DCAudioDIY.com http://www.dcaudiodiy.com/phpBB3/ |
|
AK4490 DAC Upgrade http://www.dcaudiodiy.com/phpBB3/viewtopic.php?f=4&t=1873 |
Page 1 of 2 |
Author: | Stuart Polansky [ December 24th, 2020, 10:37 am ] | |||
Post subject: | Re: AK4490 DAC Upgrade | |||
New components
|
Author: | tomp [ December 24th, 2020, 1:31 pm ] |
Post subject: | Re: AK4490 DAC Upgrade |
Not surprised there was no sonic difference when replacing the Op Amps. Both are way better than what is needed for audio work. |
Author: | David McGown [ December 24th, 2020, 2:49 pm ] |
Post subject: | Re: AK4490 DAC Upgrade |
Tom, Stuart, In my personal experience, the LME49720 is just OK, there are better sounding op-amps. The TO-99 metal can version of the same op-amp is much better sounding, I replaced the 8pin DIP with a metal can version on a Browndog adapter in a Topping D50S in my music studio, and it had a more relaxed, tonally balanced sound with better detail and more depth of image. I suspect the shielding of the metal can helps significantly. I have also had good luck with the Muses 8820 (bipolar) and Muses 8920 (J-fet input), both of which are now available pretty cheap at Mouser. I used these to replace LM49720 in a Marchand electronic crossover. The well healed can try the Muse02 and Muse03, but they are pretty expensive in comparison, one can get discrete opamps like the Burson or Sparkos for not much more. It does get crazy, like chasing after 6SN7s. I think the AK4490 is a voltage output DAC, so the op-amp is a unity gain differential to single-ended converter, with an LP filter network. Should be able to do that in tubes pretty easily. David |
Author: | SoundMods [ December 24th, 2020, 2:59 pm ] | ||
Post subject: | Re: AK4490 DAC Upgrade | ||
Just a note to say that operational amplifiers can be designed and built with vacuum tubes. Yes -- tubes take up a lot of space -- but OH the sound! Here is but one example:
|
Author: | David McGown [ December 24th, 2020, 4:00 pm ] |
Post subject: | Re: AK4490 DAC Upgrade |
SoundMods wrote: Just a note to say that operational amplifiers can be designed and built with vacuum tubes. Yes -- tubes take up a lot of space -- but OH the sound! Here is but one example: Something is not right about that circuit as drawn. Perhaps I do not understand it, but there seems to be missing plate resistors. Also, as drawn, the grids are tied together, probably a graphics error. Broskie has a Unbalancer circuit. Note that both plates of the input section are loaded. The foll http://www.tubecad.com/2011/03/blog0203.htm User's guide: http://glass-ware.com/User_Guides/Unbal ... %20-PS.pdf This might do what is needed, though it is not unity gain. David David |
Author: | SoundMods [ December 24th, 2020, 7:04 pm ] |
Post subject: | Re: AK4490 DAC Upgrade |
David McGown wrote: SoundMods wrote: Just a note to say that operational amplifiers can be designed and built with vacuum tubes. Yes -- tubes take up a lot of space -- but OH the sound! Here is but one example: Something is not right about that circuit as drawn. Perhaps I do not understand it, but there seems to be missing plate resistors. Also, as drawn, the grids are tied together, probably a graphics error. David You'll notice that it is an op-amp with complimentary +/- 300-vdc power supplies. Different rules. |
Author: | David McGown [ December 24th, 2020, 7:44 pm ] |
Post subject: | Re: AK4490 DAC Upgrade |
Walt, Still, unless the inputs are cap coupled to the preceding stage, the +/- grid inputs need to be at 0VDC. The cathode will be at a slightly positive voltage, with the 220K resistor to the -300V supply (resulting in 1.33mA current draw for both sections). So, in the first section, the plate for the - input is tied directly to +300V plate supply, and the + input has a 220K plate resistor. Lets assume that the grids tied together is an error, since the cathodes ARE tied together. So how does the circuit work from a DC perspective and keep the inputs at 0V? My first thought was that the schematic is in error. The circuit would be in DC balance IF there is a similar 220K plate resistor for the (-) input side, which means that both sections would have 150V drop through the plate resistors, 150V across the tube, the input grids at 0V. Both sections would have the same operating point (given matched sections). However, I did check with other schematics online, and am chastised that the schematic you posted IS correct for the Philbrick K2-W. I am stumped on what is going on. David |
Author: | FerdinandII [ December 24th, 2020, 9:13 pm ] |
Post subject: | Re: AK4490 DAC Upgrade |
What most of us see as "Diff Amps", in this circuit are actually "Current Mirrors". Pretty common in the op amp world, and you're right, they just don't look right in a tube circuit.... David McGown wrote: Walt,
Still, unless the inputs are cap coupled to the preceding stage, the +/- grid inputs need to be at 0VDC. The cathode will be at a slightly positive voltage, with the 220K resistor to the -300V supply (resulting in 1.33mA current draw for both sections). So, in the first section, the plate for the - input is tied directly to +300V plate supply, and the + input has a 220K plate resistor. Lets assume that the grids tied together is an error, since the cathodes ARE tied together. So how does the circuit work from a DC perspective and keep the inputs at 0V? My first thought was that the schematic is in error. The circuit would be in DC balance IF there is a similar 220K plate resistor for the (-) input side, which means that both sections would have 150V drop through the plate resistors, 150V across the tube, the input grids at 0V. Both sections would have the same operating point (given matched sections). However, I did check with other schematics online, and am chastised that the schematic you posted IS correct for the Philbrick K2-W. I am stumped on what is going on. David |
Author: | SoundMods [ December 24th, 2020, 9:14 pm ] |
Post subject: | Re: AK4490 DAC Upgrade |
David McGown wrote: Walt, Still, unless the inputs are cap coupled to the preceding stage, the +/- grid inputs need to be at 0VDC. The cathode will be at a slightly positive voltage, with the 220K resistor to the -300V supply (resulting in 1.33mA current draw for both sections). So, in the first section, the plate for the - input is tied directly to +300V plate supply, and the + input has a 220K plate resistor. Lets assume that the grids tied together is an error, since the cathodes ARE tied together. So how does the circuit work from a DC perspective and keep the inputs at 0V? My first thought was that the schematic is in error. The circuit would be in DC balance IF there is a similar 220K plate resistor for the (-) input side, which means that both sections would have 150V drop through the plate resistors, 150V across the tube, the input grids at 0V. Both sections would have the same operating point (given matched sections). However, I did check with other schematics online, and am chastised that the schematic you posted IS correct for the Philbrick K2-W. I am stumped on what is going on. David Any problem -- including electronics design -- can be dealt with at least six different ways -- chances are that all of them will work. |
Page 1 of 2 | All times are UTC - 5 hours [ DST ] |
Powered by phpBB® Forum Software © phpBB Group https://www.phpbb.com/ |