Hi All,
I purchased a DMC-10 a few months ago (GAMMA series), and it did not come with the power transformer. Through some help on this forum, it's been determined that the original power supply was a dual secondary 24VAC transformer. Suitable replacements would be: 50VA Green square 50W Sealed toroidal transformers M50-06 AC dual 24V(2*24V) | eBay or 30VA (115V-230V) Dual 24V / 30W R-type transformers | eBay
I took the top cover off and found the blue wire pair to be connected to pins 1&2 and the red wire pair to be connected to pins 4&5. It is my understanding that one pair is for the left channel, the other pair for the right channel. There is a black wire connected to pin 3, I'm assuming for ground. I wired up my input cable accordingly. The other end of my input cable was connected to a Variac set to about 20V, and checked with a multimeter to confirm it was the correct voltage output, as to avoid damaging anything. At this point, I wanted to confirm the preamp was functional before buying additional parts, so both power pair were connected in parallel to the Variac.
This was the first time I've ever seen the unit powered up. The good news is that both front panel indicators lit up green. The bad news is that I noticed a high current draw from the Variac so I had to shut it down.
I did not notice any mechanically failed capacitors and the rectifier diodes checked OK in circuit with my multimeter set to diode check. No further testing has been done. If you have any ideas, let me know. I'm looking forward to experiencing the quality of this piece in my system.
Thanks
I purchased a DMC-10 a few months ago (GAMMA series), and it did not come with the power transformer. Through some help on this forum, it's been determined that the original power supply was a dual secondary 24VAC transformer. Suitable replacements would be: 50VA Green square 50W Sealed toroidal transformers M50-06 AC dual 24V(2*24V) | eBay or 30VA (115V-230V) Dual 24V / 30W R-type transformers | eBay
I took the top cover off and found the blue wire pair to be connected to pins 1&2 and the red wire pair to be connected to pins 4&5. It is my understanding that one pair is for the left channel, the other pair for the right channel. There is a black wire connected to pin 3, I'm assuming for ground. I wired up my input cable accordingly. The other end of my input cable was connected to a Variac set to about 20V, and checked with a multimeter to confirm it was the correct voltage output, as to avoid damaging anything. At this point, I wanted to confirm the preamp was functional before buying additional parts, so both power pair were connected in parallel to the Variac.
This was the first time I've ever seen the unit powered up. The good news is that both front panel indicators lit up green. The bad news is that I noticed a high current draw from the Variac so I had to shut it down.
I did not notice any mechanically failed capacitors and the rectifier diodes checked OK in circuit with my multimeter set to diode check. No further testing has been done. If you have any ideas, let me know. I'm looking forward to experiencing the quality of this piece in my system.
Thanks
I know nothing about this unit, although I believe the designer does post here. Spectral DMC-10 mods might give you some more info.
Attached is the essential supply for the DMC-10. The original transformer was special since at the time we could not get toroids. It also was made with multiple shields and very low flux density. Toroids should work fine. The secondaries should only connect through to the board. Any other connection will short something out or cause hum.
The link to the "mods" article was interesting. Some (most) of the ideas are not wise. The black caps are MIT Multicaps so replacing them with something better is not easy. A multiturn pot for the bias means you will fry the series resistors (10 Ohms) on the collectors of the outputs as you try to turn down the bias. Its just not that critical to set precisely. The output caps on the AC coupled outputs are for convenience, Use the DC outputs. The offset should be less than 1 mV if set correctly and the max drift over temp is still only a couple of mV. I set the offset using a sheet of plex or similar with holes over the pots and test points so the temperature stabilizes. It takes at least 15-20 minutes for the temperature to stabilize.
The output can withstand continuous shorting but the whole circuit was a band-aid to correct a problem that a previous product had and this one never had. I recommend pulling the comparator out of the board socket. It's just not needed.
I did create a very ambitious set of mods for the DMC-10 years ago. Crosby did maybe 10 of them. That had airplane boards and other complex tweaks.
The NuForce P9 is a direct descendant of the DMC-10 line stage with a lot of improvements.
The link to the "mods" article was interesting. Some (most) of the ideas are not wise. The black caps are MIT Multicaps so replacing them with something better is not easy. A multiturn pot for the bias means you will fry the series resistors (10 Ohms) on the collectors of the outputs as you try to turn down the bias. Its just not that critical to set precisely. The output caps on the AC coupled outputs are for convenience, Use the DC outputs. The offset should be less than 1 mV if set correctly and the max drift over temp is still only a couple of mV. I set the offset using a sheet of plex or similar with holes over the pots and test points so the temperature stabilizes. It takes at least 15-20 minutes for the temperature to stabilize.
The output can withstand continuous shorting but the whole circuit was a band-aid to correct a problem that a previous product had and this one never had. I recommend pulling the comparator out of the board socket. It's just not needed.
I did create a very ambitious set of mods for the DMC-10 years ago. Crosby did maybe 10 of them. That had airplane boards and other complex tweaks.
The NuForce P9 is a direct descendant of the DMC-10 line stage with a lot of improvements.
Attachments
Thank you for the additional information. I found that one of the AC input pairs was out of phase with the other in my cable. I switched one and no more high current draw. Next step will be finding a fully isolated transformer, as my regular Variac couples line noise into the circuit. I'll post further updates after obtaining the correct transformer configuration.
I was able to obtain 2 separate single secondary transformers with the correct specs. I wired them up, confirmed the voltages, and the preamp sounds good. I did notice a slight channel volume imbalance. I measured each channel with a 1kHz signal into my scope, and set the balance control accordingly. It ended up about 1/2 way between straight up and the first black dot to the right. Now it sounds great. I'm attributing this volume discrepancy to the transformers. Even though they are the same model, the voltage is slightly different (about 0.3 V) between them. But now that there is no extraneous noise, I could finally experience the outstanding sound quality of this piece. I will be ordering the actual correct transformer listed above, now that I know there are no other gremlins to discover/fix in a few months when it arrives. If there are any other thoughts or things for me to check, please let me know. Thank you for all the help!
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.