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Old 3rd April 2005, 08:50 PM   #291
ergo is offline ergo  Estonia
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A lack of time has again kept me from posting my results, but I have been messing with a "clever" volume for DCX also.

I have at the moment 2 channels of DCX running so that I take a signal out right after DAC. Then I have a series resistor in both + and - output pins. Between the other ends of resistors is a 10k pot wired as an "adjustable resistor" from about 60ohm to 10kohm.

Check figure 2 on
http://borbelyaudio.com/upgrades.asp

This means that I'm running a balanced shunt regulator right after the DAC. From + terminal after the series resistor I have a 10u cap and then the RCA socket. Additonally I have only a 1000p cap in parallel with pot contacts to form a low pass filter.

That is it - this signal then goes to amp and speakers - and there seems to be no ill effects with more HF crap etc. And the sound is very good. I dragged the unit yesterday to local HiFi shop to get a second opinion and it seems I'm not alone appreciating the sound

I will post details during next week I hope (and also connect the last 4 channels to my 6ch pot )

Regards,
Ergo
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Old 4th April 2005, 06:51 AM   #292
AR2 is offline AR2  United States
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Quote:
Originally posted by ergo
That is it - this signal then goes to amp and speakers - and there seems to be no ill effects with more HF crap etc. And the sound is very good. I dragged the unit yesterday to local HiFi shop to get a second opinion and it seems I'm not alone appreciating the sound
Regards,
Ergo
That certainly is it. I did the similar thing - ran the signal straight from DAC to my passive preamp and than to amp. What a difference comparing to existing opamps. This certainly is the best mod that could be done. I am sure your simple filter does all it is needed to filter out HF.
Enjoy
AR2
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Old 4th April 2005, 05:36 PM   #293
petervv is offline petervv  Netherlands
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Default re: Pspice simulation

hi Miguel,

Some time ago I made a simulation using Microcap of the input stage of the DCX, and found that there was a build-in treble preemphasis.

I haven't checked the schematic for this, but it would surprise me if there was a treble preemphasis buildt in, I can't think of any reason doing so.

Have you made a frequency measurement after the mod?.

No I haven't, I only use the analog input for tuner en turntable, the tuner (Pioneer 8500II) sounds better than ever, most of my 45's will never sound good :-)

regards, Peter
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Old 4th April 2005, 08:54 PM   #294
Mentero is offline Mentero  Spain
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Hi Peter,

Quote:
I haven't checked the schematic for this, but it would surprise me if there was a treble preemphasis buildt in, I can't think of any reason doing so.
Don't ask me. In any case, good to know its working for you.

All the best
Miguel mentero
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Old 5th April 2005, 10:08 AM   #295
Lyckman is offline Lyckman  Sweden
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I got the same behauvior simulating with Electronic workbench. When connected as the DCX schematic, a 5db peak is present with it's centerpoint at about 5khz (if I remember correctly)....

// Lyckman
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Old 5th April 2005, 04:18 PM   #296
Davey is offline Davey  United States
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Regarding a treble pre-emphasis: I think you fellas need to recheck your simulations.

I did a simulation a long time ago when the schematics first became available and I see no pre-emphasis....only filtering above and below the audio band and DC blocking. I later confirmed this with actual measurements and everything looked as expected.

Cheers,

Davey.
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Old 5th April 2005, 08:33 PM   #297
Lyckman is offline Lyckman  Sweden
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Well.. Maybe I got something wrong. I double checked the circuit and could not find any errors. The frequency of the emphasis changes when changing the values of the components in the bias section. Trying to attach a screenshot.

// Lyckman
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File Type: jpg input_stage_sim (small).jpg (80.6 KB, 1832 views)
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Old 5th April 2005, 09:13 PM   #298
Mentero is offline Mentero  Spain
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Hi Lyckman,

do me a favour (if easy), delete C4 and R5 and retry.

Ill try to find my simulation file tomorrow.

Regards,
Miguel mentero
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Old 5th April 2005, 10:17 PM   #299
Davey is offline Davey  United States
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Fredrik,

Yes, I stand corrected. My simulation also shows this...(I had the wrong component value entered for C4.)

However, an actual in/out transfer function measurement (DCX programmed flat) doesn't show this shelved response. There must be an equivalent "de-emphasis" somewhere along the way that counters it. I took a quick look at the remaining analog circuitry and I don't see it. Maybe something in the DSP?

Cheers,

Davey.
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Old 5th April 2005, 11:13 PM   #300
Lyckman is offline Lyckman  Sweden
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Filter caps removed and as suspected, the response is now flat in the sim.

This is wierd. Why would one do an analog preemphasis and then (probably) a digital de-emphasis in the DSP? And further, you'll need a different "program" in the DSP when using the digital input instead (eg no filter).

I've done some measurements some time ago, bypassing the input stage with analog signal straight to the ADC's, but I can't remember that I noticed anything similar to what the schematics is applying. (one should notice a 5dB loss of the signal above 5khz, right )

Could the schematics be wrong? Can anyone confirm the values on the components? Maybe you accidently inserted the correct values for the cap(s) in your sim, davey?

// Lyckman
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File Type: jpg input_stage_sim_no_bandpass (small).jpg (79.7 KB, 1587 views)
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