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Old 29th October 2006, 06:37 PM   #771
AR2 is offline AR2  United States
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Default Re: DC vs AC coupling on DCX kit

Quote:
Originally posted by pgruebele
If I supply a high signal to the amp, the speakers stay at the given excursion and do not return to the neutral position. If I understand correctly, this means that the amp is not AC coupled but DC coupled.

What I don't understand is why this then requires the DCX to have AC coupling on the outputs. What is wrong with DC->DC?

Also, my main concern with the AC coupling option is how it will affect the sound. Can anyone comment on this?

Thanks

Philip Gruebele
The fact that your speaker doesn't return to the neutral position is shure sign that you have DC voltage applied to your amp. In order to avoid that, capacitors are inserted in the signal path to block DC signal. Your music should be only AC without any DC in it. Another way to remove DC from the signal is line level transformer with ballanced signal that canncel itself, but lets leave that alone for now.
DC in signal path is something you should avoid unless you enjoy burning your speakers. In order to simplify your question on AC or DC coupling - you just have find out that you do not have DC blocking in the input of your amp. So you must have something that will stop +-2.5V DC coming out of AKM DAC. With nice pp cap in the way of signal you will still have dramatical improvement over stock opamp output board.
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Old 29th October 2006, 07:01 PM   #772
Ryssen is offline Ryssen  Sweden
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Quote:
So you must have something that will stop +-2.5V DC coming out of AKM DAC
So this means that there is always 2,5v DC out from the DAC?
Then its not a god idea to leave the capacitors out as I had planned.
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Old 29th October 2006, 07:02 PM   #773
giulio is offline giulio  United Kingdom
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Quote:
Does anyone have any details on this board?

It looks very interesting and the price is right, but I could not find any details on how the analogue output is done here...

Any info on this would be most welcome!

Regards,

Lucas
I have bought one. It implements a 1st order low pass to filter out digital noise and a 1st order high pass with Mundorf MKP cap to keep DC out.

Jan's pcb has 2nd order low pass. I use transformer volume controls after the DCX so 1st order is enough for me. The board is single ended but can be easily modded to run balanced. Given that you cannot do w/out the coupling caps anyway I fail to see (in my setup) the advantage of running balanced ahead of the TVCs.

The board has two analogue (buffered, DC coupled) and 1 digital input. It was the two analogue inputs (I wanted to be able to use a microphone for the autoalign) that prompted me to go for this one.

All in all, they both look very good products to me. Horse for courses I would say.

Hope it helps
Giulio
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Old 29th October 2006, 07:19 PM   #774
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Quote:
Originally posted by Ryssen

So this means that there is always 2,5v DC out from the DAC? [snip]

Correct. As is shown in the data sheet, the DAC has a power suplly voltage of 5V. The signal is centered in the mid point between this 5V and ground (there is no neg supply). Full scale signal is IIRC 2.4V peak, meaning that the signal output can swing between 4.9 and 0.1 V with respect to ground. This is a 2.4V peak signal (or 4.8 pk-pk, or 1.68V RMS) riding on a 2.5V DC component.
And almost all DACs work like this.

Now, it is interesting how Behringer handle this. First they take the difference between the + and - output of the DAC in a standard balanced to single-ended conversion opamp circuit. That means they get rid of the 2.5V DC [(2.5 + signal) - (2.5 - signal)] = 2 * signal. Then they put a transistor to ground for the mute. Then they convert the single ended signal back to balanced with another couple of opamps. And ohh, yes, there are a couple of electrolytics in the signal path...
The circuit is in the AX article.


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Old 29th October 2006, 07:34 PM   #775
Ryssen is offline Ryssen  Sweden
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Quote:
Then they convert the single ended signal back to balanced with another couple of opamps. And ohh, yes, there are a couple of electrolytics in the signal path...
No wonder they don´t sound so god when original..
Then I better have the capacistors on the DCX board,cause my amps is DC coulpled,unless I don´t want my speakers demoluted.
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Old 29th October 2006, 07:53 PM   #776
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Default DC vs AC coupling on DCX kit

What I forgot to mention is that this apparent DC coupling is when my preamp is connected to my power amp. This means that both my preamp and amp are using DC coupling, so why should it be so bad to DC couple the DCX to my amp? Sorry if I'm ignorant, I'm not an EE.

On another note, there are lots of people who are trying to implement post-DCX volume control. It seems to me that it would be fairly simple to add the following to the Jan Didden PCB:

1. pads for 3 daisy chained CS3310 chips (or another single 6 or 8 channel volume control chip). Pads could be added for a couple of different chips for flexibility.

2. option to jumper these chips into the signal path (directly at the incoming DAC output)

3. a header with the serial lines as well as digital VCC/GND which can then be used to connect external equipment in order to implement volume control.

In my case I am using a B&K Reference 30 preamp and would tap into the serial lines of the CS3310 chips that it uses in order to control the CS3310 (or other) chips in the DCX. This would allow me to integrate the volume control of the B&K with the DCX. A perfect solution. Many other preamps use similar volume control chips and could be modded to work with DCX colume control as well.

Anyway, this is the solution I would ultimately like to implement. I think it's quite elegant because I am also planning on tapping into the AKM4393 inputs in the preamp in order to get S/PDIF output to feed the DCX digitally. This way I can use the same setup for DTS, Dolby, or PCM inputs...

Philip Gruebele
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Old 29th October 2006, 07:53 PM   #777
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Default Re: Scheme of Audiophonics

Quote:
Originally posted by Lucas_G


http://www.audiophonics.fr/carte-ent...96-p-1081.html


Does anyone have any details on this board?

It looks very interesting and the price is right, but I could not find any details on how the analogue output is done here...

Any info on this would be most welcome!

Regards,

Lucas
For the outputs, this is the same schematics that dcx2496.fr

Full asymetrical passive output with RC filter.
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Old 29th October 2006, 08:02 PM   #778
Ryssen is offline Ryssen  Sweden
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Quote:
so why should it be so bad to DC couple the DCX to my amp? Sorry if I'm ignorant, I'm not an EE.
If you get DC at your amp in,and it is DC coupled,the amp will amplify the DC,and the speakers will not be centered at no AC signal.Did I get that right?
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Old 29th October 2006, 10:51 PM   #779
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Default DC offset

Isn't there another way of getting rid of the DC offset without placing large caps in the signal path?

I'm not an EE but I can think of the following as possibilities:

1. change analog VCC and GND on the AKM4393 to 2.5v and -2.5v instead of 5v and 0v (if this is possible it would require modification of another PC board, of course).

2. Use of an opamp to offset signal (?). This would once again require a negative power supply.

Would these 2 methods (especially numer 1) not be better sonically than using large output caps? Even an opamp would be better than the cap I would think. Option 2 would probably be less expensive than the caps as well.

Any thoughts?

Philip Gruebele
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Old 30th October 2006, 01:16 AM   #780
AR2 is offline AR2  United States
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Default Re: DC offset

Quote:
Originally posted by pgruebele
Isn't there another way of getting rid of the DC offset without placing large caps in the signal path?

Philip Gruebele

There is another way and I have such a great success with it. As I mentioned earlier line level transformers are another way to go. It is much more expensive than caps, but the result is outstanding. Here is the link on my set up.
I would avoid any opamps as they are not as good as jFets or Mosfets. I tested highly praised BB 627 with and without BB buffer and it is no where close to preamps executed with jFets or Mosfets. If I could I would run only Lundahls after DACs, I would do it, but my amps are not in agreement with it. I did it for a while and that is the ultimate solution, as long as that provide enough drive for your amp.
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