04-10-2017, 11:37 AM
There are two separate things here: the dummy load, and the divider. Don't confuse them.
The problem you're having is because of the resistor values. Why use such large values? Think about stray capacitance, cable capacitance, and input capacitance (of the measurement gear) - all will play havoc. Also, they will have a lot of Johnson (thermal) noise.
Next, the input impedance of your measuring device will load down the output. If it's got an input impedance of 1M, it might not be significant. But 10k - typical for a soundcard - will. In effect, the input impedance is in parallel with the lower resistance value in your potential divider.
I'd build a simple 100:1 attenuator based on the lowest value resistors I can get away with. The lower you use, the less "loading effect" you'll get.
Here, "loading effect" means the change in output voltage with the input impedance of the measuring device.
Perhaps make the lower resistor 100 ohms. Then the upper resistor would need to be 9.9k. Obviously, you'd have to put some in series and/or parallel to get to this value - you might even include a pre-set to calibrate it.
The downside of low value resistors is that they will dissipate some heat. Overall, those resistances will dissipate about 1 watt with a 100V RMS input. But given that you're using a number of them to get to 9.9k (say, a pair of 4k7 and a 500 ohm - a 1k pre-set resistor would be my choice there), then standard 0.5W resistors will survive. Upgrade to bigger ones if you can. Prefer series combinations rather than parallel, as resistors have a thing called voltage coefficient.
The output impedance of such a network is essentially 100 ohms (actually 99), so a change in Zin of your measuring equipment from 1M to 10k will make about 0.08dB of difference.
In terms of high frequency performance, because the divider has such a low source impedance, stray C won't have much effect. Even if you had 100pF at the output (cable plus Cin of the measuring device), the -3dB point would be 16MHz.
Needless to say, putting 10k in parallel with your existing dummy loads will have no significant effect on the power amplifier. Don't worry about it.
Finally, if you want to make it variable from 0 to 1V, replace the bottom 100 ohm resistor with a 100 ohm potentiometer. Simple! Personally, I like calibration, so I'd retain the 1k pre-set, and calibrate it so that it was definitely 100:1 when at the top end of the track.
The problem you're having is because of the resistor values. Why use such large values? Think about stray capacitance, cable capacitance, and input capacitance (of the measurement gear) - all will play havoc. Also, they will have a lot of Johnson (thermal) noise.
Next, the input impedance of your measuring device will load down the output. If it's got an input impedance of 1M, it might not be significant. But 10k - typical for a soundcard - will. In effect, the input impedance is in parallel with the lower resistance value in your potential divider.
I'd build a simple 100:1 attenuator based on the lowest value resistors I can get away with. The lower you use, the less "loading effect" you'll get.
Here, "loading effect" means the change in output voltage with the input impedance of the measuring device.
Perhaps make the lower resistor 100 ohms. Then the upper resistor would need to be 9.9k. Obviously, you'd have to put some in series and/or parallel to get to this value - you might even include a pre-set to calibrate it.
The downside of low value resistors is that they will dissipate some heat. Overall, those resistances will dissipate about 1 watt with a 100V RMS input. But given that you're using a number of them to get to 9.9k (say, a pair of 4k7 and a 500 ohm - a 1k pre-set resistor would be my choice there), then standard 0.5W resistors will survive. Upgrade to bigger ones if you can. Prefer series combinations rather than parallel, as resistors have a thing called voltage coefficient.
The output impedance of such a network is essentially 100 ohms (actually 99), so a change in Zin of your measuring equipment from 1M to 10k will make about 0.08dB of difference.
In terms of high frequency performance, because the divider has such a low source impedance, stray C won't have much effect. Even if you had 100pF at the output (cable plus Cin of the measuring device), the -3dB point would be 16MHz.
Needless to say, putting 10k in parallel with your existing dummy loads will have no significant effect on the power amplifier. Don't worry about it.
Finally, if you want to make it variable from 0 to 1V, replace the bottom 100 ohm resistor with a 100 ohm potentiometer. Simple! Personally, I like calibration, so I'd retain the 1k pre-set, and calibrate it so that it was definitely 100:1 when at the top end of the track.
