Make yourself a Chill-amp!

Updated on: February 14th 2005

On this page I will discuss some topics of DIY gaincloning that I encountered during the building and tweaking of my gainclone. A part of the information comes from the Amp Chip DIY Forum, thanks guys! Please see this information as a guide, not as the absolute truth. If you have built a Chill-Amp, I would enjoy to hear about your results!
The LM3875 opamp
Let's start with the most important component, the opamp. I chose the LM3875 opamp because it has quite a lot of power: 56W continuous average power and 100W peak when used with a power supply of +/- 35V. Two versions exist of the opamp, the LM3875T and the LM3875TF. The latter is the isolated version. The LM3875T needs a silicon or mica isolation pad, because the case is connected to the negative supply voltage. The opamp has output protection for a short to ground or one of the supplies, current limiting and is protected for output over-voltage, turn-on and turn-off transients and thermal runaway. The opamp is in a 11 lead TO-220 package. I did cut off the six unused pins (NC in the datasheet) of the opamp. I advise to cut only the NC pins ;-)
The supply
I use two Amplimo 2x25Vac 160VA toroid transformers for dual mono. I would now choose 2x18Vac or 2x22Vac, because the opamp will drive 4 ohm speakers better with a lower voltage.I plan to do some experiments with four double secondary transformers. This way, you can use only the half of the diodes! Both channels have its own 2A fuse (a 1A fuse was blown once) and internal net cable. I advise to use thick (solid core) net cable and to solder it directly to the input socket. Ensure good safety!!! Isolate the cable and it's connections very well. I use fast MUR860 diodes as rectifiers, eight per channel as in the Thorsten schematic.
The capacitors
I use two 1000 uF Panasonic FC capacitors per channel for the supply. The panasonic FC is the successor of the FA caps suggested by Thorsten and is equal in performance. You can try to use 2200 uF or 2x1000 uF in parallel, potentially this is better for subwoofer amps. The original 50W gaincard also uses 2000 uF. Some people say that the capacitance is a trade-off. Increasing the capacity will improve the bass but decrease the mid and high frequency quality. I removed the plastic covers from the capacitors, that seems to improve the sound quality. Also, you can try poster buddies (similar to blue-tac) to increase the mass of the capacitors. If the plastic covers are removed, the capacitors should not touch other metal surfaces (or capacitors)! The poster buddies can help here again to isolate.

I use anti-parallel BG-N 4.7 uF caps for filtering the DC from the input. You can also try 2-10 uF film caps (eg. polycarbonate). If no DC is present at the input you can try to omit the input capacitor, but this is less safe for your speakers! (first measure the DC at the output when you try this, without speakers connected, because you can destroy your speakers with DC on the output).
The case and heatsinks
Currently, I use a 19" case. This is very practical if you want to keep everything in one case. It is better to keep the opamps far away from the transformers and 230 Vac cables. Opamps can pick up a lot of HF and 50 Hz/100 Hz! Good shielding of the opamps is essential!

As you can see on the pictures, I use a brass heatsink. The size is 20x4x1 cm. With my high efficiency speakers, I hardly use more than one watt, so this is more than sufficient. If you really want to get 50W out of the amp, you need better heatsinks. Thorsten recommends brass or bronze heatsinks. Their resonances are better to put an amp on. Aluminium would also be acceptable but steel should be avoided. Resonances have influence on the sound of equipment, especially in the case of opamps and other integrated circuits. Opamps consist of many small transistors. When there are resonances, the internal capacitance of these transistors can change due to the movement of their n and p layers. I used to be sceptical about this issue, but I have heard changes in the sound myself by putting weights etc. on amplifiers and DACs (actually, the difference is quite shocking!). I plan to try compact and rigid DIY aluminium cases soon...
The cabling
I use CAT5 cables for connecting the supply to the opamps. The opamps are connected by four cables to the supply bridges. Two double CAT5 wires carry the supply voltages (+35Vdc and -35Vdc) and the other two cables are for returning the current of these supplies back to the two bridges. Double CAT5 is better because a single wire can easily break off, and we don't want to happen that with one of the supply sides! For the (internal) output signal cables and (external) speaker cables I use 0.4mm 47 Labs OTA cable, I like that cable very much! The input signal cables are still CAT5, but that will be changed soon to OTA cable too. Also, my interlinks are from OTA cable.
The layout
Essential for good performance is the layout. Compactness is the key element of the gaincard design. If the layout is compacter, the amplifier has less trouble with RF and oscillations. It's very important that the 22k feedback resistor is mounted as closely as possible to the opamp!!! I soldered it directly to the leads of the opamp, very near to the plastic case. Also, the supply capacitors (1000 uF) should be very close to the opamp. I now use compact PCB's for a compact layout and a high similarity of channels!
Earthing, hum and noise
The grounding layout is important in order to prevent hum and to reduce the noise of the amplifier. It is a good idea to have two star ground points per channel. One for low power signals (the ground for the input, the ground from the potmeter, the ground to the resistor and capacitor at the + input) and one for high power signals (the ground of the speaker, the two ground cables from the bridges and the 1000 uF supply capacitors). These two start ground points have to be connected. There should be no ground loops! I did not connect the grounds of the two channels, they are connected only in the DAC. If you use the PCB's, you can worry less about grounding :-)

I have connected the ground of the left channel with a paralled 220 ohm resistor and 220 nF cap to the case. This sounds better than connecting it directly. For optimal safety, it is wise to connect the IEC earth connection to the case. Regrettably, a lot of noise will enter the case then too. If you make the supply in a separate case, the noise of the net earth will not influence the circuit.
Switches and connectors
Use some nice isolated gold phono chassis sockets and speaker connectors, otherwise ground loops will occur through the case! Use a good input selector, preferably four deck to be able to switch earths too. You can put a switch on the output to turn the speakers off during powering on. The amp produces very low noise and hardly uses power in rest, so usually I just leave it on.
The resistors and the volume adjustment
The resistors can be changed to alter the character of the gainclone. I currently use Welwyn resistors. Riken Ohm or Allen Bradley resistors will likely sound warmer. For a brighter sound, other brands of metal resistors can be tried. The stepped attenuator I currently use sounds much better than the Blue alps I used before. I use a 23-step four-deck switch from ELMA (see below). If you want to use a standard stereo potmeter, linear ones supposedly sound better. Thorsten recommends cermet 100 K linear potmeters.
In the chill-amp, I use a four-deck ELMA 23-step attenuator with the following resistor values:

Position Row A Row B
Step 1 49.9k 316
Step 2 49.9k 402
Step 3 49.9k 499
Step 4 49.9k 634
Step 5 49.9k 825
Step 6 49.9k 1k
Step 7 48.7k 1.21k
Step 8 48.7k 1.5k
Step 9 48.7k 2.21k
Step 10 47.5k 2.43k
Step 11 47.5k 3.16k
Step 12 46.4k 3.92k
Step 13 45.3k 4.99k
Step 14 43.2k 6.19k
Step 15 42.2k 8.25k
Step 16 39.2k 10k
Step 17 37.4k 12.1k
Step 18 33k 15k
Step 19 30.1k 20k
Step 20 24.3k 24.3k
Step 21 18.2k 32.4k
Step 22 10k 37.4k
Step 23 0 49.9k

The same values are used for row C and D. These values generally work quite well, though I plan to change the first five steps for very low volume listening at night (I plan to start at 100 ohm). Instead of a stepped attenuator, you can also use stereo potmeters. I would recommend stereo potmeters in the range of 30k to 100k.
Parts list for stereo Chill-amp
ID Used parts # of parts Comments
U1 LM3875T(F) 2 Power opamp
D1-16 MUR860 16 Ultra-fast diodes
C1,C2 4.7 uF 4 Blackgate BG-N 50V capacitor
C3 0.1 uF 2 WIMA MKS2 63V
C4,C5 1000 uF 4 Panasonic FC 50V
P1 20k-100k 1 Stereo potmeter/stepped attenuator
R1,R3 22k 4 Resistor
R2 680 ohm 2 Resistor
R4 2.7 ohm 2 Resistor (1/4W)
Div. Div. x Connectors, case, wires, transformers, fuses, switches, opamp insulators, PCB's...
The ultra cheap gainclone
If you are willing to sacrifice some performance, you could build a very cheap gainclone. I think you can build one for under 150 euro, maybe even 100 euro.

Some ideas to save money:
  • Use only a single transformer (>=225 VA)
  • Use only one bridge and of standard quality
  • Buy cheapo resistors, capacitors and connectors
  • Make a case of wood (internally shielded with foil) or build it in a tin can!
  • Use an old piece of metal as heatsink
  • Use a cheap stepped attenuator from the dump
    Final checks
    After assembling the whole thing, some important things have to be checked before turning the thing on. It's very important to measure if the case of the (LM3875T) opamps doesn't touch the heatsink! And the case of the diodes shouldn't touch each other or the case too. And the ground of the connectors shouldn't.... well, you got the picture? The 22 K feedback resistor should be mounted well, you get a large DC voltage on the output if it's loose! Check if the output is not shorted to the ground.

    If you have checked the whole schematic and the *safety* of the whole thing you can turn it on (without speakers attached). The output of the amp should have a very small DC voltage and hardly any AC voltage. If you have a scope you can test the output with music or test signals on the input (using a 8 ohm high power resistor on the output). Check if the opamps are not overheating during playback of the test signals.

    Probably, after all these steps, it's safe to put your expensive speakers on the amplifier. Check if the phase of the net supply is optimal (use your ears or a polarity checker). And check also if the phase of the speakers is connected properly. Remember that the amp is non-inverted, so you only have to connect the speakers with inverted polarity if your DAC produces an inverted signal (this the case for non-oversampling DACs like the Nonoz III). The author is not responsible for any damage to yourself, your equipment and your cat...
    The listening
    Now comes the best part... the listening!!! Just put some nice music in your cd-player and use a comfortable chair. Take your favorite drink. And get ready for some serious chillin'...

    Keep the amp on (and playing if possible) for a few days to let it burn-in. The sound of gainclones changes quite dramatically during the burn-in period. The sound will get more open and musical after some time. And don't forgot to let me know your results!

    Send me back to the Chill-amp page please!

    Show the other audio projects!!!