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Old 20th September 2005, 02:03 AM   #11
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Location: Puget Sound
Thanks Davey. That's exactly what I needed. Will try to test in the next day or two.
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Old 20th September 2005, 08:29 AM   #12
AndrewT is online now AndrewT  Scotland
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Hi,
my CD player, outputing a 0db maximum of 2Vac, needs +8db on the input gain setting to be just short of clipping on the output leds (+9db clips occasionally on some CDs but I do not hear the clip?). This is way below 9.75Vac.
Q1.
When DCX is set to 0db on all ins & outs.
Is the gain 0db? i.e. times one.
Q2.
When set to +8db on the input.
Is this an output of 6Vac when driven to 0db?
Q3.
The maximum input and output is +22dbu (9.75V). Is this across pins 2 & 3 or pin 1 to 2 and pin 1 to 3?
Q4.
Is the +22dbu = 9.75Vac or is it = 9.75Vpk or is it = 9.75V peak to peak?
__________________
regards Andrew T.
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Old 20th September 2005, 05:08 PM   #13
Davey is offline Davey  United States
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Andrew,

You've got a lot of questions there.

Let's talk just AC volts RMS. If you program the DCX with no processing at all and 0db settings on all sliders the DCX gain will be unity (0 db) if utilizing balanced inputs/outputs. It will also be unity in an unbalanced configuration, but only if you use an XLR/RCA adaptor on the output jack that connects pin 1-3. If you pickup the signal directly from pin 2 (leaving 3 floating) and pin 1 you'll drop 6db going through the DCX. Example: 2 volts RMS input yields 1 volt RMS output.

The "clipping" point of the DCX is a bit less than the rated specification....at least on my unit. The output waveform will start clipping with inputs of approximately 8.4 volts RMS (unbalanced) which yields about 4.2 volts RMS on the outputs. (The red clipping indicators are fairly accurate and they'll be flashing or close to it at this point.) This "clipping" is not originating in the analog output sections, but farther upstream.

So, if you apply a 6db external pad (as ultrachrome did) you should see a 12db total reduction relative to the input.


In your case I suspect your CD player is outputting a bit more than 2 volts RMS for 0dbFS recorded data. (One of my Sony players is like this.) However, assuming a 2 volt signal you should be able to bump up the input gain settings by at least 12db before the onset of clipping. So, your situation doesn't quite make sense to me. Hmmm.

Anyway, initially it seemed most users were operating the unit well below the levels they should have, but now some users are obsessing a bit too much and pushing it to the limit. I don't think you need to set your input gains anywhere above 0db. It really doesn't help the situation much anyway.

+22dbu is 9.75 volts RMS.....13.78 volts peak.....27.5 volts peak-to-peak.

I hope that answers your questions.

Cheers,

Davey.
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Old 20th September 2005, 05:42 PM   #14
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I think one other thing you need to consider is that you're slicing the energy of th incoming signal into three seperate bands.

If you have 0 dbU input on channel A, and split it into 3 channels the sum of the 3 channels will add up to 0dbU, not 0dbU on each channel. Do a simple experiment, send in pink noise to your DCX, with all the channel gains set to 0db. Adjust the level till you get 6 green bars on A/B inputs. You're outputs will be around 2 green leds (assuming almsot even split octaves)

I've been trying to work on this a bit on my own.. I think you might be able to get better signal to noise levels but adjusting the individual gains on the DCX so that channel 1-6 is at 0dbU with a 0dbU input signal on A/B (this would give you the best signal to noise ratio/dynamic range at the D/A convertor.. You would just have to really turn down the gains on your amplifiers

--Chris
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Old 20th September 2005, 07:00 PM   #15
Davey is offline Davey  United States
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Yeah, when you start applying filters and looking at the indications from the green and amber leds it can get a bit more confusing, but the red, clipping indicators are fairly well defined however....at least the output ones. They're simply looking for peaks within the waveform that are outside the defined limits. It doesn't matter how you have the unit setup or whether you're using noise or sinusoidal inputs. If that red light is flashing the waveform is clipping.

Also, on a side note, filters change the crest factor of waveforms (higher peak level, same average level) so it's possible that when you apply a filter or switch to a steeper one you might transition into a clipping condition. It's something to be aware of, but probably not a concern for most users.

Cheers,

Davey.
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Old 7th October 2005, 03:08 AM   #16
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Finally, a night off!

So I measured my outputs. Here are the conditions:

- DCX settings all zeroed out. Input/Output levels at 0dB
- Input A routed to outputs 1-6. (also tested using input B to 1-6)
- TrueRTA sending 1kHz sine wave to preamp
- Preamp feeding input A using RCA-XLR cable with tip on pin 2 and ring on 1 and 3.
- DMM on ouput pins 1-2 and alternately 1-3

Output 1-2 / 1-3 (VAC)
1 .590 / .608
2 .642 / .553
3 .603 / .593
4 .618 / .574
5 .614 / .583
6 .587 / .607

Tip to ring of sound card output was .332 VAC.

I don't have a frame of reference. Is this unit an RMA candidate?

Sanity Checking:

After noticing this problem a month ago, I changed my input/output settings from stock. Since I could not figure out how to reset the device I decided to drastically change the config. So instead of my tweeters being on 3 and 6, they are on 5 and 6. In other words, the left channel feeds the odd outputs, right feeds the evens.

So using TrueRTA I measured pink noise with a mic but used the same preamp channel and the same amp channel and tweeter for each test, setting the output levels for 5 and 6 both to 0dB. Formerly they were 5dB apart. So I physically moved the input and output cables for each test.

Looking at TrueRTA, the output of 6 is lower than 5 by:
6dB @ 5kHz
5dB @ 10kHz
3dB @ 20kHz

So it's not just a level issue but also looks like a frequency response issue.

I feel like I'm going crazy or doing something wrong. Is this normal? Should I expect better performance than this?
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Old 7th October 2005, 05:02 AM   #17
Davey is offline Davey  United States
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I don't see any problem with those readings. The worst case channel difference is between 2 (.642) and 6 (.587)....that's 20*LOG(.642/.587) equals 0.78db. That does seem a bit worse than expected, but still way less than what you noted previously. (I'll check mine and see if it's similar.)

It's still not clear to me where your problem is.

Why don't you PM me and we can talk further and try to figure it out? Or if you're in the Bremerton area sometime stop by and I can check it for you.

Cheers,

Davey.
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Old 7th October 2005, 07:06 PM   #18
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So I regrouped this morning and used TrueRTA and the frequency sweep function.

Sound card out to Preamp
Preamp set to 0dB to inputs A or B
DCX zeroed out with on input feeding all outputs
Same output cable used on each output, tested one at at time.

I calibrated with my preamp in the loop as I found that my preamp is -3db @ 20kHz. I'll have to figure out why later.

With the preamp in the loop but calibration only involving the sound card, the sweeps of outputs 5 and 6 match what I saw when I did the same test with pink noise and a mic.

THe attached screenshot is with the preamp response calibrated so this should be a better representation of the DCX's true performance.

Here you can see the vast difference between output levels that I've been fighting.

Again, the same cables were used. I ran the sweep, saved the results and then moved the cable to the next output and repeated.
Attached Images
File Type: png dcx2496_freqsweep_preampcalib.png (31.5 KB, 109 views)
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Old 8th October 2005, 12:22 AM   #19
Davey is offline Davey  United States
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Yeah, you've got some kind of a problem with your testing setup because those curves don't agree with the numbers you were getting from your DMM.

The frequency response should be ruler flat and there shouldn't be any of that "rippling" between 100-300Hz.

Cheers,

Davey.
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Old 9th October 2005, 12:13 AM   #20
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I ignored the ripple. I was concentrating on the relative difference between the levels and the high end response variations.

However, I wouldn't doubt my test is flawed. However the tests are all done using the same cables and are repeatable.

I just got an RMA number from Behringer so I'll take it the local service center next week and see what comes of it.

Thanks for thelp.
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