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Old November 27th, 2003, 12:35 PM   #1
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How does limiter in the MM-1 work?

I've been experimenting with trying to get the optimum settings with my MM-1 and minidisc, and I've noticed that the limiter gets progressively more active as you increase the gain. Anybody know why this is? How does it work?
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Old November 28th, 2003, 09:54 AM   #2
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Hi Marco,

I don't have an MM-1, but it uses an opto-isolator in the limiter circuit - and those have been used 'since time immoral' in some of the finest audio equipment.

Any limiter will be more active as the gain is turned up: it is preventing the output signal from exceeding the specified maximum, so the louder the signal coming in, the more it has to squash down the loudest parts.

Normally I'd just use a limiter as a safety net, not as something that is continuously active.

Ask if you'd like more details (and I'll let someone else answer!).

Best,
Helen
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Old November 28th, 2003, 10:57 AM   #3
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Thanks Helen.

To make this question more complicated, the specs for the unit read:

Output clipping level:
+22 dBu minimum with 100k ohm load.
+20 dBu minimum with 600 ohm load.

Input clipping:
+4 dBu minimum at the 0 or 18 dBu setting.

What does all this mean exactly?
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Old November 28th, 2003, 02:02 PM   #4
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Oh my, that's just the thing to answer the day afer Thanksgiving.

Here goes - I'll do my fuzzy-headed best and hope that somebody else will jump in and correct the mistakes.

dBu is a measurement of voltage. Nowadays audio signal strengths at mic and line level are measured by voltage.

600 ohms is the standard low impedance load for professional equipment that goes way back to the telephone in Frankenstein's lab or something. 0 dBu is the voltage required to drive 1 mW of power into 600 ohms, which is 0.775 V.
10 k ohms is the standard for high impedance loads (consumer equipment).

So the output figures show that the amp will achieve a maximum signal strength of 22 dBu when facing a high impedance load and 20 dBu facing a lower impedance. The lower impedance load draws more current, so the voltage falls a little because the amp output circuit has internal impedance (130 ohms) and so the more current flowing, the greater the voltage drop within the amp itself.
The limiter, when it is switched on, limits the output to +17 dBu by squashing the signal down with a 10 to 1 ratio.

The mic input clipping level of +4 dBu indicates the maximum signal coming from the mic that it can handle without clipping. +4 dBu means that it will cope with very high mic outputs with room to spare.

The limiter switch on the MM-1 also activates an input limiter - this controls what is going into the amp. The input and output limiters have a combined maximum limiting capacity of 50 dB. Plenty!

Line level for a lot of consumer/prosumer equipment is -10 dBV (oh no not another unit). 0 dBV is one volt. -10 dBV is 0.32 V or -7.8 dBu (confused? you should be). Some consumer equipment has lower line level reference voltages. Line level for most professional equipment is +4 dBu, or 1.23 V.

How are you connecting the MM-1 to your minidisc? Which minidisc are you using? Do you have the input specs?

Best,
Helen
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Old November 28th, 2003, 04:24 PM   #5
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I'm using a Sony MZ-N10 minidisc through either the line input by attenuating the cable 14dBs, or through the mic input by flipping the line switch on the XLR adapter, which gives I think 54dBs of attenuation. Sounds fantastic. I'm just trying to get a better handle on what exactly the equipment is doing though. I've noticed that with more gain the sound gets a lot fatter and less realistic if the limiter is on and wanted to know why. I've been trying to learn more about electronics, but I have to confess I don't have much of a handle on impedance yet. Thanks for taking the time. This helps a lot. I didn't realize that the limiter used ratios to do its thing. I am actually pretty confused about the rest of it. :) Hopefully with more reading it'll make more sense.
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Old November 29th, 2003, 07:37 AM   #6
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<<<-- Originally posted by Marco Leavitt : ... I've noticed that with more gain the sound gets a lot fatter and less realistic if the limiter is on... -->>>

Yes, a limiter is similar to a good ol' compressor.

Best,
Helen
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