Pulse transformer
Does anyone know why MF chose to only put a pulse transformer (PT) on this DAC's output? See georgehifi post of the circuit schematic for details:
http://www.diyaudio.com/forums/showthread.php?postid=1105266#post1105266
(the PT and S/PDIF area is on top of this schematic)
Another modder on this forum commented favorably upon adding a PT on the input. Noting that, I also added a scientificonversion.com PT (SC947-02); however, this yielded only a very subtle improvement. Franlkly I was expecting more given the (a) "reputation" of this brand (Scientific Conversion, SC; tho' not all agree that SC is all that great), and (b) the generally-favorable consensus for PT mods (additions where none existed prev.).
Any hints/suggestions as to tweaking the PT with respect to the input (load/resistance/capacitance/reactance-wise)? Also, I've noted in some comments that BNC or AES/EBU works better than RCA coax. How true is this?
The PT vendor suggests that I won't notice much of an improvement unless I also add a PT to the S/PDIF output of the CD transport (player). Not sure how true this is.
Does anyone know why MF chose to only put a pulse transformer (PT) on this DAC's output? See georgehifi post of the circuit schematic for details:
http://www.diyaudio.com/forums/showthread.php?postid=1105266#post1105266
(the PT and S/PDIF area is on top of this schematic)
Another modder on this forum commented favorably upon adding a PT on the input. Noting that, I also added a scientificonversion.com PT (SC947-02); however, this yielded only a very subtle improvement. Franlkly I was expecting more given the (a) "reputation" of this brand (Scientific Conversion, SC; tho' not all agree that SC is all that great), and (b) the generally-favorable consensus for PT mods (additions where none existed prev.).
Any hints/suggestions as to tweaking the PT with respect to the input (load/resistance/capacitance/reactance-wise)? Also, I've noted in some comments that BNC or AES/EBU works better than RCA coax. How true is this?
The PT vendor suggests that I won't notice much of an improvement unless I also add a PT to the S/PDIF output of the CD transport (player). Not sure how true this is.
a3.24 clock upgrade
Dear georgehifi (also hollowman and guido)
I followed with interest your report on the mods of your Musical Fidelity a3.24 dac. Following your advice, I got rid of the caps, and swapped the stock opamps for the ad825. All this with great results, thanks ! Also, I changed the regulators 78xx for better ones, and this further improved the performance. Now I have the following problem. I went next for the clock upgrade using a 45.1584 MHz instead of the stock 50Mhz according to the exchange you had with hollowman and guido (see in http://www.diyaudio.com/forums/showthread.php?postid=1105283), this time with less success : although I followed the instructions carefully, it just does not work. The dac is locking to the signal (well, with some initial flickering), but when converting, there is a totally inaudible mess coming out as a result. Now, the clock I used is not Guido Tent's one, but the dexa-clock. However, according to my scope measures, it outputs a "correct" signal (wave-shape, amplitude, etc. in the sense of comparison with the stock one, except of course for the frequency 45.1584 instead of 50 Mhz). So let me first ask for this confirmation : is it ok to swap the 50 Mhz with a 45.1584 ? Next, is there anything special to be considered when implementing the swap on the a3.24, something that Guido might have done since he reports "wonderful" results (ok, I know he used HIS clock, but I guess this should not matter at this point, or does it...?). The 3.24 has a dip8 oscillator and I simply replaced it with the signal of the new clock, but maybe because of the upsampling, something else is to be thought of ? Thanks in advance for any suggestion !
Dear georgehifi (also hollowman and guido)
I followed with interest your report on the mods of your Musical Fidelity a3.24 dac. Following your advice, I got rid of the caps, and swapped the stock opamps for the ad825. All this with great results, thanks ! Also, I changed the regulators 78xx for better ones, and this further improved the performance. Now I have the following problem. I went next for the clock upgrade using a 45.1584 MHz instead of the stock 50Mhz according to the exchange you had with hollowman and guido (see in http://www.diyaudio.com/forums/showthread.php?postid=1105283), this time with less success : although I followed the instructions carefully, it just does not work. The dac is locking to the signal (well, with some initial flickering), but when converting, there is a totally inaudible mess coming out as a result. Now, the clock I used is not Guido Tent's one, but the dexa-clock. However, according to my scope measures, it outputs a "correct" signal (wave-shape, amplitude, etc. in the sense of comparison with the stock one, except of course for the frequency 45.1584 instead of 50 Mhz). So let me first ask for this confirmation : is it ok to swap the 50 Mhz with a 45.1584 ? Next, is there anything special to be considered when implementing the swap on the a3.24, something that Guido might have done since he reports "wonderful" results (ok, I know he used HIS clock, but I guess this should not matter at this point, or does it...?). The 3.24 has a dip8 oscillator and I simply replaced it with the signal of the new clock, but maybe because of the upsampling, something else is to be thought of ? Thanks in advance for any suggestion !
I replaced the orig 50.000 with a Tent 45.xxx osc. Later, I built a dedicated regulator (PFM Flea) around the Tent 45.xxx, complete with its own transformer. No problems here with any of these mods.
I can't see your problem being the osc. frequency: there is nothing special about the A324's orig. 50.000 Mhz other than it was fast (at the time this DAC was designed, anyway, which was early 2000's), and very fast is supposedly best for async. reclocking. IAC, the 45.xxx is v. close to the orig., and if it's a Tent, it's almost certainly low-jitter.
I can't see your problem being the osc. frequency: there is nothing special about the A324's orig. 50.000 Mhz other than it was fast (at the time this DAC was designed, anyway, which was early 2000's), and very fast is supposedly best for async. reclocking. IAC, the 45.xxx is v. close to the orig., and if it's a Tent, it's almost certainly low-jitter.
Re: a3.24 clock upgrade
the clock also clocks the DAC, hence the quality (jitfer) affects the conversion
Later 3.24 DACs use 24.576 MHz, if I recall correctly
Replacing the DIP8 is half of what you can achieve: Since the 3.24 onboard clock supply is far from clean, one should pay attention there too......
succes
Guido
jplch said:
the clock also clocks the DAC, hence the quality (jitfer) affects the conversion
Later 3.24 DACs use 24.576 MHz, if I recall correctly
Replacing the DIP8 is half of what you can achieve: Since the 3.24 onboard clock supply is far from clean, one should pay attention there too......
succes
Guido
Hi George,
I went and bought a bunch of LME49713s without reading enough on this thread. The LME49713 is a current feedback opamp with an inverting input impedance of 58 ohms. However, its input bias current is 1.8uA, which is far greater than the AD825, AD844 and NE5532. Will this cause too much DC offset to be trimmed out with the 5K trimmers? (er, input bias current is the stat that's relevant in this case, isn't it? newb here.)
thanks,
-j
I went and bought a bunch of LME49713s without reading enough on this thread. The LME49713 is a current feedback opamp with an inverting input impedance of 58 ohms. However, its input bias current is 1.8uA, which is far greater than the AD825, AD844 and NE5532. Will this cause too much DC offset to be trimmed out with the 5K trimmers? (er, input bias current is the stat that's relevant in this case, isn't it? newb here.)
thanks,
-j
Very nice opamp for the I/V, but I think you will need to capacitor couple their outputs with a nice quality cap because I think it will have large amounts of dc offset. And the coupling cap will need some sort of resistive reference to ground after it, the one with the 2.2k resistor that gets changed for the 5k trimpot will be ok 20uf Blackgate on that one will give -3db at 7hz with the 2.2k to ground, but the other opamp has no ground reference this needs to be addressed. All too hard just get the AD825's they are cheap these days.
Cheers George
the type number of IC10 isn't DS266L532CM. It is DS26LS32CM from National Semiconductor (Quad Differential Line Receiver). Datasheet you can download about
http://www.national.com/mpf/DS/DS26LS32C.html
I use for replace the Motorola version AM26LS32
http://www.alldatasheet.com/datasheet-pdf/pdf/2854/MOTOROLA/AM26LS32.html
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