The Gainclone. Introduction: Background Info: Construction Notes. Introduction. The model Gaincard from 47 Laboratories is a 25 Watt stereo amplifier. Check for Samples: LM The LM is a high-performance audio power LM to reduce the effects of PCB trace resistance and inductance. PCB for 56W Audio Amplifier using LM HiFi Amp. For LM IC. High quality FR4 double-sided, plated-thru and tinned holes, with conformal coating.
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The model Gaincard from 47 Laboratories is a 25 Watt stereo amplifier about the size of a video cassette, powered by a large external supply. km3875
It amplifier uses pcbb power op-amp and a minimum of parts, connected with the shortest possible signal path, particularly the feedback loop. In the years since this started, pccb op-amps have become a major part of the DIY scene with dozens of different circuits and hundreds if not thousands of builders worldwide. This page dates back to the beginning when the Gaincard was literally a black box.
The non-inverting circuit shown here is, for better and for worse, the most faithful rendition of the original Gaincard possible given what was then publicly known about it. The rest is a simple matter of deduction. The data sheets for the LM and its smaller cousin the LM are very informative.
LM Gain Clone Amplifier Project – Electronics Projects Circuits
LM product folder and LM Please note that I will be using the component numbering used here throughout this page, so refer back to this diagram as needed. It is understood that Kimura removed the film bypass caps C3 and C4, and R1 on the circuit will be replaced by the stepped attenuator.
C2 is most likely also omitted. This gives the stated nine part circuit. This result is shown below:. The output signal must not share any of the inverting input ground path. I recommend a binary star ground as follows: Two ground points are defined, the base of R2 is the signal ground S and the km3875 of R3 the power ground P.
Pch all the S grounds together, and all the P grounds together, and then connect the two grounds by a single pdb. The RCA input ground can go to the base of the attenuator, rather than R2. The case can be left floating or attached to the signal ground.
The chips should be well heatsinked, but electrically isolated from the heatsink. It may look ugly at first, but its really very smart and compact. R4 is soldered directly between the leads of the IC, which is itself tucked underneath the board together with R2 and R5.
km3875 All the capacitors and R3 are on the top side. All the unused pins are cut off except for pin 11 which is kept for mechanical stability. A 50 kOhm volume potentiometer could be substituted. The power supply is very simple. There are no filter capacitors. The transformer should be rated to around VA and be either a torroid, R-core or C-core type with dual secondary windings of 16 VAC.
This will provide 25 wpc with LMs. Diodes should be rated to at least 5A. I prefer two bridge rectifiers, but one bridge or 4 individual lcb recovery diodes can be substituted.
Separate power supplies can be used for each channel if desired. I started by setting up a prototype using the LM chip with regular EI laminated transformers. The transformers hummed and so did ocb amplfier outputs. I never got the combination to work cleanly. The next build used the LM chip.
LM – Power amp kit
Both the amplifier and external power supply were cased pch Hammond series cast aluminum boxes. A VA torroid with 2x 12 V secondaries and electrostatic screen from Plitron supplied the power. Dual bridge rectifiers as shown above. C1 should be included to protect your speakers but can be omitted if you know your source components are not DC coupled at the output.
After comparing a 4. Note that C1 must be non-polar. I followed the Gaincard pretty pfb in using the mute switches, the attenuator, and choosing to have the same gain and input impedance. I also have pretty much the same short length of signal path and feedback loop, the latter obtained by soldering R4 directly across the chip leads. I used carbon film resistors for R2-R4, metal film for R1, and metal oxide for R5.
I conclude from my experiences that carbon film is the right choice for the R2-R4, AB carbon composition sounded too noisy and fuzzy, metal oxide too grainy, lm38755 from a different circuit metal film too, uh, metallic and zingy. For R1 and R5 I’m not sure.
In general, though, the capacitors affected the sound more than the resistors, the diodes more than the capacitors, and the transformer most of all. For a given capacitance, a lower voltage power supply gives a softer, more relaxed sound. Generally, the lower the impedance of your speakers, the lower the voltage should be.
I tried replacing my two 50A bridge rectifiers with 4 discrete diodes. I tried both Shottky and ultrafast types, rated 3 or 5 amps. I preferred the two big bridges, which sounded more open and powerful, but clearly there is room to experiment.
Soft recovery types are probably the best bet. This amplifier is compact, looks great, sounds great, has a few dozen clean watts per channel, and dissipates only 2. And its its an easy and safe project to build. In short, no excuses.
Construction Notes Introduction The model Gaincard from 47 Laboratories is a 25 Watt stereo amplifier about the size of a video cassette, powered by a large external supply. Top Background Information Some things we know for sure: From the 47 Labs website: Rigid and compact aluminum chassis construction to release vibrations smoothly. Separate position attenuators for each channel. Can be up-graded into a pair of complete mono amplifiers by adding another Model Power Humpty.
Attenuator is ladder type. From stereophile Z in data. Other details remain off the official record to this day: Power cord is captive four pin molex, one per channel.
Mute switch is a mechanical switch of the output. Carbon resistors are used throughout. LM chip is used. Four FE5D diodes are used for rectification. Typical operation, from the LM datasheet: This result is shown below: The actual layout of the circuit board could be something like this: The following is a suggested layout-schematic for the power supply: If you do it right the amplifier is completely quiet but grounding can be a problem.