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Mic preamp output transformer question



 
 
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  #1  
Old September 11th 18, 10:08 AM posted to rec.audio.pro
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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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  #2  
Old September 11th 18, 11:50 AM posted to rec.audio.pro
Mike Rivers[_2_]
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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
  #3  
Old September 11th 18, 05:12 PM posted to rec.audio.pro
Scott Dorsey
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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."
  #4  
Old September 12th 18, 07:36 AM posted to rec.audio.pro
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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!
  #5  
Old September 12th 18, 02:53 PM posted to rec.audio.pro
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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


  #6  
Old September 13th 18, 02:36 AM posted to rec.audio.pro
Phil Allison[_4_]
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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
  #7  
Old September 13th 18, 01:17 PM posted to rec.audio.pro
Scott Dorsey
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Posts: 16,431
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."
  #8  
Old September 14th 18, 08:26 AM posted to rec.audio.pro
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Default Mic preamp output transformer question

Thanks everyone
 




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