MF X-DAC Output stage SCHEMATICS

Will · 3rd May 2005, 11:33 AM

May I know what does the 2 opamps do (between the active i/v stage and output buffers ?)

Can I remove them ?

Will

aparatusonitus · 3rd May 2005, 12:20 PM

You can't remove them, they form a LP filter.

Elso Kwak · 3rd May 2005, 03:23 PM

Quote:

Originally posted by Will
May I know what does the 2 opamps do (between the active i/v stage and output buffers ?)

Can I remove them ?

Will

FDNR low-pass filter

Werner · 4th May 2005, 05:59 AM

Am I correct in that they use a dual opamp with one half for
the IV conversion, and the other half for the output buffer?
I knew recent MF stuff was of low design quality, but this ...

Will · 4th May 2005, 07:00 AM

Quote:

Originally posted by Werner
Am I correct in that they use a dual opamp with one half for
the IV conversion, and the other half for the output buffer?
I knew recent MF stuff was of low design quality, but this ...

All in all they used 4x dual op-amp for left and right channels !!

This dac design really suck, especially on the PSU section where they use a 12vac and convert to a dual +/- 15 v regulated for the op-amp, thru the implementation of half wave rectification.

will.

jean-paul · 4th May 2005, 08:03 PM

I wouldn't remove them as they function as a lowpass filter with R101. This filter ( forgot the name of it but acts more or less like an inductor ) is connected in parallel with the signal and not in series as usual. It was used by many manufacturers in their higher range cdplayers in the past.

It could be a drawing error but normally the output pin 7 of U100b would be connected to the non inverting input at pin 5. A 100 k resistor to ground from pin7 to avoid plops when cables are disconnected and then the stop-resistor R107 from pin7 to output. I also fail to see why C109 has to be an electrolytic of 10 uF with R106 being 10 k when a quality film cap could have been used with a higher value resistor !?!?

It could be that my theory is getting rusty, I know.

dhaen · 4th May 2005, 08:10 PM

Looks like a resonant gyrator to me. It mimics a tuned circuit.

rfbrw · 4th May 2005, 08:17 PM

Or to quote Burr-Brown " A frequency dependent negative resistance realized using a generalized immittance converter."

rfbrw · 4th May 2005, 08:24 PM

Quote:

Originally posted by jean-paul
I also fail to see why C109 has to be an electrolytic of 10 uF with R106 being 10 k when a quality film cap could have been used with a higher value resistor !?!?

Pehaps the op-amp in question has a significant input bias current.

jean-paul · 4th May 2005, 08:37 PM

Aha, that could be the reason although I would think MF used FET input opamps as usual. Too bad, the type is not mentioned in the schematic.

From reading your post I just realised the filter topology was called GIC filter by japanese brands.

4th May 2005, 08:03 PM	#6
jean-paul diyAudio Moderator Emeritus Join Date: Sep 2002 Location: The Netherlands	I wouldn't remove them as they function as a lowpass filter with R101. This filter ( forgot the name of it but acts more or less like an inductor ) is connected in parallel with the signal and not in series as usual. It was used by many manufacturers in their higher range cdplayers in the past. It could be a drawing error but normally the output pin 7 of U100b would be connected to the non inverting input at pin 5. A 100 k resistor to ground from pin7 to avoid plops when cables are disconnected and then the stop-resistor R107 from pin7 to output. I also fail to see why C109 has to be an electrolytic of 10 uF with R106 being 10 k when a quality film cap could have been used with a higher value resistor !?!? It could be that my theory is getting rusty, I know. __________________ It's only audio

4th May 2005, 08:37 PM	#10
jean-paul diyAudio Moderator Emeritus Join Date: Sep 2002 Location: The Netherlands	Aha, that could be the reason although I would think MF used FET input opamps as usual. Too bad, the type is not mentioned in the schematic. From reading your post I just realised the filter topology was called GIC filter by japanese brands. __________________ It's only audio

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3rd May 2005, 12:20 PM	#2
aparatusonitus diyAudio Member Join Date: Jan 2004 Location: Split, Croatia	You can't remove them, they form a LP filter.

4th May 2005, 05:59 AM	#4
Werner diyAudio Member Join Date: Jan 2002 Location: Belgium	Am I correct in that they use a dual opamp with one half for the IV conversion, and the other half for the output buffer? I knew recent MF stuff was of low design quality, but this ...

4th May 2005, 08:10 PM	#7
dhaen diyAudio Moderator Emeritus Join Date: Jun 2002 Location: U.K.	Looks like a resonant gyrator to me. It mimics a tuned circuit.

4th May 2005, 08:17 PM	#8
rfbrw diyAudio Member Join Date: Oct 2001 Location: .	Or to quote Burr-Brown " A frequency dependent negative resistance realized using a generalized immittance converter."

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