A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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

Post by mediatechnology » Fri Feb 08, 2019 11:06 am

JR. wrote:
Fri Feb 08, 2019 10:48 am
I am trying to remember if I even use a local feedback cap across that op amps inner loop.

The obvious thing to do is try it, and then bang the input with square waves (that don't exist in the real world).
You didn't here but used R133 and C199 instead: https://proaudiodesignforum.com/forum/p ... 8&start=10
In the P10 no local Cfb but some inductance in the C2/C3 might have helped along with low output capacitance in the JFET: https://proaudiodesignforum.com/forum/p ... amps#p1244
In the P100 there is an RIAA network: https://proaudiodesignforum.com/forum/p ... 151&p=1625
Anywhere else I can look?

If the transistor output capacitance is low a local Cfb may not be needed.
The ZTX851 is about 45 pF IIRC with the PNP even more.
It would seem to follow Cfb would at least have to be that big.

When I use square waves, or look at something in the time domain as I always do, people laugh at me. :lol:
JR. wrote:
Fri Feb 08, 2019 10:48 am
PS: I still like shorting out the blocking caps with a switch.
What do you short that blocking capacitor out with? A screwdriver?
Getting the charge out of it without welding the relay contacts or adding Rsource is not trivial.
I did one where I had the relay simply switch between a capacitor-coupled path and a direct path both fed by the same tip and ring.
https://ka-electronics.com

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

Post by JR. » Fri Feb 08, 2019 11:31 am

mediatechnology wrote:
Fri Feb 08, 2019 11:06 am
JR. wrote:
Fri Feb 08, 2019 10:48 am
I am trying to remember if I even use a local feedback cap across that op amps inner loop.

The obvious thing to do is try it, and then bang the input with square waves (that don't exist in the real world).
You didn't here but used R133 and C199 instead: https://proaudiodesignforum.com/forum/p ... 8&start=10
In the P10 no local Cfb but some inductance in the C2/C3 might have helped along with low output capacitance in the JFET: https://proaudiodesignforum.com/forum/p ... amps#p1244
In the P100 there is an RIAA network: https://proaudiodesignforum.com/forum/p ... 151&p=1625
Anywhere else I can look?
There was a MC version of P10 using 2sb737 and IIRC that did not use feedback caps.
If the transistor output capacitance is low a local Cfb may not be needed.
The ZTX851 is about 45 pF IIRC with the PNP even more.
It would seem to follow Cfb would at least have to be that big.

When I use square waves, or look at something in the time domain as I always do, people laugh at me. :lol:
From years of bench time designing NR systems, transient response matters, but reading too much into signal stimulus that don't exist in nature can be unproductive. For a while it was fashionable to give audio paths RF response bandwidths :roll:
JR. wrote:
Fri Feb 08, 2019 10:48 am
PS: I still like shorting out the blocking caps with a switch.
What do you short that blocking capacitor out with? A screwdriver?
Getting the charge out of it without welding the relay contacts or adding Rsource is not trivial.
I did one where I had the relay simply switch between a capacitor-coupled path and a direct path both fed by the same tip and ring.
I never reduced this to practice... I tried to get JP to consider this, but he declined. :D

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by mediatechnology » Fri Feb 08, 2019 12:14 pm

From years of bench time designing NR systems, transient response matters, but reading too much into signal stimulus that don't exist in nature can be unproductive. For a while it was fashionable to give audio paths RF response bandwidths :roll:
I like your characterization of slew rate limiting as "clipping in the time domain."
I tend to roll my eyes too at 1000V/µs slew rate requirements for audio.

Square wave testing was particularly useful in designing the DCAO-II because it has nested op amps and feedback loops.
The NE5532 has this curious kink in phase response between 200-500 kHz depending on manufacturer and lot.
When nested, there is a "snap oscillation" only a few kHz wide where the kinks of nested stages combine to produce positive feedback.
Though it can be found by slowly sweeping in the ultrasonic range with a sinewave the pulse response with a low frequency square wave made it really obvious.
The pulse response made me look for the kink using a sine wave.

Though the 5532 can be used in the DCAO-II it has to be over-compensated far more than other op amps which are not kinky.

So yes, banging the MC preamp with a square wave makes perfect sense and, in the un-equalized world of flat phono preamps, the edge rates particularly with MC carts can be quite high.
https://ka-electronics.com

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Project Veritas: Google "Machine Learning Fairness" Whistleblower Goes Public, says: "burden lifted off of my soul." https://www.youtube.com/watch?v=g1VeElB ... e=youtu.be
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by JR. » Fri Feb 08, 2019 1:44 pm

mediatechnology wrote:
Fri Feb 08, 2019 12:14 pm


So yes, banging the MC preamp with a square wave makes perfect sense and, in the un-equalized world of flat phono preamps, the edge rates particularly with MC carts can be quite high.
That was one thing I really liked about my phono preamp, to end all phono preamps (P-100).

The drain of the open loop input JFET gain stage is loaded by the 75uSec RIAA eq pole (roughly 2kHz LPF). I could hit that input with a square wave level equivalent to what would clip the RIAA EQ output at 20kHz, with the square wave is harmlessly LPF.

I actually had circuit improvements on the bench to reduce the input stage open loop THD but it was already respectably low distortion and the preamp was far better than the rest of the medium (and the market did not appreciate how good it was already, bad marketing on my part.) I left the audiophools alone and never looked back. :lol:

===

I am a firm believer in BP filtering all audio paths. Even a DC coupled mic preamp needs a HPF after the front end using a quality film cap. I believe you are already doing that.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by billshurv » Fri Feb 08, 2019 6:14 pm

The power amp I am currently using is 300V/uS slew rate. Completely pointless, but it works :)

My MM phono goes one further and puts a pole in passively before the first stage. Unusual but hopefully will work nicely :)

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

Post by JR. » Fri Feb 08, 2019 8:06 pm

billshurv wrote:
Fri Feb 08, 2019 6:14 pm
The power amp I am currently using is 300V/uS slew rate. Completely pointless, but it works :)
300V/uSec is not completely crazy for high enough power points since big power amps can swing 20x small signal level... that said slew limit could be a tell for questionable design (but consumers demand slew rate numbers), so at least one reputable mfr defeated their input lpf to deliver a big slew number for the data sheet to keep the sheeple happy... With a suitable LPF input the amp cannot be slew limited.

The best way to characterize an audio path IMO is "rise time". Dr Leach (RIP) wrote a classic paper about how to incorporate that LPF into a typical active front end.
My MM phono goes one further and puts a pole in passively before the first stage. Unusual but hopefully will work nicely :)
Adding a passive filter at the cartridge termination could interact with cartridge inductance.

My real pole at roughly 2kHz seems adequate, and it precedes any NF.

JR
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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

Post by billshurv » Fri Feb 08, 2019 8:15 pm

It USES the cartridge inductance to create the LPF. Load it down hard enough to create an LR filter and watch people tell you it won't work :D

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

Post by JR. » Sat Feb 09, 2019 9:37 am

billshurv wrote:
Fri Feb 08, 2019 8:15 pm
It USES the cartridge inductance to create the LPF. Load it down hard enough to create an LR filter and watch people tell you it won't work :D
It bothered me no end that hot shot magazine reviewers would audition phono preamps with no attempt to properly dial in the cartridges.

I used to also sell a small kit using a 4 station dip switch, with 24pF, 47pF, 100pF, and 200pF polystyrene caps to cover a wide range of termination capacitance. I stuck with the nominal 47k resistance for MM carts.

===

Some people mess around with low resistance terminations for microphones, they will sound different. I have never been a fan of deviating from standard bridging terminations, that deliver good voltage transfer.

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

Post by billshurv » Sun Feb 10, 2019 9:36 am

It also bothers me that almost no current manufacturers of MM phono stages publish the input capacitance of their circuits, just the added C. Add to that the ignorance that propagates around the WWW and no wonder some people think MM are bright and others think the same are dull.

So as not to dilute this thread any more I'll fire up another to discuss alternative MM loading schemes as its an interesting subject that turns up a lot of things that are taken as gospel but have never been demonstrated. most of these are things that you couldn't put in a commercial product unless you were selling a matching cart and preamp.

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

Post by JR. » Sun Feb 10, 2019 11:36 am

billshurv wrote:
Sun Feb 10, 2019 9:36 am
It also bothers me that almost no current manufacturers of MM phono stages publish the input capacitance of their circuits, just the added C. Add to that the ignorance that propagates around the WWW and no wonder some people think MM are bright and others think the same are dull.
Some consumers are bright and others dull..... :lol:
So as not to dilute this thread any more I'll fire up another to discuss alternative MM loading schemes as its an interesting subject that turns up a lot of things that are taken as gospel but have never been demonstrated. most of these are things that you couldn't put in a commercial product unless you were selling a matching cart and preamp.
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
Visit https://circularscience.com to hear what properly "cleared" drums sound like.

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