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Using an 8903B to analyze power audio amp
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Hi Group,
I've done quite a bit of searching and so far I haven't been able to find how to exactly connect a power amp to an 8903B to make some basic measurements. I know that the 8903B's signal generator can be connected to the audio amps input but what I have not seen explained is how to connect the amp's output to the 8903B input. Am I correct in assuming the right way to do it is to have an 8 ohm or 4 ohm load on the output of the amp and connect the 8903B input in parallel with the load (at 600 ohms impedance)? Thanks so much, Victor |
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Yes, you are correct, but the 8 ohm or 4 ohm load has to be rated for high power, especially if it is only cooled by ambient air. For a 10W amp we use a 100W non inductive restive load. You also need to get the ground / float stuff right. I don't have one in front of me to see what that is and it depends on the amplifier characteristics. In some cases a coupling capacitor may be needed. You will likely want to use a flat filter setting. If I remember correctly some of the impedance settings are in special menus and you may get erroneous readings if they are off. I haven't used an 8903B in a few years. I've used the Audio Precision stuff lately, so I don't remember everything about the operation. Regards, Mark On Fri, Dec 13, 2019 at 12:01 PM victor.silva via Groups.Io <daejon1=[email protected]> wrote: Hi Group, |
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Re: Connect 8903B to an audio amp
Victor: Some things.... * The input impedance of the 8903B is 100K if I remember correctly.? * The FLOAT input can be used in your amplifier does not have a common ground on the 'speaker' outputs. Else, the shell of the BNC input is connected to power line and case ground. * It is OK to supply up to 300V AC+DC to the inputs. You can select either on the front panel. Be careful that some amplifiers do not have outputs that are chassis common. Sometimes they are actively driven on both the + and - speaker binding posts. Also, it is all to common that the Left and Right channels do not share the -? terminals. You can check those with an ohm meter. Be careful not to hook grounded test equipment to those amplifiers that are not already referenced to the chassis, like an oscilloscope probe. The FLOAT switch on the input of the audio analyzer will add 100K impeadance to the shell of the BNC connector in these cases. Dan in Chandler, AZ |
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Dan,
Yes, thank you. I was mistaken about the input impedance. The 600 ohms is the output impedance of the internal signal source, can be also set to 50 ohms. The input side on the left is 100K ohms. I looked at the amp I want to analyze, the NAD 2600, and the two output channels do have a common chassis ground for left and right. This amp can be bridged into one mono amp in which case the Right Red jack become (+) and the Left Red jack becomes (-) and I believe in this case I would have to use the float switches on the 8903B. --Victor |
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I have a 8903 and have used it with home-built audio amps. Many years ago did I build a dual (as in stereo) dummy load. It is just rated at 21W but that is usually sufficient. It has a bunch of wirewounds and banana jacks so I can configure it for 4 or 8 ohm and it has convenient BNC-outputs on the back.
I wrote a small macro in Excel/VBA to step the frequency and record the distortion values. |
Anders
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