I assembled this preamp, and built it into a metal box. The power transformer is outside this housing (approx. 1 m. away).
The maximum amplification is 15x (R21 68k and R22 1M). After switching on the mains clearly audible hum is coming out of the speakers of the end amplifier.
It makes no difference when I have connected the earthing connection of the PCB to the metal housing or not. The housing is not connected to the earth pole of the 230 V. AC mains.
Apart from the humming everything else is as wished.
Most likely you have made a hum loop somewhere in your assembly.
Sorry, this forum does not allow picture uploads
If you can provide LINKS (e.g. by using a free online picture base) to HI-RES and SHARP pics of your assembly (both solder- and component side)
then we can take a look and provide some feedback.
Soldering looks OK.
A gain of 15 is kind of high. For such a gain everything need to be extremely correct.
You did not use the original connectors. By using the original connectors, a lot of troubles can be avoided.
It is possible that a hum loop is created because the metal parts of the 3.5mm jacks are connected to the enclosure.
Hum loops are difficult to solve and even more difficult to solve from a distance, so you will have to perform some experimenting.
Will a gain of 11x makes the circuit less critical? If so, I can increase the value of R21 to 100k.
(I need rather much gain because the end amplifier needs a minimal signal voltage of 1,5V. in order to reach its maximum output).
I didn’t use the original connectors because I wanted to use signal cables with 3,5" jack plugs (my cd player and also my mp3 player use this kind of connection).
Should I make sure that the 3,5" connectors are isolated from the metal housing?
At this time we have to guess where the problem is coming from, so yes, please experiment by isolating the connectors from the enclosure and by reducing the gain.
So you have to pay attention.
There are to types of ground.
A GND inside the circuit (the 0V…middle line of the secundairy winding)…wich one grounds the connectors and things like that…
A GND after R12(1K) that In my case, connects the circuits ground to mains ground.
This solved my noise problem (but only by putting away the transformer a great distance…naaaaah :D…not elegant). Therefore I used a better one for audio purposes.
You wrote: Make sure the ground of the connectors are not connected to the metal housing.
I unscrewed both connectors (IN and OUT) from the metal housing. The humming was still there.
Then I decided to connect the earth pole of the PCB to the mains earth: the humming was 1000 times stronger. Isn’t it so that the mains earth is ínfected by the 50 Hz. hum from the mains 230V AC?
Now the situation is as follows: I have a stereo end amplifier, powered by a toroidal transformer and built into a wooden housing. The transformer is connected to the mains by a 2-wire lead. Nothing is earthed. The signal cable is shielded at both ends. The 3,5" jack signal connector is attached to the wooden housing. When power is on there is no humming
The pre amp is built into an aluminum housing. The IN and OUT connectors are screwed onto this housing. By means of short shielded cable the signal is connected to the PCB. The connectors make contact with the housing via the nut that touches the housing and also via the shield of the signal cable that runs to the pcb. So far it looks that this is not the cause of the humming.
2x12V. power comes from a transformer (not toroidal), meant to solder onto a pcb. This transformer is outside the housing of the pre amp. I used a cable with 4 wires in it (former telephone cable) to feed the pre amp (3 wires for 12-0-12V AC and 1 wire for connecting the earth pole of the pcb to the earth of the mains). This cable is not shielded.
Maybe I should try to power this pre amp out of batteries. Then the 50 Hz hum cannot come out of the mains.
The pcb fits precisely into the housing: I didn’t use bolts. Only the 3 potentiometers make contact with the housing.
can you try this=
Put a plastic whasher between the frontplate and the potentiometers-housing. so that the housing of the pot doesn’t contact to the frontplate. For the rest just screw on the axis with the metal “moertjes” and fix to the frontplate. So only an isolated whasher between the front plate and the metal housing of the potentiometers.
If you don’t have isolated whashers, you can be inventive and cut something out of some old plastics;)
Decide for yourself where the STAR-point (central ground) of your whole setup (pre-amp + amp) will be. Mostly this is the 0 Volts at the capacitor bank of your power amplifier. From there ALL grounds leave (to the 0 V of the pre-amp and so on). Your shielded audio cables have their shield ONLY connected at the site of your STAR-point. This avoids earth loops, one of the most frequent causes of humming.
At last I got rid of the hum. This is what I have done:
I made a new housing for the pre amp, not of steel but of wood. Then I disconnected the shielding of the signal cables (IN and OUT) from the connectors (the shielding of the cables is only connected to the pcb of the pre amp). Finally I connected the STAR point of the pcb of the pre amp to the ground of the IN and OUT connectors.
I even incorporated the little (1,6 VA) supply transformer into this wooden housing. A mains switch switches both the pre amp and the end amp at the same time. Apart from an audible power on and power off ‘‘click’’ everything functions according to my wish.
I like to thank everybody for their valuable ideas about solving the problem. To Velleman I would like to say that the use of the 3,5" IN and OUT connectors is more in line with the signal jacks and connecting cables that are found today in contemporary music players, iphones, tablets, etc.
I will take your suggestion into consideration. Because I want to use just one switch I think I need a delayed switch-on and -off for the power amp.
I can also keep the pre amp powered, and switch the power amp on/off (Velleman mentions this in the manual of the pre amp). But then I cannot use the LED of the pre amp as an indicator (on/off) for the power amp.
Since you are using a mains switch and you would like a light to let you know when power is present (switch is on)
You might want to look at the MK181 (link below) http://www.vellemanusa.com/products/view/?id=525200
It is really possible to get a hum free operation! Without any doubt…I used a toriod transfo…not…the one provided by the electronics shop who sold me this kit.
Even with pulling + and - out of one single voltage from the toriod (half bridge rectification…and special elco structure)…it is perfect possible to get a hum free operation http://users.telenet.be/offie/k8084.html
Does anyone know the reason why Velleman recommends to keep the pre-amp powered? Sorry to hijack the thread by the way, it’s good to see that you got the hum sorted
