Archive for December, 2010

Hybrid Balance Mu-Stage preamp with subminiature tube

Friday, December 31st, 2010

This is a new made preamp – Hybrid Balance Mu-Stage.  Dual triode medium gain subminiature tube like 6N16B-V, 6N18B-V, 6N28B-V, 6111, & 6021 etc… can be used.  The choice of submini tube is due to its low noise and good microphonic effect. 


  1. Hybrid (Mosfet + Tube) mu-stage design with full balance at input and output
  2. Supply voltages: +128V, +59V, -69V and two 6.3V DC – all regulated supply
  3. Tube biased at about 1.5mA at about 60V plate voltage
  4. Mosfet IRF710 as Source Follower biased at about 7.5mA
  5. Gain about mu of the tube, about 20 to 40 times
  6. Non feedback design and thus tube may need to match for gain.
  7. Tube can be replacable using IC sockets, the submini tube is first soldered on 8-pins leads and then inserted into IC socket.

FFT measurment:

FFT Balance Mu Preamp

Update new photo on Jan 3rd, 2011 …..

FDA-1 PCM1794A DAC with Headphone Buffer (K170/J74 x 6 pairs)

Thursday, December 23rd, 2010

The is a V1 FDA-1 PCM1794A DAC board.  I have implemented a headphone buffer on to it and the result is super!  Details is on the photos.


Features of the buffer:

  1. K170/J74 x 6 pairs is used for each channel.  Idss of matched pairs is at about 6.5mA.
  2. DC supply at +/-20V DC and thus ok to drive high impedance headphones.
  3. Bias of each channel is about 40mA after warm up.
  4. Gain of buffer is x1.  The loudness is high enough for most headphones.
  5. A 100k(A) volume is used for gain reduction at the buffer input.
  6. Only SE DAC is used but the sound is transparent and full of details.

Mu-Stage with subminiature tubes – 6111, 6021 etc…

Monday, December 20th, 2010

For 9 pin tubes, I think Amperex 6DJ8 is very good performer in mu-stage pre-amp.  Thus many other triodes are evaluated in Tube post.  Now here is some photos of the mu-stage with subminiature tubes.  The way is to solder the small tube on the bottom of the tube socket and the two triodes in one tube is put in parallel.  Insulation shrink tubes is used to prevent short circuit of leads.


I have purchased one pair Sylvania 6021 and 4 pcs Raytheon 6111 tubes (non marking type).  The pair of 6021 give very good matching for the gain but only 2 out of 6111 tube are with good gain match.  The worst 6111 tube is about 1 db lower gain than the other 3 pcs. 


Note that the mu-stage is also upgraded with KBG and Telfon capacitors!

Tubes – 12BY7, 12GN7, 12HL7, 12AV7, 6BQ7A, 5670W, 6DJ8, ECC88, E88CC, 6N23P, 6021, 6111 …

Monday, December 20th, 2010

Last update: 9th Jan, 2011

Tube evaluation summary in mu stage


Last update: Dec 20, 2010:

Check the link for the evaluation of different tubes in the Mu-stage pre-amp.  Some are good and some are not that good due to microphonic problem.


Last update: Dec 3rd, 2010:

For the pass month in Nov 2010, I acquired lots of tubes from ebay and namely 12BY7, 12GN7, 12HL7, 12AV7, 6BQ7A, 5670W, 6205, 6021, 6111, 6SN7GTB & 5U4GTB.  Mostly NOS but some are used tubes.  The experience on used tube is not that good and thus I only acquire NOS in the later part.  I think the tube can be used for mu-stage or pre-amp used.  The mini tubes is also good for phono stage or micphone pre-amp which will have smaller microphonic problem.

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

Monday, 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