The quietest audio op-amp with ultra-low THD

Graham,

What resistance values are you assuming? Not one month ago, in this very boutique, I posted a link to a new part with some really cute specs--the ADA4898-1.

I gather it's probably intended for lower source resistances than yours, but its combination of low eN and low Cin is amazing.

Cheers,

Phil Hobbs

Not got round to using it yet but the ADA4899 looks tasty. 1nV, 0.7ppm,

Yes, my source resistance may dominate. It's a new module for SSL 4000E/G professional sound mixing desks to convert the 'channel buckets' into a monitor desk for Digital Audio Workstations. I was shocked at how high their mix resistor values were (15k). Crikey, even at Studiomaster in the latter days I was using 4k7 !

It's certainly amazingly quiet though.

I'm not familiar with the THD being quoted in dBc though. Can you assist there ?

Graham

Open the window, more shit flies in.

martin

It's not that 600MHz of bandwidth is directly required, but rather that all of the musical harmonics in the 20KHz to 600MHz band be passed through linearly and not (inter)modulated down into the audible band.

I guarantee that any self-respecting audiophile will be able to hear the difference -- as long as you warn him or her ahead of time.

:-|

Bob

John

noise

Yes. I was thinking more of it allowing a gain of say x5000 to 100kHz. The chip's low Vos allowing this in one hit.

A good audio mic at 100KHz and 70dB+ gain, what on earth are you are you recording?

martin

there ?

Depends on whose jfet !

The existing bus amp uses an LM394 and then destroys the noise performance by following it with a -6dB 5534 stage.

The worst of all worlds. The bus mix amp will clip before its buffer and the noise figure goes to hell.

SSL weren't noted for the best audio electronics.

Graham

Absolutely. The LME's 60 or so MHz is more than enough.

Graham

Max gain required would be about 24.

Graham

dBc = 'decibels with respect to the carrier', i.e. the undistorted output.

Cheers,

Phil Hobbs

there

Unfortunately that doesn't specify a specific voltage or power level AFAICS.

Graham

At Midas Audio in the 80's we used 12k.

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there

Yes, Aren't you going to be dominated by the current noise? Speaking of jfet opamps, Do you have a low noise favorite? I like the OPA134 from Burr-Brown (TI). But am always looking for something better. George

Yes, but you were going downhill them. KT group bought out Midas in 88 when I was there.

Dave Dearden and I were dumbstruck (and not in a good way) just with the mic input stage.

Graham

there

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, in fact I'm now thinking again of a discrete front end where en can be as low as 0.5nv/sqrt Hz with a certain relatively inexpensive device I know, widely used in direct coupled low-Z mic preamps. Time to do the sums and a balancing act !

Graham

But mind the knee which is a bit highish, see figure 13:

Since Graham does audio stuff this amp might not quite be the ticket.

How about a discrete design? Maybe with tubes ...?

Yeah but: Think what that can do in advertising: Gold plated super electrons that are made extra agile and can turn themselves around in under 2nsec. Electrons spend less time in the muddier regions in the device, therefore get less dirty and don't wear out so fast. Much less electron fatigue. Makes for extremely crisp crescendos that could never, ever be heard with an amp that is anywhere less than 500MHz :-)