Where does the hum come from?

Scott Dorsey wrote:
================
>
> If unshielded speaker wire is used for line level connections, there will
> be a hum.

** Certainly, but this can also happen with cheap RCA leads.

I found this out when repairing a Fender tube amp with a faulty reverb.
The connecting RCA leads were bad so I fitted a new pair that were the right length and had stubby plugs.
The result was buzzing noise and loud squealing if you turned up the reverb gain pot.

When cut open, the cable used was NOT co-axial !!
The ground wires just ran alongside the core in a tight bunch providing no ES shielding.

Soon as I fitted known, well shielded leads - all problems vanished.


...... Phil

On 11/02/2021 12:53 pm, Scott Dorsey wrote:
> I am -assuming- that the original poster is plugging a DAC that has a TRS
> 1/4" output into a powered speaker with a TRS input, using a proper
> TRS-TRS patch cable and that no speaker wire is involved.
>
> If unshielded speaker wire is used for line level connections, there will
> be a hum.
>
> Now.... a thing can hum for two reasons: either something is grounded in
> two places (causing a ground loop) or something is not grounded at all
> (causing something to be unshielded).
>
> We know the problem is not a ground loop because there is no connection
> to the DAC other than the speaker, and there is no connection to the
> speaker other than to the DAC and power.
>
> It's quite possible the DAC does not have a pin 1 connection to chassis and
> that therefore it is effectively unshielded until an input or power is
> connected to it. A multimeter will very quickly show if this is the case.
>
> If so, the hum will disappear when the system is configured for actual use.
> --scott
>
>


Look at the picture in the OP link. RCA unbal outputs.

geoff

On 11/02/2021 12:55 pm, Scott Dorsey wrote:
> John Doe > wrote:
>>
>> Do you want to know what a "speaker wire" is, too, you illiterate moron?
>
> Speaker wire is unshielded zip cord or SO line. There is no speaker wire
> involved in your configuration. Your use of the word "speaker wire" has
> confused people into thinking that you actually have speaker wire somewhere.
> --scott
>


LSR305 speakers are active. By 'speaker cable' he meant cable to the
speaker.

geoff

On Tue, 9 Feb 2021 16:59:09 -0000 (UTC), John Doe
> wrote:

>Obviously "the DAC having its own wall plug ground" has absolutely nothing
>to do with it SINCE I EXPLICITLY STATED THAT THE THING IS DISCONNECTED
>FROM EVERYTHING.
>
>Do you want to know what a "speaker wire" is, too, you illiterate moron?
>
>That's why I said to ask questions after the relevant part of what I said.
>You can't even count the number of fingers on your hand...

Here's the problem. Your description is confused and confusing. You
talk about speaker wire when you mean phono cable. And why are you
talking about a DAC when it isn't plugged in? It is clearly not a part
of the problem.

Now let's try to sort out what you really mean. Are you saying that
when your amplifier has an open circuit input, there is a low level
hum? If so, that is absolutely fine. That's what they do. As long as
it goes away when something is plugged in all is well. Get a phono
plug and short the live to the ground. Now plug that in. Is the noise
still there?

d

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On 11/02/2021 09:58, Don Pearce wrote:

> Here's the problem. Your description is confused and confusing. You
> talk about speaker wire when you mean phono cable. And why are you
> talking about a DAC when it isn't plugged in? It is clearly not a part
> of the problem.
>
Did you look at the picture he posted? It shows a Bluetooth to analogue
converter with an optical connection hiding round the back and a pair of
phono sockets on the front, and as Bluetooth is digital, there must be a
DAC inside it. What he claims is that the system is humming when the
receiver is not connected to anything, is not powered up and is not
plugged in to the wall.

> Now let's try to sort out what you really mean. Are you saying that
> when your amplifier has an open circuit input, there is a low level
> hum? If so, that is absolutely fine. That's what they do. As long as
> it goes away when something is plugged in all is well. Get a phono
> plug and short the live to the ground. Now plug that in. Is the noise
> still there?
>
He is, as far as I can tell, claiming that when the input to the
speakers is open circuit, there is only white or pink noise, and when he
connects to the unpowered, unearthed, DAC he gets a hum. He is going
from an unbalanced phono output to a balanced input to the active
speakers. Unless he is forgetting about a connection somewhere, I can't
see how anything can be coming out of the bluetooth receiver, which
leaves capacitive coupling to the mains somewhere in the system as the
only explanation.

The other thing I'm wondering is why this is all being copied to the
free.spam usenet group?


--
Tciao for Now!

John.

On Thu, 11 Feb 2021 11:02:07 +0000, John Williamson
> wrote:

>On 11/02/2021 09:58, Don Pearce wrote:
>
>> Here's the problem. Your description is confused and confusing. You
>> talk about speaker wire when you mean phono cable. And why are you
>> talking about a DAC when it isn't plugged in? It is clearly not a part
>> of the problem.
>>
>Did you look at the picture he posted? It shows a Bluetooth to analogue
>converter with an optical connection hiding round the back and a pair of
>phono sockets on the front, and as Bluetooth is digital, there must be a
>DAC inside it. What he claims is that the system is humming when the
>receiver is not connected to anything, is not powered up and is not
>plugged in to the wall.
>
>> Now let's try to sort out what you really mean. Are you saying that
>> when your amplifier has an open circuit input, there is a low level
>> hum? If so, that is absolutely fine. That's what they do. As long as
>> it goes away when something is plugged in all is well. Get a phono
>> plug and short the live to the ground. Now plug that in. Is the noise
>> still there?
>>
>He is, as far as I can tell, claiming that when the input to the
>speakers is open circuit, there is only white or pink noise, and when he
>connects to the unpowered, unearthed, DAC he gets a hum. He is going
>from an unbalanced phono output to a balanced input to the active
>speakers. Unless he is forgetting about a connection somewhere, I can't
>see how anything can be coming out of the bluetooth receiver, which
>leaves capacitive coupling to the mains somewhere in the system as the
>only explanation.
>
>The other thing I'm wondering is why this is all being copied to the
>free.spam usenet group?

When he shouted at me in his last post he said that everything was
disconnected, I quote

"Obviously "the DAC having its own wall plug ground" has absolutely
nothing to do with it SINCE I EXPLICITLY STATED THAT THE THING IS
DISCONNECTED FROM EVERYTHING."

So the block diagram is kind of irrelevant. Presumably nothing is
coming out of anything and it (I presume the speaker) is doing this
all on its own. In other words he has a powered speaker that is not
totally silent with an open circuit input. This is not news to me.

d

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geoff > wrote:
>
>Look at the picture in the OP link. RCA unbal outputs.

I have no graphics here. So RCA on one end, what is on the other end?

If the RCA connector is run to pins 2 and 3 of an XLR on the other end
with pin 1 lifted (as is a common and reasonable configuration), then
the case of the powered speaker won't be connected to the case of the
DAC until power is applied. So in that case the thing would hum in the
configuration the original poster describes.

But without knowing what the cable is, you don't know for sure.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

geoff > wrote:
>On 11/02/2021 12:55 pm, Scott Dorsey wrote:
>> John Doe > wrote:
>>>
>>> Do you want to know what a "speaker wire" is, too, you illiterate moron?
>>
>> Speaker wire is unshielded zip cord or SO line. There is no speaker wire
>> involved in your configuration. Your use of the word "speaker wire" has
>> confused people into thinking that you actually have speaker wire somewhere.
>
>LSR305 speakers are active. By 'speaker cable' he meant cable to the
>speaker.

Yes, I gather that by "speaker wire" he meant a line level cable but this
just added to the confusion.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

On 11/02/2021 11:29, Don Pearce wrote:

> When he shouted at me in his last post he said that everything was
> disconnected, I quote
>
> "Obviously "the DAC having its own wall plug ground" has absolutely
> nothing to do with it SINCE I EXPLICITLY STATED THAT THE THING IS
> DISCONNECTED FROM EVERYTHING."
>
> So the block diagram is kind of irrelevant. Presumably nothing is
> coming out of anything and it (I presume the speaker) is doing this
> all on its own. In other words he has a powered speaker that is not
> totally silent with an open circuit input. This is not news to me.
>
He claims that with an open circuit, he gets the normal white noise we
all expect, but when he connects the speaker input to the output on the
converter, he gets an audible hum. There is a pair of domestic standard
(And quality) RCA phono connections on the converter, so the only thing
I can suggest is that the shields on the two phono leads are making an
earth loop connecting the two speaker inputs and the two phono sockets,
which share an earth on the adaptor circuit board. The speakers will be
sharing the safety earth of the whole system with each other. Going from
a single ended output to a balanced input with no isolating transformer
won't be helping.

Either that or he thinks that turning the power off at the wall also
disconnects the safety earth for the Bluetooth adaptor.

I am using a similar Bluetooth adaptor to connect a car amplifier
feeding a pair of speakers to the computer, and have no hum problem at
all unless there is a poor connection between the amplifier and adaptor,
when the jack plug has moved a fraction. Push it back in and peace is
restored. Power for the amplifier is not connected to any earth, as it
runs off 12 volts, and the Bluetooth adaptor is also fed by an isolated
supply. The quality is good enough for an initial rough mix.


--
Tciao for Now!

John.

In article >,

John Williamson says...

> Going from
> a single ended output to a balanced input with no isolating transformer
> won't be helping.
>
I suspect the OPs "speaker wire" is probably an RCA cable with a mono 2
pole TRS adapter on the end.
What connection that is making within the 3 pole TRS input is anybodies
guess.



--
Ken