[Russellc]

I see. You don't need the tube stage then. You can't lower it's gain. Just add the I/V resistors to the board instead.

I noticed you sold your "Raindrop" DAC over on DIY, what are you listening to now?

Russellc

[wushuliu]

That was a different one. It was a TDA1541A DAC that I never got around to using. However, I have replaced the AD1865 kit w/ a used Audio-GD DAC19MK3. Love it. Would cost the same if not more to DIY something similar.

That was a different one. It was a TDA1541A DAC that I never got around to using. However, I have replaced the AD1865 kit w/ a used Audio-GD DAC19MK3. Love it. Would cost the same if not more to DIY something similar.

[lordearl]

I also just got this AD1865 DAC from Raindrop Hui.

I have a nice 6SN7 SRPP preamplifier I'd like to use, but it is in a separate box.

Is the best thing to do to use resistors to make a passive i/v stage connected to the RCA terminals in my DAC chassis, then just run normal rca interconnects to the 6SN7 preamp?

I can't tell from the photos on the forum here where to solder the resistors (from what I can tell 300R seems to be the correct value).

Is anyone able to post a close up photo of how to install the i/v resistors?

Thanks 

[jerkbag]

Hi,

Thanks for the great tutorial. Simple question: how come that hand diagram of the cap / resister filter shows the R2 resistor connected on the + side, and yet in your photo it looks very much like you have R2 going to the Neg side of the RCA? R+ in the drawing should be going to center pin of RCA, no?

Thanks!

Line Output Transformer Mod

Next up is the output transformer mod. This is also fairly easy.

1) You bypass opamps altogether.

2) Minimal contact w/ the board, reducing risk of screwing something up while (de)soldering.

2) Since you don't need the opamps you won't need to use the ~ 12v -0- 12v part of the power
supply, only the ~9v-0 part (see Transformer/PS section below). So basically you can run out to Radio Shack or digikey or wherever and use a cheap 9v transformer, buy the kit w/o transformer from ebay and save money to use towards the *line output* transformers.

Next, Getting signal from Bypass caps

This is what you're going for. Here's a great basic pic from diyaudio:




and a diagram (applies to both Left and Right):






You will need resistors and caps for the filters. You can start with ~500ohms for R1 and 1nf
for C1 and then adjust up or down to taste. I also recommend PRP or Takman resistors. They are cheap
and I think preferable to the usual dale, etc. in this application. Or better yet, compare for yourself.

Let's get started.


You want to solder in the 4 signal wires. They will go the transformers and from the transformers to your RCAs

In the Opamp mod section I showed the 4 Bypass caps by the dac chip that were tossed. Same
applies here. However, instead of jumping them we will be drawing the audio signal.

From Left to Right the signal goes: R-/R+/L+/L-

You will only need to deal with the positive (unstriped) side of each one. Like so:




I still recommend rocking the caps to break them off, thus avoiding full desoldering on the bottom side of the board. Then use tweezers to pull out the leads. Much easier and reduces iron time on the pads.

Also, the dac chip is removeable. So if you feel a little cramped during this part, gently pull up the dac chip from its sockets...

When you're done, should look like this:




That's it for the board. Phew. Now you can be butterfingers all you want with the output transformers!


Which Output Transformers?

A wide bandwidth (20hz - 20khz or higher) 500/600:500/600 is a good start. The average price for a pair is ~$100 - 150. American-made Edcors, however are a budget alternative. Not a huge amount of feedback on their quality, but it's a reputable company.

Most Popular

UTC/TRW A-20

http://cgi.ebay.com/Pair-UTC-A-20-Input-Transformer-500-500-Ohm_W0QQitemZ310206172473QQcmdZViewItemQQptZVintage_Electronics_R2?hash=item4839ba8d39



Cinemag - ~$56 ea.

CMOB - 2H

http://www.cinemag.biz/output/output.html

Altec 15356/A (on and off on ebay)




EDCOR XSM600 - ~$13 ea.

http://www.edcorusa.com/Products/ShowProduct.aspx?ID=327

These are just a few of the many to choose from. I also focused on options easily available to those
in the U.S.. Internationally there are Sowters, LLundahls, Monacors,etc. that are easier to find. You get the idea.

Now each transformer brand is different as far as their primaries (input) and secondaries (output to RCA). We want to follow the 5 or 600: 5 or 600 diagrams. In the UTC pic above for instance you can see the writing on the side:

Pins 1 and 6 = primary, signal from the board
      7 and 12 = secondary, signals to RCA
      3 and 4, 9 & 10 = Center Tap, you jump/connect these two w/ wire

So consult the transformer you have and if you have any questions just ask.

Now add the resistors and cap filter to the signal wires.

Just as in the diagram above.





then attach, solder, etc to appropriate pins on your transformer.






You can see the white output wires that are going to the RCAs. I did not take a close pic of the
1K, 1nf R-C filter but you can kind of see here. Just follow the diagram above.






There ya be. Not too tough was it?