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 Thor-Amp, I would enjoy to hear about your results!

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. For lower power amplifiers, the LM1875 could be more interesting. I never tried that chip. There are two versions of the opamp, the LM3875T and the LM3875TF. The only difference is that the second one is an isolated version. The LM3875T needs a silicon or mica isolation pad, because the case is connected to one of the supply voltages. 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 ;-) I left one mm or so out of the package.

I use two Amplimo 2x25V 160VA toroid transformers for dual mono. Maybe 225VA transformers are better. For 4 ohm speakers, using a lower voltage is better. Both channels have its own 2A fuse and internal net cable. I advise to use thick net cable and solder it directly to the input socket. Ensure good safety!!! Isolate the cable and it's connections very well. I use fast dual schottky 100V 20A TO-220 diodes as rectifiers, eight per channel as in the Thorsten schematic. The two diodes in the TO-220 package are connected in parallel for each diode.

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. I have ordered some more FC's to experiment with. It's very possible that adding two or four 1000 uF's will improve the performance of the amp. 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 cover from the capacitor, that seems to improve the sound quality. Also, I used poster buddies (similar to blue-tac) to increase the mass of the capacitors. If the plastic cover is removed, the capacitor should not touch other metal surfaces (or capacitors), the poster buddies help here again to isolate.

I use a 2.2 uF polycarbonate cap for filtering the DC from the input. Sometimes I leave it out, because my DAC already has an output cap. 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 rest are ERO 1822 MKT capacitors. They will be changed to polycarbonate too, they have a better sound. There have been some posts on the forum that you could leave out the 0.1 uF capacitor in the schematic for better sound. Again, you have to check the output DC when experimenting with this!

Currently, I use a 19" case. This is very practical if you want to keep everything in one case. It's better to keep the opamps far away from the transformers. I am planning to build the next version of the Thor-amp in three small aluminium heatsink cases. One for my Nonoz DAC with input selector and stepped attenuator and two mono amplifiers. I will keep the 19" for the supply of the opamps and the DAC (maybe with batteries!). I have to do more research on the dampening of the cases. 47 labs regards this as a very important topic. I have yet to try some feets under the amplifier.

As you can see on the pictures, I use a brass heatsink. The size is 20x4x1 cm. This is not sufficient for my speakers. They overheat when I play loudly. 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.

I use CAT5 cables for connecting the supply to the opamps. The opamps are connected by four cables to the supply bridges. Two cables carry the supply voltages (+35V and -35V) and the other two cables are for returning the current of these supplies back to the two bridges. In an earlier version of the Thor-amp I made the big mistake to use only a single CAT5 wire for these connections. That was not enough, resulting in a vague character in the mid-bass and lack of control. Now, I use four wires per connection, so two complete CAT5 cables per channel. CAT5 cables have four pairs. I chose to pair the +35V and earth in one CAT5 cable. All the coloured wires are connected to the earths on both sides and the white/coloured wires are all connected to the +35V supply. I did the same for the negative supply, the -35V and it's earth is paired.

For the signal cables (input and output) I use 0.8 mm airdieletric single core silver wire. For the input signals, it's probably better to use 0.2-0.4 mm wire but I didn't have that. Some people advise to put it a while in an oven before using. In some places, I use a teflon housing if the cables shouldn't touch other wires or the case.

The resistors can be changed to alter the character of the gainclone. I use matched cheapo 5% carbon resistors on most places for a soft sound. Allen bradley resistors could be sounding even warmer. For a brighter sound, 1% metal resistors are to be used. The stepped attenuator I currently use sounds much better than the Blue alps I used before. I use a cheapo 11 step switch with matched 8 K resistors. 11 steps works better than I had thought it would. I have ordered an Elma 23 steps four deck which will hopefully sound even better. If you want to use a standard potmeter, linear ones supposedly sound better. Thorsten recommends cermet 100 K linear potmeters. In the schematic of Thorsten is a 0.22 ohm resistor on the output. This is to ensure stability with high capacitance cables and certain speakers. In most cases it can be omitted.

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 220 K 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 and 1 uF) should be very close to the opamp. Maybe, I will add a drawing later on with the details of my layout...

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.

I have connected one of the star grounds with a 100 ohm resistor with the case. This sounds better than connecting it directly. I have heard that using a 22 nF cap in parallel with the 100 ohm resistor would even be better. Use isolated input and speaker output connectors to prevent loops through the case. 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.

Use some nice gold phono chassis sockets and 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 makes very few noise, so large parts of the day I leave it on.

Some parts of my Thor-amp that I ordered from Farnell (with order codes):
LM3875T, 295-541
0.22 ohm MPR20-TO220, 551-491
0.1 uF MKT1822, 814-120
1 uF MKT1822, 814-131
2.2 uF MKT1822, 814-143 (not advised!)
1000 uF Panasonic FC, 303-6560
Dual Schottky 100V, 20A, 996-385
Transformer 2x25V 160VA, 306-8894

Besides these components you need the resistors, a case, cabling, an attenuator, a power cord with entrance socket, a power switch, fuses with holders, input and speaker connectors, isolation sets for the opamp and schottky diodes etc. etc. I paid around 320 euro in total for my Thor-amp. I could have built it cheaper with the same quality, I bought the components in expensive shops.

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 cheap case of wood (shielded with foil internally) or build it in a tin can

Use an old piece of metal as heatsink

Use a cheap stepped attenuator from the dump

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 opamps doesn't touch the heatsink! And the case of the schottky's shouldn't touch each other or the case too. And the ground of the connectors shouldn't.... well, you got the picture? The 220 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 inverted so you have to connect the speakers with inverted polarity if your DAC is not inverted. The author is not responsible for any damage to yourself, your equipment and your cat...

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 the Thor-amp changes quite dramatically during the burn-in period. The sound will get more open and musical after some time... enjoy...

Send me back to the Thor-amp page please!


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