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-   -   Opamps for servos in tube amps - Christmas gift from LT, 12-12-12! (http://www.diyaudio.com/forums/tubes-valves/225570-opamps-servos-tube-amps-christmas-gift-lt-12-12-12-a.html)

Wavebourn 13th December 2012 02:22 AM

Opamps for servos in tube amps - Christmas gift from LT, 12-12-12!
 
Hi!

I got e-mail from LT today, looks interesting.

Since the chip draws low current, and slew rate is not very impressive, I would hesitate to spoil tube signal path by them, but it is a great candidate to be used in servos, when vacuum tubes are directly coupled, like in my Pyramids. The opamp has MOSFET inputs, and can run from up to 140V!

LTC6090 - 140V CMOS Rail-to-Rail Output, Picoamp Input Current Op Amp - Linear Technology

I ordered samples, it will be a great Christmas gift!

TheGimp 13th December 2012 03:25 AM

Interesting, but care must be exercised for the thermal pad.

gingertube 13th December 2012 03:52 AM

Quote:
"I got e-mail from LT today, looks interesting".

Ditto,
I posted the same link over at Ampage. You could use it for output tube drive (if its not against your religion) not just bias servo (10mA source/sink capability). Interest there was also in the simple 100W MOSFET Amp shown on p 19 of the datasheet. Only available in surface mount which is a bit of a bummer.
Cheers,
Ian

Wavebourn 13th December 2012 06:14 AM

My religion does not allow to use class B and 11V/uS opamps to drive tubes.

ruben alstad 13th December 2012 02:12 PM

i allways wondered about servos in tube amps,
but i had the bias in mind, a servo controlling a pot for the bias (fixed bias).
has anyone tried this?

DF96 13th December 2012 02:31 PM

Yes, people have tried this. If done the wrong way it acts like a CCS, fixing the average current instead of the quiescent current.

Wavebourn 13th December 2012 04:23 PM

I did that in class AB amp, with S/H and comparator: when signal level was above some level it switched off and kept voltage unchanged. When below, it opened the path closing servo loop.

ruben alstad 13th December 2012 04:27 PM

Quote:

Originally Posted by DF96 (Post 3283058)
Yes, people have tried this. If done the wrong way it acts like a CCS, fixing the average current instead of the quiescent current.

yeah of course, it would have to be a auto bias button or something alike, when there were no signal on the input you could press it and thus it would re bias itself. that's simpler in my opinion.

DF96 13th December 2012 04:42 PM

Wavebourn's method is better than a button. A button would require a very long-term hold (hours), or at least separate arrangements every time you switch on. At most his method need only remember the voltage for the length of a CD.

Wavebourn 13th December 2012 09:12 PM

Quote:

Originally Posted by DF96 (Post 3283227)
Wavebourn's method is better than a button. A button would require a very long-term hold (hours), or at least separate arrangements every time you switch on. At most his method need only remember the voltage for the length of a CD.

Less, actually: it had shorter time constant on soft volume than on high volume. No need for silence, about 5W for 100W amp is a good threshold to switch from 5 seconds to 300 seconds approximately. On 5W power idle and average currents are almost the same.


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