Salas DCG3 preamp (line & headphone)

Jump to Latest
#1,821
Salas said:
That's explaining the previous confusion then. And the rails look very steady with the 90mA dummy load test. You could use even more Ohm in R1 R2 for the higher rails section. Especially R2 for taming that enthusiastic -17.8. As long as the voltages are kept below 18V they can not harm the servo chip but nearer to 17.5V max is more conservative for its long term reliability. What are the drops across those 50 Ohm R1 R2 now?

Your mains voltage can go high enough as you report. On the other hand if the heat on the sinks when you test loaded with 200 Ohm wasn't too much on those mains max, no need to worry further.

Most basic check you can do on your shorted once to chassis DCG3 board is to probe with the DMM on continuity buzzer mode between + / 0 and 0 / - and not find a short. On both sides rails input connectors. Also no MOSFET tab must be buzzing continuity to the chassis. No insulation problems in other words.

You can not expect to always be so lucky especially when restoring a system from a hard fault and best is to power up with a form of mains current limiting protection during building and testing. The good old dim bulb tester can save you from lots of trouble. Very easy to make. See about it in this video: https://m.youtube.com/watch?v=wRFRwOnLsZI
Click to expand...

Thanks.

V drops across R1 and R2 at 50 ohms were .25 V and .24 V respectively. I bumped the ohms up to 100 (R1) and 133 (R2), which gave voltage drops of .37 V and .41 V. Output voltages are +17.05 and -17.08 (the other side is +17.08 and -17.05) at 121 ac V mains. I load tested the rails and voltages held firm, no issues.

Feeling somewhat confident about the PS, I hooked up the preamp board (using a dim bulb) and hit the switch. The 3 LEDs lit after 3 seconds and all mosfets warmed up for the 20 seconds I powered them. No light from the dim bulb.

I haven't attached the mosfets to the chassis...still scrounging around for 12 insulating washers. When I get the mosfets mounted I'll let you know. I'm using 8.2 ohm setting resistors.

Keith
 
#1,823
Salas said:
I listened for long enough to a friend's old white K701 in the tests phase. It sounded super good on DCG3. Also a happy surprise to him. As he always had the typical complaints you mentioned. "I bought them on reviews but never liked them, now I finally like them" he said. 😀
Click to expand...

Yes, I prefer speakers to headphones (I primarily use the DCG3 as a preamp) but my wife and kids prefer headphones. Or, I should say they prefer I use headphones and not speakers.

This amp seems to have some synergy with K701...I have a friend with lots of headphones...Audeze, Hifiman, all top end stuff. He was amazed at this amp with the AKGs. Seems a lot of people here like AKGs...they are not as popular on head-fi and other headphone forums. Perhaps that's because diyaudio.com is mostly people with speaker systems. AKGs seem more "speaker-like" than a lot of other headphones to me.
 
#1,826
Hikari1 said:
Yes, I prefer speakers to headphones (I primarily use the DCG3 as a preamp) but my wife and kids prefer headphones. Or, I should say they prefer I use headphones and not speakers.

This amp seems to have some synergy with K701...I have a friend with lots of headphones...Audeze, Hifiman, all top end stuff. He was amazed at this amp with the AKGs. Seems a lot of people here like AKGs...they are not as popular on head-fi and other headphone forums. Perhaps that's because diyaudio.com is mostly people with speaker systems. AKGs seem more "speaker-like" than a lot of other headphones to me.
Click to expand...

Maybe you have a point on AKG and their likeness to loudspeakers thing. About a month ago I helped to restore an Audio Research VT200 from a couple of output stage tubes bang that took some other parts down also. When we finished we sat to listen to it on Harbeth Super HL5s. We had some headphones around too. Grado 225e, Beyer 770-80, Sennheiser 600, AKG 612 pro. Guess which one presented almost the same tonality as the speakers at sofa distance. The K612 pro. Not the most direct one as a headphone experience vs the others but it gave the least sound changing feeling when put on after listening to the same track on the Harbeths in a living room. They could be modeling their monitor headphones series after some main studio speakers beyond mixer's vicinity near field distance.
 
#1,827
615498 said:
Thanks.

V drops across R1 and R2 at 50 ohms were .25 V and .24 V respectively. I bumped the ohms up to 100 (R1) and 133 (R2), which gave voltage drops of .37 V and .41 V. Output voltages are +17.05 and -17.08 (the other side is +17.08 and -17.05) at 121 ac V mains. I load tested the rails and voltages held firm, no issues.

Feeling somewhat confident about the PS, I hooked up the preamp board (using a dim bulb) and hit the switch. The 3 LEDs lit after 3 seconds and all mosfets warmed up for the 20 seconds I powered them. No light from the dim bulb.

I haven't attached the mosfets to the chassis...still scrounging around for 12 insulating washers. When I get the mosfets mounted I'll let you know. I'm using 8.2 ohm setting resistors.

Keith
Click to expand...

8.2 ohm resistors? (I was thinking of the buffer unit, not the DCG3).

I secured the mosfets to the chassis with the correct insulation. Unfortunately, I am getting a continuity buzz between the chassis and the D leg of an M2 and the D and S legs of an M3.

I assume I should replace these?

Keith
 
#1,828
First remove their mounting screws and let them rest on their chassis places with Silpads. While pressing them down a little buzz test again to see if there is a difference than what you got when fixed with screws. That will exclude its a hard to see mechanical short when with the screws and washers or not.

If another builder here happens to have his DCG3 in handy open box condition, can he take at those same MOSFET points the Ohmic readings to grounded chassis for our friend to have a reference? Mine is installed on the rack and interconnected. Thanks.
 
#1,829
Salas said:
First remove their mounting screws and let them rest on their chassis places with Silpads. While pressing them down a little buzz test again to see if there is a difference than what you got when fixed with screws. That will exclude its a hard to see mechanical short when with the screws and washers or not.

If another builder here happens to have his DCG3 in handy open box condition, can he take at those same MOSFET points the Ohmic readings to grounded chassis for our friend to have a reference? Mine is installed on the rack and interconnected. Thanks.
Click to expand...

Salas
Thanks for the good advice, it indeed was a hard to find insulation problem. A couple of aluminum slivers in the screw holes made bridges to ground. I powered up the pre board and all looks good. Trimmed out the DC offset to 0 volts and nothing has changed in half an hour. I'll wire in the pot and inputs tomorrow.

Again, thanks for your help and contributions to DIY.

All the best

Keith
 
#1,831
WOW!

Sorting out the builder's (that would be me) errors ( with forum help) has paid handsome rewards. If I can characterize the sound of the DCG-3 the word 'resolving' seems accurate. My system (tube phono into the DCG3 and an early transistor amp) has never sounded better. For those readers who are sitting on the fence about building this preamp I say go for it, you won't be disssapointed. Tea-Bag's current group-buy kits can be found here:

http://www.diyaudio.com/forums/group-buys/297899-gb-salas-dcg-3-dcstb-power-supplies.html

Cheers

Keith
 
#1,833
Finally finished implementing the DCSTB in double mono config. The DCG3 is now stock version with C2 being 33pF SM.

Sounds completely different from my previous version, much more relaxed and sweet. Forwardness is gone but retaining immediacy and bass control. Exquisite is the best word to define it.

Everything is in place... outstanding... than you Salas.
 
#1,837
Was it 130mA bias you set in your DCG3? DCSTB's Zo is 25 milliOhm at such a current consumption level. Remaining so, even at 250kHz (theoretically by simulation without cable elements).
 

Attachments

  • 2017-04-30_DCSTB_Zo.png
    2017-04-30_DCSTB_Zo.png
    12.3 KB · Views: 634
#1,838
Thank you Salas

It sounds really good now. I am rediscovering my LP collection (again).

Now I have two builds, one stock sounding exquisit and another one with V12r sounding extreme. maybe I could mimic the output impedance of the DCSTB by placing a series resistor between the shunts and the DCG3 ?
 
You must log in or register to reply here.
Top Bottom