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'[EE] Protecting audio inputs'
2011\10\25@132310 by Bob Blick

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Plugging an audio cable between two devices that are powered by AC can
put a lot of stress on opamps, whose inputs can sometimes be almost
directly connected to the input jack.

Is there a device that is preferred for clamping audio inputs, like a
bidirectional surge absorber is used for power input, but suitable for
this application?

A reasonable amount of capacitance can be tolerated, say up to 100 pF.

Thanks,

Bob

-- http://www.fastmail.fm - Access your email from home and the web

2011\10\25@134054 by Bob Ammerman

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> Plugging an audio cable between two devices that are powered by AC can
> put a lot of stress on opamps, whose inputs can sometimes be almost
> directly connected to the input jack.
>
> Is there a device that is preferred for clamping audio inputs, like a
> bidirectional surge absorber is used for power input, but suitable for
> this application?
>
> A reasonable amount of capacitance can be tolerated, say up to 100 pF.
>
> Thanks,
>
> Bob

Capacitively couple it on both the ground and "hot" sides to avoid common mode problems?

-- Bob Ammerman
RAm Systems

2011\10\25@141400 by Bob Blick

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On Tuesday, October 25, 2011 1:40 PM, "Bob Ammerman" wrote:
>
> > Plugging an audio cable between two devices that are powered by AC can
> > put a lot of stress on opamps, whose inputs can sometimes be almost
> > directly connected to the input jack.
> >
> > Is there a device that is preferred for clamping audio inputs, like a
> > bidirectional surge absorber is used for power input, but suitable for
> > this application?
> >
> > A reasonable amount of capacitance can be tolerated, say up to 100 pF.
>
> Capacitively couple it on both the ground and "hot" sides to avoid common
> mode problems?

Hi Bob,

It's the transient that occurs during the plugging-in. Typically an RCA
or phone plug contacts a signal to ground or a signal to signal before
the ground to ground connection is made. Imagine connecting the signal
input of a grounded device to the ground of a laptop computer, one that
has a two-prong power plug. It already is capacitively coupled, the EMI
circuitry of the laptop's power adapter has a capacitor from AC center
tap to DC ground. So you are shoving 60 VAC (120 in 240 volt lands) into
a poor little opamp. It's very lively.

Eventually the grounds of the two systems make contact and things
usually are fine from then on. Usually.

Best regards,

Bob


-- http://www.fastmail.fm - Faster than the air-speed velocity of an
                         unladen european swallow

2011\10\25@144620 by Oli Glaser

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On 25/10/2011 19:14, Bob Blick wrote:
> It's the transient that occurs during the plugging-in. Typically an RCA
> or phone plug contacts a signal to ground or a signal to signal before
> the ground to ground connection is made. Imagine connecting the signal
> input of a grounded device to the ground of a laptop computer, one that
> has a two-prong power plug. It already is capacitively coupled, the EMI
> circuitry of the laptop's power adapter has a capacitor from AC center
> tap to DC ground. So you are shoving 60 VAC (120 in 240 volt lands) into
> a poor little opamp. It's very lively.
>
> Eventually the grounds of the two systems make contact and things
> usually are fine from then on. Usually.

The "standard" couple of diodes with current limiting series input resistor is what I've usually seen implemented in audio inputs.
This page http://www.angelfire.com/sd/paulkemble/sound7.html (input protection right at the bottom) describes a few methods.
Of course you could go for e.g. an XLR connector to avoid the temporary short  problem altogether,

2011\10\25@155518 by Dwayne Reid

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At 11:23 AM 10/25/2011, Bob Blick wrote:
>Plugging an audio cable between two devices that are powered by AC can
>put a lot of stress on opamps, whose inputs can sometimes be almost
>directly connected to the input jack.
>
>Is there a device that is preferred for clamping audio inputs, like a
>bidirectional surge absorber is used for power input, but suitable for
>this application?

My preference has always been to use a pair of signal diodes from the op-amp pin to the supply rails (assuming single-ended / unbalanced input).  This, of course, requires an appropriate resistor between the input jack and the op-amp / diodes junction.

If this is something that you are modifying rather than building from scratch, varistors designed for telcom use are useful.  The ones that I have are about the size of a fat 1/2 resistor and made of green epoxy or similar material.  The markings on them are "V120-2" and they clip at about 1Vp-p.  These are nice because the onset of clipping is soft and rounded - they are significantly less audible than 4- 1n4148 diodes in series / anti-parallel at the onset of clipping.  Obviously, these varistors are bi-directional.

I purchased probably about 1000 of these green varistors about 30 years ago and often wish that I had purchased the whole (large) cardboard box - I'm pretty sure that the ones that I didn't take eventually made their way to the landfill.

Anyway, I'm pretty sure that the 1Vp-p is a fairly standard telcom varistor spec and should work nicely at the nominal -10dBu or -10dBv level that your consumer-grade audio equipment is running at.

dwayne

-- Dwayne Reid   <spam_OUTdwaynerTakeThisOuTspamplanet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\10\26@141851 by Bob Blick

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On Tuesday, October 25, 2011 1:55 PM, "Dwayne Reid" wrote:

> If this is something that you are modifying rather than building from
> scratch, varistors designed for telcom use are useful.  The ones that
> I have are about the size of a fat 1/2 resistor and made of green
> epoxy or similar material.  The markings on them are "V120-2" and
> they clip at about 1Vp-p.  These are nice because the onset of
> clipping is soft and rounded - they are significantly less audible
> than 4- 1n4148 diodes in series / anti-parallel at the onset of
> clipping.  Obviously, these varistors are bi-directional.
>
> I purchased probably about 1000 of these green varistors about 30
> years ago and often wish that I had purchased the whole (large)
> cardboard box - I'm pretty sure that the ones that I didn't take
> eventually made their way to the landfill.
>
> Anyway, I'm pretty sure that the 1Vp-p is a fairly standard telcom
> varistor spec and should work nicely at the nominal -10dBu or -10dBv
> level that your consumer-grade audio equipment is running at.

I'll look around, but it's hard to find low-voltage ones with low
capacitance. The closest I've come so far is the Littelfuse
V5.5MLA0402LN rated at 5.5 volts which clamps about 10 V (at 1 mA) up to
39V at 1 Amp and has 70 pF of capacitance. It is specifically intended
for sensors and audio. Most in the same category have ten times as much
capacitance.

But it should be OK for my needs, which are add-ons to existing gear.
Thanks!

Friendly regards,

Bob

-- http://www.fastmail.fm - IMAP accessible web-mail

2011\10\26@153735 by Dwayne Reid

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At 12:18 PM 10/26/2011, Bob Blick wrote:

>I'll look around, but it's hard to find low-voltage ones with low
>capacitance. The closest I've come so far is the Littelfuse
>V5.5MLA0402LN rated at 5.5 volts which clamps about 10 V (at 1 mA) up to
>39V at 1 Amp and has 70 pF of capacitance. It is specifically intended
>for sensors and audio. Most in the same category have ten times as much
>capacitance.
>
>But it should be OK for my needs, which are add-ons to existing gear.
>Thanks!

I wish that I could suggest a supplier of the Telcom varistors - I know they are still being used today because I see them inside recently-manufactured equipment.

Like I mentioned previously - these have a nice, soft knee which makes them significantly less audible than the 5V part you are looking at.

Actually, I just went to Digikey and pounded 'varistor' into the search field.  Then select '2 Vac' in the parametric search that comes up.

2Vac, 3.5Vdc, 100pF.  $0.57 (Canadian) in singles, $0.19 in hundreds, $0.078 in thousands.

The only problem is that they are in 0201 package size (tiny).

dwayne



-- Dwayne Reid   <.....dwaynerKILLspamspam@spam@planet.eon.net>
Trinity Electronics Systems Ltd    Edmonton, AB, CANADA
(780) 489-3199 voice          (780) 487-6397 fax
http://www.trinity-electronics.com
Custom Electronics Design and Manufacturing

2011\10\26@172207 by Sean Breheny

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I have protected an RF receiver antenna input using an SCR-based ESD
clamp before. Like these devices:

http://www.littelfuse.com/products/TVS+Diode+Arrays/General+Purpose+ESD+Protection/SP725.html

or, even lower voltage:

http://www.littelfuse.com/products/TVS+Diode+Arrays/Lightning+Surge+Protection/SP03-3.3/SP03-3.3BTG.html

This latter one seems to be based on TVS diodes alone without BJTs or
SCRs. Capacitance is still very low.

Sean


On Wed, Oct 26, 2011 at 2:18 PM, Bob Blick <bobblickspamKILLspamftml.net> wrote:
{Quote hidden}

>

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