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Author Topic: Crown CDi  (Read 2227 times)

Chris Jensen

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Crown CDi
« on: November 12, 2013, 09:35:56 pm »

I started working at a place that has some fitness studios with CDi amps installed.  Each room has 2 CDi2000s, one feeds 70V ceiling speakers and the other feeds two JBL ASB6112 subs.  After getting some complaints that audio goes out for a few seconds from time to time I looked into the set up.  What I noticed is that the celling speaker amp was not set to 70V, but set to 4/8 ohm.  I changed it via system architect but heard no difference.  Is this something that only the physical hookup affects?  I would think it should have been in 70V mode for both channels via the DSP.  The cutting out I think is from the fact that the subs are not putting out nearly enough due to being underpowered, the main gets cranked for more subs driving the main speaker amp into protection. 

Also, has anyone had any experience with the CDi amp being super power hungry?  The sub amp seems to have major power draw.  The amp is underpowered for the volume potential of the sub cabs they are paired with.  Looking at the power draw spec sheet from Crown they pull a lot of power for what they are.  Turing up the volume yields the front screen to flicker with the beat.  This seems a bit unreasonable to happen on it's own dedicated 20 amp circuit.  I will also add the building is 18 months old and in very good shape.  My thought is a bigger amp would draw less main voltage for the same amount of output all specs being somewhat equal.  I can see one amp being more efficient than another, but a CDi 2000 to a CDi 4000, the 4k would work less for the same amount, is this correct? 
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TJ (Tom) Cornish

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Re: Crown CDi
« Reply #1 on: November 13, 2013, 09:12:32 am »

I started working at a place that has some fitness studios with CDi amps installed.  Each room has 2 CDi2000s, one feeds 70V ceiling speakers and the other feeds two JBL ASB6112 subs.  After getting some complaints that audio goes out for a few seconds from time to time I looked into the set up.  What I noticed is that the celling speaker amp was not set to 70V, but set to 4/8 ohm.  I changed it via system architect but heard no difference.  Is this something that only the physical hookup affects?  I would think it should have been in 70V mode for both channels via the DSP.  The cutting out I think is from the fact that the subs are not putting out nearly enough due to being underpowered, the main gets cranked for more subs driving the main speaker amp into protection. 

Also, has anyone had any experience with the CDi amp being super power hungry?  The sub amp seems to have major power draw.  The amp is underpowered for the volume potential of the sub cabs they are paired with.  Looking at the power draw spec sheet from Crown they pull a lot of power for what they are.  Turing up the volume yields the front screen to flicker with the beat.  This seems a bit unreasonable to happen on it's own dedicated 20 amp circuit.  I will also add the building is 18 months old and in very good shape.  My thought is a bigger amp would draw less main voltage for the same amount of output all specs being somewhat equal.  I can see one amp being more efficient than another, but a CDi 2000 to a CDi 4000, the 4k would work less for the same amount, is this correct?
70v mode on an amp means something different than it did a couple decades ago.  It used to mean that there was an output transformer on the back of the amp that increased the voltage to be 70v nominal at full power, then the speaker's transformer reversed the process - lowering the voltage to the driver to apply whatever drive signal was selected by the transformer taps.  Now, where amps that make 1000w/channel are commonplace, the amp is capable of 70v output directly, with no transformer required.  "70v mode" can be as simple as a high-pass filter to roll off some of the lows that may be problematic for transformer-driven speakers.  In other words, I'm not surprised you didn't hear a difference.

As to a bigger amp drawing less power than a heavily loaded smaller amp - generally this is not the case; particularly if the two amps have the same topology.  Amplifier efficiency boils down to basically two components - idle current, and conversion efficiency.

Idle current - the power the amp draws any time it's on, is generally similar for any amp with similar design - increasing the transformers, output devices, and capacitors doesn't reduce this idle power, and may increase it slightly.

Conversion efficiency - this one is more complicated and is dependent on the design of the amp, but generally speaking, the conversion efficiency is similar at all output power levels - if an amp is 80% efficient when producing 400 watts, it will be approximately 80% efficient when producing 800 watts. 

There are nuances here - class H amps have multiple voltage rails, and some signal levels can be more efficient than others depending on the amount of power the output devices must dissipate between the voltage rail in use and the output voltage.  For example if the supply voltage rail is 100v and the output signal level is 60v, the output devices are dissipating 40v, but if the output signal level is 90v, then the output devices are only dissipating 10 volts.  Don't read too much into this though - things tend to average out.

Different amplifier topologies can have different efficiencies.  Generally speaking, class D amplifiers are more efficient than class H, but often have a higher idle current than a class AB or H.

One thing to check is to make sure there isn't a wiring error in the install.  If there is a short somewhere, your sub amp may be trying to power a load it can't handle - a 1 or 2 ohm load.  The sub you mention appears to be 8 ohms, so two of them would make a 4 ohm load if run parallel.  Bridging the amp into 4 ohms will make the amp work pretty hard - that's equivalent to a 2 ohm load per channel, and I'm not surprised that makes the amp run hot. 

If the amp is bridged, consider changing that to parallel operation, and power one sub per side of the amp.  If that's not enough volume, then it may be time for a larger amplifier.  Be careful here - if the system is regularly being driven into limit, larger amps may be just giving the kids bigger guns to play with.  If more output is really required, a full system re-design may be called for.
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Re: Crown CDi
« Reply #1 on: November 13, 2013, 09:12:32 am »


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