Archive for the ‘Pass Amplifiers’ Category

F5T Pre-amp + FDA-2A AD1955 DAC in one box

Sunday, May 11th, 2014

This is a pre-amp plus a FDA-2A in one box.  It was built in year 2012.  I use a old Yamaha DA8X box to house eveythings insides.  See the pdf files at bottom for details.


FDA-2A with USB, Optical and AES digital input.  Push button for digital input selection at the front panel.

DAC direct output is available in RCA sockets.

F5T Pre-amp:

Alps volume control from DAC output and note that there is no other analogy input for the preamp.

The pre-amp is based on Nelson Pass F5T power amp design and striped down to only one pair mosfet and lower bias current at about 200mA.  Supplier voltage is at about +/-23V with a small transformer.  Power mosfet is using Toshiba J200 / K1529 pair and casing is acted as the heatsink of power mosfet.

A headphone jack is connected in parallel with the pre-amp RCA sockets.  It can drive most headphones due to sufficient high bias of the power mosfets.


Details refer to this pdf file: F5T-Pre_DAC



F5T Turbo Power Amplifier kit for sales

Friday, November 23rd, 2012

Those who had brought my F4 kits few years ago can purchase this kit or boards for a quick conversion from F4 into F5T because all the holes and mosfet locations will be same as the F4 pcb.  The optional MUR3020W can be obmitted if not use.   If F4 buyer has the original IRF9240/240 power mosfet, it can be re-used.

The To-92D transistor are Toshiba 2SC2705Y/2SA1145Y rated at 800mW and thus it is very safe to use at 60 deg C environment for less than 200mW dissipation.  It would survive safely under a maximum supply voltage of +/-32V.  The Panasonic capacitor is rated at 35V and thus a supply voltage of no more than +/-32V is recommended for this kit.

I build a version using Toshiba Mosfet 2SK1530/2SJ201.  The supply voltage is +/-22V and thus I do not use the cascade transistors (2SC2705Y and 2SA1145Y).  I also change 3 resistors and details refer to the “F5T-BOM-cost” document.  The change on R11/R12 is to increase the thermal stability and R27/R28 is for increasing the supply voltage to jfets to be minimum 10V or above for a supply voltage of 22V.  49.9R to replace 47R do to stock issue.

Sound wise, it is more powerful than the F5 original due to higher current output and more pairs of mosfet is used.  Overall the sound is smoother and warmer than F5. Nelson said this amp sound more relax!

Please look at the F5T-BOM-cost documents for the detail kits offer – Price & BOM options.

Details: F5T-BOM-Cost_2017 (updated transistors prices)

Manual: F5 Turbo Assembly Instruction v3

Power supply ripple comparison CLC vs CRC: It can be seen that an inductor of 0.47mH air-coil has improved the ripple peak to peak level to 20 times smaller.  See attached: F5T ripple

After building another set with Cascade transistors, the noise level with grounded input is about half of that without cascade transistors.  The reason is that the filtering of the transistors at the base is much better and thus the jfets is supplied with a more stable supply with lower noise level.  Here is the measurement comparison.

Notes for F4 up-graders:  For the MUR3020W, I have no pre-tap holes for them and thus I use a L-Bar to fix the 3 power mosfet and MUR3020W as shown.

I have built the third set with 32V supply as suggested by Nelson Pass.  Here is the measurement of distortion vs Power Output at 8.2 ohm load.  With higher supply voltage, the distortion is lower at 0.004% 1W.  The non-cliped output is about 50W.  For this measurement, the bias is about 1.6A to 1.7A  for each channel.  The conclusion is that the distortion is lower at 32V and higher at 22V supply.  With no surprise, this set sound the best after adding the Solen 1mH coil in  the power supply (CLC) filter.

F5 – Amplifier

Thursday, October 8th, 2009

The is my new project in Oct 2009.  Making use of my original F4 casing and power supply, I can complete the two sets in about a week.   One set is using IRFP240/9240 mosfets and another set is using Toshiba 2SK1529/2SJ200 mosfets.  I only adjust the bias to about 1.2A which give about 20W at 8 ohm load.  Also I do not get the super low distortion that Nelson Pass got in his measurement. According to Nelson Pass, the distortion can be trimmed using a source resistor at the input jfets but I am too lazy to do that as I am very happy with the sound!

The sonic signature is very much similar to F4 but with a bit better control on bass.  I do not hear much difference between IRF and Toshiba F5 but the distortion of Toshiba set is almost double of that of the IRF set.

Here are some pictures of the IRF F5 and some measurements that I have.

F5 FFT IRF at 1W 8 ohm

F5 FFT IRF at 20W 8 ohm

F5 IRF distortion vs input level

X2 – Ver 3

Wednesday, February 13th, 2008

This is not a real Nelson Pass X2 as the output use transistors.  This is actually the ver 3 of Cheffdegaar in diyaudio post.

I do not like the sound and may be in future I shall build NP original X2 again.

UGS_V3a circuit

F4 – Amplifier

Wednesday, August 8th, 2007

This is my F4 amplifier completed in August 2007:

  1. I designed the PCB and Casing myself and get China factory to custom make for me.  The casing size is 350mm (W) x 420mm (D) x 200mm (H) with front plate 10mm larger than the casing.
  2. R3 and R4 still 22 ohm, and Q1 and Q2 are using 2SK170BL and 2SJ74BL for matched Idss to within 9.8 to 10mA.
  3. C1 to C4 use 330uF 35V Panasonic FC, gate resistors R10 to R15 are 100 ohm, Source resistors R16 to R21 are 0.51R 3W.
  4. IRFP240 and IRFP9240 are matched Vgs within 5mV which give variation of bias current within 5% (measured voltage at 0.51R 5% resistor) among all the mosfets.
  5. Use two 18V x 2 250VA transformers and 4 birdge rectifiers to parallel the secondary voltages to about +/-21.8Vdc with 1.45A bias for each channel.
  6. Filtering capacitors are Nichicon 68,000uF 63V x 2 + 0.47R//4 pcs + Mallory 33,000uF 35V x 4.
  7. Heat sink size is 420mm x 200mm x 50mm for each channel (side).  Temperature rise is about 17 deg C (i.e. 47 degC for room temp of 30 degC).
  8. The DC offset is stable to be within 10mV after warm up for 1 hour.
  9. Measured distortion is 0.02% at 1W output for both Channels.  Noise is mainly at low frequency band from 25Hz to 1kHz.
  10. The F4 amplifier has been playing for 4-5 hours a day in the pass week and the sound is already stable.  The sonic is skid to soft type bass (due to limited power) and clean and clear mid and high frequency.
  11. Overall the sound is very good and satisfying.

Some photos below:

FFT measurement at 1W and 20W:

F4 FFT2 1W 8ohm

F4 FFT 20W 8ohm