Some advice on my plan to build a passive preamp

Do It Yourself
1

Roon only has native output for DSD without volume control unless it is converted to PCM.

So I want to build a preamp to connect it to my DAC’s line out XLR.

I am thinking of something like this: Goldpoint SA2X Level Controls / Passive Preamps
A passive one. Goals are simple:

  1. Dual XLR input
  2. Dual XLR output
  3. One step attenuator ~ pot

Planning to jank it on a breadboard to test the idea. I have the following components in mind:

  1. Solder Iron
  2. Solder paste
  3. Breadboard
  4. 2 XLR female panel connector
  5. 2 XLR male panel connector
  6. Step attenuator: Prices - V24-2- (value) (Stereo): $165
  7. Some Cables

Please let me know what I am missing here and which products to choose would be best.
I am thinking of $100 total is parts, and $200 in the paste and iron. The step attenuator does seem very expensive, so if there is a good pot sitting at ~$50 would also be a great recommendation!

Thank you!

1 Like
2

You need the 4 deck attenuator as you want stereo balanced (279 dollars!).
Each leg of a balanced connection needs its own pot.
I have had great success with Alps 4 and 6 deck pots, and as a bonus you can get motorised versions.
Motor control can be done without IR and microcontrollers if you are happy witha wired remote.
I have done this, it works well and the 5 volt to drive the motor can come from a USB charger :slight_smile:

3

Thank you for the advice, I have not thought of that at all! (Noob mistakes)

This is what I am looking at: Search results for: RK27 Potentiometer Potentiometers – Mouser

How should I calculate the K number and choose from slotted or flat?
Apologize for the ignorance, I really have no idea what those parameters mean.

The device I am using for line out is a Khadas tone 2 pro. https://www.khadas.com/product-page/tone2pro

Can’t find any data about output power. Would you be able to help me on this?

Thanks!

P.S. For my breadboard experiment I would not need remote control yet.

EDIT 1: Slot or flat has to do with the volume cap you want to use.
K is the resistance. Still figuring out the math.

4

Couldn’t see an output impedance spec for the Kardas, but I have had success with 25k 50k and 100k - as long as it is higher than the output impedance of the Kardas by a factor of ten or more there should be no issue.
Check google for best way to wire a balanced feed through a volume control :slight_smile:

5

Thank you, then I think the 100K should do just fine.
One of the specs for the alps pot is detent, without detent, 11 detent or 41 detent, and googling didn’t help very much. Do you happen to know what that means?

6

Detent refers to mechanical steps in the attenuator that you can feel as you turn the device. 11 and 41 refer to the number of steps / clicks through the full mechanical travel which is usually around 270 degrees.

NOTE: I see that you want a passive preamp, but think you’d wind up with much better sound with a simple analog buffer at the output of the attenuator. I realize that means a power supply, etc. and maybe you want something super simple.

1 Like
7

I just found out after a proper google session, they could have put one more line on the data sheet explaining detent. But thank you nonetheless for the answer!

I do want something simple for now to test out the idea. I will google what a buffered output is though just to see if it is worth it on my first build. Thank you for the reccomendation.

2 Likes
8

my first was passive, and in a cardboard case. worked great.
i have since added a unity gain buffer (simple op amp) so i could drive longer cables /lower impedance power amps.
an improvement, but not essential :slight_smile: