| DCAudioDIY.com http://dcaudiodiy.com/phpBB3/ |
|
| Experimenting with digitizing phono http://dcaudiodiy.com/phpBB3/viewtopic.php?f=5&t=2019 |
Page 2 of 2 |
| Author: | David McGown [ June 21st, 2021, 10:43 am ] |
| Post subject: | Re: Experimenting with digitizing phono |
Roscoe, et. al., I think th Tascam is going to become a project. I found some information concerning a modification to the input stage made by Dave Garrelson on Audio Asylum. https://www.audioasylum.com/audio/vinyl/messages/116/1163057.html He is using Panasonic FM capacitors, and bypassing them with film capacitors. These 8 capacitors (47uF/35V) are coupling capacitors in the ADC input. Frankly, I am not sure if they are actually required, but there may be some issues with DC on the input or output of the differential amplifier stage. So my inclination is to use ELNA Silmic II (RFS) capacitors that actually sound great without a bypass. They are a little larger (10mm dia), but if I stagger the caps, they should fit fine. The other change is to increase the DC voltage capacitors for each opamp, from 22uF to 100uF. Althogh theses are 50V rated, the circuit does not see anything this high, so a 35V capacitor should do fine. Will probably use ELNA Silmics here as well, though very low ESR caps would help out. Actually, the caps I am removing from the signal path would do fine if save to recycle, they are a higher performance cap than installed already. I have to think about the opamps used in the circuit. I can get SMD (SOIC to DIP) adapters, and using flux and ChipQuik, remove the opamps on the board and replace with the adapters. But that may be later. David |
|
| Author: | tomp [ June 21st, 2021, 2:30 pm ] |
| Post subject: | Re: Experimenting with digitizing phono |
A suggestion if you are going to use SMD adapters with high performance Op Amps. In addition to the local bypassing for each original Op Amp at the power terminals on the main board, you should use bypass caps on the SMD adapter right at the power connections to the Op Amp. With the high frequency capabilities of the better devices, stray inductance and capacitance from the amp connections to the main board can be a problem. Tom |
|
| Author: | David McGown [ July 4th, 2021, 4:15 pm ] |
| Post subject: | Re: Experimenting with digitizing phono |
Tom, Roscoe, Over this weekend I replaced all the electrolytic caps in the A/D converter stage of the Tascam, using ELNA Silmic II 47uF/25V in the audio path and Panasonic FM 100uF/50V in the local power supply reservoirs at the opamps and A/D chip. Tricky going since the solder on the board is difficult to melt for removal via solder wick. The best method was to melt just a little solder containing lead on to the solder pad, then dip my solder wick into flux, tin the iron, and then remove the solder using the solder wick. Then work the old caps out heating alternate pads and rocking the old cap out until free. Clean up afterward with alcohol and Q-tips. Tiny pads but didn't lift one (success!). Probably replaced about 30 capacitors. Both the Silmic and Panny caps were 8mm diameter, the old caps were 5mm diameter, but got them to fit fine. At some point I will do the DAC section. Probably replaced about 30 capacitors in all. I gave it a quick listen just to verify it is working, and it does (so no harm done!). The Silmics are a bit dark at start, so need to break in for a couple hundred hours before I really sit down and listen. Putting my old phone to work playing music into it for a few days. David |
|
| Author: | Cogito [ July 4th, 2021, 7:29 pm ] |
| Post subject: | Re: Experimenting with digitizing phono |
David, Did you measure the AC noise in the Tascam Power Supply output? Tackling the noise in PS might pay off in a big way. |
|
| Author: | David McGown [ July 4th, 2021, 7:53 pm ] |
| Post subject: | Re: Experimenting with digitizing phono |
Shashi, I already upgraded the LM7812/LM7912 regulators with Belleson super regulator replacements. That should reduce PS noise and improve regulation considerably. Considering that devices such as OpAmps already have good PSRR, I believe that PS noise in the analog (or as converted to digital) signal is not measurable with the equipment I have. I can only tell indirectly by the impact on the sound quality (blacker background, improved detail, more spatial information). David |
|
| Page 2 of 2 | All times are UTC - 5 hours [ DST ] |
| Powered by phpBB® Forum Software © phpBB Group https://www.phpbb.com/ |
|