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Old 14th October 2012, 10:45 PM   #1681
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Quote:
Originally Posted by bcarso View Post
Finally set up to screen some today off a strip and got through 19 pieces.
It's good to people willing to spend a little of their free time contributing to this.
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Old 14th October 2012, 10:49 PM   #1682
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Originally Posted by EUVL View Post
> I wonder if anybody's interested in a parallel effort to design a dedicated unity gain follower to complement the general purpose op amp?

I already have umteen versions of those, some published and some not.
Jung diamond buffer, source follower with or without cascode, .....
Also DIP8 compatible and pin compatible with single or dual opamps on 10x10mm footprint.
You just need to tell me what you want.


Patrick
Hi Patrick,
Which one sounds best and which one is most stable with various loads?
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Old 14th October 2012, 10:51 PM   #1683
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Here is a first shot at a blow by blow description of my op-amp idea. No guarantee that it is clear to everyone so important clarifications can be added.
Hi Scott,
Thank you very much, your explanation is illuminating.
I still miss FETs and transistors part numbers.
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Old 14th October 2012, 10:59 PM   #1684
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Hi Scott,
Thank you very much, your explanation is illuminating.
I still miss FETs and transistors part numbers.
There are several possible parts lists, I think the biggest issue is the SMD vs through hole versions. Frankly most of them will make a functioning amplifer so we will have to tolerate a little indecision for a while.
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Old 14th October 2012, 11:13 PM   #1685
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Originally Posted by scott wurcer View Post
There are several possible parts lists, I think the biggest issue is the SMD vs through hole versions. Frankly most of them will make a functioning amplifer so we will have to tolerate a little indecision for a while.
Okay, understood and accepted.
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Old 14th October 2012, 11:35 PM   #1686
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Mean value of Idss: 15.693mA. Standard deviation 1.567mA.

Mean Vgs for 100uA Id: -634.3mV. Standard deviation 43.9mV.

Brad: So how many should we order to get a matched pair after matching. So out of 19 units the drift is 1.567 ma. Should we consider a larger sample than 19? Of the 19 how many hit the mean Vgs within 2 %? What % precision should we shoot for? Thanks, Ray
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Old 14th October 2012, 11:47 PM   #1687
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Originally Posted by scott wurcer View Post
I was thinking more of pennies for the resistors.
indeed so would I be initially. I was just countering the statement that for highest performance SMD resistors should be avoided. I wont use them for all positions. i'll build with susumu RG thin films initially till the opamps find a final resting place and a final set of operating conditions.

Once all is said and done, then i'll choose a few in critical positions in the circuit and replace with the ASMP (actually at these currents the ASM will likely do) to sooth my audio-nervosa

it might be worth narrowing down a few positions and using group purchasing power to get the price down if anyone else is interested
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Old 15th October 2012, 12:36 AM   #1688
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Originally Posted by Wavebourn View Post
I would say, for audio wider flat loop gain matters more, and +30 dB on 20 kHz is a huge advantage.
I also agree. but a chk on the settling time affects is needed for 'due diligence.' It could limit some apps - such as, with fast converters.

Last edited by RNMarsh; 15th October 2012 at 12:39 AM.
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Old 15th October 2012, 12:49 AM   #1689
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for a given spread what affect does the spread (mismatch) have on thd/harmonics of X10 amp and X100 SW-OPA? From worse case measured from the 19 pieces to tight spread of maybe 2%. I suspect it might be a small difference because of the high olg/fb available. What does sim say?
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Old 15th October 2012, 12:51 AM   #1690
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Quote:
Originally Posted by grhughes View Post
Mean value of Idss: 15.693mA. Standard deviation 1.567mA.

Mean Vgs for 100uA Id: -634.3mV. Standard deviation 43.9mV.

Brad: So how many should we order to get a matched pair after matching. So out of 19 units the drift is 1.567 ma. Should we consider a larger sample than 19? Of the 19 how many hit the mean Vgs within 2 %? What % precision should we shoot for? Thanks, Ray
It's not clear to me yet how close they have to be matched. Gut feeling is that out of the 19 I could get two or three decent pairs. By tweaking the source resistors in the lower devices in SW's design, [edit: post 1675] one can accommodate mismatch there more readily.

Stay tuned. Except for the tedium and however one fixtures them for testing, at least they are cheap and widely stocked.

The general impression I get talking to others about the few remaining Toshiba parts, they tend to be more tightly distributed. The few 2SK2145 duals I tested so far were quite tightly matched. According to a friend, the singles comparable to the 2145, the 2SK117 and whatever the SMD version is are pretty tightly controlled in his experience. The 2SK2145 is not usable in the SW design owing to the hardwired connection between channels, and the transconductance is not as good as the BF862.

I noticed that the leads on the BF862 are ferromagnetic. This may disturb some, but it makes it easier to retrieve ones that go tiddly-winking out of the tweezers and onto the floor, using a permanent magnet.

I am also planning to parallel a bunch of them by soldering their gate leads to a piece of wire, which will serve as a heatsink, and seeing if the resulting assembly is sufficiently stable against oscillation to require only one gate-stopper resistor or lossy inductor. In that situation the matching among the parts need not be as tight, as the characteristics will benefit somewhat from the effect of averaging.

Brad

Last edited by bcarso; 15th October 2012 at 12:53 AM. Reason: reference
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