FET Audio | Hi-End Audio & Electronics

December 20, 2010

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

Filed under: Tube — Spencer @ 4:15 pm

Last update: 9th Jan, 2011

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.

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December 6, 2010

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

Filed under: DAC — Spencer @ 10:59 am

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:

L5 is removed for both pcb
C51 change to 560uF
Add two ground wires 1 and 2 (RED) as shown in connection diagram
Use separate 9Vac windings for each pcb from R26-10 r-core transformer
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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December 3, 2010

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

Filed under: DAC — Spencer @ 6:29 pm

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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November 8, 2010

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

Filed under: Tube — Spencer @ 10:13 am

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!

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October 18, 2010

Tektronix TDS744A – Repair LV supply

Filed under: Test Equipment — Spencer @ 3:26 pm

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.