How choose a pre-amp to digital x-over?

[the_bf_swede]

I will soon, I hope, own a pair of beautiful speakers designed for active crossover between LF and MF/HF.The crossover supplied is an XTA DP200. Digital, pro unit.

What should I think about when choosing a pre-amp?
How do I know the pre can feed the crossover unit without problem?

Normal way is to set up like this, I think.
source -> pre -> x-over -> amps.

Is there any use consider an alternative set-up like this because of bit-reduction when feeding x-over with week signal?
source (perhaps digital feed) -> x-over -> 4ch pre -> amps?

Will the quality XTA crossover suffer from bit-loss or do they have a way of preventing it?


Chris

[boputnam]

Hey, Chris...

source -> pre -> x-over -> amps.
This is the normal way, for an analogue pre-amp. Good choice might be the Adcom GFP-750 - they work great for me.

That XTA DP200 looks sweet - mysteriously similar to KT's DN9848. Somebody's copying somebody!! But, I don't see a gain control on it - something that would supplant a pre-amp. But, I don't know the unit.

[the_bf_swede]

Yes, it has gain control both in and out. /Chris

[boputnam]

Yea, all DSP's do, but is it functional enough to serve as your only gain control? If so, then you would be able to avoid using a pre-amp.

My DSP has those for sure, but they are not intended for that type of use - they are for level setting(s) to optimize speaker response. The entire unit responds to the gain given it from the console (pre-amp).

[the_bf_swede]

No it will need a pre if I understand everything correct. Sucessors to this model are called "speaker management systems". I think this XTA is very similar to JBL DSC-260 and 280 units, wouldn't you agree?

(Bo, as you have been kind to answer my thread ...or anyone else...)
What is the risk of bitreduction when feeding the crossover a weak signal?
What should I consider when choosing a pre for this one?

Thank you for helping me,
Chris in Sweden


Extracts from manual:
"Double precision processing plus 40bit internal data path for exceptional dynamic range and sonic quality."
"Dual 16 / quad 8 band parametric eq and dual 14 / quad 5 band parametric eq with limiters"
"AES/EBU digital input / outputs as option, as well as optical I/O."

Specs:
Input inpedance <10kohms
Outputs, source impedance <60ohms
Min. load 600 ohm
Max level +20dBm at 600 ohm
Input gain adj. 0 to -40dB
Output gain adj. +15 to -40dB and mute (crossover modes only)

Dynamic Range >103dB 20-20kHz unwtd
CMMR >65dB 50-10kHz
Distorsion <0.02% @ 1kHz +18dBm

[boputnam]

I think this XTA is very similar to JBL DSC-260 and 280 units, wouldn't you agree?

My impression is the XTA is better than the JBL units - better (less audible and more expensive) A/D and D/A converters.

What should I consider when choosing a pre for this one?

Choose something horribly expensive... You may want a model that has a "direct" option, that allows you to go straight through from CD player to the XTA.

You certainly are going to want a balanced unit to interface nicely with the XTA. The Adcom I mentioned does really well for me - Widget introduced me to them, and now I have three for the various home systems. There are others that cost much, much more, but this model has fantastic sound for the price point.

[frank23]

I'd say that you want to skip the preamp and only use digital-in on the digital x-over and change the volume on that machine [it should be possible to set a general level??]

why first go from digital to analogue, determine "a certain level with your preamp", then from analogue to digital to do the x-over, than to analogue again to drive the power amps

there is one step to many for my taste when using a preamp AND a digital crossover

and if you state that the DACs in the x-over might be worse than the DACs in your cd-player, remember that they have to go through the x-over dacs anyway

greetings, Frank

[the_bf_swede]

You may want a model that has a "direct" option, that allows you to go straight through from CD player to the XTA.

The Digital path (The future is digital... with numerous formats and encrypted signals. )
CD Drive (digital signal) -------------------> digital pre -> digital in on the XTA
Analogue source -------> A/D conversion -> digital pre -> digital in on the XTA

What sort of pre's could do this without adding a heap of DSP I don't need (because it's all in the XTA...) Much of the marketing of digital gadgets is about adding DSP-power, I think.

The Analogue path (More to choose from and probably less expensive all the way.)
Digital source ---> D/A conversion -> analogue pre -> analogue in on the XTA
Analogue signal -------------------> analogue pre -> analogue in on the XTA

This would call for a good digital source. Then I'd just have to accept the triple D/A - A/D - D/A converison.


The sum of it all...
By looking at the smilies I'd say the analogue path is the one to prefer...

Comments?

[stevem]

I've been wrestling with this problem for the past year or so. It seems like an elegant solution to feed the XTA (in my case a BSS) with the digitial out of a CD player and leave everything in the digital domain untill you convert to analog and send it to your power amps. The problems with this approach are several. I found that if I use more than a little bit of EQ, the BSS clips with a full scale digital signal using the digital input. The other probelm is that you have is not having a way to control the volume at the analog outputs (of which there are 6 in a 3-way active speaker pair). I had to find a 6 channel surround speaker controller to do this. Now I want to go to a home theater surround system, so I'll need another 6 channels of gain control. there is really no way to control all these channels together, and never mind having a remote control!

You can use a digital preamp at the front end (Meridian, etc.) and some of these will give you digital volume control. The problem with this approach is that (in my experience) it doesn't sound very good. I'm using an older Meridian 518 for attenuation, and maybe the newer stuff sounds better and has solved the problem of loss of resolution (but from what I've read so far, the issue still remains).

At this pont I'm thinking I'm better off just using a good DA converter and analog surround preamp and going into the BSS analog inputs. The BSS also processes at a slightluy higher 96Khz this way, as opposesd to an upsampled 88khz with the direct CD feed. If anyone has some ideas about anoother apporach, I'd love to hear them.

[Ian Mackenzie]

Buy a DEQX....it woukd appear to have all the features you are looking for incl a preamp otherwise stick with analogue crossovers.

Ian

[stevem]

Buy a DEQX....it woukd appear to have all the features you are looking for incl a preamp otherwise stick with analogue crossovers.

Ian

Thanks, Ian. I have actually looked at them already, and you're right, they do address some of my issues. They deal with the clipping by knocking down the digital input by 6db (which does cause some resolution loss). The preamp model also has gain controls on the anolog outs, with a remote. My only problem here is that one (at the present time) cannot control multiple units (of 6 channels each) with the remote and have them stay syncronized.

One of the things I like about these digital units is the parametric EQ. I can EQ each individual driver so that it is flat over its passband, and then dial in the crossover. Do you feel that an analog crossover with global analog EQ would sound any better?

[Ian Mackenzie]

Best you ask The Widget has he has a DEQX and been fiddling with it for some time now.

Ian

[boputnam]

I found that if I use more than a little bit of EQ, the BSS clips with a full scale digital signal using the digital input. The other probelm is that you have is not having a way to control the volume at the analog outputs (of which there are 6 in a 3-way active speaker pair). I had to find a 6 channel surround speaker controller to do this.

I should stay outa this, but...

Have you tried mostly cutting, and less boosting of the EQ? That should moot the clipping worry.

And, the BSS FDS-336T has six outputs, each variably assignable to the two inputs, and each individually controllable. Maybe this would address your needs?

Again, I should stay out - I don't go this way, and "live" with a stereo analogue pre-amp before my crossover(s).

[stevem]

I should stay outa this, but...

Of course you shouldn't stay outa this, I really appreciate the input. I do have a bad habit of applying boost instead of cut. You are right; this does help the clipping issue. I do have a question regarding this, however. Is cutting with a digital EQ the same as digital volume attenuation? What I mean is, do you lose bits, and is this a problem?



I don't follow what you mean about the six assignable outputs. They do have individual level controls, but there is really no way to use them as a global volume control. When I mentioned the surround controller, I meant I was using it at the analog outputs of the BSS as a volume control. It's simply a 6-channel line stage with a single volume knob. The six channels feed the power amps for a 3-way stereo pair. When I go to surround for home theater, I will need more channels, especially if I go with all active speakers. How do you handle the issue of the volume control?



Widget, can you weigh in with some comments about the DEQX?

[ralphs99]

Hi Steve,

Multi-channel volume control is a problem with digital devices. There are only 2 places to do volume control in a system that uses digital devices: before the DAC(s) or after. If you control volume before the DAC (anywhere in the chain) you will lose resolution. The only way to maintain the full resolution of a DAC is to feed it with a full level signal.

Mitigating the loss of resolution is the fact that the loss tends to be buried in noise as the volume is reduced. The issue really needs to be addressed at a systems level. Given that most of the people on this forum use horn loaded speakers with incredible dynamic range, it can be argued that the loss of resolution is audible.

If we take the view that we wish to maintain the full resolution of our DAC's then we are left with analogue volume control after the DAC's before the power amp inputs.

This can be achieved with VCA's, passive or active switched attenuators, or potentiometers. The trade-offs are that VCA's and active switched attenuators (digital pots) introduce extra noise and distortion at a place in the signal chain where there is no further attenuation. The noise and distortion of such devices is fed straight into the power amps.

Passive switched attenuators and potentiometers introduce no such problems. Potentiometers do not track each other perfectly however. This creates a unique form of musical distortion where different channels change their level at different rates. This problem is just a function of the accuracy of the pots used, and can readily be reduced under 0.1dB in the middle of the pots adjustment range if high quality pots are used. Ganged motorised pots can automate any number of channels.

Passive switched attenuators are probably the ultimate soltion in terms of noise, distortion and tracking. The problems are cost, limited number of steps and the difficulty of automation.

I hope this gives you a bit of background as to the trade-offs involved.

Cheers, Ralph.