Bench Multimeter for Mic Preamp design

[Ronnie4416]

I want to measure DC bias (current and voltage), input and output impedance, gain, & frequency response.

[audiomik]

For a bench DMM with the full Audio bandwidth for AC measurements and maths functions for dB and dBm measurements - perhaps the Fluke 8846A

However for Noise and low level AC measurements a good Analog true rms AC millivolt meter is probably best - if you find a good one with an AC output, you can then use this to act as a 'pre-amp' for your 'scope. Look as a starting point also for an instrument with bandwidth and weighting filters for Noise Measurements built in. The Radford ANM2 would be my first choice.

Personally I have instruments very similar to both, amongst many others, together with Tek 7000 series 'scopes and various plugins as suggested earlier in this thread for measuring Audio Circuitry and there really isn't a 'one stop solution' to do all of what you are asking!

Mik

[Conrad Hoffman]

IMO, a meter is rarely the right choice for these measurements. I'd consider building a nice preamp for your scope, maybe differential and maybe with some built in filters, to duplicate the scope plug-in functions, so you can get down to the noise floor and so you can make accurate small signal response measurements.

Conrad

[audiomik]

Originally Posted by Conrad Hoffman

IMO, a meter is rarely the right choice for these measurements. I'd consider building a nice preamp for your scope, maybe differential and maybe with some built in filters, to duplicate the scope plug-in functions, so you can get down to the noise floor and so you can make accurate small signal response measurements.

Conrad

well Conrad, how do you think the OP, Ronnie4416, will be able to adequately build, test and calibrate such a 'preamp' which fundamentally is very similar to the reasons for this thread being posted?

I'm sure He and others following this thread will be very interested in your response
Mik

[Conrad Hoffman]

Not terribly difficult really. Start with a low noise opamp setup for some fixed gain and flat response. There's a plethora of them out there these days. Check the noise with the input shorted and with various values of input shunts. Check the response with a divider. Speaking of dividers, making up some decent dividers would probably be extremely useful for the mic preamp too. Now, I am assuming the OP has a scope to use the preamp with, as a meter alone isn't sufficient to design signal amplifiers with a great deal of confidence. Since we're talking response, hopefully I'm not assuming too much by assuming a signal generator is also available. Once a flat amplifier is available, some simple RC filtration should be added, since specifying noise without specifying bandwidth tends to be a meaningless exercise. IMO, the main purpose of the preamp isn't so much looking at the output signal of the mic amp, which should be of reasonable amplitude on its own, but for looking at output noise, supply noise and signals on what is supposedly "ground".

CH

[Ronnie4416]

Originally Posted by audiomik

For a bench DMM with the full Audio bandwidth for AC measurements and maths functions for dB and dBm measurements - perhaps the Fluke 8846A

8846A price is $1,395 almost same price as Agilent 34410A ($1,298 @ Test Equipment Depot) these meters have 6.5 digits resolution. If this is overkill for mic preamp design what about considering the Agilent U3402A 5.5 digit resolution for $600 (Agilent Technologies Test Equipment - U3402A - Multimeters - Test & Measurement - Allied Electronics) which is less than half the price of 8846A or 34410A.
If 5.5 digit is still too much maybe I'll just settle with my basic Fluke 179 (6,000) handheld meter or find a better handheld meter like the Fluke 287/289.

[Conrad Hoffman]

The 34410A is a really nice meter; we have a couple at work. I'd love to have one on my bench, but there's simply nothing in audio that requires 6.5 digits of resolution or justifies the cost of the thing. The useful features are things like programmable filtration, zeroing and offset. I think it also measures capacitance and maybe inductance. The other place it shines is resistor matching/measurement. If you want to make precision dividers (you do, whether you know it yet or not), the 4-terminal resistance measurement is excellent. OTOH, for far less money you can get one of the old HP 345x meters with similar accuracy. They are, however, way bigger than the new meters. You just need to be sure it works and is calibrated. I use a 3455 and am just as happy as if I was in my right mind.

Conrad