've been able to find youtube clips of even my more obscure albums, but I don't actively listen to recordings any more. Having live bands playing my living room at my beer parties in the 80s-90s kind of ruined the old hifi experience for me. :roll:
I'm not paying £30 for a vinyl mastering of a digital source, even if it seems the vinyl is a little less squashed in mastering. I do however enjoy the ritual and exploring dead ends from the past that might improve replay. But I freely acknowledge that, at the push of a button my server delivers far more accurate reproduction.

(aside: its also fun to see the indignation of the true believer when you say you are doing RIAA digitially. I'm trying to build at least one setup that has the highest heresy possible).
I have been defending digital since the 80s in my old "Audio Mythology" column... I never expected to be coopted by digital. :lol:


JR

billshurv
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Sun Feb 10, 2019 5:17 pm

By Beginning I meant beginning of my music collecting habit* as compared to those who have 'got into' vinyl having grown up with CD/ipod and now think 'vinyls' are cool. But I used too few words :).

* I got better around 2000, then got a server to put my music on and got worse again.

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mediatechnology
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Tue Feb 12, 2019 5:31 am

My whole reason for doing a preamp was to get those records onto a server.

But I found I enjoyed the ritual.
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terkio
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by terkio » Tue Feb 12, 2019 7:48 am

I think Hfe un balance should be considered as well as Vbe un balance.
Assuming ZTX851 Hfe=150 5% accuracy, with R4=R5=499 and Ic=5.5mA: Delta Vb=499x( 5.5/150)x0.05 mV.
That is about 1mV Delta Vb, to compare to a lower than 1mV Delta Vbe achieved by matching.
Who knows ZTX851 Hfe tempco ?
About small signal Bjt, I found Hfe tempco: From + 0.3%/°C to +0.7%/°C.
This gives about the 1mV Delta Vb an estimated Vb tempco: + 0.1 mV/°C.
This is swamped by the typical Vbe tempco: - 2mV/°C
The temperature stability issue is definatly from Vbe and not from Hfe. However Hfe accuracy that is very loose before screening, is an issue in this preamp circuit.

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JR.
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by JR. » Tue Feb 12, 2019 10:02 am

terkio wrote:
Tue Feb 12, 2019 7:48 am
I think Hfe un balance should be considered as well as Vbe un balance.
Assuming ZTX851 Hfe=150 5% accuracy, with R4=R5=499 and Ic=5.5mA: Delta Vb=499x( 5.5/150)x0.05 mV.
That is about 1mV Delta Vb, to compare to a lower than 1mV Delta Vbe achieved by matching.
Who knows ZTX851 Hfe tempco ?
About small signal Bjt, I found Hfe tempco: From + 0.3%/°C to +0.7%/°C.
This gives about the 1mV Delta Vb an estimated Vb tempco: + 0.1 mV/°C.
This is swamped by the typical Vbe tempco: - 2mV/°C
The temperature stability issue is definatly from Vbe and not from Hfe. However Hfe accuracy that is very loose before screening, is an issue in this preamp circuit.
What issue are we expecting from beta? From the data sheet Wayne posted on the first page of this thread the beta for these parts is typically 200 with max of 300, while the low noise transistors I used back in the day were higher beta than that (like 2x).

I never noticed an issue on the bench related to beta with this topology, while I did see measurable distortion from poorly matched JFET Vgs (like in one of my earlier phono preamps). I still have bags of matched JFETs with almost no present value, since they have been eclipsed by newer much lower noise JFETs.

JR

terkio
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by terkio » Tue Feb 12, 2019 11:52 am

I suspect the Beta mismatch to make trouble of the same kind and amplitude as the Vbe mismatch.
This has to do, about how much the servo will have to compensate to balance the circuit.
I am using a similar design in mike preamps using two LM394 where balancing is done with a trimmer. They work perfectly, but I am interested to use a servo in a newer design.

billshurv
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Tue Feb 12, 2019 12:51 pm

@JR: I think you would be suprised. Getting decent low noise JFETS in through hole is hard these days. What FETs do you have?

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mediatechnology
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Tue Feb 12, 2019 3:16 pm

It also bothers me that almost no current manufacturers of MM phono stages publish the input capacitance of their circuits, just the added C.
This one just did.
Just measured the Balanced Flat Moving Magnet Preamplifier stray PC board capacitance without any added termination.

Differential: 4.6 pF (plus op amp typically 2-5 pF).
Common Mode: 9 pF ±0.1 pF per leg (plus op amp typically 2 pF).

With the cart connected to the input using a balanced/floating configuration the load would be about 10 pF plus the op amp.
Jumpers can then add 50, 100, 150 and 200 pf.

I notice that when I unload MM carts completely they can have large amounts of HF distortion on sibilants, high hat etc. and you can see it in the resulting distortion in the goniometer vectors becoming s-shaped (odd-order) or comma-shaped (even-order).
https://ka-electronics.com
Learn to code. Recognize media gaslighting.
Sandmann v. Washington Post Complaint https://proaudiodesignforum.com/images/ ... plaint.pdf
CBS News chief foreign affairs correspondent Lara Logan: "That’s not journalism, it’s horseshit" https://www.zerohedge.com/news/2019-02- ... ts-are-now
"Nick Sandman: The Truth in 15 minutes" https://youtu.be/lSkpPaiUF8s

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