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I need help in remembering how to determine an amplifier's actual input impedance. I recall it involved using a signal generator, DVM, and maybe an oscilloscope. I would appreciate it if someone could
refresh my memory or give me a site that would have this information.Thanks in advance.
Follow Ups:
Why do you care? Input impedance is not of particular concern. Simulations that I have performed to determine sensitivity of output impedance, input impedance and cable characteristics show that input impedance really does not matter much.Measuring this accurately requires you to drive the inputs with a signal generator, apply frequencies across the audio range and measure the current into the input.
The reason I need to determine the input impedance is that I'm using an active two-way cross over and the high pass section is just a series capacitor that works with the input impedance of the high frequency amplifier to determine the crossover point. As in 1/(2pi*R*f)
If you could, I'd appreciate a more detailed explanation of the process.Thanks again for your response.
The amplifier in question is a Borbely DC100 I built 13 yrs ago.
I installed a .078mF capacitor for the input filter which means that the input impedance should be 10,200 Hz for a cut-off of 200 Hz. The actual input bypass resistor to ground is 100k which implies that the input impedance should've been a lot higher. I wanted to check the impedance to make sure I did it correctly back then. Thank you all for your responses.
Viktor's procedure seems to ring a bell. I'll check the impedance this weekend.
I meant to say CAPACITOR below:Since the input impedance of the receiving component is not that critical, why don't you calculate the 3 dB point of the filter when it drives into 1K ohms. Then connect a 1K ohm resistor in parallel inside the connector at the destination and a series CAPACITOR inside the connector at the source. This is what I would do. The 1K will swamp out the input impedance of the destination component and you will have an accurate, tunable filter.
A 1 k ohm load is to much for most equipment, only a handful of high end sources/preamps can handle that amount of loading without audible distress or poor overall sound.For most preamps sources, a 10 k ohm load is all they are comfortable with, and some tube units can not even handle this amount of loading.
The lowest I would go with a known output device, say one capable fo a nominal 600 ohm loae drive, would be 2.2 k ohms, and for most other unknown output stages, no lower than 4.7 k ohms.
Jon Risch
Since the input impedance of the receiving component is not that critical, why don't you calculate the 3 dB point of the filter when it drives into 1K ohms. Then connect a 1K ohm resistor in parallel inside the connector at the destination and a series resistor inside the connector at the source. This is what I would do. The 1K will swamp out the input impedance of the destination component and you will have an accurate, tunable filter.
Connect a 1kHz generator to your amp through a say, 470k pot - in series. Set the pot to zero and measure output voltage. Using the pot, set the voltage now to 1/2 of what it used to be. Measure the pot resistance - it is equal to your input resistance.Once you know it, put a fixed resistor of the same value in place of the pot and run freq scan. If you are lucky the response will not change much with frequency.
What is the design of the input stage of your amplifier? In most tube amps, the grid resistor of the first stage determines the input Z for audio frequencies (to a first, and usually accurate approximation). In solid state amps, a similar situation usually exists. Input Z is relevant not only for passive crossovers, as in your case, but also for many other reasons. If you can tell me a bit about the schematic I'm sure I can help you figure out what the input Z is. hth, Harry
input impedance for op-amp based amplifiers like the 47labs gaincard is really high or really low ?i remember that it is at the extreme end of 1 of the directions but cant remember which.
thanks.
.
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