# Audio 100V signal to line level

• posted

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:

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:

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:

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

• posted

It looks more like pickup/interference than distortion. 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.

• posted

Remember that RMS (Root Mean Square) is 70.71% of the peak value. So your peak would actually be 141.4 Volts.

• posted

I agree with George. It looks like line interference. Use line sync on the scope to confirm. It appears it is a floating signal. Use a transformer.

• posted

It depends on how the 100V system is grounded, or maybe not grounded. 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  ```
• posted

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.

• posted

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).

• posted

Crank your timebase down so you can see 60 Hz, that may tell you something. If you see it try a shield.

• posted

John,

Here is the schematic of the circuit.

The output of the PA system goes directly to P1. The oscilloscope is going directly to the connections at OUTPUT and GND.

• posted

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.

• posted

whit3rd,

Right now for my testing I have very short wires (1 foot) coming from the PA to my resistor divider.

• posted

1 foot of wire is a big antenna, perhaps. The first thing to do is find the frequency of the interference. that will lead to the source...etc.

George H.

• posted

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.

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.

• posted

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.

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.

• posted

Don't know if anyone else has suggested this, but you could just split 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```
• posted

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```
• posted

Or both could just be earthed to different places, with current in the earth wire creating voltage drop.

NT

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