IMD measurement plugin?

From the Analogue Productions test record:

"Using an IMD tester, adjust VTA by raising or lowering the tonearm for minimum distortion."

Anyone know of an IMD tester plugin I could use for this? TIA!

 

How about using a spectrum analyzer display to look at the mix product? Isn't that a function in Cool Edit?

If you can't view real time, it might be kind of slow to record, play and look at it. But you could see if you were moving in the right direction if it has enough resolution to see down to the mix product.

CoolJazz

 

Hmm. Not sure how to do that. I can look at the spectrum analyzer display, but what am I looking for?

 

I looked up the record and it's VTA adjust is using 60hz and 4k. So you'd be trying to peak down at what kind of bumps you see come up 60hz above and below the 4k.

IMD or intermodulation distortion is looking for the two tones beating against each other from non-linearities and creating mixing products. Try looking at some of the testing graphs printed in Stereophile for an idea of what it looks like from professional test equipment.

Again, I don't know if you'll have the resolution to see it or not with a computer soundcard. But you might...

CoolJazz

 

Thanks for the advice! Using the Voxengo SPAN plugin, I see large peaks at 60Hz and 4kHz, and a small peak at 8kHz. I assume that's the mixing product you're referring to. Now to try to reduce it as much as possible by adjusting VTA. Right?

 

EDIT. Or is the 8kHz peak not a mixing product, but merely a partial of the 4kHz tone?

 

EDIT EDIT. Also, the 1kHz to 10Hz sweep shows a tonearm/cartridge resonance at about 70Hz. Any way to fix this?

 

To measure IMD you take two tones and mix them. Next you filter the original tones out. What is left is the IMD. It's easy to measure as a numeric value. From there it's simple math to figure the percentage.

The filtering is critical. If you don't remove all of the original tones the reading will not be accurate. Most IMD analyzers have null circuits to minimize the residual fundamentals.

 

Oh. So using an ordinary analyzer plug-in is not going to work, it seems.

 

Hi Ron,

I think you're correct.

Best regards,
Barry
www.soundkeeperrecordings.com
www.barrydiamentaudio.com

 

Hi Ron,

I don't believe this is tonearm/cart resonance; I believe it is one or more of the following:
1. the resonance of the turntable itself
2. the resonance of the shelf the 'table sits on
3. the resonance of the room in which the shelf with the turntable sits

The second can partially be remedied by properly isolating the turntable. The third can be pretty completely remedied (at least, as far as the turntable is concerned) by isolating the turntable.

Assuming an unsuspended turntable, an inexpensive way to find out is to place a minimally inflated inner tube on the turntable shelf (inflated only enough to keep the load off the inflation valve; not one pump more or the sound will suffer and the bass will bloat). Atop the inner tube, place a dead "platform" (3/4" or better, 1" plywood works well). Place the turntable atop the platform. Listen/measure again.

Note:
1. use a thin walled inner tube that describes the largest circle that will fit on your shelf. (This makes it easier to balance the load. Place the turntable's center of gravity in the center of the platform mentioned above.)

Hope this helps.

Best regards,
Barry
www.soundkeeperrecordings.com
www.barrydiamentaudio.com

 

You can lower the resonant frequency by adding mass at the cartridge. Blu-tac, or add some metal above the headshell. Re-balance and reset the VTF with every change! The 8khz peak is harmonic distortion. Less is better.

 

Thanks, Barry. I thought the turntable was pretty well isolated already - it's a SOTA Star, and has AFAIK a pretty good suspension. Tapping on the plinth when the needle is on the record produces no sound at all. Still, it couldn't hurt to try your experiment. If I do I'll let you know the result.

 

Adding mass at the cartridge is a no-go, I think. The cartridge is very high compliance, and the arm is ultra-low mass. Wouldn't more mass mess things up?

 

No significant bumps between 60Hz and 4kHz. I guess I'm OK as far as VTA goes.

 

If you have resonance at 70hz, the arm/headshell are too low of mass to match with the cartridge. This is a pretty bad mismatch if it's an accurate. measurement. You should be able to hear that, I'd suppose that it would sound awful, with muddy and exaggerated bass around 70hz. If you're not hearing anything like that, them maybe the measurement isn't wotrking correctly for you. Anyway, you need that resonance to be below 20hz. Adding mass will accomplish this. I've done this myself when setting up a low mass tonearm with a lower compliance cartridge, and the resonance changes just as it should.

 

Hi Ron,

Yes, adding mass would lower the resonant frequency. But whatever is going on at 70 Hz, it isn't the arm/cart resonance.

Best regards,
Barry
www.soundkeeperrecordings.com
www.barrydiamentaudio.com

 

Hi Ron,

Is that a suspended turntable? I believe it is. (I saw Nakamichi "analog front end" in your profile and missed the "turntable" entry ).

Then my suggestion would change. If the turntable has a vertically compliant suspension, I would not add a second vertically compliant component.

So, I'm wondering if the suspension might need an adjustment. Perhaps its own resonance is too high (and might be the culprit in the 70 Hz issue).

Best regards,
Barry
www.soundkeeperrecordings.com
www.barrydiamentaudio.com