My QUAD 405-1 sounds great but when I measured the distortion there is a lot of noise on both channels. What is the cause?
Looks like mains hum with lots of harmonics rather than noise. Crosstalk from the mains rectifier current to the signal path maybe?
For example, if you changed the wiring such that the ground current between mains transformer and supply capacitors has a common section with the ground of the signal inputs or outputs, the spiky charging current waveform will be superimposed on the signal and cause lots of mains harmonics to appear.
For example, if you changed the wiring such that the ground current between mains transformer and supply capacitors has a common section with the ground of the signal inputs or outputs, the spiky charging current waveform will be superimposed on the signal and cause lots of mains harmonics to appear.
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Nope, can’t hear a hum. I think one of you two told me my grounds were not well laid out. My speaker returns are going to 0v and I need to move them to case. Then only a trans to 0v and 0v to case, like it was originally.
Speaker returns should go to the power supply's main capacitors common connection point,
not to the case. That is where the speaker current goes, and the path should be direct.
not to the case. That is where the speaker current goes, and the path should be direct.
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Then I wouldn't worry... really 🙂
Its true that speaker grounding needs to be correct but that is also as much for best distortion as well as hum. There are various easy tests you can do but if its silent hum wise and you are enjoying as it is then I would leave it be rather than go down the rabbit hole chasing things.
I will put the grounding wires back to their original positions. Trans to 0v. 0v to case (next to rectifier). Speakers to case. Input to din plug. It is only the speaker returns that are incorrect at the moment.
It is now wired as a standard QUAD. There is no improvement. Also the distortion increases when I increase the volume. It is not audible, could the transformer or the supply caps be breaking down?
I would doubt the transformer (or caps) are a problem. Shared PSU's are always susceptible to interactions.
For example if you set one channel of the amp up to deliver lets say 10 volts peak to peak output @ 1kHz (no load) and have the other channel input shorted at the input socket then you should see no output from the that channel.
Now apply a low load of 4 to 8 ohms. If the shorted channel now shows an output (when the other channel is under load) then that is because of grounding interaction between the channels caused by where you have returned the speaker grounds to.
For example if you set one channel of the amp up to deliver lets say 10 volts peak to peak output @ 1kHz (no load) and have the other channel input shorted at the input socket then you should see no output from the that channel.
Now apply a low load of 4 to 8 ohms. If the shorted channel now shows an output (when the other channel is under load) then that is because of grounding interaction between the channels caused by where you have returned the speaker grounds to.
It might not be my amplifier, it could be the setup I am using to measure. I can’t hear any distortion when I turn up the volume. I will need to test something else to be sure.
Many forms of distortion have to be pretty bad before you can hear them and not all distortion is bad from the perspective of how good something sounds.
The artefacts in your graph are at 50Hz mains frequency, not 100Hz. It is therefore transformer primary hum or a ground loop, not anything after the rectifier.
Yes it now has a TLE2071 opamps, gain reduction to 1.5V and new electrolytic caps. I have not changed the 100000uf caps yet. I removed C3 when I changed the opamps.
Do you have an oscilloscope? It maybe high frequencies oscillations. Yes, nobody can hear frequencies with 200kHz, but the frequency is not stable. It vary chaotically a little bit, like FM, and the modulation is what you hear.
I do have a bit of improvement in the RH channel below 1kHz. This side is closer to the transformer, further from the caps and also longer wires going to and from it.
If you don't have an oscilloscope, you can use a high-frequency rectifier and measure the DC voltage with a DMM.