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Demagnetizing micpre transformers?

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Anonymous
September 11, 2004 10:48:31 PM

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

Ok, I read somewhere comments to the effect that transformers such as one
might find on the input or output stages of micpres become magnetized over
time. And might sound better after demagnetizing, Never heard of that in my
training, what is the story with this? Is it neccessary, does it really
improve things (or not?) It is helps how often, and what is the procedure on a
console's micpres? What about microphones with transformers in them? Would
emaging them destroy the "vintage color", if so, would magnetizng make them
sound more colorful?

And is there a way to do the audio version of geomagnetic tilt correction
like on my Flat Screen TV? <G>


Will Miho
NY Music & TV Audio Guy
Off the Morning Show! & sleepin' In... / Fox News
"The large print giveth and the small print taketh away..." Tom Waits
Anonymous
September 11, 2004 10:48:32 PM

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

In article <20040911144831.09347.00000740@mb-m05.aol.com> willstg@aol.comnospam writes:

> Ok, I read somewhere comments to the effect that transformers such as one
> might find on the input or output stages of micpres become magnetized over
> time. And might sound better after demagnetizing,

I suppose it's possible. I heard that story about the transformers in
ribbon mics (being the REAL [someone else's emphasis] problem with
"phantom powering"a ribbon mic. This was debunked fairly convincingly
based on the remanance (if that's the right parameter) of the core
material not making a very good permanent magnet.

Ask Phil. He'll know (something).


--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
Anonymous
September 12, 2004 12:07:47 AM

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

WillStG <willstg@aol.comnospam> wrote:
> Ok, I read somewhere comments to the effect that transformers such as one
>might find on the input or output stages of micpres become magnetized over
>time.

This MIGHT happen with some kinds of core materials, if there is DC offset
on the transformers. It doesn't sound like a very likely thing to me, but
I could see it happening.

And might sound better after demagnetizing, Never heard of that in my
>training, what is the story with this? Is it neccessary, does it really
>improve things (or not?)

If there is residual magnetism in the core, you can measure it with the
same magnetometer you use to check tape heads. If it exists, it will
result in poorer low end response (and that is an easy thing to measure).

It is helps how often, and what is the procedure on a
>console's micpres? What about microphones with transformers in them? Would
>emaging them destroy the "vintage color", if so, would magnetizng make them
>sound more colorful?

Magnetizing them will reduce the low end but it will also change the low
end character a little.

You could run a very loud 60 Hz signal into the input if you wanted to try
and demagnetize the core. Or you could use an Annis Han-D-Mag like you use
on larger tape heads. I can't imagine it would be a big deal in a typical
installation, though I could see it being a major problem in a place with
poor maintenance and lots of leaky capacitors that might throw DC on
transformers.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
September 12, 2004 12:50:01 AM

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

On 11 Sep 2004 18:48:31 GMT, willstg@aol.comnospam (WillStG) wrote:

> Ok, I read somewhere comments to the effect that transformers such as one
>might find on the input or output stages of micpres become magnetized over
>time. And might sound better after demagnetizing, Never heard of that in my
>training, what is the story with this? Is it neccessary, does it really
>improve things (or not?) It is helps how often, and what is the procedure on a
>console's micpres? What about microphones with transformers in them? Would
>emaging them destroy the "vintage color", if so, would magnetizng make them
>sound more colorful?

If you want to try it, Sheffield Lab made a "Test & Burn-In" CD that
included sweep signals intended for low-level demag, among other
oddities. 10014-2-T.

I couldn't tell a real difference on phono cartridges, but it's
easy enough to try for yourself. You'll need to patch a small pad
between the output of a CD player and the transformer, then just
play the track.

Chris Hornbeck
Anonymous
September 12, 2004 12:50:02 AM

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

"Chris Hornbeck" <chrishornbeckremovethis@att.net> wrote in message
news:4qo6k0pofn5k2rrnkvak8877ufdll2sipe@4ax.com
> On 11 Sep 2004 18:48:31 GMT, willstg@aol.comnospam (WillStG) wrote:
>
>> Ok, I read somewhere comments to the effect that transformers
>> such as one might find on the input or output stages of micpres
>> become magnetized over time. And might sound better after
>> demagnetizing, Never heard of that in my training, what is the
>> story with this? Is it neccessary, does it really improve things
>> (or not?) It is helps how often, and what is the procedure on a
>> console's micpres? What about microphones with transformers in
>> them? Would emaging them destroy the "vintage color", if so, would
>> magnetizng make them sound more colorful?
>
> If you want to try it, Sheffield Lab made a "Test & Burn-In" CD that
> included sweep signals intended for low-level demag, among other
> oddities. 10014-2-T.

AFAIK, in order to demagnetize something that is seriously magnetized, you
have to magnetize it beyond saturation in both directions, and then let the
demagnetizing field die out slowly. The probability of a test record be
stimulating a transformer or other audio component strongly enough to
accomplish this seems unlikely.
Anonymous
September 12, 2004 12:50:02 AM

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

Phono pickups do get magnetized. I discovered this by accident ca. 1984, but
didn't understand what I had found.

I would be cautious about demagnetizing VR or IM pickups. But you're safe with
MM (take off the stylus) and MC pickups.

As for transformer preamps... I'd short the input and output, then bring a
really powerful bulk-demagnetizer near them.
Anonymous
September 12, 2004 4:16:15 AM

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

On Sat, 11 Sep 2004 17:31:22 -0400, "Arny Krueger" <arnyk@hotpop.com>
wrote:

>> If you want to try it, Sheffield Lab made a "Test & Burn-In" CD that
>> included sweep signals intended for low-level demag, among other
>> oddities. 10014-2-T.
>
>AFAIK, in order to demagnetize something that is seriously magnetized, you
>have to magnetize it beyond saturation in both directions, and then let the
>demagnetizing field die out slowly.

That's also been my understanding, except the saturation part, for
which I'll take your word. The Sheffield CD has a constant amplitude
sweep tone from 40 Hz to 19 kHz track and a conventional declining
amplitude fixed (low) frequency track.

Strangely, both are specified for demag.

> The probability of a test record be
>stimulating a transformer or other audio component strongly enough to
>accomplish this seems unlikely.

An ordinary CD player should have plenty of output to shake,
rattle and roll a phono cartridge, but transformers vary a lot.

If you're certain about the need to saturate first, then an
amplifier in line might be needed for bigger cores. Maybe with
a series resistor to observe current.

Then just a constant tone and a hand on the volume control. Set
it for the center of the Sun.

Chris Hornbeck
Anonymous
September 12, 2004 10:17:50 AM

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

"Chris Hornbeck" <chrishornbeckremovethis@att.net> wrote in message
news:jd47k0thlq1rvpfm7r8h1lgupmbkuerv52@4ax.com
> On Sat, 11 Sep 2004 17:31:22 -0400, "Arny Krueger" <arnyk@hotpop.com>
> wrote:
>
>>> If you want to try it, Sheffield Lab made a "Test & Burn-In" CD that
>>> included sweep signals intended for low-level demag, among other
>>> oddities. 10014-2-T.
>>
>> AFAIK, in order to demagnetize something that is seriously
>> magnetized, you have to magnetize it beyond saturation in both
>> directions, and then let the demagnetizing field die out slowly.
>
> That's also been my understanding, except the saturation part, for
> which I'll take your word.

This looks like a good reference:

http://home.flash.net/~mrltapes/mcknight_demag.pdf

It says:

"Demagnetizing: If the demagnetizer is to demagnetize
the core laminations, then the field that it produces must be
large enough to cause the induction (flux density) in the core
to approach saturation."

>The Sheffield CD has a constant amplitude
> sweep tone from 40 Hz to 19 kHz track and a conventional declining
> amplitude fixed (low) frequency track.

> Strangely, both are specified for demag.

>> The probability of a test record be
>> stimulating a transformer or other audio component strongly enough to
>> accomplish this seems unlikely.

> An ordinary CD player should have plenty of output to shake,
> rattle and roll a phono cartridge, but transformers vary a lot.

The reference above talks about how you can determine that you are applying
enough current for demagnetizing things.

> If you're certain about the need to saturate first, then an
> amplifier in line might be needed for bigger cores. Maybe with
> a series resistor to observe current.

That might be necessary.

> Then just a constant tone and a hand on the volume control.

Fade out.
Anonymous
September 12, 2004 10:37:03 AM

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

On Sun, 12 Sep 2004 15:46:39 +1000, Tony <tony_roe@tpg.com.au> wrote:

>Mu-metal cores can indeed become magnetized, and it seriously degrades
>their performance. I have no idea how easily that can happen, but in
>principal it might happen if you short one signal line to ground with
>the other open, and apply phantom power. Trouble is, it can NOT be
>reversed by a demagnetizer (eg by deliberately applying a diminishing
>AC flux) - you need to disassemble the transformer and thermally
>anneal the core (not usually a practical option). IOW, don't touch it
>or you'll go blind.

IOW, we've only been kidding ourselves all these years demag'ing tape
heads and their shields and such. Bummer.

Chris Hornbeck
Anonymous
September 12, 2004 12:38:11 PM

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

WillStG wrote:

> Ok, I read somewhere comments to the effect that transformers such as
> one might find on the input or output stages of micpres become magnetized
> over time. And might sound better after demagnetizing, Never heard
> of that in my training, what is the story with this? Is it neccessary,
> does it really improve things (or not?) It is helps how often, and what
> is the procedure on a console's micpres? What about microphones with
> transformers in them? Would emaging them destroy the "vintage color",
> if so, would magnetizng make them sound more colorful?

In December 1998 there was a detailed discussion of this topic on the
blessedly moderated Pro Audio mailing list which the late Gabe Wiener
founded when he fled this place.

Given the rather limited current spikes that can flow in any real-world
(i.e. partly capacitive) microphone cable even if 48 V phantom powering
is connected one pin at a time, transformer magnetization was described
as "not a common problem." Nonetheless the effect does exist, and can
be seen as an increase of a few hundredths of a percent in THD at the
lowest audio frequencies.

Since we tend to be insensitive to harmonic distortion at low frequencies,
and since no such increase in distortion could be found at midrange or
higher frequencies at all, I think a fair summary of the discussion would
be (with contributions from David Josephson and Bill Whitlock among others)
that it's not worth the worry as far as audible "coloring" is concerned.

But this is disgraceful. You've allowed yourself to doubt the logic which
says, "I can imagine how a certain effect _could_ occur, and therefore it
is a serious, hitherto unacknowledged problem in real-world audio."

Don't you realize that when a person can reach a conclusion intuitively,
without relying on the poor crutches of mathematics or physical evidence,
he is being much more brilliant than any scientist? And what a miracle
it is that unattributed, unproven allegations can be passed down a wire?

The very worst form of thought crime, which you have committed here, is
to raise questions about the _degree_ or _extent_ of any effect that is
authoritatively believed to perhaps be audible, or which cannot be proved
conclusively to be inaudible (which of course nothing ever can be). That
would imply limits to our ability to hear things which can be measured!

No idea could be more horrible in its implications. If widely believed
or even considered, it would stand the entire theological structure of
the audiophile universe on its head. You should confess and apologize
to this group forthwith, young man.

--Seriously, it was noted that some transformers could become microphonic
if their mu-metal cases were to become magnetized, but that isn't something
that would occur from ordinary use either--more likely from some mishap
during maintenance or repair work.

--best regards
Anonymous
September 12, 2004 1:37:11 PM

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

Chris Hornbeck <chrishornbeckremovethis@att.net> wrote:
>On Sun, 12 Sep 2004 15:46:39 +1000, Tony <tony_roe@tpg.com.au> wrote:
>
>>Mu-metal cores can indeed become magnetized, and it seriously degrades
>>their performance. I have no idea how easily that can happen, but in
>>principal it might happen if you short one signal line to ground with
>>the other open, and apply phantom power. Trouble is, it can NOT be
>>reversed by a demagnetizer (eg by deliberately applying a diminishing
>>AC flux) - you need to disassemble the transformer and thermally
>>anneal the core (not usually a practical option). IOW, don't touch it
>>or you'll go blind.
>
>IOW, we've only been kidding ourselves all these years demag'ing tape
>heads and their shields and such. Bummer.

For the most part, yes. Lots of people demagnetize heads unnecessarily
and wind up doing more harm than good. That is why Annis sells a little
magnetometer (and I think Tape Warehouse still stocks it), so you can check
the field strength on your heads and tape path, and demagnetize only when
necessarily. It's a little on the expensive side as I recall, but a whole
lot less than the cost of a new set of heads even on a 2-track.

It wouldn't hurt to check transformers the same way.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
September 12, 2004 1:37:12 PM

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

"Scott Dorsey" wrote ...
> For the most part, yes. Lots of people demagnetize heads
> unnecessarily and wind up doing more harm than good.
> That is why Annis sells a little magnetometer (and I think
> Tape Warehouse still stocks it), so you can check the field
> strength on your heads and tape path, and demagnetize only
> when necessarily. It's a little on the expensive side as I
> recall, but a whole lot less than the cost of a new set of
> heads even on a 2-track. It wouldn't hurt to check
> transformers the same way.

Doesn't that assume you can get close enough to the transformer?
Many (most?) are enclosed/potted so that the best you can do
is to get a few mm away from any part of the core. And the
better ones are magnetically shielded (mu-metal, etc.) so that
even if there were a residual mag field, seems unlikely that
you could measure it externally.

And besides if the transformer is reasonably efficient, it is a
closed magnetic circuit and has no point where the field can
be measured. Mag tape heads are designed with the mag
circuit open at the gap where the tape passes (and where it
is convienent to measure with the magnetometer, etc.)

It seems to me that any kind of electrical demag attempt
would put the adjacent components at risk from the significant
power required. A transformer that is not mu-metal shielded
might be be degaussed with a bulk eraser (like we use for
tape reels & cassettes), but it would certainly require taking
the case apart to gain direct access to the transformers.

Seems like more trouble (and potential damage) than it is worth.
Anonymous
September 12, 2004 8:21:18 PM

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

Richard Crowley <rcrowley7@xprt.net> wrote:
>
>Doesn't that assume you can get close enough to the transformer?
>Many (most?) are enclosed/potted so that the best you can do
>is to get a few mm away from any part of the core. And the
>better ones are magnetically shielded (mu-metal, etc.) so that
>even if there were a residual mag field, seems unlikely that
>you could measure it externally.

This is often true, yes.

>And besides if the transformer is reasonably efficient, it is a
>closed magnetic circuit and has no point where the field can
>be measured. Mag tape heads are designed with the mag
>circuit open at the gap where the tape passes (and where it
>is convienent to measure with the magnetometer, etc.)

This is also true, although if the core is magnetized in any direction
other than the normal direction of flux, it may leak severely.

The other handy way is to run a 1 KHz square wave through the thing, and
increase the level until you start seeing the top and bottom of the waveform
slanting downwards (due to poor low frequency response at high levels). If
the top end bottom angles are different, you have an asymmetry of some sort,
possibly due to magnetization.

>It seems to me that any kind of electrical demag attempt
>would put the adjacent components at risk from the significant
>power required. A transformer that is not mu-metal shielded
>might be be degaussed with a bulk eraser (like we use for
>tape reels & cassettes), but it would certainly require taking
>the case apart to gain direct access to the transformers.

Maybe, but even mu-metal isn't a perfect shield.

>Seems like more trouble (and potential damage) than it is worth.

I agree, and having measured lots of junk box transformers for an article
on transformer identification recently, I have not found any that have
any core magnetization problems. This doesn't mean they don't exist, though,
just that I don't have any in my garage.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
Anonymous
September 12, 2004 9:18:43 PM

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

Scott Dorsey wrote:

> Lots of people demagnetize heads unnecessarily
> and wind up doing more harm than good. That is why Annis sells a little
> magnetometer (and I think Tape Warehouse still stocks it), so you can check
> the field strength on your heads and tape path, and demagnetize only when
> necessarily. It's a little on the expensive side as I recall, but a whole
> lot less than the cost of a new set of heads even on a 2-track.

Before I got the Annis rig with the magnetometer I would ritually
"demagnetize" the machines' heads. After I got the measurement device I
found I very rarely needed to demag, and I could also tell whether I had
done that or just more severely magnetized a head. The Annis kit paid
for itself in time not wasted "demagnetizing" that which wasn't
magnetized in the first place.

--
ha
Anonymous
September 13, 2004 3:46:01 AM

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

"hank alrich" <walkinay@thegrid.net> wrote in message
news:1gjzvtf.2qpchfub7kcgN%walkinay@thegrid.net...
> Before I got the Annis rig with the magnetometer I would ritually
> "demagnetize" the machines' heads. After I got the measurement device I
> found I very rarely needed to demag, and I could also tell whether I had
> done that or just more severely magnetized a head.


We were told about the Annis rig by an Ampex technician at the time we got
our MM-1000s. It seems there was a big problem with people ritually
magnetizing their new MM-1000's heads using their demagnetizers!

The technician also sheepishly told us that the head demagnetizing ritual
had been created by Ampex as a solution to the problem that an Ampex 300's
heads would become magnitized if the machine were to lose power while it was
in record. It was an ultimate case of a bad engineering design being fixed
using a revision in the manual!


--
Bob Olhsson Audio Mastery, Nashville TN
Mastering, Audio for Picture, Mix Evaluation and Quality Control
Over 40 years making people sound better than they ever imagined!
615.385.8051 http://www.hyperback.com
Anonymous
September 13, 2004 4:04:29 AM

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

"Chris Hornbeck" <chrishornbeckremovethis@att.net> wrote in message
news:0dr7k0ps29bo0q2ua4o9206nic593amabf@4ax.com...
> On Sun, 12 Sep 2004 15:46:39 +1000, Tony <tony_roe@tpg.com.au> wrote:
>
> >Mu-metal cores can indeed become magnetized, and it seriously degrades
> >their performance. I have no idea how easily that can happen, but in
> >principal it might happen if you short one signal line to ground with
> >the other open, and apply phantom power. Trouble is, it can NOT be
> >reversed by a demagnetizer (eg by deliberately applying a diminishing
> >AC flux) - you need to disassemble the transformer and thermally
> >anneal the core (not usually a practical option). IOW, don't touch it
> >or you'll go blind.
>
> IOW, we've only been kidding ourselves all these years demag'ing tape
> heads and their shields and such. Bummer.



** Yeah - the story from Tony is bogus.

Mu-metal needs to be re-annealed if it is worked - ie bent, drilled, spot
welded etc.



............ Phil
Anonymous
September 13, 2004 7:42:03 AM

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

<< DSatz@msn.com (David Satz) >>
<< In December 1998 there was a detailed discussion of this topic on the
blessedly moderated Pro Audio mailing list which the late Gabe Wiener
founded when he fled this place.

Given the rather limited current spikes that can flow in any real-world
(i.e. partly capacitive) microphone cable even if 48 V phantom powering
is connected one pin at a time, transformer magnetization was described
as "not a common problem." Nonetheless the effect does exist, and can
be seen as an increase of a few hundredths of a percent in THD at the
lowest audio frequencies.

Since we tend to be insensitive to harmonic distortion at low frequencies,
and since no such increase in distortion could be found at midrange or
higher frequencies at all, I think a fair summary of the discussion would
be (with contributions from David Josephson and Bill Whitlock among others)
that it's not worth the worry as far as audible "coloring" is concerned.

But this is disgraceful. You've allowed yourself to doubt the logic which
says, "I can imagine how a certain effect _could_ occur, and therefore it
is a serious, hitherto unacknowledged problem in real-world audio."

Don't you realize that when a person can reach a conclusion intuitively,
without relying on the poor crutches of mathematics or physical evidence,
he is being much more brilliant than any scientist? And what a miracle
it is that unattributed, unproven allegations can be passed down a wire?

The very worst form of thought crime, which you have committed here, is
to raise questions about the _degree_ or _extent_ of any effect that is
authoritatively believed to perhaps be audible, or which cannot be proved
conclusively to be inaudible (which of course nothing ever can be). That
would imply limits to our ability to hear things which can be measured!

No idea could be more horrible in its implications. If widely believed
or even considered, it would stand the entire theological structure of
the audiophile universe on its head. You should confess and apologize
to this group forthwith, young man. >>

You misunderstand me Sir. I consider your post confirmation that such a
problem is indeed possible, and possibilty drives all high end audio. Now I
have a new procedure to offer in Audio Consulting and Services, and I think
this could well be as popular a services demagnetizing Hi-End Speaker Cables so
their directional characteristics function properly again...

<g>


Will Miho
NY Music & TV Audio Guy
Off the Morning Show! & sleepin' In... / Fox News
"The large print giveth and the small print taketh away..." Tom Waits
Anonymous
September 13, 2004 9:38:11 AM

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

"hank alrich" <walkinay@thegrid.net> wrote in message
news:1gjzvtf.2qpchfub7kcgN%walkinay@thegrid.net...

> Before I got the Annis rig with the magnetometer I would ritually
> "demagnetize" the machines' heads. After I got the measurement device I
> found I very rarely needed to demag, and I could also tell whether I had
> done that or just more severely magnetized a head. The Annis kit paid
> for itself in time not wasted "demagnetizing" that which wasn't
> magnetized in the first place.

Mine paid for itself by telling me what I *really* needed to demagnetize --
razor blades.

Peace,
Paul
Anonymous
September 13, 2004 12:56:54 PM

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

This is the likely source of the concept... and indicates that it should
only be required if you do something stupid to cause a high DC current to
flow through the transformer.

Note: the suggested levels may not be appropriate for equipment other than
the Studer 169/269...

The following is quoted from the Studer 169/269 manuel...

"Section 2.6

Attention:
Do not use unbalanced sources on the microphone inputs with the phantom
power ON.

Any grounding of an imput lead draws DC current through the transformer. It
goes into magnetic saturation and so stops behaving as a transformer.

After accidental magnetizing of the transformer demagnetizing procedure will
be necesary:

Apply a sine wave generator of min. 3V at 30 Hz to the microphone input and
reduce the level continuously with at least 30 s to zero. Make sure the
generator has no DC-coupled output (see fig. 223). To prevent any
accidental DC current, apply a blocking circuit."

Figure 2.23 shows a 100uf 10V electrolytic in series with the generator.
followed by a 600 ohm terminating resistor at the transformer input.


Section 7.4/3

"Attention:

Excesive second harmonic distortion may be caused by premagnetized cores.

The input transformers should therefore be periodically demagnetized.

Demagnetizing
Apply a low frequency signal to the input (30 Hz). Increase voltage until
saturation occurs and decrease slowly to zero. See also operating
instructions 2.6"

Rgds:
Eric
www.webermusic.com
Anonymous
September 13, 2004 1:00:44 PM

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

In article <Zc51d.355097$OB3.30405@bgtnsc05-news.ops.worldnet.att.net> olh@hyperback.com writes:

> The technician also sheepishly told us that the head demagnetizing ritual
> had been created by Ampex as a solution to the problem that an Ampex 300's
> heads would become magnitized if the machine were to lose power while it was
> in record. It was an ultimate case of a bad engineering design being fixed
> using a revision in the manual!

Interesting, but a bit imaginitive on the tech's part. It was very
common for home recorders that were contemporary with the Ampex 300 to
pop at the start and end of a recording because the bias was just
slammed on and off. This is what caused their heads to become
magnetized. While Ampex should have thoght harder about this, it was
just the way things were done back then.

Did the 200 have bias ramp-up/down? That is, did they recognize this
as a problem and took care of it in an earlier version? Or did it not
occur that a pop would be a problem (since all tapes were edited in
production anyway) until someone came up with the concept of the
punch-in?


--
I'm really Mike Rivers (mrivers@d-and-d.com)
However, until the spam goes away or Hell freezes over,
lots of IP addresses are blocked from this system. If
you e-mail me and it bounces, use your secret decoder ring
and reach me here: double-m-eleven-double-zero at yahoo
Anonymous
September 13, 2004 10:19:21 PM

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

"Mike Rivers" <mrivers@d-and-d.com> wrote in message
news:znr1095075384k@trad...
> Interesting, but a bit imaginitive on the tech's part. It was very
> common for home recorders that were contemporary with the Ampex 300 to
> pop at the start and end of a recording because the bias was just
> slammed on and off.

Home recorders contemporary with the Ampex 300, which would have to mean
early to mid-1949?

I have no doubt 200s probably had the same problem but I can see no reason
to question the credibility of an Ampex factory technician who was teaching
us how to maintain and repair our new machines in 1968. I've also never
found a magnetized head although my experience is almost entirely with pro
tape machines.

--
Bob Olhsson Audio Mastery, Nashville TN
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Anonymous
September 13, 2004 10:19:22 PM

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

In article <Jwl1d.586530$Gx4.397872@bgtnsc04-news.ops.worldnet.att.net> olh@hyperback.com writes:

> Home recorders contemporary with the Ampex 300, which would have to mean
> early to mid-1949?

I had a VM in the mid '50's, and my buddy had a Webcor.


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I'm really Mike Rivers (mrivers@d-and-d.com)
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Anonymous
September 14, 2004 11:52:35 AM

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

"Eric K. Weber"

(snip)
>
> The input transformers should therefore be periodically demagnetized.
>
> Demagnetizing
> Apply a low frequency signal to the input (30 Hz). Increase voltage until
> saturation occurs and decrease slowly to zero. See also operating
> instructions 2.6"
>


** That is how you would expect to demagnetise a core.

The same thing happens every time you switch a AC transformer on - during
the first half cycle the current surges, saturates and hence magnetises the
core, then the following 20 -50 AC cycles demagnetise it.





.............. Phil
!