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.

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

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.

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

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.

In article , Dave Plowman (News)
wrote:
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.

I'd suspect they are chosen as a "reasonably value" much as many circuit
examples tend to use, say, 10k resistors or other round numbers. I'd guess
the aim is that these resistors have values much lower than any 'internal'
leakage resistances to ground in the chip

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.

How have you altered the gain?

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.

So you've switched from 'floating' (i.e. 'very high' resistances to ground)
to 'balanced' with relatively low values to ground.

That can mean you're getting more actual induced current flow in the leads.

Are the leads co-ax or twisted pair?

Coax will probably have a different series resistance for its sheild than
for its inner. Which could mean the two currents end up being imbalanced.
However as before I'd need to know more.

When making ultra-low-noise amps for far-infrared sensors it could be a
pain dealing with such matters as the signals are so tiny and even small
'imbalances' can cause pickup.

FWIW in pre-amps I've sometimes found being able to vary the resistances
(inc having a resistor in the dc ground return to 'soak up' the hum in some
cases) helps. Very hard to diagnose as when the signal levels are small it
is easy to miss the presence of effects if you take for granted the problem
is the classic 'hum loop' when in reality it is some other mechanism.

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

In article , Dave Plowman (News)
wrote:

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.

I don't know the 2017 but I've been looking on the web. e.g.s

http://www.dz863.com/datasheet-81085...-Preamplifier/

http://wiringdiagramcircuit.com/mono...7-and-ssm2142/

and found a data sheet.

I assume you have changed the gain by changing the resistor across the two
input devices?

Two things struck me.

1) That one of the above shows much higher value grounding resistors - 6k8
I think. That would tend to reduce any common mode induced currents
compared with 51 Ohms. And probably also do a better job of swamping any
difference in series resistance of the leads.

2) The CMMR varies with the gain ( que surprise! :-) )

Does it matter to the hum if the MC is actually connected or not?

Also, the 2017 on a quick look seems to have a very high input resistance.
I'm wondering if using a shunt to give the MC some loading might be a good
idea just as a general thought. If the 51s are really meant to look like a
100 Ohm load this might be a better approach at least in principle.

Its also possible you're getting more hum just because the lack of loading
is allowing the MC to give a bigger level.

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

"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

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

"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