GAINCLONE Amplifier build

It all started in 1999.

47 Laboratory (Junji Kimura) introduced the 4706 Gaincard amplifier, which produced 25 Watts per channel from a single IC chip amplifier per channel.

The special features of this amplifier design were:

  • A ‘high-end’ dual-mono amplifier designed around a single power IC (per channel)
  • The world’s smallest number of parts in an amplifier (9 per channel)
  • The world’s shortest signal path length (32mm including parts lengths)
  • The world’s shortest negative feedback loop (9mm)
  • Smallest possible Power Supply filter capacitance (1,000uF) combined with a high capacity power transformer

STEREOPHILE magazine in 2004 said:

” …a sense of transparency: a crystalline clarity that seemed to extend from the lowest bass to the highest treble. The resolution of fine detail was quite extraordinary… The sound had a directness, a feeling that music was being reproduced with a minimum of artifacts getting in the way. “

The 47 Laboratory Gaincard amplifier subsequently inspired and spawned numerous ‘Gainclones’.

One of the most well-known is by AUDIOSECTOR (Peter Daniel), who offers PCB kits.

(Note – the above is a ‘Classic’ (basic) kit – which was upgraded later with the ‘Premium’ Kiwame and Caddock resistors.)

Jeremy Young’s project here is based on an AUDIOSECTOR ‘Premium’ LM3875 amplifier kit.

This kit consists of 2 x amplifier PCBs with 2 x LM3875 power amplifier chips (Texas Instruments), resistor components, PSU smoothing capacitors, 1 x PSU PCB with rectifier diodes.

The Premium LM3875 kit price is US$98 and is upgraded with gold-plated PCBs, Kiwame and metal film resistors.

The Classic (basic) LM3875 kit price is US$68. Both kits are supplied with Panasonic FC PSU caps and MUR ‘ultrafast recovery’ rectifier diodes.

(PSU rectifier board, +/- supply x 2 channels.)
(Amplifier board, 1 channel, with PSU caps located very close to the LM3875 chip.)

Power output is around 50 Watts per channel (depending on supply voltage), into 4 – 8 Ohms.

To the above kit, you need to add:

  • Power transformer
  • Chassis
  • Volume control 25k – 50k Ohms
  • Input and output socket hardware
  • Mains power input connection parts
(300VA / 2 x 0-22v / 6.82A toroidal power transformer from Tortech in Sydney, Australia.)
(Testing the transformer.)

Jeremy elected to build into a Walnut wood case again.

(Simpler mitred glued joints this time – no dove-tailing.)
(Front and rear panels.)
(Aluminium brackets and plates to support internal components.)
(Power transformer and rectifier board installed adjacent)
(LM3875 amplifier boards, 2 channels assembled with a solid copper Ground bus between.)
(Solid-core 0.7mm diameter copper hook-up wiring, keeping signal paths short.) (Spot the deliberate Earthing error!)

Note – refer to Dr Malcolm Hawksford for ideal conductor wire size for AC signals.

(Volume pot & Input Selector switch are mounted inboard with shaft extensions.)
(The volume potentiometer used is a TKD 2CP601 50k)
(Getting ready for the first power-up test. And crucially checking for low DC-offset at the direct-coupled outputs..)

The heatsink design for this LM3875 amplifier required quite a few hours of CNC machining:

(Initial test-run, driving Spendor SP1 loudspeakers.)
(Knobs, escutcheon-rings, top plate and heatsink – all bead-blasted ready for anodising.)
(Top plate anodised with CNC-engraved lettering.)

CNC-engraving the front panel escutcheon rings:


The Sound:

Right from the first piece of music – wow!

This LM3875 ‘Gainclone’ sounds REAL good – hooked up to Spendor SP1 loudspeakers (87dB/W). So transparent, so clear, clean, effortless, ‘direct’ and ‘natural’.

Wide subjective bandwidth with ‘oodles’ of harmonic richness and ‘decay’.

Bass is delivered with deep extension (when it exists in the recording) and power.

Stereo ‘imaging’ is ‘out-of-the-speakers’ and ‘open’. And with absolutely zero hours of ‘running-in’.

Upper frequencies – jury is still out until we can hook this up into the big system with a variety of source material. So far, very elegant, uniform tone, no complaints.

There’s a lot to be said for simplicity and minimal signal path.

For US$98, I’m shaking my head.

I’m looking forward to hooking up this Gainclone LM3875 amplifier to my big Compound Mid-Bass Horn loudspeaker system.

I feel that this modest little amplifier will create a sound quite different (but equally interesting) compared to my Output transformer-coupled 6SN7 linestage with 2A3 triode push-pull power amplifiers.


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