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Old 19th January 2010, 07:02 PM   #2441
mige0 is offline mige0  Austria
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Gary - you have any indicators for transformers using nickel core other than a data sheet if I want to do a search on surplus too? At what signal levels did you audition the TRW UTC you were happy with?

Michael
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Old 19th January 2010, 07:05 PM   #2442
Legis is offline Legis  Finland
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Quote:
Originally Posted by Cobra2 View Post
dbx 260 is no better than DCX...have tried, even a full opamp & cap -upgrade on the dbx.

Arne K
DCX is by no means bad, it's is actually very good piece of equipment. Even unmodded DCX is very transparent in the signal path of a hi-resolution system. Modding DCX besides op-amps and (someway) reduced hiss might be totally waste of time.
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Old 19th January 2010, 07:13 PM   #2443
SQLGuy is offline SQLGuy  United States
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Originally Posted by Legis View Post
Yes, the hiss reduced by half by setting the input mode from analog to ASE/EBU. This narrows the hiss to analog in op-amps, ADC and analog input buffer?

The hiss might be completely normal in DCXs. It might be a feature of a unmodded DCX? I find out when my other DCX arrives later this week.
The half that goes away when switching to AES/EBU will only be reduced in a modded DCX that changes the input buffer, like maybe to an OPA1632, or maybe if the DCX has an upgraded power supply. Personally, I'd bet on the OPA making a bigger difference.
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Old 19th January 2010, 07:22 PM   #2444
Legis is offline Legis  Finland
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Quote:
Originally Posted by SQLGuy View Post
The half that goes away when switching to AES/EBU will only be reduced in a modded DCX that changes the input buffer, like maybe to an OPA1632, or maybe if the DCX has an upgraded power supply. Personally, I'd bet on the OPA making a bigger difference.
Is the input buffer right in the next to the power supply in the PSU(dip8 op-amp looking thing)? I will change that also when I change PSU's capasitors. OPA1632 is the best one available? Or do you mean the op-amps in the I/O board?

edit: I am going to change all the 17 op-amp of the DCX to LM4562s, also input op-amps.

Last edited by Legis; 19th January 2010 at 07:37 PM.
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Old 19th January 2010, 07:40 PM   #2445
Pano is offline Pano  United States
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Default Gain Structure

OK, so now we know that part of the hiss is coming from the analog input and part from elsewhere. As Michael says, once you get the levels set properly, you shouldn't get any hiss. This is known as "Proper Gain Structure."

Here is one method of setting it up properly:
  1. Choose a piece of dynamic music, about the most dynamic you usually listen to. A track with at least an average to peak value of 16dB is needed.*
  2. Set all gains in the DCX to 0dB (or at nominal, if you use different gains for each section)
  3. Play this track at the loudest level you think you would normally listen to. Check the input and output VU meters. Note what they say.
  4. Where are the peaks on the inputs? They should be just hitting the yellow LEDs. If they are not, go to next step.
  5. Reduce the gains as much as possible on the outputs. -15dB. Now bring up your preamp volume until you hit the yellows on the input VU meters - or - you get to the same high acoustic level as before.
  6. If the track is now hitting the inputs and lighting the yellows LEDs, your input level is good. If it's still below that, you'll need more than 15dB attenuation on the DCX outputs.
  7. If you are not at the loud music level from before, bring up the gains on the outputs until you hit the same volume. Note this setting and note where the peaks are on the output VU meters.
  8. If you succeed in obtaining a strong level on the inputs and a "loud enough" volume with the outputs attenuated by the DCX menu, then you will know that you need the same amount of attenuation as found in the DCX output menus.
  9. If -15dB on the outputs is not enough to give you a strong level on the outputs and not blow up your speakers, you'll need more than 15dB attenuation between the DCX and your amps.

I have to run now - but will continue later.

* If you don't know how to measure this, I can help.
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Old 19th January 2010, 08:02 PM   #2446
Xoc1 is offline Xoc1  United Kingdom
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Default Optimising the Gain Structure

Legis
The best results with the DCX have generally been acheived by using a digital source in to the DCX so that it is running flat out and then attenuating the outputs using a six channel volume control.
You are on the other hand using the DCX as a conventional active crossover.
In order to optimise the gain structure, you need to drive the input harder (to pro audio levels) and then attenuate the DCX outputs into your amps.
Simple resistive divider networks on your signal leads between the DCX and your power amps will give you a more optimal gain structure, and reduce the hiss. I think that in this case this would give you the performance gains that you are after with least effort / cost.
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Old 19th January 2010, 08:03 PM   #2447
SQLGuy is offline SQLGuy  United States
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Quote:
Originally Posted by Legis View Post
Is the input buffer right in the next to the power supply in the PSU(dip8 op-amp looking thing)? I will change that also when I change PSU's capasitors. OPA1632 is the best one available? Or do you mean the op-amps in the I/O board?

edit: I am going to change all the 17 op-amp of the DCX to LM4562s, also input op-amps.
OPA1632 is a replacement for the arrangement of JRC4580's and other components used in the input section between the A/B/C inputs and the ADC's. It requires circuit redesign to swap in.

Jan's I/O board uses the OPA1632.

Cheers,
Paul
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Old 19th January 2010, 08:45 PM   #2448
Legis is offline Legis  Finland
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Quote:
Originally Posted by Xoc1 View Post
Legis
The best results with the DCX have generally been acheived by using a digital source in to the DCX so that it is running flat out and then attenuating the outputs using a six channel volume control.
You are on the other hand using the DCX as a conventional active crossover.
In order to optimise the gain structure, you need to drive the input harder (to pro audio levels) and then attenuate the DCX outputs into your amps.
Simple resistive divider networks on your signal leads between the DCX and your power amps will give you a more optimal gain structure, and reduce the hiss. I think that in this case this would give you the performance gains that you are after with least effort / cost.
What I have read, changing all the 17 I/O board's the op-amps will reduce the hiss/noise significantly. If the changing won't help at all, I'll have to look for those attenuator to be put after DCX.

My main plan was to use DCX with analog inputs as I have read that it actually performs better (excluding louder hiss) with them than digital inputs. Analog inputs are also more convenient than digital inputs since I can control the volume with my preamps (especially with my tube preamp).
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Old 19th January 2010, 08:51 PM   #2449
Legis is offline Legis  Finland
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Quote:
Originally Posted by SQLGuy View Post
OPA1632 is a replacement for the arrangement of JRC4580's and other components used in the input section between the A/B/C inputs and the ADC's. It requires circuit redesign to swap in.

Jan's I/O board uses the OPA1632.

Cheers,
Paul
Yep, I read that OPA1632s perform well, but, as you said, they don't suit well in the original desing as they run too hot. LM4562s should work fine based on what I have read. Do you have experience of how much changing the input and output op-amps to OPA 1632/LM4562 or equivalent will reduce the hiss?
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Old 19th January 2010, 08:51 PM   #2450
SQLGuy is offline SQLGuy  United States
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Quote:
Originally Posted by Legis View Post
What I have read, changing all the 17 I/O board's the op-amps will reduce the hiss/noise significantly. If the changing won't help at all, I'll have to look for those attenuator to be put after DCX.

My main plan was to use DCX with analog inputs as I have read that it actually performs better (excluding louder hiss) with them than digital inputs. Analog inputs are also more convenient than digital inputs since I can control the volume with my preamps (especially with my tube preamp).
Maybe, but that's a bit brute force. There are other circuit optimizations that can be done besides just swapping different op amps into the existing topology. For instance, switching from the existing input buffer to the OPA1632 reduces the input path from six op amp stages to the one OPA1632, simplifies the addition of the 2.5V offset, and allows for DC coupling.

Similarly, the output side can be simplified and DC coupled as well. If you're looking at such wholesale changes as swapping out all the 4580's, I'd really consider swapping in Jan's I/O board instead.

Cheers,
Paul
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