Measurements Power and Sound

Using a old Motu 828 Mark II I bought many years ago. It uses a firewire 400 interface, so my options were to use an old, had been retired Macbook (mid 2010 Macbook pro) that was old enough to have a firewire interface.

Could also get adapters to go from new M1 Macbook Air (2020)

Thunderbolt to firewire. $30. MD464LL/A

USB C to thunderbolt. $50. ‎MMEL2AM/A

I already have a firewire 400 to 800 cable

It works on my old 2010 mac, but is slow. Took a long time to install REW, but eventually installed and runs.

I also tried another sound card I picked up to replace the 828, and Prosonus, but the noise floor was pretty high in it so went back to the 828.

Wiring up for measurements

Diyaudio Thread for harmonics measurement

There are figures in the thread describing how to wire to amp to make measurements

They used a 8 ohm load resistor because that is somewhat typical for a speaker.

I used a 50 ohm load because my HE6 is 50 ohm impedance. I had two 25 ohm, 20w load resistors

EBG UXP-300 non-inductive resistor is recommended load. But after reading this post I decided to use the cheap resistors I had found and not change to a non-inductive one.
Figured that the actual load you drive, speaker or headphone, is far from an ideal resistor, so that is not real life.

made by wtnh at diyaudio

If using a XLR connector

Pin 1 = ground or shield

Pin 2 = tip blue wire

Pin 3 = ring white wire

Used R1=R2=14.9k, R3=R4=150 ohms

Using R1=R2= 10k R3=R4=100 ohms Gives 100x attenuation

"This gives a 100:1 voltage reduction, which seems to work OK for amp testing up to about 30 Volt p-p without overloading the EMU's inputs."

Levels

2.83 vac at load is 1w into 8ohms. Note that 2.83 is sqrt(8). p=1watt=v2/r. So if r=8, v2=8

for a 50 ohm load and 1 watt, 1=v2/50 gets v=sqrt(50) = 7.07 vac

with 100 to 1 resistor divider, voltage at input is 0.0707 vac

803 mv at signal generator and -3db main out on 828

https://www.superbestaudiofriends.org/index.php?threads/measurement-setups-post-your-rig.141/page-6#post-38555

Instead of a CD glued to the business end of the 4.5" ABS tube, I swung by TAP Plastics and got two 6"w x 7.25"h sheets of 1/8" ABS plastic and glued them to the ends of the 6" long 4.5" ABS tube using the cement recommended by the dude at TAP Plastics. Protip: cutting ABS plastic with a jigsaw is a disaster. TAP Plastics is the answer to my jigsaw skills.

I used three yellow poly foam car-wash sponges from the hardware store, cut into the right shape to fit snugly into the ABS tube with pacman-shaped cutouts to allow the MiniDSP measurement mic to pass through on an angle. (thanks for the tip, @Thujone).

https://www.superbestaudiofriends.org/index.php?threads/measurement-setups-post-your-rig.141/page-7#post-50142 rew settings

https://www.homedepot.com/p/QEP-7-1-2-in-x-5-1-2-in-x-1-7-8-in-Extra-Large-Grouting-Cleaning-and-Washing-Sponge-3-Pack-70005Q-3VP84/203260188

http://www.changstar.com/www.changstar.com/index.php/topic%2c2305.0.html

Sound card OLD

Using a Presonus Audiobox USB Audio Interface. Bought it used from Guitar Center fro $40 + tax and shipping, total just under $60

After installing REQ, just plugged into my macbook, and it was recognized.

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