Home > Amplification > First One Amplifier Module

First One Amplifier Module

October 31, 2014 Leave a comment Go to comments

The First One Amp module [link] is a high performance (High Fidelity?) and yet very affordable class AB current-feedback amplifier module.  It establishes a benchmark for price/performance.

Developed by “Lazy Cat” (LC) at diyaudio, it is the big-brother commercial version of the DIY VSSA (“Very Simple Symmetric Amplifier) [link] and incorporates all the knowledge obtained from that project. Whereas the VSSA was fully open and fully diy, the First One amplifier is available as a factory built and tested module. Available for diyers as well as OEM to manufacturers, the module has been seen in a finished amplifier for a road show in Slovenia [link].

Photo of First One’s little brother: completed VSSA module (built by LC):



Since I am new to this module and was not aware of the VSSA, I’ll use this and following post to gather my knowledge for my amplifier build. The information is mostly from the diyaudio threads, but there it  is spread out all over the place and hard to find.

Photos of the First One Amplifier Module.


Use of name-brand “audio grade” components…


The current version is V1.2. There is a V1.3 that has been developed but not quite yet available for sale. For those of us with V1.2, LC has promised to send modding instructions but only to those that have completed the build of the amp.


Thermal coupling for these two transistors. The schematic is not public since this is a commercial product.


Notice the adjustment pots (TRx) and the measuring points (TPx)


Output Power transistors are Semelab “ALFET” double die MOSFET N and P-channel pair, rated at 250 W and 16 Amp continuous current. These are specially designed for audio applications [link]:

  • The N-channel device is: ALF16N16W/ALF16N20W [link]
  • The P-channel device is: ALF16P16W/ALF16P20W [link]




Module Size

100 x 50 x 40 mm (W x D x H). [link]

Supply Voltage

+/-40 V to +/-63 V

DC coupled

The amp modules are DC coupled, no capacitor in front of the input stage.


I’ll compare the specifications of the First One module (FO) [link] vs an old Hitachi amp [link] and an Adcom amp:

Parameter First One
Hitachi HMA-7500
Adcom GFA-5300
Max Power 8 Ohm 150 Watt 0.05 THD 80 Watt 0.005 THD 80 Watt 0.018 THD
Max Power 4 Ohm 230 Watt 0.05 THD 80 Watt 0.005 THD 125 Watt 0.018 THD
Bandwidth 3 Hz to 3 MHz (-3dB) 5 Hz to 100 KHz (-1dB) 3 Hz to 130KHz (-3dB)
THD 0.0034% (100 Watt) <0.005% (80 Watt) 0.02% (125 Watt, 1KHz)
IMD 0.003% <0.008% <0.07%
SNR 110 dB 118 dB >100 dB
Input Impedance 10 Kohm 47 Kohm 50 Kohm
Damping Factor >2000 (4 ohm) 100 (8 ohm, 1KHz) >350
Year Introduction 2014 1980 1995

According to published specifications, the First One amp has very impressive specifications and overall best of the bunch. The old Hitachi has still has very impressive specifications (but at a much lower max power).

Damping factor

Measurements performed in order to determine Zout and consequently the damping factor (DF). A sinusoidal signal of 100 Wrms at 20 Hz, 1 kHz and 20 kHz was passed onto a 4.08 Ohm load resistor, measured with FLUKE 289 True RMS Multimeter and here are the results. [link]

20 Hz, 100 Wrms/4.08 Ohm:

  • DF(20 Hz)=Rload/Zout=4.08 Ohm/0.00121 Ohm=3372

1 kHz, 100 Wrms/4.08 Ohm:

  • DF(1 kHz)=Rload/Zout=4,08 Ohm/0,0004 Ohm=10200

20 kHz, 100 Wrms/4,08 Ohm:

  • DF(20 kHz)=Rload/Zout=4,08 Ohm/0,00162 Ohm=2519

Very large damping factor by itself likely means that the amp itself would not be the limiting factor for controlling the oscillations in the speaker. This means that other factors (such as speaker cable impedance) would contribute more to the damping factor seen by the speaker. The speaker’s own impedance is the mayor contributor…


Seems a perfect match for the upcoming discrete R2R DAC. The amp being single-ended (and DC-coupled) can take the output signal straight out of the resistor ladder. In addition, being wide-band would further benefit from R2R conversion (as opposed to delta-sigma) because the R2R DAC does not generate high frequency noise.

  1. Matthias
    August 4, 2015 at 17:10

    Thanks for your blog, I like to read of other people’s opinions and how they realize them.
    As to the noise, I think the DAC has a output filter, and/or the Pre and Amp an input filter, so that the noise, if there is theoretical any, wouldn’t pass and be amplified.
    Look for low pass filter.


  2. Frank
    July 13, 2016 at 16:18

    Is this available?

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