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DL Preamp A/D D/A Limits

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WK154:
RR you had to ask! Well rather than dump it into meters although related I started a new thread just for the gain structure within the DL. Normally one would just be dealing with gain structure between external equipment but it also pertains to the internals of a piece of gear. In analog one of the concerns was clipping on the mix bus (CR1604 myths has some keeping input levels low to prevent the mix buss from clipping).
So to the rescue comes the Behringer EU1202 mixer with a output spec of 22 dBu as a input source to the DL which is supposed to be good to 21 dBu. The first thing of course is to check out the signal source (EU1202) to avoid garbage in garbage out issues. It passed all the way and a little beyond 22 dBu certainly enough for the DL's 21 dBu. So what happens when we put a 21 dBu signal on the input of the DL?  That's a sine wave with a RMS value of 8.69 V at 1kHz . Since we needed to see the output of the A/D converter we had to have the ability to measure it's output. Well the iPad audio-stream out bypasses both the D/A converter and the output circuitry. I have AudioTools from Studio 6 and it has a scope that looks at the audio-stream out, how convenient. Judging from the sine wave form it barely makes the 21dBu before distortion (visual not measured but plainly visible). OK so the input is to spec but no margin. With both the faders at 0 what is the output? All of 5.15V RMS or 16.5dBu. The waveform looks fine. We know we can get more output so what happened. Let's work this backwards set main fader to max (10) let's see what it takes for clip/distortion. The output is maxed at 21.5dBu to cause clipping.  Well  we really don't have input devices this hot and the preamps were at unity gain so why not apply more real world input values such as the output of a SM58. At his point we will have to let the preamp do it's work and well look at how many dB it would take for clipping or distortion after the preamp/A/D chain. At -25 dBu input it takes 50dB to clip the A/D chain. I used -25dBu to cover most dynamic mics and screamers, other mics may be too hot (condensers). The output at (10) main fader is 21.5dBu the max again before clipping. Remember your music is not a sine wave so peaks will be OK.  The only instrument I know that produces sine wave music is a Armonica (a Ben Franklin invention) yes I do know about Fourier.  ::)
So what does this all mean. You can push you SM58 to the limit and be OK condensers not so much. But then you already knew all this just had no numbers from Mackie to place the limits.
Corrections applied.

RoadRanger:

--- Quote from: WK154 on February 25, 2014, 02:06:09 AM ---In analog one of the concerns was clipping on the mix bus (CR1604 myths has some keeping input levels low to prevent the mix buss from clipping).

--- End quote ---
That was true of the first gen 1604's I don't remember which gen "fixed" that.

WK154:

--- Quote from: RoadRanger on February 25, 2014, 03:19:04 AM ---
--- Quote from: WK154 on February 25, 2014, 02:06:09 AM ---In analog one of the concerns was clipping on the mix bus (CR1604 myths has some keeping input levels low to prevent the mix buss from clipping).

--- End quote ---
That was true of the first gen 1604's I don't remember which gen "fixed" that.

--- End quote ---
The VLZ series when they switched op-amps.

RoadRanger:

--- Quote from: WK154 on February 25, 2014, 02:06:09 AM ---The output is maxed at 15.5dBu to cause clipping.
--- End quote ---
Are you sure you weren't measuring one pin to ground? That would reduce the measured by 6 dB so the real would be the expected 21dB. Is the output meter tickling the red?

WK154:
Checked the cable and yes it was signal return. Thanks correction coming. Rechecked  voltages stands as original. Found the problem had to recheck cable and found 1&3 connected. Now were OK corrections reapplied.

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