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Old 11th April 2010, 10:39 AM   #1
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Default Diff(erent) input stages, simple is not better

My views on different differential input stages in folded cascode headphone amps, both AD797 and AD829 types. Current through input stage is 2 - 4 mA. All amps are tuned for a good square response. Voltage supply 10 or 18 V depending on battery or wall-power operation. In spite of great difference in THD there's very little or no audible difference with increased supply.

You probably know I'm not an engineer, so these are of course an amateurish view, but I do as good as I can with LTSpice, RMAA and an oscilloscope.

1. JFET (SK170): Good tonality but warmer than neutral. Lack of definition and dynamics. Fuzzy soundstage. Typical JFET graininess.

2. SK170 cascoded with SK246: leaner and probably brighter than neutral, increased clarity with better details and soundstage, less but obvious JFET sound.

3. CFP with SK170 and SA1016: better clarity than 1 but not as good as 2. Soundstage is also better in 2. Tonality closer to 1. Grainy.

4. SK170/SA1016 CFP and SK246 cascode: to my ears much better than the three above. Clean non-fatiguing sound with very little JFET character.

5. Bipolar (SC2362): tonality is OK, so is dynamics and soundstage. Instead of JFET graininess there's a metallic "solid state" sound. With SC2240 sibilant and even more "metallic". With BC550 the tonality is close to SC2362 but with less clarity.

6. SC2362 cascoded with SK246: better clarity and brighter but not really an enjoyable sound.

7. SC2362/SA1016 CFP: much more enjoyable than 5 and 6. Tonality warmer than neutral but it doesn't lack details or airiness.
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Old 11th April 2010, 11:02 AM   #2
AndrewT is offline AndrewT  Scotland
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that must have taken a fair bit of time with all those different prototypes.
Thanks for all the effort & report.
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Old 11th April 2010, 11:16 AM   #3
Bonsai is offline Bonsai  Taiwan
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How did you arrive at these conclusions? Can you share what you used?
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Old 6th June 2012, 07:47 PM   #4
xslavic is offline xslavic  Moldova
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Hi
whats the perfect value for base resistor of SA1016 to use as amplifier ?
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Old 6th June 2012, 08:44 PM   #5
Boscoe is offline Boscoe  United Kingdom
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Quote:
Originally Posted by nelsonvandal View Post
My views on different differential input stages in folded cascode headphone amps, both AD797 and AD829 types. Current through input stage is 2 - 4 mA. All amps are tuned for a good square response. Voltage supply 10 or 18 V depending on battery or wall-power operation. In spite of great difference in THD there's very little or no audible difference with increased supply.

You probably know I'm not an engineer, so these are of course an amateurish view, but I do as good as I can with LTSpice, RMAA and an oscilloscope.

1. JFET (SK170): Good tonality but warmer than neutral. Lack of definition and dynamics. Fuzzy soundstage. Typical JFET graininess.

2. SK170 cascoded with SK246: leaner and probably brighter than neutral, increased clarity with better details and soundstage, less but obvious JFET sound.

3. CFP with SK170 and SA1016: better clarity than 1 but not as good as 2. Soundstage is also better in 2. Tonality closer to 1. Grainy.

4. SK170/SA1016 CFP and SK246 cascode: to my ears much better than the three above. Clean non-fatiguing sound with very little JFET character.

5. Bipolar (SC2362): tonality is OK, so is dynamics and soundstage. Instead of JFET graininess there's a metallic "solid state" sound. With SC2240 sibilant and even more "metallic". With BC550 the tonality is close to SC2362 but with less clarity.

6. SC2362 cascoded with SK246: better clarity and brighter but not really an enjoyable sound.

7. SC2362/SA1016 CFP: much more enjoyable than 5 and 6. Tonality warmer than neutral but it doesn't lack details or airiness.
You do realise clarity/brightness/warmth etc are just your ears listening to distortion at different frequencies? For example warmth usually means lack of control in bass drivers in a hifi system and then added clarity/brightness is higher frequency distortion.

What I say above matches all of your conclusions and surely is vague proof that different amplifier characteristics are just distortion in different places which is something we can measure and improve.
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Old 7th June 2012, 03:18 AM   #6
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Are you able to measure distortion, bandwidth, noise, etc. to corroborate what you hear with the different topologies? Did you also do any blind testing or have others listen during your testing to validate your findings?

Do you have schematics or pictures of the circuits?

Thanks for your insight!
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Old 7th June 2012, 03:37 AM   #7
CBS240 is offline CBS240  United States
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Quote:
Typical JFET graininess.
I do not find this to be the case. I suppose it depends on the circuit they are in. It is my experience they can be quite crystal clear and balanced in tone. I find cascode (with proper devices) is good and J-fets are swell to use in partnerships with BJT's.
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Old 7th June 2012, 01:24 PM   #8
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You people hopefully do realize this is a necro'd thread? Posts #1..3 are 2 years old...
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Old 7th June 2012, 02:29 PM   #9
Bonsai is offline Bonsai  Taiwan
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Ha ha.

Yes, seems we are trying to raise it from the dead!

;-)
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Old 7th June 2012, 03:32 PM   #10
juma is offline juma  Germany
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Quote:
Originally Posted by nelsonvandal View Post
...Current through input stage is 2 - 4 mA....

1. JFET (SK170): Good tonality but warmer than neutral. Lack of definition and dynamics. Fuzzy soundstage. Typical JFET graininess....
Typical result for starved 2sk170. Raise Id to 70-90% of Idss and you'll get very different results. Study the datasheet - transfer characteristic (Id vs. Vgs).

Quote:
Originally Posted by sgrossklass View Post
You people hopefully do realize this is a necro'd thread? Posts #1..3 are 2 years old...
Doesn't matter. People still can learn from it.

Last edited by juma; 7th June 2012 at 03:35 PM.
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