Audio precision in London

[jackinthebox]

Does anyone in the London area have an audio precision or similar testing rig? I have a batch of microphones on which I will be performing various upgrades and I would really like to be able to test the Effect of these circuit changes with something very accurate. I would like to initially test the frequency response and THD of the amplifier circuit to find out how flat it is

[Bill@WelcomeHome]

Have you considered something like the SMAART system? It would not capture all the data one might like, but you can check the effect of a mod with ease. Run and save the tests, do the mod, run them again and compare or overlay the graphs. I've not done it but Dr Don was promoting it to me highly.

[jackinthebox]

i have fuzz measure pro and spectre with usual measurement mic i would really like to get some very accurate measurements of the circuit, post capsule. Just so i know how the electrical circuit behaves. This would make it easier to ascertain the effect of different capsules.

[Jim Williams]

A System One or Two is fine, you don't need the digital interface options. Avoid the Porta One set, not enough resolution for you.

I use mine with a very high quaility input cap wired to the input circuits under test. A 1000 pf Rel-Cap polystyrene at 600 volts is ideal. You will need to build some sort of screening rig to completely shield the DUT and that cap too.

Next, you need a very pure 48 volt psu. Double regulation is recommended. Also, pre select the 6.81K phantom resistor pair. I use either a pair of Vishay S-102 bulk foils or a pair of Dale CMF60 resistors matched to one ohm with a 4 or 5 digit multimeter. That match is critical for best CMRR specs.

Also post filter the phantom power feed at the DUT as those wires can pick up more external interference. You also must follow AP's unbalanced wiring instructions or you will skew the results. I made up special cables for unbalanced testing, if not done their way, the tests are all off.

As you can see, the results can be easily skewed if any of these steps are missed. Attention to the details will give you the best real world results.

[jackinthebox]

They are tube microphones so the phantom power regulation wouldn't be necessary I imagine. I would be testing the electronics as a whole system including the power supply. I do need to check the voltages from several of the power supplies to make sure they are consistent. I will be using gotham gac-7 between power supply and mic. Would i need a capacitor to act as a dummy capsule?

[Jim Williams]

Yes, if the bias is blocked by it. I made a mistake about using 1000 pf Rel-Caps. Capsules are typically around 47~80 pf, I would use a quality polystyrene film cap of about the same value as your capsule. You can test the capsule on a decent LCR meter to determine it's self capacitance, then you want to match that as the input source.

[jackinthebox]

Originally Posted by Jim Williams

Yes, if the bias is blocked by it. I made a mistake about using 1000 pf Rel-Caps. Capsules are typically around 47~80 pf, I would use a quality polystyrene film cap of about the same value as your capsule. You can test the capsule on a decent LCR meter to determine it's self capacitance, then you want to match that as the input source.

Thanks jim, that's very helpful. Just one more question, would i need to test the capacitance of the capsule while it is in the circuit and charged? Or will it have fixed capacitance as a stand alone component, not wired in to the circuit.

[Jim Williams]

Good question. They do have a fixed capacitance just from the plate and diaphram. That might change under charge a bit, I'd check with and without to be sure. Just use a blocking cap as the AP may not like seeing 60 volts.