The quietest audio op-amp with ultra-low THD

[Martin Griffith]

Those need to be transported in a humidor

Ever time I hear the word "Humidor" I think of a Belgium dude,
Hercules Parrot, and over-acting, and an authoress who had a hangover
and went missing for a couple of days, and made the UK "Daily Mail"
headlines( maybe)

martin

 

[Eeyore]

Speaking of dirty... I'm waiting for the marketing types to sell some
super-expensive cans of compressed air to clean out heat sink dust.

Improve heat transfer! Make your heat sinks sparkling clean with
AudioPhool(tm) Compressed Air! Crisper highs! Tighter Bass! Make
your power transistors happy! Give your electrons a bath! We blow
away the competition! (in cubic feet per minute that is)

When we did a re-furbishment at my local venue last year we blew them out with
compressed CO2 that they normally use for pressure delivery of the beers. Did a
fantastic job. Make a few people quite dusty though.

Graham

 

[Eeyore]

15K? Sounds like jfet turf.

That's the feedback resistor value in an inverting summing amp thay may be fed from
up to 32 sources simultaneously, giving an Rin of 400 ohms approx.

Just done the sums. Does anyone want to slap me round the face with a wet fish ?

I make 400 ohms 2.6nV/sqrt Hz ! I wasn't expecting it to be that noisy. I didn't
mess up did I ?

Graham

 

[Eeyore]

Dave Dearden, that name rings a bell.

Tech Director then of DDA that had just merged with Klark Teknik. Now in the same
poition at Audient. He was with Soundcraft previously. I worked alongside him for a
while in design.

I jumped ship to Brit Row around
87. It was mental at Midas then, bank holding the purse strings, full
order book but no cash flow. Pro40 was crazy, so much more electronics
crammed into the same old Pro4 frame, Geoff just wouldn't contemplate
having a larger frame. They got seriously hot.

Weren't XL's being shipped then ? That's where it seems it all went wrong.

I can recall seeing a heatsink drawn on one schematic of theirs !

Graham

 

[Eeyore]

Eeyore wrote:

Dave Dearden, that name rings a bell. I jumped ship to Brit Row around
87. It was mental at Midas then, bank holding the purse strings, full
order book but no cash flow. Pro40 was crazy, so much more electronics
crammed into the same old Pro4 frame, Geoff just wouldn't contemplate
having a larger frame. They got seriously hot.

You know I interviewed for a post at Midas once. Would have been around 1980.

I don't recall the names now but would Geoff have been the tech bod ?

I took down some of my work to show him including a pcb layout and an actual pcb IIRC.
Do you know what he said ? He said I should start up my own business !

I simply wasn't financially in that position, having lost my parents at age 14 and 18
and gained a truly evil (wanted to be a black witch) stepmother. Had my father still
been around I have no doubt he would have backed me. He was that kind of guy. It's so
sad both my parents had to die so young, both from cancer.

So there you go. Maybe if Geoff had taken me on. I'd be running Midas (at least as tech
Dir) now !

Graham

 

[Eeyore]

Martin Griffith wrote:.

Dave Dearden... Many Confused Individuals (MCI consoles). them where
the days, when SMPTE timecode was new

Maybe Dave was with MCI before Soundcraft ?

Taling of SMPTE etc, do you remember Audio Kinetics and their synchroniser / locators ? I
worked for them for a while too as a summer holiday job when I was at college.

Graham

 

[Martin Griffith]

Martin Griffith wrote:.


Maybe Dave was with MCI before Soundcraft ?

Taling of SMPTE etc, do you remember Audio Kinetics and their synchroniser / locators ? I
worked for them for a while too as a summer holiday job when I was at college.

Graham

Yep DD was with MCI with another guy called bernie.
Qlock , strange company, badly organised. Lot of systems, walked out
of the backdoor, as spares. Opposite side of the road from FWO Bauch
in Boringwood, where I did some time, a year and a day I think, fixing
Studers.
Do you remember Werner?

martin

 

[[email protected]]

That depends how many inputs are assigned. As these are going only to be monitoring
desks, the number of inputs will be 24 or 36 typically.

"That depends how many inputs are assigned. As these are going only
to be monitoring
desks, the number of inputs will be 24 or 36 typically.
So the suming node will see ~ 400-600 ohms. Current noise shouldn't be
a problem
here,"

Ahh I see it's one big summing junction. But what is the feedback
resistor? (I know nothing about high end audio stuff.)

Georgw

 

[Eeyore]

"That depends how many inputs are assigned. As these are going only
to be monitoring
desks, the number of inputs will be 24 or 36 typically.
So the suming node will see ~ 400-600 ohms. Current noise shouldn't be
a problem
here,"

Ahh I see it's one big summing junction. But what is the feedback
resistor? (I know nothing about high end audio stuff.)

Its value, correcting for the gain structure mess-up of the original design will be 7k5,
i.e a voltage gain of around -18x , but must be stable with nothing routed which equates
to a stable gain of -0.5x. I don't see a problem with the stability issues, I thought it
out on the walk back from the supermarket.

Graham

 

[Eeyore]

Yep DD was with MCI with another guy called bernie.
Qlock , strange company, badly organised. Lot of systems, walked out
of the backdoor, as spares. Opposite side of the road from FWO Bauch
in Boringwood, where I did some time, a year and a day I think, fixing
Studers.
Do you remember Werner?

That's after my time. Audio Kinetics was in beautiful converted church in St Albans when I
worked for them. Must have been 77 ish.

I personally wire-wrapped some of the very first Q-locks. We used Ampex ATR 100 series as test
machines. Woo-hoo - were they something else !

Graham

 

[Eeyore]

That's about right. So the summing resistors are about 400*32 = 12.8K
each. Each input has a gain to the output of a bit over 1. Noise gain
is close to 40.

I wonder if multi-group summing would be any better.

That would be ideal and is a method implemented in some more modern products. In this
case we're stuck with what we have i.e. a single mix bus.

Graham

 

[Les Matthew]

Martin Griffith wrote:.


Maybe Dave was with MCI before Soundcraft ?

Taling of SMPTE etc, do you remember Audio Kinetics and their synchroniser / locators ? I
worked for them for a while too as a summer holiday job when I was at college.

Graham

That must be it as I was with Soundcraft before Midas.


les...

 

[Les Matthew]

Yep DD was with MCI with another guy called bernie.
Qlock , strange company, badly organised. Lot of systems, walked out
of the backdoor, as spares. Opposite side of the road from FWO Bauch
in Boringwood, where I did some time, a year and a day I think, fixing
Studers.
Do you remember Werner?

martin

Didn't MCI end up in Stanhope St? Midas and Martin Audio were in the
yard and MCI had the building at the front and over the top of the
entrance to the yard.


les...

 

[Les Matthew]

Tech Director then of DDA that had just merged with Klark Teknik. Now in the same
poition at Audient. He was with Soundcraft previously. I worked alongside him for a
while in design.



Weren't XL's being shipped then ? That's where it seems it all went wrong.

I can recall seeing a heatsink drawn on one schematic of theirs !

Graham

XL's were after my time, so maybe I left earlier. It's all a bit hazy now.

les...


les...

 

[Les Matthew]

You know I interviewed for a post at Midas once. Would have been around 1980.

I don't recall the names now but would Geoff have been the tech bod ?

I took down some of my work to show him including a pcb layout and an actual pcb IIRC.
Do you know what he said ? He said I should start up my own business !

I simply wasn't financially in that position, having lost my parents at age 14 and 18
and gained a truly evil (wanted to be a black witch) stepmother. Had my father still
been around I have no doubt he would have backed me. He was that kind of guy. It's so
sad both my parents had to die so young, both from cancer.

So there you go. Maybe if Geoff had taken me on. I'd be running Midas (at least as tech
Dir) now !

Graham

Geoff was the owner, iirc it was Jim Cousins doing most of the design
work and some other guy who I cant remember at this time. This was after
Chas Brookes had left.


les...

 

[[email protected]]

Its value, correcting for the gain structure mess-up of the original design will be 7k5,
i.e a voltage gain of around -18x , but must be stable with nothing routed which equates
to a stable gain of -0.5x. I don't see a problem with the stability issues, I thought it
out on the walk back from the supermarket.

Graham- Hide quoted text -

- Show quoted text -

" Its value, correcting for the gain structure mess-up of the original
design will be 7k5"

OK I'm still kinda new to noise calculations, but won't the opamp
current noise of 1.5pA/rtHz give you something like 10nV/rtHz at the
output with the 7.5k ohm feedback resistor. Not that bad really given
the voltage noise of the resistor.

George

 

[[email protected]]

" Its value, correcting for the gain structure mess-up of the original
design will be 7k5"

OK I'm still kinda new to noise calculations, but won't the opamp
current noise of 1.5pA/rtHz give you something like 10nV/rtHz at the
output with the 7.5k ohm feedback resistor.  Not that bad really given
the voltage noise of the resistor.

George- Hide quoted text -

- Show quoted text -

"OK I'm still kinda new to noise calculations, but won't the opamp
current noise of 1.5pA/rtHz give you something like 10nV/rtHz at the
output with the 7.5k ohm feedback resistor. "

Opps, please ignore that last post. I see the current noise gets
divided by the source and feedback resistors .. my mistake!

George

 

[Eeyore]

Transformer + jfet!

Do you know how much good audio transformers cost ! I like Lundahls but you get such a
little thing for so much money. Say $80 ea. The client has indicated btw that cost is a
major factor so it might be back to 5532/4s (good enough - the rest of the desk is stuffed
with them) and a discrete front end.

Check out that 2SA1083/4/5 btw. www.profusionplc.com has them in stock (they can be tricky
to get hold of) and a link to the data sheet.

Graham

 

[Eeyore]

" Its value, correcting for the gain structure mess-up of the original
design will be 7k5"

OK I'm still kinda new to noise calculations, but won't the opamp
current noise of 1.5pA/rtHz give you something like 10nV/rtHz at the
output with the 7.5k ohm feedback resistor. Not that bad really given
the voltage noise of the resistor.

The noise current will also flow in the input resistors.

Graham

 

[Eeyore]

"OK I'm still kinda new to noise calculations, but won't the opamp
current noise of 1.5pA/rtHz give you something like 10nV/rtHz at the
output with the 7.5k ohm feedback resistor. "

Opps, please ignore that last post. I see the current noise gets
divided by the source and feedback resistors .. my mistake!

Nevertheless shows how critical resistor values are in high-performance audio. This mixing desk
didn't have the best reputation in that regard and I can see why now. I was using low R values
in really low cost equipment not long back. Usually a case of make it as low as possible
compatible with what the op-amp will drive happily.

Graham