The All Purpose Advice Thread - Part 2

Shotgun mics were designed for boompole operators to capture actor dialog while keeping the mic out of the camera frame. Typical operating distances are 1 ~ 1.5 meters (39 to 59 inches).

I use a stand similar to this:
https://www.sweetwater.com/store/detail/KDStand--audix-stand-kd
I don't have issues requiring a shock mount. A shock mount can always be added later if necessary.

I have attached a file of me speaking 1 meter in front of an AT875R in our kitchen.
Downsampled from 44KHz to 22KHz for smaller size.

 

For a properly rigorous evaluation we need another with you snoring and/or purring.

 

I'm gonna add this just for fun. Wife is asleep in the next room so I'm speaking relatively quietly.
Shure SM7b 10" from face at a diagonal (I said 6" in the recording but I was mistaken), with background blur on it is usually invisible in my team calls unless I lean in and my shoulder pulls the mic/arm into focus.

edit: yes this is a completely useless comparison... different voice, different setup, different everything blah blah

 

basic question... probably dumb... but that's the way I roll:

Unity gain - I've read in the past that you want to set your amps at unity gain and use your preamp to control volume. Since I never really had a preamp before, I never really worried about it. But now things are changing and I'd like to understand how to find/determine/divine where unity gain is on the volume controls of my various amps (eg., Studio B, ECP 3F, BH Crack, etc, etc).

Any helpful advice appreciated. Unhelpful advice also appreciated... just not as much.

TIA

 

Three ways to look at this:
1) max out the amp so it mostly takes the pot out of the circuit, this is easily done by just turning the knob all the way to the right, then fully use your preamp to control your volume. Sometimes if your amp has a very high gain then you don't have much room to work with on the preamp
2) find the exact spot on your amp pot where it has attenuated enough that after it goes through the gain stage what you get out is equal to what goes in. This requires fudging around with a multimeter playing sine waves
3) who cares where "unity" gain is and go for where you get the most control with your preamp, which is usually somewhere around the 10am-2pm range on the knob. Set the preamp to 2 o'clock, turn up the amp to the point where it hurts. Leave it there, now use the preamp to control volume.

 

Satisficing: ask the amp manufacturer/distributor "roughly, where is unity gain on the volume dial?"

(on background, on another forum this question was asked of Victor Kung and he happily obliged by providing a picture of the volume knob at measured unity gain on the Elekit tu-8600)

 

To me it seems kinda silly to do it just for "unity" gain on the amp because all you're doing is chopping the input signal down with the amp pot then amping it back up, THEN using the preamp to chop your input signal further. Really all this does is introduce two pots into the equation with potential for more noise. The only reason to do it is if the amp gain is too high for your application and just the preamp pot control isn't enough to get you down to comfortable range.

 

I set my amps to unity gain so their output matches that of the DAC being used when preamp or attenuator is at 0 dB attenuation / gain. This makes it easier to know what Vrms will be produced at 0 dBFS. Very useful for my lab work.

Otherwise, setting amplifier to full gain and using the external stepped attenuator as the primary control is another consideration.

 

Pardon the noob question but how does one measure 0db? Can you do this with a common DMM?

 

It can be done with a cheap DMM. Play a sine wave (say 60Hz), measure the dac output, plug into amp, measure the amp output, adjust pot until you get the same number.

 

@zonto and others
Limited Comparison of Microphones for Video Conferencing

 

as @Armaegis posted, use a DMM. I prefer to use 400 Hz as it is a more pleasant tone and also doesn't run into any issues for amplifier with small output transformers. Input Vrms should equal output Vrms with the intended load attached. Do this at a volume loud enough to hear but not loud enough to be annoying.

 

Really noob here, but I’ll ask anyway because others might want to know too …do I set the Fluke/DMM to V and insert the one probe into the RCA out any the other touches the RCA external part?

 

Most DMMs have both a DC voltage setting and AC voltage. Usually the AC one has a little squiggle and straight line and typically only has two choices, while the DC is two lines and has maybe 4-6 choices.

Stick one end of the probe on the RCA pin and the other on the RCA sleeve.

 

Thanks. I take it that as I would be using 2 probes, that’ll be DC. I’ll try this on a cheap DAC just to satisfy my curiosity. I don’t even have a need to set unity gain

 

Random image and mspaint skills. The V~ is for AC, the V= is for DC. The numbers represent the max range that you're looking at, and typically you set it at the lowest one closest to (but above) your expected signal.

On a dac, put it on V~ (40).
On an amp... probably the same.

edit: you use both probes regardless if you're measuring AC or DC.

 

@joch if you've got a Fluke meter like this:



Use the V with the ~ over it as your selection, and attach the black probe to the COM plug and the red plug to the rightmost plug, which is labeled V. You'll want to use the Range selection to ensure you're measuring in volts, not tens or hundreds of volts, or millivolts.

If you're confident that your amp's output impedance is low-to-zero and that it's happy running without a load, you can skip attaching the load and just measure at the speaker terminals or the other end of the wire. I'll typically skip it when measuring headphone amps as (OTL amps aside) even a couple of ohms of output impedance aren't going to make a huge difference compared to the typical headphone impedance.

I'll second the suggestion of 400Hz (I usually pick 440Hz) as a lot of speakers and headphones have an impedance peak or dip in the low bass. You don't want to go too high since most multimeters won't measure accurately above 1KHz. I'd recommend using REW's generator for this. Make sure the volume is set to maximum on the host side and that you're measuring what you expect to see on the DAC output, usually 2 or 2.1Vrms.

 

@joch use the AC scale and follow advice from @Armaegis and @darmok above.
Nice instructive pictures.

 

Thanks gentlemen!

And since we’re using the DMM, would this be the same way to check for impedance (but switch to the Omega sign)?

 

No, Volts again - but you'll also need a resistor:

https://www.superbestaudiofriends.org/index.php?threads/measuring-amplifier-output-impedance.1329/