Rui, start to rule out things first.
Have nothing connected to the input but short them. Hum now?
MC cartrigdes produces uV signals so I would advice to use shielded cables everywhere.
Hi I've tryed short the input, and still a lot of hum?, I don´t know if it is hum (at least the sound of the loudspeakers is like that), or DC because I don't have any measuring equipment.
When playing the sound seems good, but between tracks we can easly ear the hum.
I need Help, (don't forget that I have all jumpers on), and that R2,R51 are wire jumpers
You must rule out if the hum is picked up from the cartridge, the wires or from the amp itself.
So disconnect the cartridge and short the input or have 100 ohms load there (MC load). Do you have hum now?
I've tryed what you sugest and still Hum (with the volume control on pre-amp turned to maximum, but in the midle position the is audible without getting close to the loudspeaker).
I've disconnected all jumpers (turning the riaa into an MM), and in this way the hum is hardly audible even with the volume at its maximum.
But I have MC cartdridge, so what do you sugest next?
Do you have shielded cables from the QSXM3? If you disconnect the RIAA amp, do you then get rid of the hum?
Do you have the QSXM3 grounded? How?
How does you supply voltage look like? Transformer connection? You should not mix protective earth with signal ground.
Yes I have Shielded cables from and into the QSXM3, Yes when disconnected Hum is "zero".
I have the QSXM3 not grounded, I have it connected to my preamp, and the ground is from the preamp (through interconnects)
My supply is transformer connection, and to prevent any noise pick-up I have tryed lifting the earth from the supply chassis in order to avoid any gound loop.
Inside the QSXM3 I have one single connection to the chassis.
Put 100 ohms in series with a pair of headphones and connect it directly to the phono amp output and have nothing else connected. Do you now get hum? I'll suspect a ground loop problem of some kind.
What happens if you only use your preamp and headphones and not your power amp?
How about strong magnetic fields? How near the phono amp do you have transformers?
Do you have any mains filter? Does this filter have a connection to protective earth? Is it connected to earth via the mains connector?
Is the hum soft like 50 Hz hum or is it hard 100 Hz hum?
Hi, I am back in business, I have been way on vacation on the Caribean watters.
In reply to your last post:
1-When connected, the hum on the headphones is very low, but the output volume is also low, because I´ve connected the turntable and the sound was low.
2-Magnetic fields is not the issue, the transformers are in a separate box way from the amp chassis, and connected to this buy an unbilical cord carrying the ac supply to the amp.
3-No mains filters, just a Supra mains block.
4-It seams to be 100Hz
Do you have one or two transformers with dual secondary windings? How many wires from the transformers do you have? 4 or 3?
I have two separate transformers with dual secondary windings, these have 4 wires.
The input shorted, only one channel powered up, headphone at the output via 100 ohms, still hum? 15 volts from the regulators? No ripple voltage from them? Make sure you have absolutely no magnetic field near the board.
The volume from the headphone when you have a record is _very_ high even with 150 ohms Sennheiser. A normal record should produce approx. 1 volts out, at least!
I will try it tomorrow, just a question more: Is it possible that perhaps I didn't used all of the components, and still the amp be playing? because apart of the hum with no record playing, the sound is quite fantastic when a record is playing, but hum is allways present.
You should not use all parts! There are lot's of options, DC-servo, output buffer and class A circuit. Maybe you could take some close up photos and I'll be able to spot something?
ok, I will send them to your email. Thanks
Hi, during this weekend I have managed to "clean" the Hum problems, buy rewiring the internal cables, I was using single core wires without screening, and with a common ground, changing these for screened high quality cables just simpy took the hum out of the equation, now this is sounding amasing... my words go out to you on this truly hi-end design.
Thanks for everything, neverteless I will send you some fotos of the finished unit.
Glad it all turned out well. Wiring is a serious business! Please do send some photos.
I have checked your pictures and so far I have hte following comments:
All skrews should be insulated so the groundplane is connected to the chassis only in _one_ single point, preferrable X1 and pin 3, "chassis".
The cartridge should be connected with a shielded wire from both inputs. The turntable you have, has probably a separate ground for each channel.
Otherwise it looks very tidy and nice. :D
All screws are insulated both from bottom, and from top with rubber o-rings.
I'm now using shielded cable as you advise.
Thanks for every thing.
PS- does this unit benefit from changing the 7815/7915 for your speacial regulators?
Quote from: rcgomesPS- does this unit benefit from changing the 7815/7915 for your speacial regulators?
It might be the case but used opamp has pretty good PSRR. If I had designed the amp today I may had made it differently but having R+C+R+C+R+C+R+C+R+C+R+C in front of the regulator is a positive thing in order to reduce hum and after the regulator I have 2.2 ohms + 470 uF which reduce the regulator noise.