Newbie question.
I'm wiring up a dual mono blocks amplifier (this board: https://www.aliexpress.com/item/1005003450663314.html) , here's what i have so far (pardon the ugly drawing):
Plugging it in without RCA input, there's hum.
Plug RCA input in, hum is reduced. It goes up when the volume stepped attenuator is turned up, but gone completely when attenuator is at 10. Reduce it to 9 and hum is back immediately.
Can someone explain why there's no noise when the volume knob is at maximum?
I've done some internet search, somewhere says it is with grounding but no details explaination.
My country uses 2-prong electric system so i don't have any earthing. All connections are depicted in the image.
Looking forward to your suggestion.
Thanks.
I'm wiring up a dual mono blocks amplifier (this board: https://www.aliexpress.com/item/1005003450663314.html) , here's what i have so far (pardon the ugly drawing):
Plugging it in without RCA input, there's hum.
Plug RCA input in, hum is reduced. It goes up when the volume stepped attenuator is turned up, but gone completely when attenuator is at 10. Reduce it to 9 and hum is back immediately.
Can someone explain why there's no noise when the volume knob is at maximum?
I've done some internet search, somewhere says it is with grounding but no details explaination.
My country uses 2-prong electric system so i don't have any earthing. All connections are depicted in the image.
Looking forward to your suggestion.
Thanks.
Attachments
It might help if you would draw in more of a schematic form how you have all the grounds connected. That includes for any differential inputs/outputs that may be connected for use as single-ended.
Sometimes it also helps to see some hi res pics of all the interconnections.
Sometimes it also helps to see some hi res pics of all the interconnections.
Probably electric (capacitive) coupling of hum into the wire from the volume control to the amplifier. When the input is shorted, the volume control's output impedance is probably practically zero at zero and maximum volume and maximal at half volume (-6.02 dB).
Is there a difference between left and right?
Do you use shielded cables from the inputs to the volume control and from the volume control to the amplifiers?
Is there a difference between left and right?
Do you use shielded cables from the inputs to the volume control and from the volume control to the amplifiers?
Try connecting transformer center tap to earth...with / without resistor, capacitor, or both.
Shield the chassis, earth that also (toroids, be careful of mounting and shorted turns).
Good advice above...also see if you can change input level, sometimes low level causes issues...rarely too high.
Shield the chassis, earth that also (toroids, be careful of mounting and shorted turns).
Good advice above...also see if you can change input level, sometimes low level causes issues...rarely too high.
It's pretty much what i have in the drawing above.
Signal input via two RCA jack, with their ground (outer shell) connected together and connect to chasis via a 3.3nF disk capacitor (as suggested here). Then via a 3-wire cable to the volume control 3 input posts (L-G-R). Output from the volume control goes into each of the amplifier board via two-wire cables (left signal and ground, right signal and ground. Two ground wires soldered to same ground post on volume control).
At the other end of amplifier pcb, as can be seen in the picture above, there are 7 connector positions, from left to right: 36v-0v-36v from toroid transformer, two for speaker wires, and two 12v input from toroid transformer.
Thats all i connect. Would you suggest doing any other grounding connection?
There's no hum when knob at 0. Hum starts at 1, increase with volume up, and disappears at 10. No difference between left and right. It's a cheap made-in-china stepped attenuator (Fake Alps like this) so i'm thinking it may be of low quality, but does that make a noise difference?
Yes i used shielded cable with twisted copper wires and foil wrap.
Foil wrap grounded? Why twisted copper wires for an unbalanced connection?
Strange that there is no difference between the hum levels left and right, considering the large difference in wire lenghts (on your sketch anyway).
Do you get hum when you disconnect the input wires at the amplifier boards? That is, run them with open inputs?
Maybe the amplifier boards inject a hum current into their own inputs. At least that would explain why left and right hum about equally loud and why you (apparently) get hum current despite using shielded cables.
Strange that there is no difference between the hum levels left and right, considering the large difference in wire lenghts (on your sketch anyway).
Do you get hum when you disconnect the input wires at the amplifier boards? That is, run them with open inputs?
Maybe the amplifier boards inject a hum current into their own inputs. At least that would explain why left and right hum about equally loud and why you (apparently) get hum current despite using shielded cables.
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MarcelvdG
No difference between left and right according to my ear, i might be wrong.Here's what i use for connection from RCA to volume control:
And this below is for from volume control to amplifier board:
I've also redraw the connection schemantic if it's any help:
I'm at work rn. Will try desoldering input at the amplifier board to see if there's hum and let you know later.
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Hi. Input level is already at maximum. I use my ipad as input.
What do you mean by connecting transformer center tap to earth? I don't have earth since my electric system is only 2-prong.
The RCAs need to be isolated from the chassis ( metal construction ) and the chassis needs to be connected to safety ground for the 3.3nF disc capacitor to be of any benefit otherwise it might even be a hindrance . The 3.3nF capacitor is intended to take RF interference to ground via the safety earth .
I may be mistaken, but the fact that hum is absent at both 0 and max volumes but appears when volume settings are intermediate suggests that the hum is introduced as a voltage field rather than any currents flowing across the chassis. What's the impedance of the attenuator? Any data?
Some scatter shot:
Where is the foil shield connected?
At mid volume with no other equipment connected, is hum affected when you touch the enclosure?
Try removing the 223 cap connected to chassis. It might be be better if connected at speaker common rather than to input signal ground.
Good luck!
Some scatter shot:
Where is the foil shield connected?
At mid volume with no other equipment connected, is hum affected when you touch the enclosure?
Try removing the 223 cap connected to chassis. It might be be better if connected at speaker common rather than to input signal ground.
Good luck!
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Safety earth is for safety, if the equipment isn't double insulated, you don't need it for shielding. The only exception I know to that rule is when you try to keep low-frequency electric fields inside a metal enclosure.
Hi,
Actually now that i listened again, there was actually a difference between left and right, probably due to the difference in cable length.
The hum does get worse when i hold the cable with my hand.
The volume attenuator is 100k, which as i've read somewhere, may be more prone to noise than one that is lower in impedance.
Since hum doesn't happens at 0 or 10 of the volume control, i decided to remove the volume attenuator. Now the hum is gone completely and the amplifier is really quiet.
So i'll go with this and not having any volume control at all in the amp. Will use volume control at the source (currently my iPad, may decide to upgrade to an external DAC/Preamp in the future).
Thanks guys for the opinions and advices.
Actually now that i listened again, there was actually a difference between left and right, probably due to the difference in cable length.
The hum does get worse when i hold the cable with my hand.
The volume attenuator is 100k, which as i've read somewhere, may be more prone to noise than one that is lower in impedance.
Since hum doesn't happens at 0 or 10 of the volume control, i decided to remove the volume attenuator. Now the hum is gone completely and the amplifier is really quiet.
So i'll go with this and not having any volume control at all in the amp. Will use volume control at the source (currently my iPad, may decide to upgrade to an external DAC/Preamp in the future).
Thanks guys for the opinions and advices.
I'm surprised hum pickup was susceptible to cable grasp. Are you certain the shield is grounded? Typically, there is a small bare wire that is enclosed within the shield that is used to make connection to the foil sheath. You have to make sure it doesn't get snipped away when you trim the foil.
BTW, if there's a significant difference in channel gain, it's doubtful that it's due to cable length.
Hi BSST,
Thanks for the suggestion.
Such a shame I did snipped away the bare wire and not soldered it to ground. As i said i'm a newbie and this is the first time i know what it's for.
A question though, searching on the internet there're suggestion to ground either one end or both end of the shield. What would you suggest?
Thanks for bearing with such a noob in me.
Thanks for the suggestion.
Such a shame I did snipped away the bare wire and not soldered it to ground. As i said i'm a newbie and this is the first time i know what it's for.
A question though, searching on the internet there're suggestion to ground either one end or both end of the shield. What would you suggest?
Thanks for bearing with such a noob in me.
Both is better.
But it might pick up noise from iPad, unlikely, but strange things do happen, so try it.
Wi fi or Bluetooth streaming from iPad may be a better option, some mobility is possible. Less chance of noise.
A simple FM module has Bluetooth built in, costs less than $2, connect it to amp. Then you can sit on the couch, and play the music over wireless connection.
Many apps will allow control over volume, music choice etc.
But it might pick up noise from iPad, unlikely, but strange things do happen, so try it.
Wi fi or Bluetooth streaming from iPad may be a better option, some mobility is possible. Less chance of noise.
A simple FM module has Bluetooth built in, costs less than $2, connect it to amp. Then you can sit on the couch, and play the music over wireless connection.
Many apps will allow control over volume, music choice etc.