FET Audio | Hi-End Audio & Electronics DIY Electronics & Repair

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

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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.

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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.

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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:

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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!

http://www.fetaudio.com/archives/388

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.

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The DAC is base on the circuit of the hardwire version with some additional features.

1. Digital Input: I2S (via pin header), SPDIF – RCA, AES and Toslink (optical); selected by two pins 00/01/10/11 for all 4 inputs.  Default input is RCA – SPDIF without selector sub-pcb.
2. Output: RCA Left and Right (about 1.5Vrms); mute by relay shorting output signal to ground
3. Display: 3 LED – Mute, 48k, 96k; Fs display is for 8416 only!
4. For Analogy Balance output, two pcb are required and one pcb jumpers will be set to Mono L and the other set to Mono R channel.  I2S flat cable is reqired to connect the source pcb to the slave pcb.  To be tested!
5. Power input: 18V x 2 0.5A and 9V x 1 0.5A AC

This is the first version and thus there are few minor errors on the pcb: Optical input reversed pin 1 and 3 connection, no mounting holes for optical receiver, no mounting holes for XLR female socket, and non-optimized ground path.  Thus some cut trace and jumper wires are reqired on the bottom of the pcb.  But anyway the FFT measurement is quite satisfactory after error corrections.   Here are the photos and FFT measurements.

1794 DAC V1 FFT

From the FFT, you can see the noise floor is about -135dBV (-138dbr).  It is about 5dB better than the hardwire version.

1794 DAC V1 FreqResponse & Distortion

Distortion and frequency is about same for both hardwire and Pcb version 1.

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Update: June 30, 2010

One pcb is install in the casing and use a better transformer.  The FFT is ploted again and 50Hz hum is not seen anymore.

D-A FFT DAC 1794 V1 log (Case set1)

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This is the PCM1794A DAC I am working on with 24bit/192k digital input format.  Thus it is good for computer, media player or CD/DVD player as digital source.

The basic structure is CS8416, PCM1794A, Discrete transistor IV/buffer, super regulator for IV and 5V/3.3V regulator ICs for digital circuit.  This is a hard wired version and measurement is done with very good results.

1. Distortion: Around 0.02% at 1kHz, 30k BW, 1.45Vrms output level

2. Frequency Response: -0.07dB at 20kHz, 20Hz to 20kHz BW

The transistor IV is a common base with class A buffer and no feedback design.  The output is OCL (output capacitor less) and this ensure very smooth sound and good dynamics of music.

Measurement Results:

D-A FFT DAC 1794 set 2 lin lite

D-A FFT DAC 1794 set 2 FR and Dist lite

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