Loudspeaker to Line Level Pad

Bennett Prescott

Bennett Prescott

Just This Guy, You Know?
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Jan 10, 2011
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I need to measure the output of an amplifier, so I would like to build an adapter from Speakon to XLR, and preferably be clever enough to fit the pad in the adapter. Assuming I want 32dB of attenuation, am I good just using 570 and 30 ohm resistors? How much power will the amplifier develop into 30 ohms (assuming I do not need to observe its behavior at maximum output)?
 
Re: Loudspeaker to Line Level Pad

Bennett Prescott said:
I need to measure the output of an amplifier, so I would like to build an adapter from Speakon to XLR, and preferably be clever enough to fit the pad in the adapter. Assuming I want 32dB of attenuation, am I good just using 570 and 30 ohm resistors? How much power will the amplifier develop into 30 ohms (assuming I do not need to observe its behavior at maximum output)?
Click to expand...

Hi Bennett,

Here are the formulas that you need and a handy web app:

Power = Voltage^2 / Resistance

Gain (dB) = 20*log10(V1/V0)

http://www.sengpielaudio.com/calculator-voltagedivider.htm

Also handy, http://en.wikipedia.org/wiki/Decibel

Jeff
 
Re: Loudspeaker to Line Level Pad

Bennett,

There are so many things that can 'go wrong' when measuring output of an amplifier...
#1 red terminal isn't always 'hot'. It is always positive but may as well be hard wired to ground.
#2 output may be 'bridged' ... both hots
#3 outputs may sit on a DC (think ICEpower)
.
.
the solution is a 'full balanced attenuator' (H bridge ???), preferably with incorporated DC blocking caps in the measuring side.
I will come up with schematic later on.

Not 'obeying' my suggestions may result in a spectacular circuit closures with almost unlimited current available to make some spectacular damage.

Loading : 30 ohms on the output of the amp reminds me of a joke about the elephant and the flee. Do a 'load duck test' separately to get the info, or connect serious dummy load on the output. It may not be as good and big as Langston's (he posted pics of his resistor bank somewhere a while ago), but still. I'd do measurements with no load.

With respect
 
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Re: Loudspeaker to Line Level Pad

Bennett Prescott said:
I need to measure the output of an amplifier, so I would like to build an adapter from Speakon to XLR, and preferably be clever enough to fit the pad in the adapter. Assuming I want 32dB of attenuation, am I good just using 570 and 30 ohm resistors? How much power will the amplifier develop into 30 ohms (assuming I do not need to observe its behavior at maximum output)?
Click to expand...
When I am looking at the output of a "modern" amplifier, (who knows what the black terminal is hooked to?) I like to stick a transformer between the output of the amp and the measuring device-for isolation reasons.

Yes at the extremes there may be some "drooping", but much better than possibly damaging the amp.

Since the measuring device should be high impedance, there will be minimal load on the transformer.

I guess it all depends on what sort of measurement you are trying to look at.

Regarding the resistors and "power" it depends on what the input (output voltage of the amp) voltage is to the circuit. We would assume that the measuring device is a high impedance, so your circuit is a 600 ohm load.

At 40V output ( 400 watts@4 ohms) the wattage (continuous sine wave) across the 30 ohm resistor would be around 0.13watt. Across the 570 ohm resistor it would be around 2.53 watts.

At 90 volts (2000watt@4ohms) the wattage across the 30 ohm resistor would be 0.675 watts. THe 570 ohm resistor would be around 12.8 watts.
 
Re: Loudspeaker to Line Level Pad

Hi Bennett,

Bennett Prescott said:
I need to measure the output of an amplifier, so I would like to build an adapter from Speakon to XLR, and preferably be clever enough to fit the pad in the adapter. Assuming I want 32dB of attenuation, am I good just using 570 and 30 ohm resistors?
Click to expand...

With two 30 ohm resistors, yes, but I'm not sure how you'd do it with one of each!

If you put R1 and R2 in series across the amp and take the output from across R2, the attenuation is 20*log10(R2/(R1+R2)) assuming the amp output Z is zero and the load Z applied infinite. For an amplifier output and line-level input as load, the impedances are pretty close to zero and infinite respectively, but if they're not, you need to include them in the analysis.

R1=570 & R2=15 ohms (or R1=1160 & R2=30 ohms) get you -32dB.

How much power will the amplifier develop into 30 ohms (assuming I do not need to observe its behavior at maximum output)?
Click to expand...

It's not the 30 ohms (R2) that's the issue; it's R1. If the amp is rated at 1000W/4ohms, it'll dissipate about 7W into the 570 ohm resistor in the 570+15 ohm case.

Excel is ideal for problems like this. For example (you fill the yellow boxes).

Ales is right is raise concerns about bridged outputs. If padding a line-level output into a line-level input, you can make the pad balanced by splitting R1 in half and putting half in each leg. But with a power amp output to a line-level input, things can easily get outside of the common-mode range of the input. A transformer after the pad is the easiest solution, though you then also have to consider the impedances the transformer wants to see hung across it to keep things linear.

A 600 ohm load is really neither here nor there to a power amp, so I'd be inclined to scale everything up so that the power in R1 diminishes. With a high attenuation pad, the output Z remains nicely low.

I did have a plan to build a similar amp pad with switchable attenuation (and a pot to give something like a -20dB to -60dB range). Never got round to building it!

Nick
 
Re: Loudspeaker to Line Level Pad

Bennett Prescott said:
I need to measure the output of an amplifier, so I would like to build an adapter from Speakon to XLR, and preferably be clever enough to fit the pad in the adapter. Assuming I want 32dB of attenuation, am I good just using 570 and 30 ohm resistors? How much power will the amplifier develop into 30 ohms (assuming I do not need to observe its behavior at maximum output)?
Click to expand...

Aren't there DIs with a "speaker" mode meant for this exact purpose? I guess the DI may color the signal though.
 
Re: Loudspeaker to Line Level Pad

Ferrit,

What should the value of R1 and R2 be, or am I building a T-pad like suggested above and adding these Zener diodes?

I totally do not understand how the higher value resistor is drawing the most current... I regret not taking more electronics classes (well, any!).
 
Re: Loudspeaker to Line Level Pad

Bennett Prescott said:
Ferrit,

What should the value of R1 and R2 be, or am I building a T-pad like suggested above and adding these Zener diodes?

I totally do not understand how the higher value resistor is drawing the most current... I regret not taking more electronics classes (well, any!).
Click to expand...

Two resistors wired in series will have the same amount of current flowing through them. V=I*R (Voltage=Current*Resistance). Since R is higher on the bigger resistor, it also has a correspondingly larger voltage drop across it. P=I*V (Power=Current*Voltage) or substituting in the formula from V above, P=I^2*R. Thus the bigger resistor will dissipate more power.

http://en.wikipedia.org/wiki/Voltage_divider
 
Re: Loudspeaker to Line Level Pad

Bennett Prescott said:
Ferrit,

What should the value of R1 and R2 be, or am I building a T-pad like suggested above and adding these Zener diodes?

I totally do not understand how the higher value resistor is drawing the most current... I regret not taking more electronics classes (well, any!).
Click to expand...
Since both resistors are the effective load on the amp-you add them together for 600ohms (I am assuming that your measuring circuit has a high impedance-so the "load" will still be 30 ohms-or the total seen by the amp will be 600 ohms

. Then figure the current from the applied voltage.

Then you use the current to figure the voltage drop across each one (it will be proportional to the value of the resistor).
 
Re: Loudspeaker to Line Level Pad

As a side note on this subject, if anyone was interested in building a properly functioning V-I box for loudspeaker testing, they may have realized there is a high common mode voltage on the current sense resistor. In fact it has always amazed me that the commercial V-I boxes act as if it’s fine to plug in them into a sound card even though the common mode rejection is at best 50V for most differential inputs even on very high end equipment. I found a part from Texas Instruments a while ago that makes this measurement safe as it has a CMR of 200V. And it comes in a DIP package. http://www.ti.com/product/ina117
 
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