Salas DCG3 preamp (line & headphone)

Member
Joined 2003
Paid Member
dbis, same problem here with the 10mm fronts. This was someone else suggestion. Ideally a drillpress and a one inch cheap or used spade bit that normally would be used for wood.
Drill about half way through from the back side, perhaps a little more, then drill your hole for the selector and/or volume. You can also drill a small index hole to keep the selector switch from accidentally rotating.
You made to use a spade bit larger than one inch.

I just used an extender kit from parts connexion whose bearing was just was just long enough to protrude all the way through the 10mm face plate with enough room left for the tightening nut on the inside.

Failing that, I was going to use a 7/8ths inch bit to drill the backside a bit.

On my phone now, I have pics in my computer, will post one tomorrow.


Russellc
 
Last edited:
Hi Magicbus how are you using DCG3 in balanced mode? just connecting two preamp for + and - inputs? which balanced amp are you using?

Hi Rhythmsandy,
Yes, I used two DCG3 boards, one (double) DCSTB and two 100VA transformers. I wired it with XLR for gear compatibility. Output XLR pin 1 is connected to chassis as ought to be. But input XLR pin 1 is connected to circuit ground because it serves as signal returning path since this configuration is not a differential amp. See schematic. In my build, I left the circuit ground floating (chassis always to mains ground) because I take signal from a PC. Other sources may require different approach.

I use diy tube amps. Currently a balanced KT88 PP. Input XLR pin 1 to chassis.
 

Attachments

  • Wiring.jpg
    Wiring.jpg
    64.9 KB · Views: 515
  • Uncovered.jpg
    Uncovered.jpg
    630.2 KB · Views: 511
Reading again my previous posts I felt like making some comments. Still, I think balanced mode sounds better than SE - that is in my humble system. But I started revising my opinion that this is due to differential amps present a constant load to the psu. It seems more plausible that the difference comes from the grounding scheme that a balanced system allows. Provided that pin 1 is connected to chassis, components' circuit ground don't mix. Just my thoughts...
 
diyAudio Chief Moderator
Joined 2002
Paid Member
Hi Salas
how important is it to mach the BC327-40

no need to change resistor or so ??

are it easy to mach them ?..maybe you have a little schematic to show how

i am ready to try it,,,,because i am waiting to get my component to the pre

Best Bjarne

No need to change anything in the circuit or in orientation. Their bias is defined elsewhere and they are pin compatible transistors to the BC560C. This pre isn't picky about exact Hfe and Vbe matching, just pair per channel those that appear closer to each other between your samples. The particular type is consistent enough when from the same batch. Don't mix brands. The working bias will be between 2-2.5mA DC. There are some relatively easy methods to evaluate them: Transistor matching
 
Member
Joined 2003
Paid Member
Orientation of the LED bars question here. I have the Teabag kit and boards, I see a heavy line on one end of the trace on the board. I know I remember reading here how they are oriented, either a flattened corner or #1 pin where a small embossment on the part...I can not find this.

Can someone tell me how to orient these pieces?

Thanks,

Russellc
 
diyAudio Chief Moderator
Joined 2002
Paid Member
The beveled edge mark should suffice. A dot on the silkscreen is situated opposite to that beveled side as well. This one marks cathode in case you want to confirm the LEDs bar part electrically also. In the attached pictures a red probe is connected to the beveled edge's pin and a black probe to its opposite pin. The DMM's diode function lights up and measures the bottom LED. Goes correctly to the PCB with the the black probed pin at the dot marked pad.
 

Attachments

  • IMG_20180117_224531.jpg
    IMG_20180117_224531.jpg
    377.2 KB · Views: 420
  • IMG_20180117_230712.jpg
    IMG_20180117_230712.jpg
    519.4 KB · Views: 419
Member
Joined 2003
Paid Member
The beveled edge mark should suffice. A dot on the silkscreen is situated opposite to that beveled side as well. This one marks cathode in case you want to confirm the LEDs bar part electrically also. In the attached pictures a red probe is connected to the beveled edge's pin and a black probe to its opposite pin. The DMM's diode function lights up and measures the bottom LED. Goes correctly to the PCB with the the black probed pin at the dot marked pad.

Thanks. I see it now. The bevel is very slight!

Russellc
 
Member
Joined 2003
Paid Member
I intend to use as line stage and Preamp. Question as to selection of gain resistors and current setting resistors.

1. I will be using different power amps, want enough gain to drive M2 to full power. I will also be using a BA3 complementary power amp with much more gain. (not at same time, obviously!)

As to headphones, Audioquest Nighthawks, Impedance: 25 ohms, sensitivity: 100 dBSpl/mW.

2. I have some little sockets that solder in the holes for resistors, allowing easy changes to both gain and current. Is it possible to use them here, or are there other changes that would have to be made?

3. Is there any downside to using the high current resistors even if they are not always needed? (but may with other headphones) Does this affect the sonics of use as line stage? Assuming plenty of sink and box volume...

Thanks for any help here,

Russellc
 
Last edited:
diyAudio Chief Moderator
Joined 2002
Paid Member
I intend to use as line stage and Preamp. Question as to selection of gain resistors and current setting resistors.

1. I will be using different power amps, want enough gain to drive M2 to full power. I will also be using a BA3 complementary power amp with much more gain. (not at same time, obviously!)

As to headphones, Audioquest Nighthawks, Impedance: 25 ohms, sensitivity: 100 dBSpl/mW.

2. I have some little sockets that solder in the holes for resistors, allowing easy changes to both gain and current. Is it possible to use them here, or are there other changes that would have to be made?

3. Is there any downside to using the high current resistors even if they are not always needed? (but may with other headphones) Does this affect the sonics of use as line stage?

Thanks for any help here,

Russellc

Those three uses will probably end up to different enough optimum gain settings. From driving some insensitive amp to driving 110dB at 1V RMS Nighthawks* there will surely be changes.

You would only need to socket R6 (gain setting resistor) which is not a negative thing if the little sockets are both clean and hold the resistor's pins mechanically very well.

There is no downside regarding line level use when the bias is set at the higher circa 150mA level for low impedance headphones. To the contrary one or two members maintained that it was subjectively little better overall although I could not support that by THD and IMD SMPTE measurements alone on loads > 65 Ohm.
 
I intend to use as line stage and Preamp. Question as to selection of gain resistors and current setting resistors.

1. I will be using different power amps, want enough gain to drive M2 to full power. I will also be using a BA3 complementary power amp with much more gain. (not at same time, obviously!)

As to headphones, Audioquest Nighthawks, Impedance: 25 ohms, sensitivity: 100 dBSpl/mW.

2. I have some little sockets that solder in the holes for resistors, allowing easy changes to both gain and current. Is it possible to use them here, or are there other changes that would have to be made?

3. Is there any downside to using the high current resistors even if they are not always needed? (but may with other headphones) Does this affect the sonics of use as line stage? Assuming plenty of sink and box volume...

Thanks for any help here,

Russellc

Personally, I would choose 3x especially if you get another set of headphones. 3x is just right with f5 turbo (20db gain in amp) a little to much gain for grados. 3x is perfect for akg 7xx. Headphones vary a lot. Those audioquest headphones are meant to be driven from a phone...so not much gain needed.

Mosfets theoretically sound best with more current so I’d max it out if you have the heatsink and your power supply will accommodate it. And with 25ohm headphones you want as much current as you can get.