INA103 differential stage

[Guest]

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

My idea is to use a INA103 but without the second differential stage as
a mic-pre. One possibility would be of course to replace the
differential stage it with another OP or a discrete setup but my
question goes in another direction:

Since I want the output to be symmetrical anyway wouldn't it be possible
to amplify each phase separately with a single OP and use that as a
symmetrical output? Sounds a little to simple, I know. Am I missing
something here?


Thanks,
Rob

 

[mark]

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

>
> Maybe I am not completely aware of how the input stage of an
> instrumentation amp works. Is it possible to benefit from the balanced
> first stage when I use only one of the differential outputs and the
> other is grounded?
>
> >

Yes you get the full benefit of hum rejection of balanced input
realtive to the mic cable regardless of how you connect the output of
the pre-amp.


If you connect the output of the pre-amp as single ended, then you get
no benefit of hum rejection at the output cable but that does not
detract fomr the hum rejection at the balanced input. Think of it as a
new signal source starting a new trip.


The INA103 has a single ended output.

Mark

 

[Guest]

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"Robert Angst"
>
> My idea is to use a INA103 but without the second differential stage as a
> mic-pre. One possibility would be of course to replace the differential
> stage it with another OP or a discrete setup but my question goes in
> another direction:
>
> Since I want the output to be symmetrical anyway wouldn't it be possible
> to amplify each phase separately with a single OP and use that as a
> symmetrical output? Sounds a little to simple, I know. Am I missing
> something here?


** No - all you need is two op-amps.

Eg. a NE5532 dual op-amp configured as two inverters with 4.7 kohm input
and feedback resistors.

Balanced low noise in - balanced line level out.




............ Phil

 

[Guest]

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

>
> ** No - all you need is two op-amps.

Ok, another thougt:
if I connect the symetrical output of the two OPs into a single ended
input (e.g. RNC) with + hot, - and shield grounded, I will loose the
entire benefit of the differential construction between mic and output.
If I'd leave the shield unconnected it should work. Is this correct?

I sure want to be fully balanced but want to keep compatibility with
single ended gear as well. Do I in that case have to go the classic
route: first differential amp and then splitting it up again in two
differential pairs? In that case I could include an insert jack for my
RNC fairly easy..

As an OP I would use an LT1469/8. I plugged them in my SX202 (as
suggested in this NG) and realy like the results.

 

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

Phil Allison wrote:

> "Robert Angst"
> >
> > My idea is to use a INA103 but without the second differential stage as a
> > mic-pre. One possibility would be of course to replace the differential
> > stage it with another OP or a discrete setup but my question goes in
> > another direction:
> >
> > Since I want the output to be symmetrical anyway wouldn't it be possible
> > to amplify each phase separately with a single OP and use that as a
> > symmetrical output? Sounds a little to simple, I know. Am I missing
> > something here?
>
> ** No - all you need is two op-amps.
>
> Eg. a NE5532 dual op-amp configured as two inverters with 4.7 kohm input
> and feedback resistors.
>
> Balanced low noise in - balanced line level out.

Phil ! Are you feeling unwell ? He said a mic pre ! That configuration will be
noisy as hell.

Using two op-amps like that destroys CMRR too - unless you use say 0.1%
tolerance resistors.

The long tailed pair input has far better inherent common mode rejection.

Graham

 

[Guest]

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

"Robert Angst"
>>
>> ** No - all you need is two op-amps.


** You trimmed out my name and everything but for one line.

That is very bad manners.



............ Phil

 

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

Phil Allison schrieb:

> "Robert Angst"
>
>>>** No - all you need is two op-amps.
>
>
>
> ** You trimmed out my name and everything but for one line.
>
> That is very bad manners.
>
>
>
> ........... Phil
>

Sorry, won't happen again.

 

[Guest]

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

"Robert Angst"
> Phil Allison
>>
>>>>** No - all you need is two op-amps.
>>
>>
>> ** You trimmed out my name and everything but for one line.
>>
>> That is very bad manners.
>
>
> Sorry, won't happen again.


** OK.

As for your additional questions:

1. Shorting one side of a balanced line to ground is bad practice.

2. The majority of the CMRR of the INA103 mic-preamp comes from the first
(differential) stage - a common mode signal gets only unity gain.

3. The vast majority of external hum field rejection obtained with balanced
*mic* lines is due to the two wires inside the cable being twisted.


Sorry if this sounds like heresy.

It isn't.


.......... Phil

 

[Guest]

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

Phil Allison schrieb:

> As for your additional questions:
>
> 1. Shorting one side of a balanced line to ground is bad practice.

Sure, I am just trying to make the box more idiot proof for 'emergency'
situations when only a TS cable is at hand (or the person who built it
isn't).

> 2. The majority of the CMRR of the INA103 mic-preamp comes from the first
> (differential) stage - a common mode signal gets only unity gain.

Maybe I am not completely aware of how the input stage of an
instrumentation amp works. Is it possible to benefit from the balanced
first stage when I use only one of the differential outputs and the
other is grounded?

> 3. The vast majority of external hum field rejection obtained with balanced
> *mic* lines is due to the two wires inside the cable being twisted.

That sounds like even if differential input is compromised it's still
good enough for these 'emergency' cases.

> Sorry if this sounds like heresy.
>
> It isn't.
>
>
> ......... Phil

I guess I'll build it the way you suggested and use proper cables if I
want to compress while tracking. That should make a nice two-chip pre.

Thanks
Rob

 

[Guest]

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

Robert Angst wrote:
>
> Maybe I am not completely aware of how the input stage of an
> instrumentation amp works. Is it possible to benefit from the balanced
> first stage when I use only one of the differential outputs and the
> other is grounded?
>

Don't actually connect the other output to ground, but the first stage
of a classic instrumentation amp has whatever differential gain is
defined by the gain-defining resistor, and a common mode gain of 1
always. So for all gains above 1 you'll get some common mode rejection,
equal to the differential gain.

I don't know why you wouldn't want to use the second stage of the
INA103, which is a differencing amp with unity gain and CMR as good as
resistor tolerances will allow. The main limitation of a whole mic
preamp constructed that way is that the output stage of the INA103 isn't
the best thing for driving arbitrary loads on the end of possibly long
cables. (but then the input stage isn't any better...)

--
Anahata
anahata@treewind.co.uk -+- http/www.treewind.co.uk
Home: 01638 720444 Mob: 07976 263827

 

[Guest]

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

On Tue, 16 Aug 2005 17:04:38 +0200, Robert Angst
<robert.angst@tu-berlin.de> wrote:

>
>>
>> ** No - all you need is two op-amps.
>
>Ok, another thougt:
>if I connect the symetrical output of the two OPs into a single ended
>input (e.g. RNC) with + hot, - and shield grounded, I will loose the
>entire benefit of the differential construction between mic and output.
>If I'd leave the shield unconnected it should work. Is this correct?
>
>I sure want to be fully balanced but want to keep compatibility with
>single ended gear as well. Do I in that case have to go the classic
>route: first differential amp and then splitting it up again in two
>differential pairs?

I was thinking there was something 'bad' about your original idea
of keeping the positive and negative signals separate, and now you've
mentioned it.
Another reason(s) to combine them is to cancel out any common-mode
signal from the input. Passing on such a common-mode signal reduces
the maximum signal (if the signal path can handle 15V peaks and your
common-mode noise is 10V peak, your biggest signal is 5V before
clipping. It also relies on the next device having good CMRR to cancel
common-mode stuff, which if the next device has an unbalanced input,
it obviously won't have.

You absolutely need the differential-conversion stage (with
highly=precise, matched resistors to get good CMRR) for a mic preamp,
as the mic signal could be a few millivolts and the common-mode signal
can be several volts. To turn the situation around so that the mic
signal is much larger instead, you need to cancel the common signal
with a very high precision.
The instrumentation amplifier configuration helps by giving (up to
about) 1,000 times gain for differential signals, and only a gain of 1
for common-mode signals, but you still need the differential stage to
cancel out the common-mode signals.

>In that case I could include an insert jack for my
>RNC fairly easy..
>
>As an OP I would use an LT1469/8. I plugged them in my SX202 (as
>suggested in this NG) and realy like the results.

-----
http/www.mindspring.com/~benbradley

 

[Guest]

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

In article <ddt5tu$ise$1@mamenchi.zrz.TU-Berlin.DE> robert.angst@tu-berlin.de writes:

> > 1. Shorting one side of a balanced line to ground is bad practice.
>
> Sure, I am just trying to make the box more idiot proof for 'emergency'
> situations when only a TS cable is at hand (or the person who built it
> isn't).

This is why the single-ended so-called "impedance balanced" output
configuration is so popular. It doesn't matter if you short the
non-driven side to ground because there's no signal on it. Much
cheaper than a transformer, simpler than a cross-coupled see-saw
output stage, and good enough for even some of the finest microphones.



--
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

 

[Guest]

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

"Pooh Bear"
> Phil Allison wrote:
>> "Robert Angst"
>> >
>> > My idea is to use a INA103 but without the second differential stage as
>> > a
>> > mic-pre. One possibility would be of course to replace the differential
>> > stage it with another OP or a discrete setup but my question goes in
>> > another direction:
>> >
>> > Since I want the output to be symmetrical anyway wouldn't it be
>> > possible
>> > to amplify each phase separately with a single OP and use that as a
>> > symmetrical output? Sounds a little to simple, I know. Am I missing
>> > something here?
>>
>> ** No - all you need is two op-amps.
>>
>> Eg. a NE5532 dual op-amp configured as two inverters with 4.7 kohm
>> input
>> and feedback resistors.
>>
>> Balanced low noise in - balanced line level out.
>
> Phil ! Are you feeling unwell ? He said a mic pre ! That configuration
> will be
> noisy as hell.


** Try reading the context - dickhead.

The INA103 allows access on pins 5 & 12 to the differential stage's outputs.

http/focus.ti.com/lit/ds/symlink/ina103.pdf

The NE5532 would merely buffer those outputs.




........... Phil

 

[Guest]

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

Phil Allison wrote:

> "Pooh Bear"
> > Phil Allison wrote:
> >> "Robert Angst"
> >> >
> >> > My idea is to use a INA103 but without the second differential stage as
> >> > a
> >> > mic-pre. One possibility would be of course to replace the differential
> >> > stage it with another OP or a discrete setup but my question goes in
> >> > another direction:
> >> >
> >> > Since I want the output to be symmetrical anyway wouldn't it be
> >> > possible
> >> > to amplify each phase separately with a single OP and use that as a
> >> > symmetrical output? Sounds a little to simple, I know. Am I missing
> >> > something here?
> >>
> >> ** No - all you need is two op-amps.
> >>
> >> Eg. a NE5532 dual op-amp configured as two inverters with 4.7 kohm
> >> input
> >> and feedback resistors.
> >>
> >> Balanced low noise in - balanced line level out.
> >
> > Phil ! Are you feeling unwell ? He said a mic pre ! That configuration
> > will be
> > noisy as hell.
>
> ** Try reading the context - dickhead.
>
> The INA103 allows access on pins 5 & 12 to the differential stage's outputs.
>
> http/focus.ti.com/lit/ds/symlink/ina103.pdf
>
> The NE5532 would merely buffer those outputs.

Ok, I follow you now. From what he said ( about simply amplifying each leg of
the mic input separately ) I got a different picture of what you meant.

Why not buffer those points using a voltage follower configuration though ?
Slightly less noisy without those 4k7s.

Graham

 

[Guest]

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

"Pooh Bear"
> Phil Allison wrote:
>>>
>> >> ** No - all you need is two op-amps.
>> >>
>> >> Eg. a NE5532 dual op-amp configured as two inverters with 4.7 kohm
>> >> input and feedback resistors.
>> >>
>> >> Balanced low noise in - balanced line level out.
>> >
>> > Phil ! Are you feeling unwell ? He said a mic pre ! That configuration
>> > will be
>> > noisy as hell.
>>
>> ** Try reading the context - dickhead.
>>
>> The INA103 allows access on pins 5 & 12 to the differential stage's
>> outputs.
>>
>> http/focus.ti.com/lit/ds/symlink/ina103.pdf
>>
>> The NE5532 would merely buffer those outputs.
>
> Ok, I follow you now. From what he said ( about simply amplifying each leg
> of
> the mic input separately ) I got a different picture of what you meant.
>
> Why not buffer those points using a voltage follower configuration though
> ?
> Slightly less noisy without those 4k7s.



** Are you feeling well - Pooh ??

The reason for preferring unity gain inverting is that it produces less
HD - since there is no common mode voltage at the inputs.

The self noise from a unity gain inverter, using an NE 5532 and 4.7 kohms is
about 3 uV rms ( 1 uV from the resistor, 1 uV from the op-amp & noise
gain of 2 ).

Relative to a 1 volt output level, this is a s/n ratio of 110 dB -
exceeding the spec for the INA103.

The noise from the preceding op-amp is gonna be more in any case - even at
unity gain the INA103 is speced at 10 uV output noise ( 70 nV rt Hz) .



............ Phil

 

[Guest]

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

Phil Allison wrote:

> "Pooh Bear"
> > Phil Allison wrote:
> >>>
> >> >> ** No - all you need is two op-amps.
> >> >>
> >> >> Eg. a NE5532 dual op-amp configured as two inverters with 4.7 kohm
> >> >> input and feedback resistors.
> >> >>
> >> >> Balanced low noise in - balanced line level out.
> >> >
> >> > Phil ! Are you feeling unwell ? He said a mic pre ! That configuration
> >> > will be
> >> > noisy as hell.
> >>
> >> ** Try reading the context - dickhead.
> >>
> >> The INA103 allows access on pins 5 & 12 to the differential stage's
> >> outputs.
> >>
> >> http/focus.ti.com/lit/ds/symlink/ina103.pdf
> >>
> >> The NE5532 would merely buffer those outputs.
> >
> > Ok, I follow you now. From what he said ( about simply amplifying each leg
> > of
> > the mic input separately ) I got a different picture of what you meant.
> >
> > Why not buffer those points using a voltage follower configuration though
> > ?
> > Slightly less noisy without those 4k7s.
>
> ** Are you feeling well - Pooh ??
>
> The reason for preferring unity gain inverting is that it produces less
> HD - since there is no common mode voltage at the inputs.

That's an interesting assertion. I recall that being said in the 70s. The
reason being supposed 'common mode failure' of the input pair.

For a while I followed the 'inverting is best' suggestion but eventually
abandoned it since it generally leads to increased noise. I see no sign of this
issue with modern op-amps.


> The self noise from a unity gain inverter, using an NE 5532 and 4.7 kohms is
> about 3 uV rms ( 1 uV from the resistor, 1 uV from the op-amp & noise
> gain of 2 ).
>
> Relative to a 1 volt output level, this is a s/n ratio of 110 dB -
> exceeding the spec for the INA103.
>
> The noise from the preceding op-amp is gonna be more in any case - even at
> unity gain the INA103 is speced at 10 uV output noise ( 70 nV rt Hz) .

I'm just getting very fussy over *all* noise contributions these days. As you
correctly point out, even 4k7 makes a significant difference to the overall
noise of a 5532.

Graham

 

[Guest]

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

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:4302B5E7.BE5703E0@hotmail.com...

> For a while I followed the 'inverting is best' suggestion but eventually
> abandoned it since it generally leads to increased noise. I see no sign of
this
> issue with modern op-amps.

It's there, although more with FET-input op-amps than bipolars like the
5532. Try high-frequency IM tests on an OPA-604 or 2604 (same design, dual
package) in non-inverting mode, first with a low-impedance source, then with
something like 25k source impedance. More distortion.

Peace,
Paul

 

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

Phil Allison wrote:

> Graham Stevenon - Studiomaster's biggest Enemy
> >
> > Phil Allison wrote:

< snip Phil's insults >


> >> All you are doing is mindless TROLLING and NETSTALKING me.
> >
> > If you think I'm trolling or stalking you - lol - then you clearly have a
> > serious mental problem.
> >
> > I'm contributing to a thread in a *public* newsgroup. Get over it.
>
> ** You will STOP posting your mindless garbage under every post I put up.

I do *not* post every single time you do. Just take a look and see. It does
happen often though since we both have similar expertise in the audio area -
and in turn you often reply to one of my posts. Entirely normal behaviour in a
newsgroup !


> That is criminal behaviour.
>

I suggest you go look up the meaning of criminal. You just make yourself look
stupid by posting such daft claims.

> That is netstalking.
>

If I wanted to netstalk you, trust me you'd know about it. I don't have that
mindset nor the interest, inclination, time, money whatever anyway !


> The one with metal issues is YOU - Graham Stevenon of Studiomaster UK.

Lol @ metal. Thankfully I'm able to shrug off such nonsence. I only wish you'd
grow up and get over your persecution complex.

Come on Phil, I'm sure you're capable of better than this ?


Graham

 

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Paul Stamler wrote:

> "Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
> news:4302B5E7.BE5703E0@hotmail.com...
>
> > For a while I followed the 'inverting is best' suggestion but eventually
> > abandoned it since it generally leads to increased noise. I see no sign of
> this
> > issue with modern op-amps.
>
> It's there, although more with FET-input op-amps than bipolars like the
> 5532. Try high-frequency IM tests on an OPA-604 or 2604 (same design, dual
> package) in non-inverting mode, first with a low-impedance source, then with
> something like 25k source impedance. More distortion.

More THD with the high Z source ?

Ah well, I keep signal impedances low too ! ;-)

Graham

 

[Guest]

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

"Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
news:43030A15.E99D08CC@hotmail.com...
>
> Paul Stamler wrote:
>
>> "Pooh Bear" <rabbitsfriendsandrelations@hotmail.com> wrote in message
>> news:4302B5E7.BE5703E0@hotmail.com...
>>
>> > For a while I followed the 'inverting is best' suggestion but
>> > eventually
>> > abandoned it since it generally leads to increased noise. I see no sign
>> > of
>> this
>> > issue with modern op-amps.
>>
>> It's there, although more with FET-input op-amps than bipolars like the
>> 5532. Try high-frequency IM tests on an OPA-604 or 2604 (same design,
>> dual
>> package) in non-inverting mode, first with a low-impedance source, then
>> with
>> something like 25k source impedance. More distortion.

> More THD with the high Z source ?

IME that's how it usually works.

> Ah well, I keep signal impedances low too ! ;-)

...but not too low. ;-)