Dolby A

[Klay Anderson[_2_]]

A good friend needs to transfer some tapes encoded with Dolby A for restoration. His A units are a bit noisy (germanium and age...?). Anyone have the ability to A) transfer, B) repair his units, or C) rent?

Yours truly,

Mr. Klay Anderson, D.A.,Q.B.E.

[Scott Dorsey]

Klay Anderson > wrote:
>A good friend needs to transfer some tapes encoded with Dolby A for restoration. His A units are a bit noisy (germanium and age...?). Anyone have the ability to A) transfer, B) repair his units, or C) rent?

Send the tapes to Steve Puntolillo. He does good work.

That said, I can probably rent some 280 cards if that is what he needs.
--scott

--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Trevor]

"Klay Anderson" > wrote in message
...
>A good friend needs to transfer some tapes encoded with Dolby A for
>restoration. His A units are a bit noisy (germanium and age...?).

Never saw a Dolby A unit that used germanium transistors, I think everyone
had moved to silicon long before then. IC's of the time were pretty noisy
however, and capacitors are likely to need replacing. Any carbon resistors
will also add to the noise problem, but the old tapes themselves are likely
the biggest problem.

Trevor.

[William Sommerwerck]

"Trevor" wrote in message ...

> Any carbon resistors will also add to the noise problem, but the
> old tapes themselves are likely the biggest problem.

I can think if reasons for replacing them with metal-film resistors, but noise
isn't one of them.

Carbon resistors generate no more thermal noise than any other kind of
resistor. But they do generate more excess noise. This is generally a problem
only in vacuum tube equipment, with its higher supply voltages.

[Scott Dorsey]

In article >, Trevor > wrote:
>
>"Klay Anderson" > wrote in message
...
>>A good friend needs to transfer some tapes encoded with Dolby A for
>>restoration. His A units are a bit noisy (germanium and age...?).
>
>Never saw a Dolby A unit that used germanium transistors, I think everyone
>had moved to silicon long before then. IC's of the time were pretty noisy
>however, and capacitors are likely to need replacing. Any carbon resistors
>will also add to the noise problem, but the old tapes themselves are likely
>the biggest problem.

There is germanium in the A301, but the A361 is all silicon. They are
both filled with millions of tantalum caps that are all leaky by now, BUT
the manuals detail the test jigs and setup procedures for the cards which
the more recent Dolby hardware does not do. There are a lot of trimmers in
there to tweak.

Because the setup procedures are documented you can at least tell if yours
is performing to spec or not with little more than a scope, a signal
generator, and a VTVM.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Scott Dorsey]

William Sommerwerck > wrote:
>"Trevor" wrote in message ...
>
>> Any carbon resistors will also add to the noise problem, but the
>> old tapes themselves are likely the biggest problem.
>
>I can think if reasons for replacing them with metal-film resistors, but noise
>isn't one of them.
>
>Carbon resistors generate no more thermal noise than any other kind of
>resistor. But they do generate more excess noise. This is generally a problem
>only in vacuum tube equipment, with its higher supply voltages.

It's can sometimes be a problem in high gain circuits, such as in the
feedback loop of op-amps.

But if you look inside the Dolby decoders, they have carbon resistors in
most places, but film resistors in the few places where the noise would be
an issue. They were designed by actual engineers who calculated noise
budgets rather than being thrown together randomly like many products are.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[William Sommerwerck]

"Scott Dorsey" wrote in message ...

> [Noise] can sometimes be a problem in high gain circuits, such as
> in the feedback loop of op-amps.

Do you have a specific example? I just happen to have the Philbrick
"Applications Manual for Operational Amplifiers" * on my desk, and the section
on Noise doesn't even hint at problems with thermal noise from resistors in
the feedback loop. Op amps have relatively low output impedances, so it's
rarely necessary to use huge values of feedback resistors.

* This is the finest piece of technical writing/documentation -- by light
years -- of any I have ever seen. Consider yourself blessed if you are
fortunate enough to have a copy. A fellow engineer gave me a spare copy a few
years ago. If I had to rush out of the house, and could take only a few things
with me, this would be #3 (#1 and #2 being bears). I loaned it to an EE
student (who worked at Costco), and it provoked many "ooohs and aaahs" in his
class.

[Scott Dorsey]

William Sommerwerck > wrote:
>"Scott Dorsey" wrote in message ...
>
>> [Noise] can sometimes be a problem in high gain circuits, such as
>> in the feedback loop of op-amps.
>
>Do you have a specific example? I just happen to have the Philbrick
>"Applications Manual for Operational Amplifiers" * on my desk, and the section
>on Noise doesn't even hint at problems with thermal noise from resistors in
>the feedback loop. Op amps have relatively low output impedances, so it's
>rarely necessary to use huge values of feedback resistors.

The bible on this stuff is Motchenbacher's book, Low Noise Electronic
System Design. It has a bunch of examples discussing noise voltage and
noise current sources in typical circuits.

>* This is the finest piece of technical writing/documentation -- by light
>years -- of any I have ever seen. Consider yourself blessed if you are
>fortunate enough to have a copy. A fellow engineer gave me a spare copy a few
>years ago. If I had to rush out of the house, and could take only a few things
>with me, this would be #3 (#1 and #2 being bears). I loaned it to an EE
>student (who worked at Costco), and it provoked many "ooohs and aaahs" in his
>class.

The Philbrick book is an excellent introduction to op-amps and to a lot of
design practices, but it comes from an era when nobody would ever be using
those things for low-noise designs. But it _is_ great for discussions about
how to use the op-amp modules and how to figure out when they cease being
op-amps.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[Edi Zubovic]

On Thu, 12 Jun 2014 06:37:15 -0700, "William Sommerwerck"
> wrote:

>"Scott Dorsey" wrote in message ...
>
>> [Noise] can sometimes be a problem in high gain circuits, such as
>> in the feedback loop of op-amps.
>
>Do you have a specific example? I just happen to have the Philbrick
>"Applications Manual for Operational Amplifiers" * on my desk, and the section
>on Noise doesn't even hint at problems with thermal noise from resistors in
>the feedback loop. Op amps have relatively low output impedances, so it's
>rarely necessary to use huge values of feedback resistors.
>
>* This is the finest piece of technical writing/documentation -- by light
>years -- of any I have ever seen. Consider yourself blessed if you are
>fortunate enough to have a copy. A fellow engineer gave me a spare copy a few
>years ago. If I had to rush out of the house, and could take only a few things
>with me, this would be #3 (#1 and #2 being bears). I loaned it to an EE
>student (who worked at Costco), and it provoked many "ooohs and aaahs" in his
>class.

--Is it this one, downloadable here in two parts?
http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=569

-- Is it some kind of a general op-amp tester besides that
Philbrick-Nexus (quite a) desktop one of the '70s? I think once I've
came across one handy British-made analog IC tester, but it's price
around 450 GPB isn't something I could spend for the occassional use
unly.
Sometimes, though, I'd like to do a go-no go test without
breaboarding.

Edi Zubovic, Crikvenica, Croatia

[Edi Zubovic]

On Thu, 12 Jun 2014 17:52:53 +0200, Edi Zubovic <edi.zubovic[rem
> wrote:

#####Bummer##### Two parts only....

Edi Zubovic, Crikvenica, Croatia
>--Is it this one, downloadable here in two parts?
>http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=569
>
>-- Is it some kind of a general op-amp tester besides that
>Philbrick-Nexus (quite a) desktop one of the '70s? I think once I've
>came across one handy British-made analog IC tester, but it's price
>around 450 GPB isn't something I could spend for the occassional use
>unly.
>Sometimes, though, I'd like to do a go-no go test without
>breaboarding.
>
>Edi Zubovic, Crikvenica, Croatia

[William Sommerwerck]

"Scott Dorsey" wrote in message ...

> The Philbrick book is an excellent introduction to op-amps and to
> a lot of design practices, but it comes from an era when nobody
> would ever be using those things for low-noise designs. But it _is_
> great for discussions about how to use the op-amp modules and how
> to figure out when they cease being op-amps.

And a good deal more than that. Those who have not seen it should do so.

[William Sommerwerck]

"Edi Zubovic" wrote in message
...

> Is it this one, downloadable here in two parts?
> http://www.proaudiodesignforum.com/forum/php/viewtopic.php?f=12&t=569

Yes, but parts 3 and 4 are not yet on this site.

[Mike Rivers[_2_]]

On 6/12/2014 9:37 AM, William Sommerwerck wrote:
> Do you have a specific example? I just happen to have the Philbrick
> "Applications Manual for Operational Amplifiers" * on my desk, and the
> section on Noise doesn't even hint at problems with thermal noise from
> resistors in the feedback loop.

Back when Philbrick was making op amps, the threshold for noise to worry
about was considerably higher than it is today.

--
For a good time, visit http://mikeriversaudio.wordpress.com

[Scott Dorsey]

In article >, Mike Rivers > wrote:
>On 6/12/2014 9:37 AM, William Sommerwerck wrote:
>> Do you have a specific example? I just happen to have the Philbrick
>> "Applications Manual for Operational Amplifiers" * on my desk, and the
>> section on Noise doesn't even hint at problems with thermal noise from
>> resistors in the feedback loop.
>
>Back when Philbrick was making op amps, the threshold for noise to worry
>about was considerably higher than it is today.

Also, doesn't the Philbrick book have a circuit with a low noise front end
tacked in front of an op-amp with negative feedback going through the front
end, to get better noise performance than the opamp alone?

Maybe that was Opamp Labs...
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."

[[email protected]]

http://www.siongboon.com/projects/2008-04-27_analog_electronics/op%20amps%20for%20everyone%20third%20edition%20200 9%20%28Texas%20Instrument%29.pdf


TI handbook "Op Amps for Everyone"

see Chap 10

Mark

[William Sommerwerck]

"Scott Dorsey" wrote in message ...

> Doesn't the Philbrick book have a circuit with a low noise front end
> tacked in front of an op-amp with negative feedback going through
> the front end, to get better noise performance than the opamp alone?

I'm not sure that makes much sense. Can I interest you in a perpetual-motion
machine?

By the way, the "point" of the Philbrick book is not to provide a bunch of
cookbook circuits, but to explain "how and why". It's 45 years old, but it's
still useful. And I think it will be 45 years from now.

[PStamler]

On Friday, June 13, 2014 10:20:08 AM UTC-6, William Sommerwerck wrote:
> "Scott Dorsey" wrote in message ...

> > Doesn't the Philbrick book have a circuit with a low noise front end
>
> > tacked in front of an op-amp with negative feedback going through
>
> > the front end, to get better noise performance than the opamp alone?
>
>
>
> I'm not sure that makes much sense. Can I interest you in a perpetual-motion
>
> machine?

Don't tell that to Greg Mackie; he's made his living from a circuit like that for several decades, going back to the input stage he designed for the E-V Entertainer.

Peace,
Paul