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Old 23rd March 2006, 03:19 PM   #11
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IMHO the DCX has the capability to sound as good as a good passive crossover, but it has some key advantages that you cannot get with a passive solution if you think a little differently.

First of all, in a typicall Hi-Fi system people attenuate the signal with a pre-amp before the amplifers, and then the full output signal is split by the passive crossover inside the speaker. Plugging a DCX into this kind of system, especially with high gain amplifers is not going to sound very good.

The main reason for this, is that with the passive system your source is running at full swing, for CD players this means you are using 16 bits to recreate the analog waveform, then attenuating it with a pre-amplifier, the analog signal after the pre-amplifier is now much smaller than the full swing signal.

When you insert a DCX after the pre-amp as most people try to do you end up loosing resolution!. The reason for this is that if you feed analog inputs to the DCX it has to run them through an A-D conversion and then back through D-A convertors on the output. If the swing is much lower than maximum you will use very few bits to sample the analog waveform. For example lets say you're pre-amp attenuated the signal only 6db, you would loose "2 bits" of resultion on the A-D Convertor.. 12 db = 4 bits etc...

You also add noise to the signal. Any time you go through an A-D or D-A stage the system noise will be added to the output signal. For this reason it is imparative to keep the analog swing as high as possible. For example, if the DCX has 1 mv of noise in the D-A stage, and you output 7V the noise is a very small portion of the signal giving a high signal to noise ratio. If you're only outputting 1V then the 1mv is a much larger portion of the output therefor lower signal-noise ratio..

I have a DCX and would never go back to a passive crossover based on the featrures and flexability available in the DCX. You just have to be carefull, if you have high power amplifiers (75W+), you will need attenuators on the DCX outputs to keep the swing level high. Or better, use amplifiers which have gain controls.

There are also other benefits to using the DCX
- Drive digital inputs from your CD player directly to the DCX for better noise imunity/accuracy
- Time align your drivers digitally
- Adjust cross over frequencies and slopes in realtime while listening for changes!
- Mute individual drivers to hear the pass band of a driver by itself...

HTH

--Chris
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Old 23rd March 2006, 03:42 PM   #12
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Yes, you really want to feed the DCX with a digital signal, then fit a six-channel stepped attenuator after it as the volume control. The huge advantage is that if you want to try something like altering the crossover frequency, you can do it in less than a minute whereas changing a whole load of components on a breadboard (analogue active) takes ten times as long. The ability to compensate for timing errors is a huge advantage. Frankly, at the price, the DCX is an excellent tool for working out precisely what sort of analogue crossover is needed, let alone as a crossover in its own right.
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Old 23rd March 2006, 07:03 PM   #13
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If you have volume control after the DCX, how do you coordinate volume control in a 7.1 ch HT system, with the DCX being used to biamp the LCR's?

Thanks
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Old 23rd March 2006, 08:01 PM   #14
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Life gets harder if you insist on 7.1. For one thing, you're going to need three DCX if each of the major channels is a two-way system. You simply (!) need a fifteen channel stepped attenuator. Rather boggles the imagination, really. Some serious mechanical engineering is required to fit fifteen switch wafers onto one shaft. Or perhaps you could fit seven on one shaft, seven on another, then gear them off the centre shaft (which would carry the sub and have an inverse logarithmic law).

Two channels is enough for anybody. Or was it 640k?
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Old 23rd March 2006, 08:08 PM   #15
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I usually watch movies at reference (0dB on my processor vol control) and listen to music around 15dB lower. I don't bother with attenuation after the dcx at the moment, but when I get round to it I'll just use a 2 position volume pot on each channel - 0dB attenuation and -15dB attenuation. Then I can fine tune with my processor master volume.

Rob.

btw my amps all have volume pots. If they didn't then I'd have to work out resistor values to give me the 0dB attenuation setting. Hope that makes sense
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Old 23rd March 2006, 08:10 PM   #16
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Quote:
Originally posted by EC8010
Life gets harder if you insist on 7.1. For one thing, you're going to need three DCX if each of the major channels is a two-way system. You simply (!) need a fifteen channel stepped attenuator. Rather boggles the imagination, really. Some serious mechanical engineering is required to fit fifteen switch wafers onto one shaft. Or perhaps you could fit seven on one shaft, seven on another, then gear them off the centre shaft (which would carry the sub and have an inverse logarithmic law).

Two channels is enough for anybody. Or was it 640k?
Alternativly and not as good as post-DCX volume control is to use fixed attenuators after the DCX to set the loudest volume you plan to listen at. Then only slightly attenuate the volume pre-dcx to adjust. The less you attenuate pre-DCX the better the sound quality will be.

Multiple volume controls are not that bad post-DCX. I'm using 3x A500 amplifiers for my 2-challe 3-way project. Thats 6 gain knobs to adjust. The truth is I just set the volume once and listen for as long as I want..

I don't mind adjusting 6 gain controls at the start of listening session to ensure the highest quality sound out of my setup

--Chris
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Old 23rd March 2006, 08:11 PM   #17
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Quote:
Originally posted by EC8010


Two channels is enough for anybody. Or was it 640k?

2.1 channels would do me fine. Could easily live without the surrounds and centre, but I'd hate to lose the lfe channel.

Rob.
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Old 23rd March 2006, 08:42 PM   #18
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Originally posted by RobWells



2.1 channels would do me fine. Could easily live without the surrounds and centre, but I'd hate to lose the lfe channel.

Rob.
Much easier to use the DCX to generate the LFE channel and run 2-way mains...

Set it up like this :

A -> output 2 and output 3 : high/mid-woofer left channel
B -> output 5 and output 6 : high/mid-woofer right channel
Sum (A:B) -> C
C -> output 1 and output 4 : dual subwoofers, or use only output 1 for a single sub

Set crossovers to "un-linked"

This way you can cross over to your subwoofer at any frequency you want and with any slope you want. You can also use asymetrical freq/slope crossovers

--Chris
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Old 23rd March 2006, 08:54 PM   #19
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Hi Chris,

I was actually refering to the lfe channel encoded on dts / dolby digital dvd's that is a discreet channel in itself- if you run your mains as large you'll lose the lfe unless you redirect it to the mains.

I run 3 way mains, designed to roll off at 12dB/octave at 80Hz so that they work with my processors xo when set to small.

I use 2 dcx's - 1 does my left / right 3 ways, the other does my subs and leaves enough for me to add a 3 way centre channel if I want to.

Cheers,

Rob.
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Old 24th March 2006, 02:03 AM   #20
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My take on the DCX kind of goes like this… Hope I don’t offend too many folks with my opinion of the stock DCX analog circuitry… it sucks!

I’m only using the DCX for Sub control. The main signal to the main speakers bypasses the DCX. The subwoofer drive goes through the DCX for crossover, OB EQ, and room EQ.

When I first started this project just before Christmas last year I was quite disappointed with the sound. Everything sounded off including the full range that did not go through the Behringer. The new U-Baffle speaker system was using the same drivers as the old vented box system so I was very familiar with what the speakers should have sounded like. After a bunch of messing around I found the analog input circuitry of the Behringer was so poor that the backwash from the Behringer was messing up the full range sound. I found this quite amazing as both my digital and analog sources have low output impedance and feed into a balanced autotransformer volume control that lowers the impedance even further.

So, the answer to this problem was to design a new analog input circuit for the Behringer. The original analog input uses cheap op-amps, 3 47uf electrolytic capacitors, and runs the op-amps at very low impedances, a nice recipe for mediocre sound at best.

The new input uses a Burr Brown OPA2134 configured as a DC coupled differential amplifier with a gain of 3 feeding a 2K: 600 UTC PCO-25 nickel interstage transformer. No coupling capacitors! The bias for the ADC inputs is provided via the secondary centertap. The transformer provides the common mode isolation, provides a easy load for the op-amps and low drive impedance for the ADC. The only mod to the digital board is to remove the 333 ohm resistors that are between the + and – inputs of the ADC’s.

The output analog circuitry is almost as bad as the input circuitry was. Currently I’m directly driving 1:1 interstage transformers with the DACs.

With the new analog input and output circuitry the DCX is a really nice sounding unit. Their digital work seems to be pretty good. They just need some good analog engineers on their design teams. Seems that analog design is not practiced anymore, just copy from the cookbook and call it good.

Here is a link to the schematic of the new input circuit.
input circuit

Gary
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