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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 8th September 2016 04:33 PM

Quote:

Originally Posted by Tham (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4824324.html#post4824324)
Just a bit of my listening experience with regards to imaging capability of DCG3...

I put on Amused To Death CD yesterday and the effect is akin to listening through a huge pair of headphone. A totally enveloping sound field; very good sense of depth, well separated images front and back and even above and behind the listening position, and all those (surround) sound effects contained in the recording are well spread and mostly discernible.

It is one of the most difficult test I have put the DCG3 through so far and I must say that this CD has never sounded better before.

Quote:

Originally Posted by AndrewT (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4824365.html#post4824365)
If there is such a big difference between then and now with a good intermediate stage between Source and amp, then that points me to conclude there is something lacking in the drive between your Source and your Power Amplifier.

Tham you think that you may had system gain dynamic deficiency before from CD source with buffer control preamp as Andrew suspects? You have very efficient speakers I believe.

Tea-Bag 8th September 2016 10:20 PM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4824359.html#post4824359)
Tea has Lowthers. They can be picky. I propose he tries U1 AD823, MKP2 C5, R1 R2 33R in DCSTB, shorter signal cable runs, preferably shielded coax. RG174 is handy. Or Mogami.

I can make those mods over time. Including moving R4,R5 in DCG-3 to 33R as well from 51R.

MKS2 is currently used as C5. It is what fits :)

With my system, it simply does not need a lot of gain, so not being able to open up the ALPS pot all that much may be limiting too (plus it's a fairly inexpensive pot. But I honestly have no complaints at this point.

Salas 8th September 2016 11:36 PM

1 Attachment(s)
MKP2 63V DC or 100V DC for C5 also fits. I have the 100V 5% type in my hand and I can see it does because there is some free space around the MKS2. Indeed fatter and taller but same pitch. 5.5x11.5x7.2mm. No real problem with MKS2 but no problem investigating for better too.

Had also moved Cb Cc decoupling caps underneath PCB and soldered from above to could extract and change op-amps during the many done tests without drama. TL072 TLE2082 OPA2604 OPA2134 LM4562 NE5532 NJM4580 OP275 AD823 :D

*1206 alternative surface pads for 0.1uF 50V C0G Cb Cc are already added for the future.

Tham 9th September 2016 02:27 AM

Hi Andrew, Salas

With my high efficiency speaker system, I was struggling with just enough gain with my last uni-gain preamp. Agreed that the gain provided by DCG3 is among the most important features to me in a system context.

Tham 9th September 2016 03:43 AM

DCG3 P2P Built
 
1 Attachment(s)
P2P DCG3 built ready for boxing :).

vgeorge 9th September 2016 06:40 AM

I also built a prototype, as Salas showed, with capacitor outputs for line and headphones.
I have to confess that due to work and kids, had not a lot of time for testing.
Initially tested as line amp with 3X gain and liked it a lot! Of course I could hear the coloration of the capacitor coupling (russian petp bypassed by FT_-3) but as I use long cables to drive two amplifiers, dcg3 could cope with cable capacitance with ease.
I have now change the gain to 2X (increased the feedback) and put a bigger capacitor for headphones, but I am still short of time. I also now have all the parts to built the servo and hope to find some time next month to play around.
Some new headphones are on their way too!

Salas 9th September 2016 09:53 AM

You will only need 1.78V RMS for 110dB with your AKG K712 PRO (50mW for 62 Ohm 105dB/V transducer). So 2X may serve you right for line and hp duty. Tea could also do that to can move up his Alps pot a little further. By making R6 ~500R he will have 2X gain. Still a welcome boost for when using the turntable but it could compromise his headphones loudness range on the other hand if they are not as sensitive as yours.

Then again because this preamp can drive low impedance with ease, an output voltage divider can cater for both situations. Keeping the internal gain as is but making the 50R series line output resistor (R15) to 100R and moving that original 50R across the output RCA will divide the line signal by 3 or -10dB. That restores his system gain to unity and keeps 3X for his headphones too. Will cut the line out noise level by 10dB as well. Can bring more THD though. If audible and how only he could tell.

Salas 9th September 2016 10:00 AM

Quote:

Originally Posted by Tham (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4825173.html#post4825173)
P2P DCG3 built ready for boxing :).

Ayrton-Perry hand wound R6 and R15 speak traditional DIY :up:

Tea-Bag 9th September 2016 10:30 AM

Quote:

Originally Posted by Salas (https://www.diyaudio.com/forums/analog-line-level/296406-salas-dcg3-preamp-line-headphone-post4825346.html#post4825346)
You will only need 1.78V RMS for 110dB with your AKG K712 PRO (50mW for 62 Ohm 105dB/V transducer). So 2X may serve you right for line and hp duty. Tea could also do that to can move up his Alps pot a little further. By making R6 ~500R he will have 2X gain. Still a welcome boost for when using the turntable but it could compromise his headphones loudness range on the other hand if they are not as sensitive as yours.

Then again because this preamp can drive low impedance with ease, an output voltage divider can cater for both situations. Keeping the internal gain as is but making the 50R series line output resistor (R15) to 100R and moving that original 50R across the output RCA will divide the line signal by 3 or -10dB. That restores his system gain to unity and keeps 3X for his headphones too. Will cut the line out noise level by 10dB as well. Can bring more THD though. If audible and how only he could tell.

Can do!

A few questions, for R4,R5 - I get the impression now they are part of the signal path and maybe should be decent resistors?
Also, how limiting is the Alps pot here? It's worth purchasing a Goldpoint or better attenuator, I am interested in that as well.

Salas 9th September 2016 11:19 AM

R4 R5 are gate-stoppers i.e dampers. Signal is there but no real current and no drop due to high MOSFET gate impedance. On the oscilloscope 33R gives slightly smoother corner on very high frequencies and amplitudes square wave tests than 51R, but generators, probes, and layouts can change this. Even with 10R it was stable and even smoother but I prefer bit more anti layout inductance damping to be there as good general practice. I got Dale RN60s for all signal associated resistors to use at a point but I always keep the initial test board as generic as possible in order to make sure its basically alright without fancy parts. No difficult tonal problems as for now. Stick an AD823 first which is very easy to do. Tham says he can easily prefer his DIY LDR over a Tocos. I use $1.5 carbon Alpha pot for tests :D


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