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Old 2nd March 2004, 02:20 PM   #1
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Join Date: Feb 2004
Location: Würzburg
Default behringer dcx + passive preamp + studio pa

- I just tried to post, may be it worked, cant say so, my computer messed a little bit around, so I try again

Hi,

this is my first thread, so please be patient with me ... like many, I am not an EE or alike, but I sometimes wish I would be

I am thrilled by these digital XO and their possibilities, yet they need some stup changes. due to my present economic status it's probably the behringer dcx going to be.

To sum up what I have read so far in your posts: I need to drive the dcx digitally with volume control post dcx for optimal results. there are two possibilities for volume control, if I am not mistaken, passive and active, and there are two possibilities for amplifying: studio/PA and hifi amps.

personally, I tend to choose the passive-studio combination... sounds easier..

there have been a lot of threads on passive preamps, even on balanced, yet leaving some things unexplained to me:

first of all, "impedance" still makes me feel uneasy, is it the resistance between signal an ground jack?

there was a post on impedance requirements; the behringer has a 160 ohm impedance and my in-mind hafler P1500 amp has an impedance of 47k; folks used the HH shunt appearently succesful, but I didn't manage to reconstruct their impedances, is my setup in the right or wrong track for possibly working?

If I got everything right, the problem could be the formation of an highpass filter via the interaction of impedance and cable or other capacitances (true?). there was a post and a link for a circuit to eliminate these problems by http://mitglied.lycos.de/Promitheus...sive_preamp.htm, would that be a reasonable improvement?

It was stated, that "fake-law" passives improve tracking between different channels when a multi-gang poti is used, is this, basically, an error calculation problem - half the error?


thank you so much for any help or comment, I'll appreciate any idea on my idea very much


thanks

leif
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Old 2nd March 2004, 07:52 PM   #2
Previously: Kuei Yang Wang
 
Join Date: Nov 2002
Default Re: behringer dcx + passive preamp + studio pa

Konnichiwa,

Quote:
Originally posted by leifislive
To sum up what I have read so far in your posts: I need to drive the dcx digitally with volume control post dcx for optimal results.
Not neccesarily. The DCX has > 112db dynamic range, meaning you can attenuate around 20db on a CD Signal in the digital domain before you run into reduced resolution. As long as you make sure that you set the channel gains suitably and adjust the input sensitivity of your poweramplifiers suitably, this will work fine.

You need to first make sure the gain on all output channels is unity, then simply adjust the level to each amp p[air with it's own passive volume control (10k stereo log pot) and later replace the Potentiometers with fixed resistor pairs and fineadjust with each channels digital gain. Then simply adjust the volume ahead of the DCX in the Analoge domain.

Quote:
Originally posted by leifislive
there are two possibilities for volume control, if I am not mistaken, passive and active,
Volume control itself tends to be passive, but it MAY be followed by a buffer or amplifier stage, making an active "preamplifier".

Quote:
Originally posted by leifislive
and there are two possibilities for amplifying: studio/PA and hifi amps.
There is no real difference between these. The best Studio Amp's are often similar to top of the range HiFi Amplifiers, in both sound and implementation and the worst Studio Amp's are just as bad as crappy HiFi Amp's.

Quote:
Originally posted by leifislive
personally, I tend to choose the passive-studio combination... sounds easier.
Good inexpensive studio amp's tend to be rarer than good affordable HiFi Amp's, at least in England, Germany and Japan.

Quote:
Originally posted by leifislive
first of all, "impedance" still makes me feel uneasy, is it the resistance between signal an ground jack?
Not really. Impedance as such is mearly a measure of the Resistance for AC signals. It cannot usually be measured using a VOM (VoltOhmMeter).

Quote:
Originally posted by leifislive
there was a post on impedance requirements; the behringer has a 160 ohm impedance and my in-mind hafler P1500 amp has an impedance of 47k;
Well, you might as well hink "Output Resistance" and "Input resistance". In typhical Audio application the input impedace or resistance should be large compared to the output resistance.

Quote:
Originally posted by leifislive
folks used the HH shunt appearently succesful, but I didn't manage to reconstruct their impedances, is my setup in the right or wrong track for possibly working?
Using a shunt system is excellent for the Behringer for a number of reasons, among them is of course that it is easy to make a balanced shunt attenuator. The big problem is making a 4 or 6 Channel one, actually.

Typhically the DCX has much more output level than most amplifiers require to deliver full power. Thus you can "design in" a reasonab;e amount of "always on Attenuation" to match the DCX output to normal amplifiers.

Quote:
Originally posted by leifislive
If I got everything right, the problem could be the formation of an highpass filter via the interaction of impedance and cable or other capacitances (true?).
If you use reasonable values for shunt and series resistors this should not be an issue, but ideally you avoid long cables after the attenuator section.

Quote:
Originally posted by leifislive
http://mitglied.lycos.de/Promitheus...sive_preamp.htm,[/url]
Is dead. Any which way, the Behringer will happily drive an attenuator with a 10k log potentiometer as shunt and a pair of 10k series resistors (balanced) and the result will be happy with cables < 1m after the attenuator.

Quote:
Originally posted by leifislive
It was stated, that "fake-law" passives improve tracking between different channels when a multi-gang poti is used, is this, basically, an error calculation problem - half the error?
No, it si down to the way log potentiometers are manufacturerd. "Law faked" also sounds better. But "law faking" is not as easy using a shunt arrangement. I would recommedn using a 6-Way Elma 24-Pole switch and individual resistors to make a 6-Way passive balanced attenuator if money was an object or I would use 6 pcs S&B TX-102 attenuator transformers (sandwich type for balanced operation) if it was not.

Sayonara
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Old 3rd March 2004, 09:07 AM   #3
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thank you very much for your answer


I am glad to be basically in the right direction
no impedance problems to be expected... great!!!

the point my thinking goes now to is (of course) how to implement...

scenario 1: using a mutlichannel hifi amp: I will outmost likely not have balanced inputs, therfore I can build an ordinary shunt by using the positive line (do you say line for that in english, I am not sure...?!?) of the balanced signal and ground and leave the negative line unaffected , can't I?

Or do I have to build an balanced attenuator and then pick up only the positive signal (for whatever reason)?

scenario 2: straight forward balanced attenuator... as mentioned before.

switch poti vs. ordinary multi gang log poti and the "error-reduction-thing":

yes, switch poti would be a good choice, since I can calibrate the attenuation-switch-number-curve as I want to and I won't have inter channel tracking problems significant for my ears when using better then <1% resistors...
alternatively, there is a affordable multi gang log pot available stating to be within a 1dB max/ average 0,5 dB interchannel range. To make my question more precise: does shunting reduce inter channel error as well (I thought it should) and can the error-reducing-factor be calculated by dividing pot value by shunt resistor?

thanks for any comment

leif
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Old 3rd March 2004, 09:28 AM   #4
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I may be overconcerned, yet, rotel rmb 1066 6 channel amp has an input impedance of 22K... still not an issue?

thanks,

leif
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