FDA-1 PCM1794A DAC new FFT measurement for full balance mode

December 6th, 2010

After some time, it is time to disclose the FFT measurement of this wonderful DAC by AP SYS-2522.  To achieve this good measurement results, the following changes are made:

  1. L5 is removed for both pcb
  2. C51 change to 560uF
  3. Add two ground wires 1 and 2 (RED) as shown in connection diagram
  4. Use separate 9Vac windings for each pcb from R26-10 r-core transformer
  5. OCL mode is used

All input signal are throught SPDIF @ 0.5Vpp FS=44.1k 24bit data.

FDA-1 FB connection diagram

FFT FDA-1 FB measurement results

Update Dec 7th 2010: Add new FFT measurement below for distortion and frequency response at -1dbFS, -10dbFS, -20dbFS, -40dbFS and -60dbFS.  Output load at 600 ohm.

FDA-1 FFT 600 load FR and Dist

FDA-1 full balance DAC – 1st complete set sold!

December 3rd, 2010

This complete unit is built for a studio user in New York.  This is a full balance set with output select either by capacitor couple (silmic II BP) or OCL (output capacitor less) mode.  The front panel from left to right is : Input select, Error LED, 48k LED, 96k LED, Capacitor couple select switch, Power LED and Power switch.

6DJ8 / 6N23P tubes experience from ebay !!!

November 8th, 2010

Recently I bided a pair of 6DJ8 – Amperex from Germany (USD $5, see photo) ebay item number 290488910892.  This is a used tube and expect the performance is not as good as NOS.  So it ends up to be one tube with much higher noise level as shown in first 3 photos. Even the noise level is higher for one tube but I really cannot hear the difference between them.  Amperex tube sound good!!!  Thus our ears are really not able to tell much about tube variations!?


Another experience is a lot 10 pcs of Russian 6N23P (USD $10), ebay item number 260680492416.  All of them has much higher microphonic noise than US brand tubes and 7 of them with extreme high frequency buzzing noise when knock on the tube body.  The buzzing will extended few to 10 seconds!  See last photo.  The seller has refunded me partly due to this problem and I am happy with his compensation!!!


I use all the tubes in my Mu-Stage pre-amp!

Tektronix TDS744A – Repair LV supply

October 18th, 2010

I owned this Tek scope since 2007 but the scope was burnt fuse in 2009 April when I moved it to a new rack for usage.  Since then I tried to repair the power supply but was not successful without a schematic.  Later in 2010, I was able to find a TDS520B component service manual in which there is a LV supply schematic.  The schematic is not exactly same as my scope’s supply revision but is 85% similar.  Thus I am able to trouble shoot and located the faulty part Q5 – 2SK1018 booster mosfet.  In fact Q5 and Q6 are running in parallel and thus I try to replace Q5 with IRFP460 but it was burnt again after power up.  This inducated that some more parts are faulty!  Thus later I found out that R1 and R2 gate resistor of Q5 is at high resistance and thus I have to replace it with dale 1/2 W resistors 10 ohm.  I ordered some IRFP450 from RS component and is able to delivery in same day.  After replacement, the supply never burnt fuse and works again.  See some internal photo for the TDS744A.

FDA-1 PCM1794A DAC OCL mode & capacitor experiment!

September 8th, 2010

After many hours of break-in, the PCM1794A DAC can be operated in OCL mode (Output Capacitor Less) as show in the picture below.  First the Point A and B has to be ensured at near 0V by measurement with and without digital input signal and also power on and off for serveral times.  If everything is fine, then you can connect a wire to short the output capacitor locations below:

A wire is used to short the location C83 and C84.  This is the reference sound mode and it is natural and accurate.

*** Try OCL mode at your own risk *** in fact the risk is low after I try my set for 3 weeks, still safe!

You can also try KBG 10uF PIO capacitor which is expensive but it sound a bit more mellow than MBGO cap and the bass is also stronger.  I think a 0.01uF telfon capacitor in parallel with KBG is very good.  So it would be lot of fun to try different combinations of output capacitors to get the sound you want…

Silmic II Bi-Polar capacitor is custom made as shown in the pictures below.  The value of capacitor can be from 22u till 100uF with voltage minimum 35V.

Update: 22 Sept, 2010

After testing KBG at the output couple, now it is time to try it at I/V capacitor – 6800pF location.  I brought this cap from ebay again and it is a 1963 made part claimed to be KBG-I version.  I do not know what is the meaning of I but anyway I have put it in the set and listen.  I think I need more time to listen (may be break-in also as the cap is almost same age as me!).  Some pictures below:

Update: 24 Sep, 2010

I use my mu-stage preamp with the OCL DAC, it sound just right!  Mu-stage see below link with all PIO upgrade!


So if you do not have a tube preamp, use KBG cap at the DAC output.  If you have a tube preamp, use OCL mode to get best bass.