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-   -   Salas DCG3 preamp (line & headphone) (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone.html)

Salas 9th August 2017 12:01 AM

Quote:

Originally Posted by Jim50hertz (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post5152441.html#post5152441)
Thanks for chiming in guys.

I connected input and output grounds at PCB and tied all RCA grounds together, one wire to chassis from PCB.

All seems to work, DC offset very low (0.2mv).

Is it normal for the bias current on each side to differ? Voltage drop across R10 is different on each side and there's a difference of about 2ma between sides.

I hooked it up and it works and is making music (no ground issues thus far), first impressions are very good!

Jim you went by the provided PCB groundings with no issues and that's fine too. Your DC offset level and bias matching between channels results are very best. Of course the M3 MOSFETS can differ a lot for IDSS and VGS in production so they are hand picked to be predictable for bias levels with the two different R10 resistor values, and to be close enough in the DCG3 channels.

How high is your bias current beyond its well matched though?

Salas 9th August 2017 12:22 AM

2 Attachment(s)
Quote:

Originally Posted by Marra (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post5152248.html#post5152248)
Of course my pictures have disappeared because of photobucket. I will try another way. That didn't work. Pointers please.

I could only find through Google's cache just a couple of your thumbnails on pink fish media. Your four normal sized ones are broken in your thread there too.
Better use the go advanced answer button that has the attachments menu so to upload pics on our server for posts. Its the only way they are to remain with us.
You can as well link from attachments here their image URL normally to other places on the web by right clicking on them and copying their location.

Salas 9th August 2017 12:42 AM

Quote:

Originally Posted by Marra (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post5152248.html#post5152248)
Damn my ancient Leak isodynamic phones have just died on the right channel:mad:

As for the vintage Leak isodynamics, what a shame! See if it can be due to a broken connection in the cable or inside them somewhere. If nothing can be done, maybe the only 60s-70s design for looks and tonality that I can think of which is still in production and really worthwhile is the Beyer DT 150. Well priced on Amazon with free UK delivery. https://www.amazon.co.uk/d/Musical-I...set/B000LDPP1K

Jim50hertz 9th August 2017 06:28 AM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post5152590.html#post5152590)
Jim you went by the provided PCB groundings with no issues and that's fine too. Your DC offset level and bias matching between channels results are very best. Of course the M3 MOSFETS can differ a lot for IDSS and VGS in production so they are hand picked to be predictable for bias levels with the two different R10 resistor values, and to be close enough in the DCG3 channels.

How high is your bias current beyond its well matched though?

I've attached input ground at one pcb ground only and output ground at one pcb point only. Should I tie the two input grounds and the two output grounds on the pcb together?

I've got 149.7ma in one side and 147.5ma on the other. A bit high?

Salas 9th August 2017 07:49 AM

Perfect practical bias result both for nominal target and matching. You shoot for the higher bias 150mA level with 7.5 Ohm R10s and that's what you got at a hair's distance. We allow 10% from target and 5% channels mismatch for DCG3's bias acceptable tolerances to have a picture. If you are not comfortable with the heat on your sinks but to can still keep some extra bias try 8.2 Ohm R10s.

See not to make loops between grounds. If they are circling together at the RCA rings also. Use the DMM's continuity to know if a ground wire's end beeps with general ground already for example. There are many wiring ways, follow what's handier to you, but the final pass test always is -no hum & buzz-.

Salas 9th August 2017 08:17 AM

Another thing now. Your AT headphones are 470 Ohm. Some high impedance models like those are voiced with high output impedance amps also in mind. There are popular headphone drive chips in the industry recommending 10-50 Ohm series output resistors so to protect themselves oscillating on layout and headphones wire capacitance for instance. Manufacturers know that many commercial headphone amps have significant Zo either based on chips or on discrete.

The DCG3 does not care, has proper near zero Ohm output. Maybe its handy now before you finish to put female pins at the RZ pads so to can swap zero Ohm and higher resistors to investigate if your headphones like some less damping factor subjectively. Of course anybody with a finished DCG3 of zero links at RZ can also try that when in doubt by attaching resistors in series at the HP PCB connectors or at the phones output jack.

Jim50hertz 9th August 2017 08:35 AM

Thanks Salas,

Looks like another successful DCG3 build in the wild, making music.

Art Pepper and the rhythm section sounding fabulous right now with vinyl setup.

It might be a while before I button it up! I may be too busy playing music.

Thanks for the tip re headphone damping. I'll do some listening once I install the headphone jack.

Regarding power up, should I attach headphones then power up or plug headphones in once powered up. Sorry for the naive question!

Salas 9th August 2017 09:13 AM

Great. Nice to know. Its primarily meant to be a decent line preamp for the whole system after all. Post some pics also at a point if you may.

Me I hot plug and unplug my headphones collection to the DCG3 regularly without second thoughts and all of them are still alive. Power up has the DC settling delay relay anyway.

Salas 9th August 2017 09:25 AM

*Only make sure to check that the offset is still low during the finishing phase everytime before hooking up things because its in the open and you are still tweaking. Not to disconnect some wire or probe something wrongly killing a part etc. and not notice.

Marra 9th August 2017 03:31 PM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post5152616.html#post5152616)
As for the vintage Leak isodynamics, what a shame! See if it can be due to a broken connection in the cable or inside them somewhere. If nothing can be done, maybe the only 60s-70s design for looks and tonality that I can think of which is still in production and really worthwhile is the Beyer DT 150. Well priced on Amazon with free UK delivery. https://www.amazon.co.uk/d/Musical-I...set/B000LDPP1K

Thank you for the link. They look a good buy. Would I need to change anything as I am currently running[no pun intended] with three times gain and the 10R resistor giving 126 & 130 ma of current?


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