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 Input Moving Coil Preamp Using the ZTX851

Post by JR. » Thu Feb 07, 2019 11:18 am

I have long been a fan of floating, DC coupled front ends (I started following Wayne around the WWW after he talked about this), but by the time this ever gets perfected and out into the world, mics will be wirelessly talking to the cloud. :lol: Likewise you could drive pin 1 to negative voltage, but with more complexity perhaps.

I tried to get a console designer buddy to consider a simple bypass switch for input phantom blocking caps when the microphone isn't using phantom power... In fact this could be the same switch, 8-) but he had no interest and he isn't around any more (RIP).

Keep on improving it...

JR

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

Post by mediatechnology » Thu Feb 07, 2019 7:18 pm

JR. wrote:
Thu Feb 07, 2019 11:18 am
mics will be wirelessly talking to the cloud. :lol:
It's called Alexa and its very scary.
She hears and records everything from the day she was plugged in.
Show me the man and Alexa will play for you the crime.
Put your SmartPhone in the microwave and shut the door when you want to have a really private conversation.
Just sayin'

But hey - I'm gettin' that reverb done. That was in 2007 too. :lol:
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JR.
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by JR. » Thu Feb 07, 2019 8:16 pm

mediatechnology wrote:
Thu Feb 07, 2019 7:18 pm
JR. wrote:
Thu Feb 07, 2019 11:18 am
mics will be wirelessly talking to the cloud. :lol:
It's called Alexa and its very scary.
She hears and records everything from the day she was plugged in.
Show me the man and Alexa will play for you the crime.
I do not understand why people embrace that technology, they don't even have to hide it.

Put your SmartPhone in the microwave and shut the door when you want to have a really private conversation.
Just sayin'
If I had a smart phone I might
But hey - I'm gettin' that reverb done. That was in 2007 too. :lol:
I can't even finish some of my easy projects...

JR

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

Post by billshurv » Fri Feb 08, 2019 6:50 am

I would nag Wayne to get some boards out were it not for my many simple projects that need completing before I can use it. However if this weekends moving around is successful I will be a step closer to wanting it.

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

Post by mediatechnology » Fri Feb 08, 2019 7:31 am

Good to know, thanks Bill.

I have a spring reverb to ship out today and once my bench is cleared I'll move that Protoboard to the front.
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"Mr. Sociopath Goes to Washington" https://www.zerohedge.com/news/2019-02- ... washington
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by billshurv » Fri Feb 08, 2019 8:18 am

My second most embarrassing backlog item is a subwoofer I've had since 1998. It was cheap as missing the controller. Yeah I can have 3 kids and get time to build one of those up... I have at least replaced the cone foam.

Anyway miniDSP has balanced out so no extra analog required bar the power up wire as infinity chose to put the on off switch underneath. I guess I should open her up and just put 20V on the caps for a few minutes just in case as would make an awful mess of the stuffing if they did blow :).

So when I post that this is finally working you'll know I'll be nearly ready for the balanced MC and a couple of other boards :D
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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 8:54 am

Amazing to think about how those tiny wires, coils and vibrations in the cart wind up in that beast of a sub-woofer.
I can see why you'd be interested in rumble/warp reduction.

Totally off-topic but one of the big industrial uses for Crown power amps in the US, besides shaker tables, was chicken debeaking.
Dunno about debeaking but I bet you could break some eggs with that sub.
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Re: A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

Post by mediatechnology » Fri Feb 08, 2019 9:02 am

Likewise you could drive pin 1 to negative voltage, but with more complexity perhaps.
OK, I finally thought this through.
If the console is electrically grounded to Earth (with a capital "E") and pin 1 and shield tie together due to a mis-wired mic or cable out on the user end, what prevents -48V from appearing in the case of an SM57? (I realize an SM57 is a dynamic mic but you get the point.)
I realize the current is low but I feel this tingling just thinking about it.

Back to the preamp: One thing I wanted to try is a small-value resistor between the input stage collector and the op amp inverting input to see if isolating the output capacitance of the transistor a little bit from the summing node would allow the op amp's local Cfb to be lowered. I've seen this in a number of circuits recently.
https://ka-electronics.com
Learn to code.
"Mr. Sociopath Goes to Washington" https://www.zerohedge.com/news/2019-02- ... washington
"FBI Directing CNN Camera Crew During Raid" https://www.brighteon.com/6000098956001
"Knowing Keeps Us Free" https://www.youtube.com/watch?v=5lC3mpG60sw
"Nick Sandman: The Truth in 15 minutes" https://youtu.be/lSkpPaiUF8s

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

Post by billshurv » Fri Feb 08, 2019 9:49 am

I'll be honest, will have a different sub profile for vinyl! I've survived all these years without the bottom 2 octaves..

I did chat with someone who had worked for a company that made auto shakers. The sort that can turn a brand new merc into a pile of bolts in about 48 hours. Back then (around 1990) up to 50kw was solid state, over was tube. Apparantly their test cell had a small pool as the heat dump. These days class D rules for that application.

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

Post by JR. » Fri Feb 08, 2019 10:48 am

mediatechnology wrote:
Fri Feb 08, 2019 9:02 am
Likewise you could drive pin 1 to negative voltage, but with more complexity perhaps.
OK, I finally thought this through.
If the console is electrically grounded to Earth (with a capital "E") and pin 1 and shield tie together due to a mis-wired mic or cable out on the user end, what prevents -48V from appearing in the case of an SM57? (I realize an SM57 is a dynamic mic but you get the point.)
I realize the current is low but I feel this tingling just thinking about it.
The more likely spoiler is significant noise if the mic body touches 0V through another electrical path.

In practice the voltage on the mic body should not be -48V in use, but indeed not ready for prime time.
Back to the preamp: One thing I wanted to try is a small-value resistor between the input stage collector and the op amp inverting input to see if isolating the output capacitance of the transistor a little bit from the summing node would allow the op amp's local Cfb to be lowered. I've seen this in a number of circuits recently.
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).

JR

PS: I still like shorting out the blocking caps with a switch. Some premium mic preamps get used in passive mixers without phantom power. Caveat I am not remotely endorsing passive mixers... :lol:

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