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Old 2nd September 2012, 02:15 AM   #611
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several on Amazon today... reasonable, various conditions... got one myself, should be the right thickness to hold up the back of my laptop, I expect...

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lessee, everything I want to know about a type of device I can barely get a hold of now, and surely will not be able to get shortly?
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Old 2nd September 2012, 02:21 AM   #612
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Been meaning to mention this before I forget to open my mouth... I really liked this opamp for a long time, it seemed to do well when other opamps got a bit "unhappy", it was mentioned to me by an AD person that the "secret" was in the ability of the front end to handle, ummm... what did he say?... I think it was large differential signals, not sure.

http://www.analog.com/static/importe...eets/AD845.pdf

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Old 2nd September 2012, 03:13 AM   #613
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by scott wurcer View Post
Dick. you could use an avatar and nice pithy signature.
Suggestions?

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Old 2nd September 2012, 05:41 PM   #614
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Quote:
Originally Posted by john curl View Post
Before we get too confused about a lot of things, let's put things in order.
First, if you have an active load, you should try to reduce the intrinsic (noise) gain of the load by reducing Gm through any means available, usually a series R.
You have to look at the whole input device assortment to determine how much FORWARD gain you will have. It is NOT the NOMINAL voltage gain, but the effective Gm(R[load]) where R[load] will be a very high and almost undefined impedance.
So since both the active load and the input see R[load] then the only thing you have to compare is their intrinsic voltage gain, each one, and the self noise of the active parts.
The 'trick' is to lower the Gm of the active load, to be lower than the effective forward Gm, in each leg, of the source.
This is NOT as easy as it might look.
To intuit the process, think about a SUMMING amplifier, where you have 2 separate active sources contributing. IF you do not use enough emitter or source resistor degeneration, you WILL not achieve what the input stage, itself, is capable of.
John, a good post.

Cheers
Stein
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Old 2nd September 2012, 07:44 PM   #615
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How about a simple circuit with an SE output stage...

Designed to use the 2SK2145 in the front end and medium power switching complementary mosfets in a single package for the second stage. Works well without a VAS, +/-8 to +/-18v supplies, will drive to within 3 volts of the rails, +/-8 volts into 600 ohms, and makes for a small circuit layout using surface mount.

I'm happy to post part numbers and values for those who care but am interested in comments on the topology first.

Dave
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Last edited by fitzfish; 2nd September 2012 at 08:07 PM.
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Old 2nd September 2012, 08:48 PM   #616
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Default parallel Not series -

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Originally Posted by bcarso View Post
The other problem here is that the complementary series connection ideally has both JFETs operating close to their zero tempco points. But it's work just to find that point for a single FET. When we need to do it for two of them AND have that operating point be the same magnitude, the selection task is daunting. Separate current generators could be provided I suppose, and the signals combined afterwards. But despite the appeal of a push-pull stage, a single-ended one has a lot going for it, particularly as good NFET duals still exist (and can be configured successfully as low-drift single-ended stages, or of course as conventional differential pair front ends).

If we get lucky and the two track, that's another way that the phenomena can be saved. We still have high noise to deal with for the few currently-manufactured devices. With modest gains needed for some applications, perhaps the noise upstream will dominate, and of course the lower gains reduce the voltage drift concerns as well.
First to point out that the push-pull input stage has the jFETs effectively in parallel. The classic differential stage puts the two transistors in series. Thus the noise potential is lower in push-pull than the other (all things being equal).

If you select the jFETs I used and show, you wont have to worry about the issue of finding good compliments and thier TC bias etal.... I did the leg work for you. It isnt luck to get the zero TC from the circuit.

The circuit I used gets as low thd into 30 Ohms as a high neg feedback design but does so with only 8 transistors.
Their total cost is below a single BF862 diff pair. It will be hard to beat in low to moderate gain applications for performance, cost, simplicity. One would have to be cheaper and better performance for audio and use fewer parts to qualify as all around "better". It sounds great, BTW. -Dick Marsh

Last edited by RNMarsh; 2nd September 2012 at 08:51 PM. Reason: details -
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Old 2nd September 2012, 09:52 PM   #617
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Quote:
Originally Posted by RNMarsh View Post
First to point out that the push-pull input stage has the jFETs effectively in parallel. The classic differential stage puts the two transistors in series. Thus the noise potential is lower in push-pull than the other (all things being equal).

If you select the jFETs I used and show, you wont have to worry about the issue of finding good compliments and thier TC bias etal.... I did the leg work for you. It isnt luck to get the zero TC from the circuit.

The circuit I used gets as low thd into 30 Ohms as a high neg feedback design but does so with only 8 transistors.
Their total cost is below a single BF862 diff pair. It will be hard to beat in low to moderate gain applications for performance, cost, simplicity. One would have to be cheaper and better performance for audio and use fewer parts to qualify as all around "better". It sounds great, BTW. -Dick Marsh
How many of these did you build? Are you aware of the huge spread of values for the pertinent parameters of the 5457 and 5460? Would you be prepared, based on your experience, to go into production of, say, 10k pieces? That's usually my world.

The BOM is below the cost of a single BF862 pair? I buy 862s for 22 cents in 100's from Mouser, iirc, and I don't think they are doing me any special favors. Perhaps you had an old Toshiba dual in mind? Even they didn't used to be all that expensive when they were still in production, but yes, as Wurcer remarked, even the single SK170 has a die size bigger than some ADI entire video amps Even with 100% yield from a wafer you won't be getting all that many from a run.

I have nothing against the topology per se. However, the requirements on voltage drift are modest for a closed loop gain of 4.3. As well, you provide a trim adjustment for initial offset, and although useful, you thereby add a potential concern for manufacturing and reliability.

The noise remark (parallel versus series) is certainly true. However, to operate the devices at the zero tempco point requires substantial resistance in series with each source. If these were 2SK170V and 2SJ74V (the latter getting very scarce now), there's the chance that the zero tempco point would occur around Idss, and that would be fat city indeed. However, for both of the devices from process 55 and 89, zero tempco occurs far from Idss, at operating points that correspond to low transconductance and relatively high noise, even before considering the thermal noise of the series resistances.

Will such noise really be a problem? Quite possibly not, depending on the noise in the source.

Again, nothing against the topology. With a couple of better parts it could be very low noise and likely still lower distortion, higher bandwidth. I don't doubt that it sounds good.
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Old 3rd September 2012, 01:43 AM   #618
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Quote:
Originally Posted by bcarso View Post

1. How many of these did you build? Are you aware of the huge spread of values for the pertinent parameters of the 5457 and 5460? Would you be prepared, based on your experience, to go into production of, say, 10k pieces? That's usually my world.

2. The BOM is below the cost of a single BF862 pair? I buy 862s for 22 cents in 100's from Mouser, iirc, I have nothing against the topology per se. However, the requirements on voltage drift are modest for a closed loop gain of 4.3.

3. As well, you provide a trim adjustment for initial offset, and although useful, you thereby add a potential concern for manufacturing and reliability.

4. The noise remark (parallel versus series) is certainly true. If these were 2SK170V and 2SJ74V (the latter getting very scarce now), there's the chance that the zero tempco point would occur around Idss, and that would be fat city indeed.

5. However, for both of the devices from process 55 and 89, zero tempco occurs far from Idss, at operating points that correspond to low transconductance and relatively high noise, even before considering the thermal noise of the series resistances.

6. Will such noise really be a problem? Quite possibly not, depending on the noise in the source.

7. Again, nothing against the topology. With a couple of better parts it could be very low noise and likely still lower distortion, higher bandwidth. I don't doubt that it sounds good.
1. If I were building in high volume, i would have the devices sorted/graded at the factory. And, with cheap labor used, the final batch can be quickly determined. This would be true with any complimentary jFET used.

2. 22 cents is good. One builder reported finding the ones I use for 3 cents. 1/7th the cost.

3. Trim isnt needed if matched. But could also be fixed values... even if selected. Of course, you know that already.

4, 5. Other devices have been used/tried as I have mentioned before. In addition, paralleling jFETs could be done with minor R changes.

6. Noise is definately not an issue for appl at line levels.

7. My own Goal: Low parts count - no more than 8 transistors, super high performance (H2-3 below -100db re 1v AT 30 Ohms), high drive capability into low Z, no DC servo and be low cost. Then throw in that it must do this withOut high gnfb and the open loop bandwidth must be 40Khz ( I missed that mark a little) for the audiophiles. That was my goal and challenge to others. Doing "better" with more cost and complexity just didnt seem like a proper challenge to me.

Have a nice day. Got to go do some reading, now. -Dick Marsh

Last edited by RNMarsh; 3rd September 2012 at 02:07 AM.
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Old 3rd September 2012, 02:19 AM   #619
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2SJ498
2SK2880

Neither are listed as deleted, but as to a source???
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Old 3rd September 2012, 02:26 AM   #620
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-100 db of 2-3 order without negative feedback? Impossible.
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