Induced hum on an MC cart.
 

I have an Ortophon MC SL15 cart that was originally fed via a transformer
into a normal pickup input. Since 'rationalising' to a pre-amp with no
pickup input, I've fitted a pre-amp to the turntable. It's a design by
Stewart Pinkerton who used to post here. I'm pleased with the results.

However, the cartridge seems much more prone to hum pickup from external
sources. First of all from my PS for the RIAA pre-amp - sorted by changing
to a toroidal transformer. Then from a wall wart some 3 feet away. Unplug
the cart and the hum disappears. The obvious answer was to re-site the
wall wart - but I was idly wondering if the cart would be more susceptible
to hum pickup with a pre-amp rather than transformer? As it never seemed
to suffer from this before - even with a large power amp transformer close
by.

--
*I took an IQ test and the results were negative.

Dave Plowman London SW
To e-mail, change noise into sound.

Induced hum on an MC cart.
 

In article , Dave Plowman (News)
wrote:
I have an Ortophon MC SL15 cart that was originally fed via a
transformer into a normal pickup input. Since 'rationalising' to a
pre-amp with no pickup input, I've fitted a pre-amp to the turntable.
It's a design by Stewart Pinkerton who used to post here. I'm pleased
with the results.

However, the cartridge seems much more prone to hum pickup from external
sources. First of all from my PS for the RIAA pre-amp - sorted by
changing to a toroidal transformer. Then from a wall wart some 3 feet
away. Unplug the cart and the hum disappears. The obvious answer was to
re-site the wall wart - but I was idly wondering if the cart would be
more susceptible to hum pickup with a pre-amp rather than transformer?
As it never seemed to suffer from this before - even with a large power
amp transformer close by.

Was the signal transformer acting as a way to balance the input and reject
any common mode? If so, then yes I can see that the amp might not do this
as effectively. If a magnetic field is inducing similar currents on the
live and ground leads, then the amp may sink that on the ground, and sense
it on the live. Or is the amp input well balanced and differential?

Can you say more about the amp, transformer, etc?

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Induced hum on an MC cart.
 

In article ,
Jim Lesurf wrote:
In article , Dave Plowman (News)
wrote:
I have an Ortophon MC SL15 cart that was originally fed via a
transformer into a normal pickup input. Since 'rationalising' to a
pre-amp with no pickup input, I've fitted a pre-amp to the turntable.
It's a design by Stewart Pinkerton who used to post here. I'm pleased
with the results.

However, the cartridge seems much more prone to hum pickup from
external sources. First of all from my PS for the RIAA pre-amp -
sorted by changing to a toroidal transformer. Then from a wall wart
some 3 feet away. Unplug the cart and the hum disappears. The obvious
answer was to re-site the wall wart - but I was idly wondering if the
cart would be more susceptible to hum pickup with a pre-amp rather
than transformer? As it never seemed to suffer from this before - even
with a large power amp transformer close by.

Was the signal transformer acting as a way to balance the input and
reject any common mode? If so, then yes I can see that the amp might not
do this as effectively. If a magnetic field is inducing similar currents
on the live and ground leads, then the amp may sink that on the ground,
and sense it on the live. Or is the amp input well balanced and
differential?

Can you say more about the amp, transformer, etc?

Jim

The amp has a balanced input - it has an SSM2017 for the first stage. Star
grounding too. The transformer input is fully floating. It was situated
close to the amp input.

I'm not sure the whole thing is more susceptible to hum - just that it was
never a problem before. And having altered so much it's not like for like.

--
*Virtual reality is its own reward*

Dave Plowman London SW
To e-mail, change noise into sound.

Induced hum on an MC cart.
 

On 19/09/2013 10:44 PM, Dave Plowman (News) wrote:
I have an Ortophon MC SL15 cart that was originally fed via a transformer
into a normal pickup input. Since 'rationalising' to a pre-amp with no
pickup input, I've fitted a pre-amp to the turntable. It's a design by
Stewart Pinkerton who used to post here. I'm pleased with the results.

However, the cartridge seems much more prone to hum pickup from external
sources. First of all from my PS for the RIAA pre-amp - sorted by changing
to a toroidal transformer. Then from a wall wart some 3 feet away. Unplug
the cart and the hum disappears. The obvious answer was to re-site the
wall wart - but I was idly wondering if the cart would be more susceptible
to hum pickup with a pre-amp rather than transformer? As it never seemed
to suffer from this before - even with a large power amp transformer close
by.


**Could be an input impedance issue. MC transformers often exhibit a
nice low load impedance to the cartridge. This can assist with hum
rejection. Additionally, a transformer can be arranged as a true
balanced load. Can you fill us in on the configuration of the head amp?
Input impedance? Unbalanced?

--
Trevor Wilson www.rageaudio.com.au

Induced hum on an MC cart.
 

"Jim Lesurf"

Was the signal transformer acting as a way to balance the input and reject
any common mode? If so, then yes I can see that the amp might not do this
as effectively. If a magnetic field is inducing similar currents on the
live and ground leads, then the amp may sink that on the ground, and sense
it on the live. Or is the amp input well balanced and differential?

** With a "floating " signal source like a PU cartridge or a microphone -
there is no common mode signal.

External magnetic fields from transformers inject AC frequency hum in
differential mode into the coils of the transducers and nothing in the
matching device can fix that.

The connecting cable is an interesting case and one that most people get
wrong.

1. In order to pick up hum from a magnetic field, there must be a loop of
some open area. A figure 8 type cable has a small, but definite loop area.

2. Microphone cables normally used twisted pair of wires with an overall
earthed shield - the twisting creates many loops of opposing polarity and
so there is very little hum pickup.

3. A co-axial cable also rejects external magnetic hum pickup due to its
symmetry - in fact rather better than a twisted pair one does. If you doubt
this, please try it.

IOW, the whole idea that PU and mic cables and inputs need to be "balanced"
to reject AC frequency hum is a nonsense.



.... Phil

Induced hum on an MC cart.
 

In article , Phil Allison
wrote:

"Jim Lesurf"

Was the signal transformer acting as a way to balance the input and
reject any common mode? If so, then yes I can see that the amp might
not do this as effectively. If a magnetic field is inducing similar
currents on the live and ground leads, then the amp may sink that on
the ground, and sense it on the live. Or is the amp input well
balanced and differential?

** With a "floating " signal source like a PU cartridge or a microphone
- there is no common mode signal.

Yes, the intended signal isn't common mode.

However I've certainly encountered cases where you can measure common
mode currents induced by external magnetic fields. This isn't prevented by
normal co-ax or twisted pairs, etc.


1. In order to pick up hum from a magnetic field, there must be a loop
of some open area. A figure 8 type cable has a small, but definite loop
area.

That is one of the mechanisms. But not the only one.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Induced hum on an MC cart.
 

In article , Dave Plowman (News)
wrote:
In article , Jim Lesurf


The amp has a balanced input - it has an SSM2017 for the first stage.
Star grounding too.

I'd need to know details like the impedances to ground of both input leads
(+ve and -ve). Both their sizes and how similar they may be can matter.

Have you changed the grounding for the MC side? Was this grounded, or left
floating, or via a resistor (or two), or center-tapped, when using the
transformer?

The transformer input is fully floating. It was situated close to the
amp input.

The advantage of floating here is that it helps ensure similar impedance to
ground for both inputs.

I'm not sure the whole thing is more susceptible to hum - just that it
was never a problem before. And having altered so much it's not like for
like.

Hard to comment without knowing more.

The problem is that hum can arise in a number of ways.

Slainte,

Jim

--
Please use the address on the audiomisc page if you wish to email me.
Electronics http://www.st-and.ac.uk/~www_pa/Scot...o/electron.htm
Armstrong Audio http://www.audiomisc.co.uk/Armstrong/armstrong.html
Audio Misc http://www.audiomisc.co.uk/index.html

Induced hum on an MC cart.
 

In article ,
Jim Lesurf wrote:
In article , Dave Plowman (News)
wrote:
In article , Jim Lesurf


The amp has a balanced input - it has an SSM2017 for the first stage.
Star grounding too.

I'd need to know details like the impedances to ground of both input
leads (+ve and -ve). Both their sizes and how similar they may be can
matter.

Each leg of the SSM 2017 is grounded via a 51 ohm resistor. I'm not quite
sure how these are calculated - the data sheet isn't much help. The
circuit is for a different make of MC cart - but was told it should work
ok with mine. All I've done is to increase the gain of the SM2017
slightly. The cart it was designed for has a 12 ohm output.

Have you changed the grounding for the MC side? Was this grounded, or
left floating, or via a resistor (or two), or center-tapped, when using
the transformer?

The cart itself just has the normal four terminals. The transformer has
floating inputs and outputs (four input terminals and four output ones) -
although of course one leg of each output would be grounded by the pre-amp
input if a normal unbalanced type. It's marked 2 ohms input, 10-50k
output.

The transformer input is fully floating. It was situated close to the
amp input.

The advantage of floating here is that it helps ensure similar impedance
to ground for both inputs.

I'm not sure the whole thing is more susceptible to hum - just that it
was never a problem before. And having altered so much it's not like
for like.

Hard to comment without knowing more.

The problem is that hum can arise in a number of ways.

Slainte,

Jim

--
*They told me I had type-A blood, but it was a Type-O.*

Dave Plowman London SW
To e-mail, change noise into sound.

Induced hum on an MC cart.
 

"Jim Lesurf"
Phil Allison

"Jim Lesurf"

Was the signal transformer acting as a way to balance the input and
reject any common mode? If so, then yes I can see that the amp might
not do this as effectively. If a magnetic field is inducing similar
currents on the live and ground leads, then the amp may sink that on
the ground, and sense it on the live. Or is the amp input well
balanced and differential?

** With a "floating " signal source like a PU cartridge or a microphone
- there is no common mode signal.

Yes, the intended signal isn't common mode.


** There simply is no common mode signal.



However I've certainly encountered cases where you can measure common
mode currents induced by external magnetic fields.


** Impossible with a floating source like a PU or mic.

Do try to pay attention, Jim.


1. In order to pick up hum from a magnetic field, there must be a loop
of some open area. A figure 8 type cable has a small, but definite loop
area.

That is one of the mechanisms.


** There is no other - you bull****ting, over snipping fool.

Try to pay attention, I know that is hard for the senile.

But otherwise you will never learn how just how wrong you are.

And you need to - yet again.





.... Phil

Just for Jim
 

"Jim Lesurf"


** Try actually READING this in ONE piece.

Do not SNIP.

Do no pass GO.

Do not fob it off with your usual smug crapology.
----------------------------------------------------------


** With a "floating " signal source like a PU cartridge or a microphone -
there is no common mode signal.

External magnetic fields from transformers inject AC frequency hum in
differential mode into the coils of the transducers and nothing in the
matching device can fix that.

The connecting cable is an interesting case and one that most people get
wrong.

1. In order to pick up hum from a magnetic field, there must be a loop of
some open area. A figure 8 type cable has a small, but definite loop area.

2. Microphone cables normally used twisted pair of wires with an overall
earthed shield - the twisting creates many loops of opposing polarity and
so there is very little hum pickup.

3. A co-axial cable also rejects external magnetic hum pickup due to its
symmetry - in fact rather better than a twisted pair one does. If you doubt
this, please try it.

IOW, the whole idea that PU and mic cables and inputs need to be "balanced"
to reject AC frequency hum is a nonsense.

Which you have obviously swallowed whole.



.... Phil