Chill-amp Updated on: Februari 5th 2006 For almost two years, I have been experimenting with 'gainclone' amplifiers. Gainclone amplifiers use opamps instead of the tubes and transistors that can be found in most amplifiers. This project is superseded by the Chill-amp II project!   Opamps ??? I thought they sounded bad ??? 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 (called 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. With this circuit, I made my first amplifier, the Thor-amp. That amplifier sounded quite sweet and for a long time I was quite happy with the result! After a while, I found some limitations. I tried the non-inverted opamp circuit, and found it to be superior to the inverted circuit in some ways (especially near the maximum volume :-). After a bit of tweaking of the non-inverted circuit, the Chill-amp was born... Chill-amp concepts Usage of opamps. The high quality power opamp LM3875 from National Semiconductor is used in the circuit. An advantage of opamps is that the design of the amplifier can be extremely compact. The internal transistors are well matched, 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. One disadvantage is the larger number of used transistors and factors like the cross-talk between the transistors. Compact layout. The layout of the Chill-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 some components (for ensuring the stability) can be omitted. The supply. The supply of the Chill-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 comprehend the theory behind the low capacitance at this moment. 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 has 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 amplifier sounds fine with only 1000 uF per channel... The PCB's The chill-amp makes use of PCB's. This done for multiple reasons. The first reason is that it makes the amplifier much easier and faster to build. It is easier to make good solder connections, especially for the ground wires! The components are mounted more rigid, thus vibrations will have less influence on the sound. Also, the two channels are more identical. Finally, the robustness is somewhat improved... (and it even looks better IMHO!)   The schematics The circuit is very simple and consists of one power opamp per channel with a handful of resistors and capacitors. The amp is built dual mono. The amplifier circuit The supply   PCB's Two PCB's are available for the Chill-amp: The supply PCB The amplifier PCB The amplifier PCB's cost only EUR 10,00 for a pair, the supply PCB's EUR 12,00. Four PCB's cost EUR 20,- International shipping is 5 euro. Mail me for more information!!!   Speakers 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. This amplifier is not so suitable for speakers with an impedance below 4 ohm. The Chill-amp delivers about 40-50W. I use the Chill-Amp with Tannoy SGM-12X speakers (93 dB/W). This pair of equipment is usually more than LOUD enough for me!!! :-P   The sound The sound of the Chill-amp is in my humble opinion quite special. It is fast, clear and smooth. It can even be a bit warm, with the right music and source. You could describe the sound as 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. All in all, the amp makes music more interesting for me... :-P   Photos Regrettably, at this moment no photos are available of the Chill-amp! It looks similar to the Thor-amp, as it is built in the same 19" rack. I plan to build a smaller enclosure for the Chill-amp. I will make some photos when it is finished!   Datasheets LM3875 power opamp MUR860 ultra-fast diode   Build your own Chill-amp!!! Finally, the DIY Chill-amp page is finished!   Links 47 Labs, the company that makes very inspiring audio designs! Amp Chip DIY forum, forum about Gainclones and non-oversampling DACs Audio-cube, page about 47 Labs and other high-end products Nick Whetstone's gainclone page Craig Frasers gainclone page Maartens gainclone page Richard Murdeys gainclone page Show the DIY Chill-amp page please!!! Show the other audio projects!