hey guys, i notices hum in the preamp and desperate to find the fault i made the observations below.
connecting only the right channel everything is ok. once i connect both right and left channel hum appears in both channels.
also reversed the inputs to check if its the cardage but it seems to be related with the left channel of the preamp
any help with that ?
Do you use shielded wires to the cartridge? Do you have a separate ground wire from the tone arm?
yes i thing its shielded. there is a ground wire even i see no difference with it connected or not
Are the shields tied together in the turntable end?
i think so yes, any way i can check that for sure
They should not be connected in both ends. It would be good to know if the turntable is humming with an another phono amp.
the channel grounds are connected together, the cable shield/phono earth runs seperated
i find it very confusing when both channel of the preamp have ground there is hum everywere. you thinking that the cable picks the hum?
From my amp you should have a straight connection to the cartridge four connections and a separate ground wire from the tone arm.
any chance to modify that so to have a common channel ground?
I recommend that you take a look at your turntable's wiring first.
this also means that i should use isolated rca plugs so the grounds do not touch the chassis?
i mean what i dont get is that the 2 "ground" wires from the cartage will be in parallel as they are now right?
Isolated contacts is a must.
this is how my turntable is wired
u recommend separating the grounds from pin number 2.
any good isolated rca to recommend?
Wow, it's an old thing with DIN connector. Do you have some sort of adapter DIN-RCA? You have one signal ground from the cartridge and one ground for the chassis/tone arm.
its basicly a beogram 2000. yep ivegot a converter :). should i mess with its wiring or the problem maybe on the preamp?
You must somehow break the ground loop. Your RCA terminals must not be connected to the chassies.
any other things i should check for a ground loop?
Is the TT chassis going through the adapter in someway?
through? in what way?
If you measure the resistance between your ground in your phono plugs and the chassi in the TT, which value do you get, high or low resistance?
ok fixed the ground loop, it was the wire going from the rca to the input pins. now my toroidal transformer is causing hum. i have it a good 5cm away from the power supply part of the board but i can still hear it. perhaps its a cheapo transformer? any suggestions for a fix and a quality transformer ?
Good the you fixed the ground loop.
Is it mechanical noise from the transformer?
what do you mean mechanical?
Is it vibrations or can you hear in your speakers?
i can hear it in my speakers, changes with the orientation and proximity of the transformer and the board
OK, then you have magnetic coupling. A small toroidal transformer would probably do the trick.
i already have one in an aproximate distance of 5 cm ( the outer from the pcb) and its still picking up noise. maybe it is missfunctional? btw i can hear the noise only with the volume of my amp at 3;4 and above
You say noise, you don't mean hum? Does it go away if you move the transformer?
hum i mean yes. yes woth the trasnformer further away, ( out of the case) the hum dissapears. is it normal fora such a transformer to need so much distance?
A normal EI-core transformer has a magnetic leakage field but either has less. You can get rid of the hum if you have shielded wires inside and you can also insert a steal shield between the transformer and the rest.
I have shielded wires and that reduced the ham a lot , at the volume i hear it i never use the amp, its intolerable. using a steel sheet the hum gets redued even more . but there is still some there i guess the amplifier is just too sensitive at that point?
I belive that something is wrong with one of the channels lt1115. when i mesure the opamps out one of them gives me a stable voltage where the other suddenly drops to a -ve voltage. any chance that the answer? Also the output of the lt1010 on the side with the hum is 0.002v where on the silent side is ~0.6v
I am usign a single transformer so i tried moving it from one channel to the other and guess what the hum passed to the other channel. So does this mean that i need an ac line filter or something?
Is the sound quality good?
when playing music its excelent. so the ics are ok right?
maybe i should use an emi filter on the mains ?
What is the problem right now?
hum , to the channel the transformer is connected to ( the bridged channel also has hum but lots less)
Is the transformer feeding two channels? How does this connection look like?
The green wire, what is that for?
i use it ad a ground when i probe
Are you getting hum when nothing is connected at the input? Is the hum audible or do you see the hum on your oscilloscope only?
with nothing connecred on the input i het noise which u explained is from the high gain. the hum is audible only at about 3/4 and above of the volume of my amp. and its louder on the channel that the transformer is connected to
Is it 50/60 Hz hum or 100/120 Hz? If it is 50 Hz, then you have a ground loop or magnetic coupling from the transformer. 100 Hz and harmonics comes from the smoothing caps.
How high is the hum compared to the normal music level?
Does the hum go away if you locate the transformer further away?
i thing ot is 100/120 hz . the hum is very low compaired to music level. if i take the transformer further away it seems to decrease but not complitly. btw if i move my input cable over the caps it gets very loud so maybe harmonics?
If you use the MC mode the input signals are very small so you must separate input and output as much as you can.
i have a mm stylus. how does one eliminate cap harmonics? so i can check if its that
I can't really figure out your real problem. Do you have insulated phono terminals? You are using shielded wires? No antennas or loops?
insulated rca, shielded wires and no loops or antenas ( not quite sure fir the antenas). maybe there is dc in my mains ?
Dead silent if the inputs are shorted?
DC in the mains will have no influence of the electrical hum, only mechanical.
Your case, connected to anything?
Antennas means wires that can pick up electrical or magnitcal fields.
i have some unshielded wires for probing the power rails is there any chance to influende the hum?
You can try to move them and see what happens.
Cleaned up my wiring again and now am left with 50hz hum, any thoughts on that?
The left channel isn't the same as the right when it comes to wiring. Try to identify the difference. If to hum is gone with shorted inputs then you know that the problem is before the inputs.
lots of rewireing and a broken transformer later hum is 99% removed. now here is somethig crazy with my amp turned on, on full volume and the preamp off i get noise. turn the preamp on and yhe noise is still there. remove the rca which connects the preamp and the amp an the noise is gone. i tried lifting the earth connection on the preamp but no luck. i would like to point that what am hearing js noise and not hum.
Noise may come from high source impedance in your volume pot. Hard to tell.