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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

Salas DCG3 preamp (line & headphone)
Salas DCG3 preamp (line & headphone)
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Old 18th April 2018, 12:07 AM   #2931
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
Never heard it in the DCG3 but put 100pF between collector and base of the relay driver transistor and listen again. Just in case that intricate config of yours triggers some chatter around the relay circuit.
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Old 21st April 2018, 04:15 AM   #2932
takitaj is offline takitaj  United States
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Sorry it took so long, was waiting for parts before pulling it apart. I put in the 100pf cap and the sizzle is gone! I first tried disconnecting everything but the DCG3 and it was still there so I did the mod. It probably isn't anything catastrophic but something others may want to be aware of.

I tried a Wima MKP 0.1uf coupling cap on the Vellman output and didn't notice any loss in the low end and it still sounded great. I think I'll take your advice and just stick with those and save some $$. The Vellman had some 10R series resistors on the outputs. I removed those and put the caps in their place, should be ok right?

I dropped in the 7R5 source resistors to bump the bias up. I'm getting around 145ma. The power supply voltages dropped a little but still over 17v. I don't know if I'll ever need the higher bias but since I have plenty of heat sink I figured I might as well do it while I had it apart. Then I don't have to take it apart again in the future. It warmed up to about 40c so should be good there. I also have it set to 4x gain.

Got all the resistors changed to Dales in the Vellman and 12.7v zehners installed for the on-board BC550/560 "regulators". Now I have 11.8v for the opamps (was 8.5v). The Burson discrete opamps should be here next week. For the money, I sure hope they're at least a little better than the standard IC types!
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Old 21st April 2018, 11:08 AM   #2933
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
Nice to know that the collector to base compensation cap suggestion has worked. Not mentioned before by builders that the relay driver transistor can resonate in this but if it occurs again its a solution tip for other users to hold. Maybe some unlucky hfe and inductance tolerances combination. Also nice that you saved money on the coupling capacitor front regarding other sections of your combined build. The output resistor on the Velleman should be a damping resistor for better phase margin when driving longish interconnects so if it does not show problems without it in this one box build with your op-amp chip of choice then no worries.
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Old Yesterday, 09:50 PM   #2934
Kris0603 is offline Kris0603  Sweden
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Looking forward to this, came late to the party so will tag along in the next GB(thanks Tea!). Got a pair of quality cans coming late june, the Massdrop HD58X.

My current setup consists of a DCB1 feeding a Fetzilla, and i wanted a dedicated headphone amp, which can double as a pre in case i ever need more gain.

But i'm a sucker for short signal paths, so i'm considering either splitting the input signal from my selector so that it feeds into both the DCB1 and the DCG3, or splitting after the DCB1(i doubt i'd hear much difference), still signal path-wise i prefer option 1.

Should i increase the input termination(decoupling?, i forget the correct term) resistor at whichever inputs i end up "splitting"/doubling up on from 47k to 100k, the idea being that the source should see roughly 47k input impedance?

It is tempting to build everything into one enclosure, a real Salas box feeding both my cans and the Fetzy.

Last edited by Kris0603; Yesterday at 09:52 PM.
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Old Yesterday, 11:51 PM   #2935
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
DCB1 is 220k DCG3 is 330k so a Y split to them should see 132k when no pots are involved. Else it will be their pots paralleling result. Maybe the best sounding arrangement in your situation will be: input signal --> 47k pot --> DCB1--> split to DCG3//Fetzy
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Old Today, 06:29 AM   #2936
Kris0603 is offline Kris0603  Sweden
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Thanks Salas, will try!
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Old Today, 06:33 AM   #2937
Salas is online now Salas  Greece
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Salas DCG3 preamp (line & headphone)
Goo luck then Kris. Let us know when the time comes.
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