Turntable to PC, hum problem

[Guest]

Archived from groups: rec.audio.tech (More info?)

The setup:

Old BIC 960 turntable, from garage sale.
RCA SA-155 "integrated stereo amplifier" from Radio Shack
PC sound input

Ground wire hooked up between turntable and SA-155. The cables are all
shielded. Both turntable and SA-155 have 2-wire line cords, so I doubt
there are any ground loops. There is some audible hum, which also
shows up in a graph made by capturing the sound card signal using a
visual basic program. This hum appears to be a few dB above the noise
floor, but it's there.

Suggestions? Fortunately, I can measure the effect, so I will know if
I'm making any progress. Of course I could start with trial and error,
but if anybody else has been through this, it would save me time and
wasted effort.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

You need to determine if this hum is only from the turntable, the amp, or
the result of a ground loop (very common). Turn the amps input selector to
an Aux input. Does the hum persist? If so, the turntable is not the main
source, though it could still have a bit of hum - many do.
If not, then the turntable may have a bad connection internally, or bad
cables.
If there's still hum while the amp is hooked up, the amp might have a bad
power supply capacitor.

Ground loops can be a tough nut, but usually they are due to some connection
between your computer and an antenna or cable connection. Try a 3-to-2 prong
adapter from the power strip to the wall. This will lift the grounds on
everything, not just the computer box. This is perfectly safe by the way,
unless somebody has been monkeying around inside your computer introducing
AC faults.

If necessary disconnect any connection that may run from the computer area
to any other part of the house, like to your main stereo.


Mark Z.


"Detector195" <Detector195@yahoo.com> wrote in message
news:6213f73a.0411091921.79853b91@posting.google.com...
> The setup:
>
> Old BIC 960 turntable, from garage sale.
> RCA SA-155 "integrated stereo amplifier" from Radio Shack
> PC sound input
>
> Ground wire hooked up between turntable and SA-155. The cables are all
> shielded. Both turntable and SA-155 have 2-wire line cords, so I doubt
> there are any ground loops. There is some audible hum, which also
> shows up in a graph made by capturing the sound card signal using a
> visual basic program. This hum appears to be a few dB above the noise
> floor, but it's there.
>
> Suggestions? Fortunately, I can measure the effect, so I will know if
> I'm making any progress. Of course I could start with trial and error,
> but if anybody else has been through this, it would save me time and
> wasted effort.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Mark D. Zacharias" <mzacharias@yis.us> wrote in message news:<2vee2qF2kg2ldU1@uni-berlin.de>...
> You need to determine if this hum is only from the turntable, the amp, or
> the result of a ground loop (very common). Turn the amps input selector to
> an Aux input. Does the hum persist? If so, the turntable is not the main
> source, though it could still have a bit of hum - many do.
> If not, then the turntable may have a bad connection internally, or bad
> cables.
> If there's still hum while the amp is hooked up, the amp might have a bad
> power supply capacitor.
>
> Ground loops can be a tough nut, but usually they are due to some connection
> between your computer and an antenna or cable connection. Try a 3-to-2 prong
> adapter from the power strip to the wall. This will lift the grounds on
> everything, not just the computer box. This is perfectly safe by the way,
> unless somebody has been monkeying around inside your computer introducing
> AC faults.
>
> If necessary disconnect any connection that may run from the computer area
> to any other part of the house, like to your main stereo.
>
>
> Mark Z.

This helps. Now a general question. Why do turntables have the
separate ground wire? I have never seen this with any other kind of
transducer. Why not just let it be grounded through the coaxial cable?

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Detector195" <Detector195@yahoo.com> wrote in message
news:6213f73a.0411101932.12fcf7ab@posting.google.com...
> "Mark D. Zacharias" <mzacharias@yis.us> wrote in message
> news:<2vee2qF2kg2ldU1@uni-berlin.de>...
>> You need to determine if this hum is only from the turntable, the amp, or
>> the result of a ground loop (very common). Turn the amps input selector
>> to
>> an Aux input. Does the hum persist? If so, the turntable is not the main
>> source, though it could still have a bit of hum - many do.
>> If not, then the turntable may have a bad connection internally, or bad
>> cables.
>> If there's still hum while the amp is hooked up, the amp might have a bad
>> power supply capacitor.
>>
>> Ground loops can be a tough nut, but usually they are due to some
>> connection
>> between your computer and an antenna or cable connection. Try a 3-to-2
>> prong
>> adapter from the power strip to the wall. This will lift the grounds on
>> everything, not just the computer box. This is perfectly safe by the way,
>> unless somebody has been monkeying around inside your computer
>> introducing
>> AC faults.
>>
>> If necessary disconnect any connection that may run from the computer
>> area
>> to any other part of the house, like to your main stereo.
>>
>>
>> Mark Z.
>
> This helps. Now a general question. Why do turntables have the
> separate ground wire? I have never seen this with any other kind of
> transducer. Why not just let it be grounded through the coaxial cable?

I'm a repair guy not an engineer (I have to say that because if I flub an
explanation someone more knowledgeable than I will hand my a** to me).

Having said that, consider:

1. Each channel of most cartridges are essentially a coil at the end of two
wires. A "balanced connection" which allows reversing or inverting the
polarity of the two channels as desired. The phono preamp is single-ended at
it's input.
2. The separate ground wire grounds the metal tonearm pipe to prevent it's
picking up hum and inducing it to the wires inside. Balanced mics have a
shield connection for the same reason.
3. With a plastic or carbon tonearm it could be just grounded by the coax
cable, but it would probably still be good practice to run a shield the
length of the tonearm.

All of this is important because the signal from (most) cartridges is very
low, the length of tonearm wires is an excellent opportunity to pick up
induced noise, plus the RIAA equalization in the phono preamp will REALLY
aggravate any hum problem.


Mark Z.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Detector195" <Detector195@yahoo.com> wrote in message
news:6213f73a.0411091921.79853b91@posting.google.com
> The setup:
>
> Old BIC 960 turntable, from garage sale.
> RCA SA-155 "integrated stereo amplifier" from Radio Shack
> PC sound input
>
> Ground wire hooked up between turntable and SA-155. The cables are all
> shielded. Both turntable and SA-155 have 2-wire line cords, so I doubt
> there are any ground loops. There is some audible hum, which also
> shows up in a graph made by capturing the sound card signal using a
> visual basic program. This hum appears to be a few dB above the noise
> floor, but it's there.

Welcome to vinyl!

> Suggestions?

Since you are transcribing with a PC, you have the option to notch out the
hum in the digital domain. Some people worry about how narrowband filtering
affects sound quality, but if the hum is audible, getting rid of it is a
major sonic improvement. Folklore aside, IME the narrower the notch, the
less likely it will have an audible effect on the music. There are some
wonderful notch filters in Audition/CEP for example.

> Fortunately, I can measure the effect, so I will know if
> I'm making any progress. Of course I could start with trial and error,
> but if anybody else has been through this, it would save me time and
> wasted effort.

First off, you want to get rid of hum due to grounding problems. It's trial
and error time!

There has been some discussion of using phono preamps with balanced inputs
to help address this kind of problem. You could probably quickly breadboard
up something that would work with two preamps driving a line amp with
balanced inputs. Remember to parallel another 47 k resistor to return the
input impedance to standard.

The hum starts out on the AC wiring for the turntable, and the motor if its
an AC motor. Once you eliminate the grounding problems, you are stuck with
the nasty job - rerouting signal wiring and applying magnetic shielding to
get rid of hum due to magnetic coupling.

If I was going to build up a low-hum turntable today, I'd probably try to do
something with DC motors with built-in tachometers or the like.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Mark D. Zacharias" wrote:

> Try a 3-to-2 prong
> adapter from the power strip to the wall. This will lift the grounds on
> everything, not just the computer box. This is perfectly safe by the way,
> unless somebody has been monkeying around inside your computer introducing
> AC faults.

*Very* stupid advice and not true it's perfectly safe.

RF interference filtering caps in the computer PSU connect from live to ground.
If the ground isn't actually grounded then you'll have a slightly live 'ground'
on the ground lifted equipment.


Graham

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:4193F7EC.3799F50B@hotmail.com...
>
>
> *Very* stupid advice and not true it's perfectly safe.

Nonsense. No different from any other 2-prong device ever made. Been doing
this for 30 years. It was just a troubleshooting suggestion anyway.

Mark Z.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

Detector195 wrote:

> This helps. Now a general question. Why do turntables have the
> separate ground wire?

Because the cartridge cables aren't connected to the turntable chassis - they *just* go to the
cartridge.


Graham

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Mark D. Zacharias" wrote:

> "Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
> news:4193F7EC.3799F50B@hotmail.com...
> >
> >
> > *Very* stupid advice and not true it's perfectly safe.
>
> Nonsense. No different from any other 2-prong device ever made. Been doing
> this for 30 years. It was just a troubleshooting suggestion anyway.

It's *very* different due to the RFI filter incprporated in every modern
computer power supply.

The AC line is connected to chassis via an RF suppressing capacitor. Remove
the ground from chassis and you have a slightly live chassis.

Won't help ground loop problems either !


Graham

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:41952AF7.D2997CF8@hotmail.com...
>
>
> "Mark D. Zacharias" wrote:
>
>> "Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
>> news:4193F7EC.3799F50B@hotmail.com...
>> >
>> >
>> > *Very* stupid advice and not true it's perfectly safe.
>>
>> Nonsense. No different from any other 2-prong device ever made. Been
>> doing
>> this for 30 years. It was just a troubleshooting suggestion anyway.
>
> It's *very* different due to the RFI filter incprporated in every modern
> computer power supply.
>
> The AC line is connected to chassis via an RF suppressing capacitor.
> Remove
> the ground from chassis and you have a slightly live chassis.
>
> Won't help ground loop problems either !
>
>
> Graham
>

Only ELIMINATE them! This assumes the ground doesn't get picked up again via
a printer, monitor, etc. This is why I mentioned that the power strip should
have it's ground lifted temporarily, so that all components are
ground-isolated at the same time.

Just to correct another point, every modern vcr has an RFI filter, but
they're a 2-prong device.

BTW I'm still not recommending a permanent lifting of this ground - it's a
TROUBLESHOOTING STEP.

Mark Z.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

On Sat, 13 Nov 2004 05:06:14 -0600, "Mark D. Zacharias"
<mzacharias@yis.us> wrote:

>Only ELIMINATE them! This assumes the ground doesn't get picked up again via
>a printer, monitor, etc. This is why I mentioned that the power strip should
>have it's ground lifted temporarily, so that all components are
>ground-isolated at the same time.
>
>Just to correct another point, every modern vcr has an RFI filter, but
>they're a 2-prong device.
>
>BTW I'm still not recommending a permanent lifting of this ground - it's a
>TROUBLESHOOTING STEP.
>
>Mark Z.
>

Lifting the ground on a power strip does not eliminate ground loops.
They will still be there between the pieces of equipment connected to
that strip. You have to lift the grounds of the individual items
plugged into the strip.

d
Pearce Consulting
http/www.pearce.uk.com

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Don Pearce" <donaldun@spamfreepearce.uk.com> wrote in message
news:4196f090.11040046@212.159.2.87...
> On Sat, 13 Nov 2004 05:06:14 -0600, "Mark D. Zacharias"
> <mzacharias@yis.us> wrote:
>
>>Only ELIMINATE them! This assumes the ground doesn't get picked up again
>>via
>>a printer, monitor, etc. This is why I mentioned that the power strip
>>should
>>have it's ground lifted temporarily, so that all components are
>>ground-isolated at the same time.
>>
>>Just to correct another point, every modern vcr has an RFI filter, but
>>they're a 2-prong device.
>>
>>BTW I'm still not recommending a permanent lifting of this ground - it's a
>>TROUBLESHOOTING STEP.
>>
>>Mark Z.
>>
>
> Lifting the ground on a power strip does not eliminate ground loops.
> They will still be there between the pieces of equipment connected to
> that strip. You have to lift the grounds of the individual items
> plugged into the strip.
>
> d
> Pearce Consulting
> http/www.pearce.uk.com

I was in fact suggesting temporarily lifting the ground connection to the
entire system to eliminate a ground loop between a cable TV line or for
example a stereo setup in another part of the house. I can say this works
since I have done exactly this, although isolation via transformer etc is
preferable.

Mark Z.

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

"Mark D. Zacharias" wrote:

> "Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
> news:41952AF7.D2997CF8@hotmail.com...

< snip >

> > It's *very* different due to the RFI filter incprporated in every modern
> > computer power supply.
> >
> > The AC line is connected to chassis via an RF suppressing capacitor.
> > Remove
> > the ground from chassis and you have a slightly live chassis.
> >
> > Won't help ground loop problems either !
> >
> >
> > Graham
>
> Only ELIMINATE them! This assumes the ground doesn't get picked up again via
> a printer, monitor, etc. This is why I mentioned that the power strip should
> have it's ground lifted temporarily, so that all components are
> ground-isolated at the same time.

Still bad practice and potentially legally actionable if you were advising it in
a professional capacity.


> Just to correct another point, every modern vcr has an RFI filter, but
> they're a 2-prong device.

So they wire the filter differently ! Grounded equipment uses Y caps to ground !
You're not capable of correcting anything it seems.

I design this stuff ! I know !


Graham

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

On Sat, 13 Nov 2004 15:45:40 -0600, "Mark D. Zacharias"
<mzacharias@yis.us> wrote:

>
>"Don Pearce" <donaldun@spamfreepearce.uk.com> wrote in message
>news:4196f090.11040046@212.159.2.87...
>> On Sat, 13 Nov 2004 05:06:14 -0600, "Mark D. Zacharias"
>> <mzacharias@yis.us> wrote:
>>
>>>Only ELIMINATE them! This assumes the ground doesn't get picked up again
>>>via
>>>a printer, monitor, etc. This is why I mentioned that the power strip
>>>should
>>>have it's ground lifted temporarily, so that all components are
>>>ground-isolated at the same time.
>>>
>>>Just to correct another point, every modern vcr has an RFI filter, but
>>>they're a 2-prong device.
>>>
>>>BTW I'm still not recommending a permanent lifting of this ground - it's a
>>>TROUBLESHOOTING STEP.
>>>
>>>Mark Z.
>>>
>>
>> Lifting the ground on a power strip does not eliminate ground loops.
>> They will still be there between the pieces of equipment connected to
>> that strip. You have to lift the grounds of the individual items
>> plugged into the strip.
>>
>> d
>> Pearce Consulting
>> http/www.pearce.uk.com
>
>I was in fact suggesting temporarily lifting the ground connection to the
>entire system to eliminate a ground loop between a cable TV line or for
>example a stereo setup in another part of the house. I can say this works
>since I have done exactly this, although isolation via transformer etc is
>preferable.
>
>Mark Z.
>
If the ground loop was to another part of the house, then lifting the
ground to the power strip was hardly lifting the entire system. It is
just as likely that the ground loop is between two bits of kit plugged
into the one power strip - in which case you have achieved nothing.

d
Pearce Consulting
http/www.pearce.uk.com

 

[Guest]

Archived from groups: rec.audio.tech (More info?)

Don Pearce wrote:

> If the ground loop was to another part of the house, then lifting the
> ground to the power strip was hardly lifting the entire system. It is
> just as likely that the ground loop is between two bits of kit plugged
> into the one power strip - in which case you have achieved nothing.

Few ppl realise that PCs have EMI filters that have caps from line to ground.

These cause ground currents that can result in audio problems.


Graham