HP 400E VS HP 3400A Meter

[Johnnydiaz]

Can someone tell me the difference between the HP 400E and the HP 3400A meter?  I will be using this meter to measure rms voltage from my audio amplifier.

[theatrus]

Besides for the collectable value, why would you use an old HP analog bench meter?

The ability to measure high crest factor and response up to 10MHz is nice, but an audio amplifier is a 20Hz-20kHz device, and any modern DMM is going to give you better full scale accuracy (the 400E is specified to 2% full scale)

[Johnnydiaz]

Theatrus,

Thanks.  I was told to get one of the meters as they are more consistent with reading when using pink noise as the stimulus.  The Fluke 289 for example bounces all over the place but am told with the HP meters you get a more consistent reading.  Any thoughts?

[theatrus]

Are you just trying to determine the voltage gain of the amplifier (vs power)? Is there a particular reason you are using "pink" noise?

[SArepairman]

The 3400A is a thermal meter.

http://cds.linear.com/docs/en/application-note/an83f.pdf


refer to page 17 for some comparisons relating to noise measurements. The 3400A is good, I don't know about the 400E.
Keep in mind this thing will probably load down your circuit and you should build a nice low noise preamplifier for it if you want to measure noise.
But also keep this in mind, unless you are a voltnut like I, the fluke 87 is only 5% off from an expensive specialty meter, weighs about 5 times less. Sure, it won't measure 100MHz like a 3403A will, but do you really need it? And this study assumes the  worst case measurement, of noise.

*keep in mind these measurements are done in a 100KHz bandwidth. You are probably only interested in a MAYBE 20-30 KHz bandwidth. Modern meters are probobly more then good enough for your task.

If you can get it cheap and you have the space for it then I would get it, because maybe you will decide to do something that requires it if you have some buyers guilt, but it seems like extreme overkill for your situation.

[Johnnydiaz]

Are you just trying to determine the voltage gain of the amplifier (vs power)? Is there a particular reason you are using "pink" noise?

I would like to measure the voltage coming out of the amp.  SO for example if I have a woofer with a 800watt rating at 4ohms which is 56 vrms.  Im using pink noise to have equal energy throughout the frequency range when getting reading.

[schopi68]

Besides for the collectable value, why would you use an old HP analog bench meter?

The ability to measure high crest factor and response up to 10MHz is nice, but an audio amplifier is a 20Hz-20kHz device, and any modern DMM is going to give you better full scale accuracy (the 400E is specified to 2% full scale)

From where in the documentation do you have this 2% value? I can not see it.

[TimFox]

Note that the input of the thermal-RMS -hp- 3400 is grounded to its chassis, and many high-power audio amplifiers do not ground one side of the speaker, using some kind of H-bridge to drive the load.

[1audio]

The 400E measures the average voltage (rectified AC) and displays in RMS. The 3400 uses a thermal conversion and has a pretty high crest factor (7 I think at full scale) meaning it will be accurate on waveformes where the peak is 7 times the average voltage. They are both quite good at what they do and should have the individually calibrated meters and can be 1% instruments. Noise is particularly challenging for digital instruments. You need a long integration time and even then the numbers bounce a lot. Visually averaging an analog meter is pretty easy and lots quicker.

True RMS in most digital meters works well on sinusoidal waves. For high crest factor noise its not optimum but noise in this sense is rarely encountered.

For audio program there is a version of the 400 that has VU ballistics. The 3400 is much too slow for that.

I think getting a primer on audio measurements should be done before attempting any measurements. Putting 400W of noise into any loudspeaker has a half life in milliseconds and the smoke from the voice coil is not pleasant. I'm not sure what you are trying to test for. White noise and pink noise are really only useful with a spectrum analyzer to see how the response is altered by the system or for life testing of drivers. They are usually toast after the life test. An analog or digital meter is not useful in this context.