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Old 3rd October 2012, 01:18 PM   #1481
dadod is offline dadod  Croatia
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Quote:
Originally Posted by fas42 View Post
Thanks for that, dadod. I've already looked at your verson, and there's no real difference between the two, apart from the LED and single diodes, not sure what you mean about the comp cap. Yet you were getting close to a null DC output, didn't make sense.

Anyway, in the end it came down to the JFETs subcircuit being very sensitive to the source resistors, if you change a single 25R to a 26R say, then your version shows the same symptoms as mine ...

Frank
Why would you increase 4% that resistor, try 1%?
Compensation capacitor is C4 in my schematic, instead C2 in yours.
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Old 3rd October 2012, 01:41 PM   #1482
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Now I'm really confused about your comp cap, in the opamp.asc I downloaded it's C3 ...

With regard to the resistor, I'm concerned about unstable equilibrium in circuits, when everything looks and works fine if the values are "perfect", but if they vary just a little then the circuit behaves very differently. So, if a DIYer happens to use non-perfect parts, or the active devices are not 100% matched in parameters say, then unwanted, larger scale behaviours can emerge ...

Frank
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Old 3rd October 2012, 02:06 PM   #1483
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I agree with Dado,
1% tolerance on resistors is very common, Ithink it is better to specify precision where it is needed then to develope an opamp for the DIYers that use other parts then the BOM suggest.
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Old 3rd October 2012, 02:49 PM   #1484
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Quote:
Originally Posted by fas42 View Post
Now I'm really confused about your comp cap, in the opamp.asc I downloaded it's C3 ...

With regard to the resistor, I'm concerned about unstable equilibrium in circuits, when everything looks and works fine if the values are "perfect", but if they vary just a little then the circuit behaves very differently. So, if a DIYer happens to use non-perfect parts, or the active devices are not 100% matched in parameters say, then unwanted, larger scale behaviours can emerge ...

Frank
YES, I agree! There should be procedure to allow DIYers to MATCH parts. Dale Vishey RN60D, 1% thru hole resistors are readily avaliable. And I believe jfets and bjt's should be matched at the voltage they are gona work in the circuit and not a 9 volt battery. Also most of the parts in my inventory and are avaiable are THRU HOLE (to92) parts. So please don't come up with SOT parts and require a damn microscope to work with them! Just my two cents! Ray
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Old 3rd October 2012, 02:56 PM   #1485
dadod is offline dadod  Croatia
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Quote:
Originally Posted by fas42 View Post
Now I'm really confused about your comp cap, in the opamp.asc I downloaded it's C3 ...

With regard to the resistor, I'm concerned about unstable equilibrium in circuits, when everything looks and works fine if the values are "perfect", but if they vary just a little then the circuit behaves very differently. So, if a DIYer happens to use non-perfect parts, or the active devices are not 100% matched in parameters say, then unwanted, larger scale behaviours can emerge ...

Frank
Sorry, the part of zip file was Wurcer shunt compensation(C3), look at the figure at the post Discrete Opamp Open Design C4 connected between collectors of the Q8 Q9 and R1 R2.
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Old 3rd October 2012, 03:25 PM   #1486
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A comment from the sideline to Dadod.

When comparing circuits, it is important to make it fair.

If I interpret your circuit properly you are using Ccomp = 100p in Scott's circuit, while using 22p in your circuit, it is of course true that you can lessen Cc when you take it back to the input stage instead of gnd, but not as much as you do.

Here is a Loopgain comparison and a transient THD comparison, each with gain = 5

Another thing is that you use 1kohm load instead of 600 ohms and very low values in FB network.

Cheers
Stein
Attached Files
File Type: pdf AC Analysis Dado Cc 22p.pdf (46.8 KB, 32 views)
File Type: pdf AC Analysis Dado Cc 44p.pdf (47.0 KB, 17 views)
File Type: pdf AC Analysis SW Cc 44p.pdf (46.7 KB, 15 views)
File Type: pdf AC Analysis SW Cc 100p.pdf (47.2 KB, 19 views)
File Type: pdf Transient Analysis Dado Cc 22p.pdf (33.3 KB, 19 views)
File Type: pdf Transient Analysis Dado Cc 44p.pdf (33.3 KB, 11 views)
File Type: pdf Transient Analysis SW Cc 44p.pdf (33.3 KB, 23 views)
File Type: pdf Transient Analysis SW Cc 100p.pdf (33.3 KB, 21 views)
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Old 3rd October 2012, 03:56 PM   #1487
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You guys are moving fast, I would hope some kind of breadboard could be made before anyone spends time or money on parts and boards. The last one worked perfectly according to sim but no need to push our luck.

Less time for this for a few days.

EUVL - Thanks for the cascode devices I missed those (J310 which I frequently use for this is only 25V). Boards would be fine but we need a definitive schematic which will still take a while.

On the load drive, this would require a high current buffer or hefty output devices (another version). I never intended this to do 50 or 30 Ohms by itself.
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Old 3rd October 2012, 04:12 PM   #1488
EUVL is offline EUVL  Europe
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Old 3rd October 2012, 04:31 PM   #1489
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Ready when you are.
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Yes
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Old 3rd October 2012, 05:04 PM   #1490
dadod is offline dadod  Croatia
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Quote:
Originally Posted by stinius View Post
A comment from the sideline to Dadod.

When comparing circuits, it is important to make it fair.

If I interpret your circuit properly you are using Ccomp = 100p in Scott's circuit, while using 22p in your circuit, it is of course true that you can lessen Cc when you take it back to the input stage instead of gnd, but not as much as you do.

Here is a Loopgain comparison and a transient THD comparison, each with gain = 5

Another thing is that you use 1kohm load instead of 600 ohms and very low values in FB network.

Cheers
Stein
My intension was not to compare, just simulate it. First I don't know what are suggested JFETs and BJTs. I've got couple of 2sk389 and 2sj109 and I need good line preamp so I simulated it according to that. Regarding GNFB Scott did not suggested starting values, so I used very low values as it gives better response.
You are right about Cc, half than the shunt value is better(47pF) and still distortion is lower.
I noticed that the opamp distortion is quite sensitive to the source impedance.
Here, this one is with more powerful output transistors, maybe Scott can suggest correct currents for it?
dado
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File Type: jpg schematic-2.jpg (145.8 KB, 250 views)
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