After a few months of tweaking, modifying and listening to my 'gainclone' amp I decided that it was time to put some information about it on the web. Gainclone amplifiers use opamps instead of the tubes and transistors that can be found in most amplifiers.

Yes, I thought that too. My interest in trying something with opamps was born when I saw the raving reviews of the Gaincard of 47 Labs and the article "Op-amps are More Fun than Tubes". Soon, I found some pages describing various clones of the gaincard (gainclones). The most interesting was a forum dedicated to making amplifiers with opamps, the Amp Chip DIY Forum. I found an interesting circuit made by Thorsten Loesch there, an inverting gainclone. This amplifier is not really a direct clone of the gaincard amplifier of 47 Labs, but some concepts are shared between these. After enjoying listening to my freshly built amplifier based on Thorstens circuit, I decided to name my amplifier the Thor-amp. The idea of this page is to summarize some of the knowledge available on the amp chip forum and some things I have learnt by experimenting with the amp.

Usage of opamps. The high quality power opamp LM3875 is used from National Semiconductor. Opamps have the advantage that the design of the amplifier can be extremely compact. The internal transistors are matched very good, and the temperature of the transistors is about the same. Also, current mirrors are used in places where usually only resistors are used in discrete amplifiers. The disadvantage is the cross-talk between the transistors. An inverted opamp amplifier design is chosen. Thorsten chose this topology because it reduces common mode induced distortion. Anyway, it sounds better. That's enough to remember :-)

Compact layout. The layout of the Thor-amp is very compact. This has several advantages. First of all, each cable has an effect on the sound, so shorter is better. Secondly, if the power supply capacitors are close to the opamp, the impedance of the leads to the capacitors is low for high frequencies. This results in better high frequency stability. Finally, a very important concept of gainclones is that the feedback resistor is extremely close to the opamp. Because of the short feedback loop, oscillations are less likely to occur so a lot of components to ensure stability can be omitted. The Thor-amp is hardwired to have minimal distance between components.

The supply. The supply of the Thor-amp is quite special. It has two bridges per channel instead of the single one normally used. The advantage is that the current is split over more diodes. Furthermore, the return currents of the supply can be feed back to the bridges through two seperate cables. A very controversial point of gainclones is the low capacitance supply. Only two 1000 uF capacitors are used per channel. Compared to most solid state amplifiers the used capacitance is very low. I must say that I do not fully understand the theory behind the low capacitance. The idea is that the LM3875 opamp has a very high power supply rejection rate, especially below 100 Hz. This means that a lot of ripple can be tolerated on the supply without affecting the sound quality too much. The purpose of having a lower capacitance supply is that the supply gets quicker. So during complex musical passages the amplifier can have more control. One of the reasons of the quickness is the reduced impedance at higher frequencies of smaller caps. Another effect is that with too high capacitance the time that the caps are refilled is very short, creating high frequency peaks in the diodes/toroids. I must say that at present I am not fully convinced about the advantages compared to having a lot of small capacitors in a bank. But anyway, the amp sounds fine with only 1000 uF per channel...

The circuit is very simple and consists of one power opamp per channel with a handful of resistors and capacitors. The amp is built completely dual mono. Have a look at the schematic from Thorsten! Further details about building your own gainclone can be found on this page!

A very important topic regarding gainclones is the choice of speakers. Thorsten recommends speakers with 90 dB/W or higher efficiency. Also, speakers with 8 ohm input impedance (or higher) are better. Currently, I use my gainclone with 82 dB/W 8 ohm speakers. The sound is good at low volumes, but the amp loses control if played louder. At very high volumes, the amp overheats or clips. The amp is quite neutral, so neutral speakers are recommended too.

The sound of the Thor-amp is in my humble opinion quite special. It's very warm and pleasant, though also detailed and fast. Maybe it's a mix between tube amps and solid state amps. It has something of the warmth of tube amps and the control of solid state amps without the grainy and lifeless character. The bass is quite tight. Also, the stereo placing is good. I am very happy with the sound, it is a quite musical amp. I have yet to compare it to very expensive amplifiers. When compared to amplifiers in the price class of 500-1200 euro, the Thor-amp wins in many aspects and sometimes in a large amount. A downside of the amplifier is the low power capability. So to be better than other amplifiers during playing loudly, the speakers have to be of a higher efficiency.

Thanks Niels for making these photos! Regrettably, the amp looks a bit messy after a lot of tweaks. Also, at the moment the photos were taken I was trying polycarbonate capacitors at the input instead of the MKT's under them. The stepped attenuator is mounted on the back. I stil have to find a long axis to be able to control the volume from the front side. Currently, no input selector is used because I use my DAC as the only source.

My gainclone in close-up
The right channel nearby
The schottky bridges
The Thor-amp

LM3875 high-performance audio power opamp

Show me how to make a Thor-amp!

Back please!!!