He tried a lot of things. I was also thinking about wrong connections but what I have seen from pictures everything seems OK. Only part that can be wrong is (maybe) volume potentiometer. Because it is logarithmic.
For comparison I soldered my own Lovely Cube. I did few minor changes in the power supply and near BD transistors. I made three channel supply (separated channels) for output and opamp. I wanted to know what will happen. I put 1200uFon each output. Elna Silmic II in opamp power supply. Some old ERO 1841 (bypassed with russian polystyrene capacitors) on input. Polystyrene for opamp coupling. Etc, etc... A lot of capacitors in power supply.
Maybe it is silly but it sounds good. On Grado SR225 this funny hand made amp kicks ass. Bass is deep. Very very deep. Highs are mellow but enough fast. Mids are in front. Sound stage is wider and deeper than on Lovely Cube. If I compare the same OPA2604 in Lovely Cube, difference is like heaven and earth. Lovely Cube is maybe faster but also more sterile. This is more old fashion with grunt and deep power. I must admit that I do not like when people listen to oscilloscope and talking about how good is 96 Khz signal in their amplifier. I give a **** about measuring and about 96 kHz when I am able to hear only 18kHz. My ears are the best oscilloscope and they have final word.
Also what I noticed is that this amp does not get warm with stock 1.2K resistors. After turning off there is no DC over voltage on output, and without opamp, DC offset is 0. In Lovely Cube, without opamp, DC offset was 6 volts (SIX VOLTS). So you can imagine what will happen in Lovely Cube if your opamp suddenly stops to work... Here this is not the case. Also this amp has more power because it doesn't sag on very demanding parts in classical music. Bass is always here, mids are always here and highs are always here, no matter what. Drives like a tank.
There is no noise, what is really interesting, because of lots of wires and connections. I tried to implement multiple layer grounding (separate grounds for signal, opamp and supply but that did not work. Too much noise. When I had done star ground (below PCB, close to capacitors and rectifiers) noise was gone completely. 5W resistors are on borad because I didn't want to pull them out, but they are not in function.
It is not on the picture but I put a mute switch in front, that allows me to disconnect headphones without making a short circuit on BD-s. Also, for safety measures I always leave amplifier for 3-5 minutes to work without headphones (mute on)... This is still hand made amplifier. Who knows what might happen. %) If nothing goes on fire I start to listen to music.
I put another transformer for driving OPA2604 (10VA). It is not on the picture. It is added later.
Now I have plans to make another amplifier but with more changes and with less wasted space. I think that after this I would never buy an amp from manufacturer or seller. Because of three reasons: First I like to DIY, second I do not have money for buying something that is made for 300 USD and costs 1300 USD, and finally the third reason - I listen to MP3 mostly and benefit from 4000 USD amplifier, is completely gone right from start.
And fourth reason - I am sick when I see overheated amplifiers with tiny transformers.. Put that amplifier in big box, give it big transformer, and see how it sings...
Quote:
How are things, miky?
If the good advices from francisdemarte has not brought a solution, I think it might be an idea to check if the signal is present at the output of the opamp. You could use at 1k sinus wave at the input (and turn up the volume a bit). A multimeter should show the AC signal at the points shown in the picture (which are directly connected to the output pins of the opamp). You should easily be able to get lots of Volt out of the chip.
For that matter you send music into it and take the signal from here, and (together with a ground lead) connect to a pre-amp input).
Preferably connect through a 100-1000 Ohm resistor close to the output, in this case).
I strongly suspect that something is wrong with the op amp or its connections.
A pair of completely wrong resistor values is theoretically also a possibility, in a case like this.
Good luck. Olaf
