Salas DCG3 preamp (line & headphone)

[JOIMONF]

I placed the heatsinks below bottom plate for an easy access above. Now I will start populating the boards. But before, must I drill a another hole for star grounding? where? Or Is better a ground copper bar in between the channels ( no star)?

 

[drpro]

My compliments to Nick. My DCG3 is now several years old. I rotate preamps and amps around as I build new ones.
Time to bring the DCG3 back into action. So,
I left the DCG3 on overnight and measured the offset this morning. Zero on both channels, no change since I constructed it. Power supply within a .1v or so for both groups of +ve and -ve.
A really nice build.

 

[Salas]

I placed the heatsinks below bottom plate for an easy access above. Now I will start populating the boards. But before, must I drill a another hole for star grounding? where? Or Is better a ground copper bar in between the channels ( no star)?

For chassis reference grounding maybe just one wire from one zero point from the whole PCBs system will suffice.

 

[Salas]

My compliments to Nick. My DCG3 is now several years old. I rotate preamps and amps around as I build new ones.
Time to bring the DCG3 back into action. So,
I left the DCG3 on overnight and measured the offset this morning. Zero on both channels, no change since I constructed it. Power supply within a .1v or so for both groups of +ve and -ve.
A really nice build.

Satisfying indeed

 

[JOIMONF]

Thanks!

 

[JOIMONF]

Now populating Ubib boards, what values R1 current limiting to power pos. and neg. rails for a 150mA DCB3 (R10=7.5 ohms) ? I guess not the same, as pos. rail must be lower value than neg. one.

 

[Salas]

Are you going to also use headphones? If yes of what impedance?

 

[JOIMONF]

32,2 ohm headphone, so called planar magnetic I think.
Amps will be lower power Firstwatt (Aca and F2)

 

[Salas]

2R R1 in your positive UltraBiBs, 2.3R in your negatives. 1W will easily suffice, but if you have 2W prefer them for even cooler resistors temperature.

rails for a 150mA DCB3 (R10=7.5 ohms) ?

Correct

 

[Salas]

Current limit will come at about 300mA peak per channel. I hope its good enough for your headphones even when played very loud. I don't know their sensitivity, but If you will ever feel any compression from them let me know. Not to run the shunt PSUs very hot without good reason initially.

 

[JOIMONF]

Sensitivity is uncertain, I read on his chinese specifications sheet, my unit is 94db., but seller advertisement claim an 97db.
I will watch if compresion occurs when playing loud, thanks Salas. I guess same rule applies for amplifier proper driving.

 

[Salas]

Differs a lot if the quoted sensitivity is for 1mW or 1V RMS for your 32.2Ω cans

If for 1mW 94dB then near 120dB SPL with will be possible (~3.6VRMS ~400mWRMS)

Worst case insensitive is to be 94dBSPL at 1VRMS
Still moderately high 105dB SPL is achieved for about same RMS output.

 

[Salas]

I guess same rule applies for amplifier proper driving.

No, driving amps is voltage domain mainly. They don't ask for power like low Z headphones. You just don't want to clip them. In that case, sound becoming harsh is a good clue you are pushing them near clip or they clip occasionally. At sustained clip the distortion is gross and constant, you know it. Tweeters suffer first when amps clip and they are in danger.

 

[Oracle1]

Salas i have audiotechnica msr7 (35R), for RZJ what should i use ?

 

[Oracle1]

100db/mW

 

[Salas]

You have very sensitive headphones. They will reach 120dB with only 100mW at 1.87V. Even 100mA DCG3 bias is enough. But you can still apply 150mA for any future use if your sinking is ample. RZJ either zero Ohm or no more than 3.5 Ohm if you think they are bit dry in nature and you would want to offer them some under-damping (10% of their nominal Z).

 

[Oracle1]

Thank you, as always you give us the light!

 

[Salas]

In any case don't overdo it with loud headphones listening. No more than 15 minutes of 100dB continuous!

 

[Eslheadphone]

Finished the preamp. Tests good on the bench. Have not had time to listen to it yet. I removed one of the LEDs from the mute circuit and placed it on the front panel with a switch in series.

 

[Eslheadphone]

Added snubber to power transformer.