Speaker Workshop and MobilePre

[mpotoka]

If I remember correctly I have read of a couple users on here using this combination for doing speaker measurements. I have a jig I built for Speaker Workshop--but at the time I was using a jack to connect an external Microphone Preamp. Now that I have a MobilePre--I am wondering if I am able to eliminate the jig for FR measurements? Or am I going to be building a new sort of jig? I'm thinking along the REW lines where I could just use an output to the amp and have the mic in do my data--but don't you need a reference in for Speaker Workshop?

Thanks
Mike

[mpotoka]

Does anyone have this combination working???? I can NOT get this to even make Impedance measurements. I try to get it to calibrate resistors and it keeps giving me an impedance around 16 ohms (instead of 11.5) and it keeps telling me my series resistance is about 3 ohms (instead of .2)

Its weird because when I click on the "What has been measured" tab it shows that there is only a difference of .2 between what was measured and what I stated it as.

Then when I try to measure an Inductor it says its a cap, when I measure a resistor it says its a cap. When I measure a cap it gives me random values but usually around 1/2 the actual value. Any suggestions here? I have tried uninstalling/reinstalling SW but I think there is some data somewhere I am not deleting. I don't really want to start poking around the registry for information....

This has to work!

[ThomasW]

Saurav

https://www.htguide.com/forum/member.php?u=3595

Uses Speaker Workshop. I don't remember what he's using for a preamp

[mpotoka]

Should I be using a 1/4" Line Out or should I be using the 1/8" Stereo Headphone out?

[Saurav]

I think I have a Mobile Pre I haven't done anything with this since last summer. You'll still need some kind of jig... I had everything connected with alligator clips. I know that would be inaccurate for impedance measurements, but I was just trying to make sure nothing was completely out of whack, and for the most part I was only doing FR (which didn't need alligator clips).

IIRC, I was using 2 stereo-1/8" : RCA pair adapters, and I remember having to cut into an RCA cable to get access to the hot and null wires to build one of the 'jigs'. Beyond that, I don't really remember what I did. I pretty much followed the diagrams in the Speaker Workshop manual. Which is pretty helpful, it goes through a bunch of initial setup that I had totally missed, and would not have thought of doing. So I'd recommend reading that if you haven't already.

[ThomasW]

Nothing is done in stereo. The standard loopback test connects the left and right channels only for the duration of the test. After that only one channel is used.

[mpotoka]

Yep--I've read the SW manual--I've had it setup previously on a couple different computers. I just added the MobilePre to try and avoid a few power supplies and gain some portability but this is becoming just silly.

Do you use an external amp for Impedance measurements? Obviously one is needed for the FR sweeps (I haven't even attempted that).

I was thinking maybe I needed to use the 1/8" phone jack for an increased amount of voltage for the impedance measurements.

I cut the cables on my jig I have built out of frustration and gone down to the basic cables with alligator clips just to verify I had everything right. Now I'm beginning to think that the problem is needing an external amp?

So my next question is for FR measurements--do I run a 1/4" jumper cable from one output to one input for reference and just use the other channel output to go to the amp? Or isn't the reference channel required when doing frequency measurements? If I can use the mobilepre without my mic preamp then maybe I'll just pick up a WT3 to use for portable TS/Impedance measurements.

[mpotoka]

Ok--I have finally found a few answers from someone on HTShack. The line voltage sags under minor draw so the measurements get all messed up. I need to start using an external amp for all impedance measurements. As far as Frequency measurements it sounds like I just run the 1/4" loop cable from an output to an input.

Now I just need to learn about this Voltage Divider thing so I don't fry the sound card.

[Notorious_AK]

for voltage divider I use car speaker to line level converter, it can be bought for less than $10

[norcad]

Ok--I have finally found a few answers from someone on HTShack. The line voltage sags under minor draw so the measurements get all messed up. I need to start using an external amp for all impedance measurements. As far as Frequency measurements it sounds like I just run the 1/4" loop cable from an output to an input.

Now I just need to learn about this Voltage Divider thing so I don't fry the sound card.

Hello!
Im using Speaker Workshop and M-Audio Mobile Pre.
Its true that you need an external amp for impedance measurements.
But Im not using any voltage divider.
For the impedance measurements the signal is low enough, so there is no problem with the input on the sound card.
For frequency measurements you can use the input on the amp (RCA) for the second channel input on the card and not the speaker out, and instead make and use an amp calibration in SW.
The most important is to make a good volume calibration, the Mobile Pre sound card has some tricky volume pots! And after the volume calibration is done, DONT adjust the volume knobs on the Mobile Pre anymore.

Be aware that the earlier software version from M-Audio used on XP is a better choice than the new software on Vista or XP.
With Vista og new software with XP you will get some latency issues.

[mpotoka]

Hello!
Im using Speaker Workshop and M-Audio Mobile Pre.
Its true that you need an external amp for impedance measurements.
But Im not using any voltage divider.
For the impedance measurements the signal is low enough, so there is no problem with the input on the sound card.
For frequency measurements you can use the input on the amp (RCA) for the second channel input on the card and not the speaker out, and instead make and use an amp calibration in SW.
The most important is to make a good volume calibration, the Mobile Pre sound card has some tricky volume pots! And after the volume calibration is done, DONT adjust the volume knobs on the Mobile Pre anymore.

Be aware that the earlier software version from M-Audio used on XP is a better choice than the new software on Vista or XP.
With Vista og new software with XP you will get some latency issues.

Hey this is encouraging.

What level of output do you measure at the speaker terminals on your amp when you make an impedance measurement? I was told to keep it below 1 Vac.

Frequency measurements--I did not quite understand what you said about how you wired that. Can you explain a little bit more? I ran a 1/4"-1/4" cable from output 2 to input 2, and then plugged the microphone into the Input 1 (XLR plug). Maybe the 1 and 2 were reversed--and I think I switched the reference to the right channel and data on left--but I don't remember for sure. (I will have to look at my MobilePre again)

Which version of the software do you suggest?

Thanks!

Mike

[Dave Bullet]

Does anyone have this combination working???? I can NOT get this to even make Impedance measurements. I try to get it to calibrate resistors and it keeps giving me an impedance around 16 ohms (instead of 11.5) and it keeps telling me my series resistance is about 3 ohms (instead of .2)

Its weird because when I click on the "What has been measured" tab it shows that there is only a difference of .2 between what was measured and what I stated it as.

Then when I try to measure an Inductor it says its a cap, when I measure a resistor it says its a cap. When I measure a cap it gives me random values but usually around 1/2 the actual value. Any suggestions here? I have tried uninstalling/reinstalling SW but I think there is some data somewhere I am not deleting. I don't really want to start poking around the registry for information....

This has to work!

I'd get your jig gixed first. Are you using a cable jig or an EW one?

PS: Remember to disconnect your pre-amp when doing calibration setup - or you'll get whacky results too (well, I do anyway).

[mpotoka]

I was suspecting my jig had a wiring issue so I pulled it apart and went back to just the basic cable jig.

Disconnect my pre-amp? I'm not quite sure what you mean. I have the laptop, the MobilePre, and an amplifier.

Mike

[norcad]

It isnt easy to explain this in english, but I have some very helpful pictures here for you!

Calibration:




Impedance:




Frequency:




The voltage on the amp output, I dont know. But here is how you can do it:
- Set the "Device output" volume at -6dB, the "usb recording" at 0dB, connect the calibration wire, use a sine wave test signal (1khz) in SW and adjust the soundcards volume pots until you have about 16k at the SW VU-meter on both channel. This isnt easy!
- Check and adjust the Latency.
- Run the "Calibrate channel difference"
- Adjust the "Interchannel time delay"

Then connect the amp for impedance measuring with a 15ohm resistor connected to the amps output instead of the driver, use the test signal again and slowly turn up the volume on your amp when you look at the soundcards green and red leds. If you get it steady green without the red ones, you are safe!
Use a 10ohm reference resistor, and run the "Impedance jig definition" test with two resistors of perhaps 3,9 and 15 ohm.

And finally you are ready to go!

Good luck!

[BobEllis]

You definitely want to keep your signal low when using an external amp to avoid frying your inputs.

You can use four general purpose diodes 1N400x (where x is a number from 1 to 7) to provide protection. String them together band end to blank end. fold the string in half so you have two parallel strings of two diodes with the bands pointing opposite directions. Solder the loose ends together. The new ends of the string are the connection across the measurement leads.

The diodes will conduct when the signal across the string exceeds ~1.2V. Keep the measured signal below that to ensure that you don't clip the measured signal and affect apparent frequency response.

A voltage divider is nothing more than a pair of resistors. Input from DUT -- Resistor1 --test lead attach point --- Resistor2 -- Ground.

You can use something like 10K for R1 and 1K for R2 to reduce the signal to 1/11 of the signal across the device under test. I'd still put the diode string from the test lead attach to ground to be sure no transients sneak through.

[Dennis H]

You definitely want to keep your signal low when using an external amp to avoid frying your inputs.

You can use four general purpose diodes 1N400x (where x is a number from 1 to 7) to provide protection. String them together band end to blank end. fold the string in half so you have two parallel strings of two diodes with the bands pointing opposite directions. Solder the loose ends together. The new ends of the string are the connection across the measurement leads.

The diodes will conduct when the signal across the string exceeds ~1.2V. Keep the measured signal below that to ensure that you don't clip the measured signal and affect apparent frequency response.

A voltage divider is nothing more than a pair of resistors. Input from DUT -- Resistor1 --test lead attach point --- Resistor2 -- Ground.

You can use something like 10K for R1 and 1K for R2 to reduce the signal to 1/11 of the signal across the device under test. I'd still put the diode string from the test lead attach to ground to be sure no transients sneak through.

In the ARTA manual, he recommends using zener diodes along with the voltage divider to protect the soundcard. "To protect the soundcard input from high voltage that is generated by the power amplifier, it is recommended to use a voltage probe circuit, as shown in Fig. 1.3. Values of resistors R1 and R2 have to be chosen for arbitrary attenuation (i.e. R1=8200 and R2=910 ohms gives probe with -20.7dB (0.0923) attenuation if the soundcard has usual input impedance - 10kΩ)."

[kendomusic]

In the ARTA manual, he recommends using zener diodes along with the voltage divider to protect the soundcard. "To protect the soundcard input from high voltage that is generated by the power amplifier, it is recommended to use a voltage probe circuit, as shown in Fig. 1.3. Values of resistors R1 and R2 have to be chosen for arbitrary attenuation (i.e. R1=8200 and R2=910 ohms gives probe with -20.7dB (0.0923) attenuation if the soundcard has usual input impedance - 10kΩ)."

Dennis,
Is there an easy way to calibrate the sound card in Arta without having to measure the input and output voltages on the Mobile Pre USB card and a sound calibrator?

[Dennis H]

For the calibration, you need a digital voltmeter to measure the voltage.

[kendomusic]

For the calibration, you need a digital voltmeter to measure the voltage.

Ok, I guess I will need an AC Voltmeter? The voltmeter setting recommendation on the "STEPS" manual is 2V. My voltmeter that has a 200V setting sometimes read way above 2V.

Also, when I try to run a sweep while the sound card is loopback using my impedance jig(without the resistor), I get a flat frequency response and phase.
If I remove the right input jack on the jig the frequency response tilts downward.
This is the same result when I try to do a loopback from "1/4 1/L" jack to the "XLR ch2 mic" on the Mobile Pre. Should the results be that way?

Thanks.
Kendo