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Audio 100V signal to line level
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robertxm...@gmail.com
Mar 24, 2016, 12:54:31 PM3/24/16
to
All,
I am doing some testing with an audio PA system that has the output of 100V (rms). What I am trying to do is take this 100V(rms) and bring it down to an audio line level of about 280 mV (rms). I am doing this with a resistor divider.
1.) An image of 100V (rms) input signal measured on oscilloscope is below:
http://s1070.photobucket.com/user/Mrblueblue1234/media/input_100V_zpsze4bcqkm.jpg.html
2.) I am then dividing this down to ~280mV with a resistor divider,
375k and 1k. Here is the image of the signal from the resistor divider:
http://s1070.photobucket.com/user/Mrblueblue1234/media/scope_reading_zpshvc6hhbp.jpg.html
As you can see the signal here looks extremely distorted on the oscilloscope! I am not sure if this is what really is happening or if it is an artifact of the oscilloscope.
3.) We have some audio equipment from the company Audio Precision here and with their equipment the signal looks much cleaner. See image below:
http://s1070.photobucket.com/user/Mrblueblue1234/media/AP_zpsaxn9km9s.jpg.html
4.) There are some transformer options out there to take a 100V signal and bring it to a line level but I wanted to see if we could do this with a less expensive resistor divider solution.
Thank you from any thoughts you have on this.
-Robert
I am doing some testing with an audio PA system that has the output of 100V (rms). What I am trying to do is take this 100V(rms) and bring it down to an audio line level of about 280 mV (rms). I am doing this with a resistor divider.
1.) An image of 100V (rms) input signal measured on oscilloscope is below:
http://s1070.photobucket.com/user/Mrblueblue1234/media/input_100V_zpsze4bcqkm.jpg.html
2.) I am then dividing this down to ~280mV with a resistor divider,
375k and 1k. Here is the image of the signal from the resistor divider:
http://s1070.photobucket.com/user/Mrblueblue1234/media/scope_reading_zpshvc6hhbp.jpg.html
As you can see the signal here looks extremely distorted on the oscilloscope! I am not sure if this is what really is happening or if it is an artifact of the oscilloscope.
3.) We have some audio equipment from the company Audio Precision here and with their equipment the signal looks much cleaner. See image below:
http://s1070.photobucket.com/user/Mrblueblue1234/media/AP_zpsaxn9km9s.jpg.html
4.) There are some transformer options out there to take a 100V signal and bring it to a line level but I wanted to see if we could do this with a less expensive resistor divider solution.
Thank you from any thoughts you have on this.
-Robert
George Herold
Mar 24, 2016, 1:23:34 PM3/24/16
to
On Thursday, March 24, 2016 at 12:54:31 PM UTC-4, robertxm...@gmail.com wrote:
> All,
>
> I am doing some testing with an audio PA system that has the output of 100V (rms). What I am trying to do is take this 100V(rms) and bring it down to an audio line level of about 280 mV (rms). I am doing this with a resistor divider.
>
>
> 1.) An image of 100V (rms) input signal measured on oscilloscope is below:
>
>
> http://s1070.photobucket.com/user/Mrblueblue1234/media/input_100V_zpsze4bcqkm.jpg.html
>
>
> 2.) I am then dividing this down to ~280mV with a resistor divider,
> 375k and 1k. Here is the image of the signal from the resistor divider:
>
>
> http://s1070.photobucket.com/user/Mrblueblue1234/media/scope_reading_zpshvc6hhbp.jpg.html
>
>
> As you can see the signal here looks extremely distorted on the oscilloscope! I am not sure if this is what really is happening or if it is an artifact of the oscilloscope.
>
>
> 3.) We have some audio equipment from the company Audio Precision here and with their equipment the signal looks much cleaner. See image below:
>
> http://s1070.photobucket.com/user/Mrblueblue1234/media/AP_zpsaxn9km9s.jpg.html
It looks more like pickup/interference than distortion.
> All,
>
> I am doing some testing with an audio PA system that has the output of 100V (rms). What I am trying to do is take this 100V(rms) and bring it down to an audio line level of about 280 mV (rms). I am doing this with a resistor divider.
>
>
> 1.) An image of 100V (rms) input signal measured on oscilloscope is below:
>
>
> http://s1070.photobucket.com/user/Mrblueblue1234/media/input_100V_zpsze4bcqkm.jpg.html
>
>
> 2.) I am then dividing this down to ~280mV with a resistor divider,
> 375k and 1k. Here is the image of the signal from the resistor divider:
>
>
> http://s1070.photobucket.com/user/Mrblueblue1234/media/scope_reading_zpshvc6hhbp.jpg.html
>
>
> As you can see the signal here looks extremely distorted on the oscilloscope! I am not sure if this is what really is happening or if it is an artifact of the oscilloscope.
>
>
> 3.) We have some audio equipment from the company Audio Precision here and with their equipment the signal looks much cleaner. See image below:
>
> http://s1070.photobucket.com/user/Mrblueblue1234/media/AP_zpsaxn9km9s.jpg.html
What's the frequency of the pickup?
Shielded box?
Turn off room lights?
Lower resistor ratio maybe... 3.7k and 10 ohms
Transformer?
George H.
Kevin Glover
Mar 24, 2016, 1:31:05 PM3/24/16
to
On Thu, 24 Mar 2016 09:54:22 -0700 (PDT), robertxm...@gmail.com
wrote:
Remember that RMS (Root Mean Square) is 70.71% of the peak value. So
your peak would actually be 141.4 Volts.
wrote:
your peak would actually be 141.4 Volts.
tom
Mar 24, 2016, 1:45:53 PM3/24/16
to
"George Herold" <ghe...@teachspin.com> wrote in message
news:4bbcfb05-12b1-45e4...@googlegroups.com...
scope to confirm.
It appears it is a floating signal. Use a transformer.
John Larkin
Mar 24, 2016, 1:47:51 PM3/24/16
to
There could be a lot of common-mode voltage on the 100V pair, and if
you are using a single-ended divider, that noise will show up in your
output.
Post the actual schematic of your divider and its grounding.
A transformer would help. Or a diffamp. The Audio Precision thing
seems to do it right.
If the entire 100V system is floating, you could ground one side.
Maybe.
--
John Larkin Highland Technology, Inc
picosecond timing precision measurement
jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com
Oppie
Mar 24, 2016, 2:43:02 PM3/24/16
to
Whenever feeding audio from a power amp to something else - always assume
that the power amp's output is full bridge and that neither side is ground.
I would strongly recommend using a resistive divider into the input of a DI
(Direct Input) box. Go for one of the transformer coupled DI boxes so you
get a galvanic isolation between input and output.
I do concert sound as a side job.
<robertxm...@gmail.com> wrote in message
news:2d1ea202-388a-4a36...@googlegroups.com...
that the power amp's output is full bridge and that neither side is ground.
I would strongly recommend using a resistive divider into the input of a DI
(Direct Input) box. Go for one of the transformer coupled DI boxes so you
get a galvanic isolation between input and output.
I do concert sound as a side job.
<robertxm...@gmail.com> wrote in message
news:2d1ea202-388a-4a36...@googlegroups.com...
whit3rd
Mar 24, 2016, 3:51:30 PM3/24/16
to
On Thursday, March 24, 2016 at 9:54:31 AM UTC-7, robertxm...@gmail.com wrote:
> I am doing some testing with an audio PA system that has the output of 100V (rms). What I am trying to do is take this 100V(rms) and bring it down to an audio line level of about 280 mV (rms).
> I am doing some testing with an audio PA system that has the output of 100V (rms). What I am trying to do is take this 100V(rms) and bring it down to an audio line level of about 280 mV (rms).
> 4.) There are some transformer options out there to take a 100V signal and bring it to a line level but I wanted to see if we could do this with a less expensive resistor divider solution.
Your PA system has long wires, and your oscilloscope ground is probably combining with
those to make a big ground loop. Transformer isolation is your friend (you don't need
anything elaborate, a 1:1 transformer with series resistor on input side, and load resistor
on the output side, will make a transformer-isolated resistive divider).
amdx
Mar 24, 2016, 4:04:24 PM3/24/16
to
something. If you see it try a shield.
robertxm...@gmail.com
Mar 24, 2016, 5:33:44 PM3/24/16
to
John,
Here is the schematic of the circuit.
http://i1070.photobucket.com/albums/u484/Mrblueblue1234/PA_Divider_zps3fk3fahh.jpg?t=1458768680
The output of the PA system goes directly to P1. The oscilloscope is going directly to the connections at OUTPUT and GND.
Here is the schematic of the circuit.
http://i1070.photobucket.com/albums/u484/Mrblueblue1234/PA_Divider_zps3fk3fahh.jpg?t=1458768680
The output of the PA system goes directly to P1. The oscilloscope is going directly to the connections at OUTPUT and GND.
robertxm...@gmail.com
Mar 24, 2016, 5:42:52 PM3/24/16
to
Hi Oppie thanks for the post.
I took a look at some DI box videos on youtube but I don't want a XLR/balanced output. I need the signal to be single ended at 280mVrms.
I took a look at some DI box videos on youtube but I don't want a XLR/balanced output. I need the signal to be single ended at 280mVrms.
robertxm...@gmail.com
Mar 24, 2016, 5:44:53 PM3/24/16
to
whit3rd,
Right now for my testing I have very short wires (1 foot) coming from the PA to my resistor divider.
Right now for my testing I have very short wires (1 foot) coming from the PA to my resistor divider.
George Herold
Mar 24, 2016, 9:02:47 PM3/24/16
to
On Thursday, March 24, 2016 at 5:44:53 PM UTC-4, robertxm...@gmail.com wrote:
> whit3rd,
>
> Right now for my testing I have very short wires (1 foot) coming from the PA to my resistor divider.
1 foot of wire is a big antenna, perhaps.
> whit3rd,
>
> Right now for my testing I have very short wires (1 foot) coming from the PA to my resistor divider.
The first thing to do is find the frequency of the interference.
that will lead to the source...etc.
George H.
Oppie
Mar 24, 2016, 9:23:24 PM3/24/16
to
Just consider the DI box to be a transformer that can eliminate ground noise
or common mode noise. I use one all the time between my mixing board output
when I have to feed a house sound system through a mic level input. The
house sound systems almost always have 48V phantom power for condenser mics
that would damage my board's output. The transformer coupling totally
negates this and passes only a clean signal.
As you intend to couple to the output of a 100vrms rated amplifier to the
input of another amplifier, you have to pad down the input to the
transformer so it does not saturate. I have modified some boxes to replace
the 1/4" phone jack on the Hi-Z side. Normally they are TS (tip sleeve)
types for monaural use. I replaced it with a TRS (Tip Ring Sleeve) jack so I
could keep the ground on the sleeve but have the transformer between tip
and ring. Just a different way to use a fully floating Hi-Z input for signal
sources that may not work well with a grounded path. Just note that the
padding resistors should be split on each side of feed from the power
amplifier. Phone plugs will short out when being inserted or withdrawn. This
runs the possibility of damaging the amplifier.
This is a typical passive DI box schematic. The XLR output can be patched
any way you need. If you don't need balanced, tie pins 1 and 2 together for
signal ground and take your signal from pin 3.
Hope that helps.
wrote in message
news:172ff92e-f1d9-4c7d...@googlegroups.com...
or common mode noise. I use one all the time between my mixing board output
when I have to feed a house sound system through a mic level input. The
house sound systems almost always have 48V phantom power for condenser mics
that would damage my board's output. The transformer coupling totally
negates this and passes only a clean signal.
As you intend to couple to the output of a 100vrms rated amplifier to the
input of another amplifier, you have to pad down the input to the
transformer so it does not saturate. I have modified some boxes to replace
the 1/4" phone jack on the Hi-Z side. Normally they are TS (tip sleeve)
types for monaural use. I replaced it with a TRS (Tip Ring Sleeve) jack so I
could keep the ground on the sleeve but have the transformer between tip
and ring. Just a different way to use a fully floating Hi-Z input for signal
sources that may not work well with a grounded path. Just note that the
padding resistors should be split on each side of feed from the power
amplifier. Phone plugs will short out when being inserted or withdrawn. This
runs the possibility of damaging the amplifier.
This is a typical passive DI box schematic. The XLR output can be patched
any way you need. If you don't need balanced, tie pins 1 and 2 together for
signal ground and take your signal from pin 3.
Hope that helps.
wrote in message
news:172ff92e-f1d9-4c7d...@googlegroups.com...
Oppie
Mar 24, 2016, 9:38:53 PM3/24/16
to
Missed pasting the link
http://tinderwetstudios.com/blog/wp-content/uploads/2012/09/passive-di-box-schematic.gif
"Oppie" wrote in message news:bq0Jy.59145$3G4....@fx33.iad...
http://tinderwetstudios.com/blog/wp-content/uploads/2012/09/passive-di-box-schematic.gif
"Oppie" wrote in message news:bq0Jy.59145$3G4....@fx33.iad...
rickman
Mar 24, 2016, 10:19:26 PM3/24/16
to
the dropping resistor R1 into two, one on each side of the signal from
the PA. That will give you isolation and likely get rid of the common
mode noise.
R1
->>----\/\/\/----+------>>-- Out
180K |
\
/
\ R2
/ 1K
\
R3 |
->>----\/\/\/----+------>>-- GND
180K
Like someone said, you can lower the values of the resistors as well.
--
Rick
John Larkin
Mar 24, 2016, 11:00:57 PM3/24/16
to
On Thu, 24 Mar 2016 14:33:35 -0700 (PDT), robertxm...@gmail.com
wrote:
>John,
>
>Here is the schematic of the circuit.
>
>http://i1070.photobucket.com/albums/u484/Mrblueblue1234/PA_Divider_zps3fk3fahh.jpg?t=1458768680
>
>The output of the PA system goes directly to P1. The oscilloscope is going directly to the connections at OUTPUT and GND.
>
>
>
The scope is probably earth grounded, and you might not know how the
100V amp is grounded. One side of the 100V pair might be ground, or it
might be floating, or it might be opposite phases driven against
ground. There could be interesting conflicts.
A small transformer would help.
--
John Larkin Highland Technology, Inc
lunatic fringe electronics
wrote:
>John,
>
>Here is the schematic of the circuit.
>
>http://i1070.photobucket.com/albums/u484/Mrblueblue1234/PA_Divider_zps3fk3fahh.jpg?t=1458768680
>
>The output of the PA system goes directly to P1. The oscilloscope is going directly to the connections at OUTPUT and GND.
>
>
>
100V amp is grounded. One side of the 100V pair might be ground, or it
might be floating, or it might be opposite phases driven against
ground. There could be interesting conflicts.
A small transformer would help.
--
John Larkin Highland Technology, Inc
tabb...@gmail.com
Mar 25, 2016, 7:57:14 AM3/25/16
to
On Friday, 25 March 2016 03:00:57 UTC, John Larkin wrote:
> On Thu, 24 Mar 2016 14:33:35 -0700 (PDT), robertxm...@gmail.com
> wrote:
>
> >John,
> >
> >Here is the schematic of the circuit.
> >
> >http://i1070.photobucket.com/albums/u484/Mrblueblue1234/PA_Divider_zps3fk3fahh.jpg?t=1458768680
> >
> >The output of the PA system goes directly to P1. The oscilloscope is going directly to the connections at OUTPUT and GND.
> >
> >
> >
>
> The scope is probably earth grounded, and you might not know how the
> 100V amp is grounded. One side of the 100V pair might be ground, or it
> might be floating, or it might be opposite phases driven against
> ground. There could be interesting conflicts.
>
> A small transformer would help.
Or both could just be earthed to different places, with current in the earth wire creating voltage drop.
> On Thu, 24 Mar 2016 14:33:35 -0700 (PDT), robertxm...@gmail.com
> wrote:
>
> >John,
> >
> >Here is the schematic of the circuit.
> >
> >http://i1070.photobucket.com/albums/u484/Mrblueblue1234/PA_Divider_zps3fk3fahh.jpg?t=1458768680
> >
> >The output of the PA system goes directly to P1. The oscilloscope is going directly to the connections at OUTPUT and GND.
> >
> >
> >
>
> The scope is probably earth grounded, and you might not know how the
> 100V amp is grounded. One side of the 100V pair might be ground, or it
> might be floating, or it might be opposite phases driven against
> ground. There could be interesting conflicts.
>
> A small transformer would help.
NT
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