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[email protected] digitaltrousers@gmail.com is offline
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Default Mic preamp output transformer question

Hi All,
In the mic preamp circuit, about halfway down this page: https://web.archive.org/web/20170831...-jfet-mic-pre/

....should the output transformer be left floating at one end as pictured?

Also, the output transformer I have has a center-tapped output, and the core is not broken out to a pin as pictured in the circuit diagram. Should I connect the center tap to pin 1 of the XLR socket instead?
Thanks for any assistance!
-Dan.
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Mike Rivers[_2_] Mike Rivers[_2_] is offline
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Default Mic preamp output transformer question

On 9/11/2018 5:08 AM, wrote:
Hi All,
In the mic preamp circuit, about halfway down this page:
https://web.archive.org/web/20170831...-jfet-mic-pre/

...should the output transformer be left floating at one end as pictured?


No. It won't work like that. It's a error. The "floating" end of the
primary should be connected to ground.


Also, the output transformer I have has a center-tapped output, and the core is not broken out to a pin as pictured in the circuit diagram. Should I connect the center tap to pin 1 of the XLR socket instead?


No. Leave the center tap disconnected.

--

For a good time, call http://mikeriversaudio.wordpress.com
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Scott Dorsey Scott Dorsey is offline
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Default Mic preamp output transformer question

wrote:
Hi All,
In the mic preamp circuit, about halfway down this page: https://web.archive.org/web/20170831...-jfet-mic-pre/

...should the output transformer be left floating at one end as pictured?


No. It goes to ground.

I notice this circuit uses only one ground, rather than different ground and
chassis earth. This may come to bite you if you're trying to get noise down.


Also, the output transformer I have has a center-tapped output, and the core is not broken out to a pin as pictured in the circuit diagram. Should I connect the center tap to pin 1 of the XLR socket instead?


No, just let the center tap float. It's not important.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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[email protected] digitaltrousers@gmail.com is offline
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Default Mic preamp output transformer question

I notice this circuit uses only one ground, rather than different ground and
chassis earth. This may come to bite you if you're trying to get noise down.


Cool. I wondered about grounding and have drawn the power supply ground separately to the "everything else" ground in KiCAD. I suspect that's not what you're suggesting though. How would you configure the ground(s), and where would you advise connecting them together?
Thanks!
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[email protected] makolber@yahoo.com is offline
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Default Mic preamp output transformer question

On Wednesday, September 12, 2018 at 2:36:07 AM UTC-4, wrote:
I notice this circuit uses only one ground, rather than different ground and
chassis earth. This may come to bite you if you're trying to get noise down.


Cool. I wondered about grounding and have drawn the power supply ground separately to the "everything else" ground in KiCAD. I suspect that's not what you're suggesting though. How would you configure the ground(s), and where would you advise connecting them together?
Thanks!


the most important ground issues involve signals that enter and exit

1) the + and - leads from the wall wart should go directly to the 470 uf cap so that large ripple currents do not flow through any grounds. I would also use the one point to tie the circuit to chassis ground.

2) the pin 1 connections for the XLRs should go directly to the chassis so that any large ground currents do not flow through the circuit grounds. You can google "pin 1 problem" to learn more about this.

enjoy

mark




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Phil Allison[_4_] Phil Allison[_4_] is offline
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Default Mic preamp output transformer question

wrote:



the most important ground issues involve signals that enter and exit

1) the + and - leads from the wall wart should go directly to the
470 uf cap so that large ripple currents do not flow through any grounds.
I would also use the one point to tie the circuit to chassis ground.

2) the pin 1 connections for the XLRs should go directly to the chassis
so that any large ground currents do not flow through the circuit grounds.
You can google "pin 1 problem" to learn more about this.



** Mic pre-amps tend not to have that problem - since microphones are "floating" sources.

However, direct connection of pin 1 on the input XLR to case is advisable to prevent RF interference picked up on the shield of the mic cable entering the internal circuit.


.... Phil
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Scott Dorsey Scott Dorsey is offline
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Default Mic preamp output transformer question

wrote:
On Wednesday, September 12, 2018 at 2:36:07 AM UTC-4, =
wrote:
I notice this circuit uses only one ground, rather than different groun=

d and
chassis earth. This may come to bite you if you're trying to get noise=

down.
=20
Cool. I wondered about grounding and have drawn the power supply ground s=

eparately to the "everything else" ground in KiCAD. I suspect that's not wh=
at you're suggesting though. How would you configure the ground(s), and whe=
re would you advise connecting them together?
Thanks!


the most important ground issues involve signals that enter and exit

1) the + and - leads from the wall wart should go directly to the 470 uf c=
ap so that large ripple currents do not flow through any grounds. I would =
also use the one point to tie the circuit to chassis ground.

2) the pin 1 connections for the XLRs should go directly to the chassis so =
that any large ground currents do not flow through the circuit grounds. Yo=
u can google "pin 1 problem" to learn more about this.


This is a good description.

If you're hand-wiring the thing, try and have a chassis ground and an
internal ground and connect them at only one point. If you're using
a PC board, make a ground plane and connect that to the chassis ground at only
one point. You don't want circulating currents through the chassis at any
time.
--scott


--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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[email protected] digitaltrousers@gmail.com is offline
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Default Mic preamp output transformer question

Thanks everyone
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