hello everybody,
I am new to this forum and I have made a phono preamp clone of the 834 following the thread on lencoheaven,
I am very happy with the sound and all works fine with the kit I purchased from douk but I want to try another power supply,
I have bought an assembled board trying to upgrade the one coming with the kit but I have a strong hum:
first 4-5 seconds it is silent and then the hum increases for about another 4-5 seconds and gets very loud
with the other power supply it is silent but I want to know what I have made wrong in order to learn a little bit before I begin another project
here is a pic of the wirring
the transformer on the top right is HT 300-0-300, the red wires are 300 v and the black one is 0v connected to the chassis ground along with the brown wire coming from the board ground,
there is one missing wire in the picture: the ground coming from the phono board which is also connected to same point as black and brown, the heaters are also missing for clarity purpose
can anyone give some advise to wire the power supply properly to avoid the hum?
I am new to this forum and I have made a phono preamp clone of the 834 following the thread on lencoheaven,
I am very happy with the sound and all works fine with the kit I purchased from douk but I want to try another power supply,
I have bought an assembled board trying to upgrade the one coming with the kit but I have a strong hum:
first 4-5 seconds it is silent and then the hum increases for about another 4-5 seconds and gets very loud
with the other power supply it is silent but I want to know what I have made wrong in order to learn a little bit before I begin another project
here is a pic of the wirring
the transformer on the top right is HT 300-0-300, the red wires are 300 v and the black one is 0v connected to the chassis ground along with the brown wire coming from the board ground,
there is one missing wire in the picture: the ground coming from the phono board which is also connected to same point as black and brown, the heaters are also missing for clarity purpose
can anyone give some advise to wire the power supply properly to avoid the hum?
It may be that the AC voltage on the transformers is not sufficient for the regulators to work properly. i.e. your below the minimum dropout voltage. That's one of many possible explanations.
Make the wires from the transformers to pcb longer...like 50... 70cm and you'll most probably see that you need careful shielding with iron sheets of the transformers themselves!
Connect PT (CT) black wire to PCB (middle) GND. Connect there brown wire also.
The white wire (from chassis "ground" connect to 10R resistor, and resistor another leg connect to brown wire.
Is PCB insulated from chassis (no metal screws, or all of it insulated from PCB)?
Please show the input RCA connections.
The white wire (from chassis "ground" connect to 10R resistor, and resistor another leg connect to brown wire.
Is PCB insulated from chassis (no metal screws, or all of it insulated from PCB)?
Please show the input RCA connections.
The first 4-5 seconds is silent, because the tubes are cold. As soon as the emission starts, they amplify the hum.
The source of the hum can be:
The source of the hum can be:
- AC on the B+ (reason can be insufficient raw DC voltage before the stabilizer)
- Ground loop (usually because signal ground and chassis ground are connected at more than one point, see euro21 advice)
- Stray magnetic field from mains transformer
ok guys, thank you all for your inputs,
yesterday I placed the transformer outside the box but same result, there was hum + a hiss (I guess because of the wires), see pic and sorrry for the mess :
my rca connections before (it works fine this way with the power supply coming with the kit):
and the rca connections after shorting them ( I guess this is what you refer to when you say "shorten the rca inputs", it's connecting the negative input to the ground of the PCB),
and guess what, the hum was gone but the hiss was still there
so I placed the transformer inside the chassis but I get hum + mechanical hum so I switched all off I'll continue trying this afternoon
yesterday I placed the transformer outside the box but same result, there was hum + a hiss (I guess because of the wires), see pic and sorrry for the mess :
my rca connections before (it works fine this way with the power supply coming with the kit):
and the rca connections after shorting them ( I guess this is what you refer to when you say "shorten the rca inputs", it's connecting the negative input to the ground of the PCB),
and guess what, the hum was gone but the hiss was still there
so I placed the transformer inside the chassis but I get hum + mechanical hum so I switched all off I'll continue trying this afternoon
Hum can a bugger to fault find, it can be caused by loads of things or a combination. First you need to narrow things down a bit, as icsaszar says is your hum 50/60hz or 100/120hz? Have you a scope?
1) First off scope or check your PSU out with a DMM, that means black lead to ground chassis, probe/red lead to HT/B+. Make sure your scope input or DMM is rated to handle whatever volts your HT/B+ is. You should see a small triangle/sawtooth wave of about 50mV or less on a scope or 20mV ish on a DMM.
2) Do the same at the output or at various points in the circuit, EG V1 grid, V1 anode, V2 grid, etc, etc.
Let us know the results. Andy.
1) First off scope or check your PSU out with a DMM, that means black lead to ground chassis, probe/red lead to HT/B+. Make sure your scope input or DMM is rated to handle whatever volts your HT/B+ is. You should see a small triangle/sawtooth wave of about 50mV or less on a scope or 20mV ish on a DMM.
2) Do the same at the output or at various points in the circuit, EG V1 grid, V1 anode, V2 grid, etc, etc.
Let us know the results. Andy.
Sorry, but "the connecting" is wrong.
The turntable phono cable grounding lug (between RCA connectors) is tied to the PCB "ground" (whatever it's mean).
Is grounding lug insulated from chassis?
Shorting is means short RCA pins together (hot to cold).
Is RCAs "cold" pin insulated from chassis?
dear all,
after the last days checking all the connections(power and signal), shorting the inputs, connecting and disconnecting grounds on the pcb, etc..., I gave up and installed the original power supply which is very simple but does the job without hum.
I must confess it is a really annoying job to search for the cause of this ground loop, moreover when the original power supply works well, so I am puzzled.
thank you all for your inputs, I really appreciate your help.
after the last days checking all the connections(power and signal), shorting the inputs, connecting and disconnecting grounds on the pcb, etc..., I gave up and installed the original power supply which is very simple but does the job without hum.
I must confess it is a really annoying job to search for the cause of this ground loop, moreover when the original power supply works well, so I am puzzled.
thank you all for your inputs, I really appreciate your help.
Yep, it can be a pain to track hum, in your case though it sounds like the PSU is at fault with excessive ripple voltage.
Andy.
Andy.
yes Andy, I am aware of your point and because other people told me it could be the ripple:
I didn't mention that I checked the big PS caps and I replaced two caps after the rectifiers for others with higher voltage specs (600v) and higher capacitance and I also put a full bridge rectifier before the HT input and may be more things I don't remember now.
So I'm done and I give up, sorry but it drives me crazy
I didn't mention that I checked the big PS caps and I replaced two caps after the rectifiers for others with higher voltage specs (600v) and higher capacitance and I also put a full bridge rectifier before the HT input and may be more things I don't remember now.
So I'm done and I give up, sorry but it drives me crazy