First One Amplifier Module

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):

FIRST ONE MODULE

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]

SPECIFICATIONS

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.

Measurements

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…

A PERFECT MATCH WITH R2R DAC?

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.