On Fri, 20 Sep 2013 15:19:45 +0100, "Dave Plowman (News)"
wrote:

In article ,
Jim Lesurf wrote:

[1] May well be something else like a side-effect of way he changed the
gain, os some detail we don't yet know.


I don't *think* so. The hum level appeared to come up with the signal
level when I changed the gain - although I didn't attempt to measure it.
Discovered early on it was cause by the PS transformer (cured by using an
external one) so didn't do much more until the replacement toroidal (and
Eddystone box) for it arrived.

Dave. Unplug the headshell again and replace the cart connections with
a couple of shorting links. Is the hum still there then?

d

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

[1] May well be something else like a side-effect of way he changed
the gain, os some detail we don't yet know.


I don't *think* so. The hum level appeared to come up with the signal
level when I changed the gain - although I didn't attempt to measure it.

Having seen the circuit and the way you changed the gain I'd think you are
correct that the gain change hasn't caused the hum. Before I'd seen the
info I was worried as some 'differential' designs aren't that well balanced
so the gain can affect this. You can see it to some extent in the specs for
the 2017. But not to a level that looks particularly significant here.

I suspect it may be related to the fact that the system isn't now floating.
So it would be interesting to know what effect two changes make.

1) Changing the 51 Ohm resistors to something much larger - 5 to 10 k
range.[1]

2) Putting a loading resistor *between* the leads to act explicitly like a
differental signal load.

However as before it is hard to tell without having the system to examine
and experiment with! Could be something 'unexpected' like a poor connection
which arose when you changed things. :-)

Slainte,

Jim

[1] Confess I also wondered about replacing the pair of 51 Ohms with a 100
Ohm skeleton pot (ground on the finger, leads on the ends) to see if
adjusting this would 'hum buck' and let you null an imbalance that was
causing common-mode injection via imbalance to give you hum! But if this is
the problem I suspect the above is a better ploy.

--
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
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Audio Misc http://www.audiomisc.co.uk/index.html

"Dave Plowman (News)" wrote in message
...
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.

Put the preamp and as much of whatever goes with it inside a steel shield.

The last time I had problems with magnetically induced hum I found that I
could fit the device into a tomato paste can, and I found an olive jar lid
that slipped into the can for a friction fit.

"Phil Allison" wrote in message
...

"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.

I think there is - capacitance. However, rule of thumb is that signals 10
KHz don't travel well that way.

On Fri, 20 Sep 2013 14:28:20 +0100, Jim Lesurf
wrote:

In article , Phil Allison
wrote:

"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.
----------------------------------------------------------

OK. In return: Do not reply with abuse. :-)


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

Although true for the intended signal from the sensor (MC here), there can
be common mode induction from something else. Particularly in the case Dave
is describing where the leads are *not* "floating" as they each have a 51
Ohm resistor to ground.


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.

That is, indeed, one mechanism.

However another is to realise that the magnetic field is varying with time
(hum) and this will set up an alternating potential difference along even a
single wire.

Put two very similar wires along much the same path in the time-varying
magnetic field, and they will have inductions that are largely common mode.

In an ideal case - perfectly identical wires, experiencing exactly the same
field, driving a perfect and totally floating transformer, this has no
effect. But reality isn't always so perfect. And Dave's system clearly
isn't. Nor is it now floating.


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.

Afraid that won't solve the problem I'm describing and which I've had to
deal with more than once in the past.

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.

I have. :-)

You could consider why as you say, "microphone cables use a twisted pair
with an overall earthed shield". Why *both* if coax does the job perfectly?

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.

Straw man detected. :-)

I followed your requests. (Do not snip, etc). How about you following mine:

OK. NO losing your temper and becoming abusive. Just stay calm and think
about what I've actually described. I've no idea if it *is* the cause of
Dave's problem. [1] But lacking more info, it might be. Neither coax nor
twisted pair will prevent what I'm describing if the circumstances allow
it. Been there, have the Tee shirt!

Slainte,

Jim

[1] May well be something else like a side-effect of way he changed the
gain, os some detail we don't yet know.

There is also a problem with using the two 51 ohm resistors. Because
they go individually to ground, the balance of the circuit is only as
good as the tolerance of the resistors with respect to each other. A
better way would be to use a single hundred ohms between the two live
pins. If the chip needs DC ground returns, this could be done with a
pair of large value (10k?) resistors, which wouldn't impact the signal
balance.

d

On Fri, 20 Sep 2013 11:58:51 -0400, "Arny Krueger"
wrote:


"Dave Plowman (News)" wrote in message
...
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.

Put the preamp and as much of whatever goes with it inside a steel shield.

The last time I had problems with magnetically induced hum I found that I
could fit the device into a tomato paste can, and I found an olive jar lid
that slipped into the can for a friction fit.


Oxo or Altoids, I believe are the containers of choice for electronics
circuits.

d

In article ,
Arny Krueger wrote:
Put the preamp and as much of whatever goes with it inside a steel
shield.

Sadly, it's the cart itself which is picking up the hum. And I don't think
it would track too well inside a tin can. ;-)

If I remove the head shell and cart the hum stops completely. Regardless
of the input wiring being terminated or not.

--
*Would a fly without wings be called a walk?

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

In article ,
Don Pearce wrote:
I don't *think* so. The hum level appeared to come up with the signal
level when I changed the gain - although I didn't attempt to measure
it. Discovered early on it was cause by the PS transformer (cured by
using an external one) so didn't do much more until the replacement
toroidal (and Eddystone box) for it arrived.

Dave. Unplug the headshell again and replace the cart connections with
a couple of shorting links. Is the hum still there then?

No hum. If I wind the wick up pretty high so you can here the 'hiss' there
is a change in that if I short the input. At the headshell.

--
*If you don't like the news, go out and make some.

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

In article ,
Don Pearce wrote:
There is also a problem with using the two 51 ohm resistors. Because
they go individually to ground, the balance of the circuit is only as
good as the tolerance of the resistors with respect to each other. A
better way would be to use a single hundred ohms between the two live
pins. If the chip needs DC ground returns, this could be done with a
pair of large value (10k?) resistors, which wouldn't impact the signal
balance.

Right - food for thought and easy enough to implement.

The circuit is designed for a cart with a nominal 12 ohm impedance and
mine is 2 ohms. So is it worth using a lower value shunt?

--
wife.

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

On Fri, 20 Sep 2013 17:16:32 +0100, "Dave Plowman (News)"
wrote:

In article ,
Arny Krueger wrote:
Put the preamp and as much of whatever goes with it inside a steel
shield.

Sadly, it's the cart itself which is picking up the hum. And I don't think
it would track too well inside a tin can. ;-)

If I remove the head shell and cart the hum stops completely. Regardless
of the input wiring being terminated or not.

Is the deck now sitting close to the mains transformer in another
piece of kit?

d