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PostPosted: 23 Sep 2018, 00:28 
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Joined: 26 Dec 2016, 03:46
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Location: Bayarea
I can tell you jFET input stage sounds different than BJT.

I have not try MOSFET yet, that's another to do on my list. MOSFET has a completely different crossover distortion profile. I expect it to sound different on the OPS.


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PostPosted: 23 Sep 2018, 02:47 
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I'm using 2sk30 for my input stage for all op amps I design its a jfet


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PostPosted: 23 Sep 2018, 03:13 
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Location: Bayarea
You worry about offset and drift using single jFET. They drift a whole lot more than BJT. I would want to get dual jFET which is much harder and a lot more expensive. That's the reason I look at Dual MOSFET.

I am more looking at using MOSFET as output stage. Problem is using as source follower will eat up a lot more headroom than BJT ( Vth=3V instead of Vbe=0.62V of BJT).

I don't want to use common source output, output impedance is too high, only way to lower output impedance is through global feedback. That's a bad practice. On top, common source has gain, more load on driver stage ( high Cdg and miller effect). Other than Nelson Pass using common source, Everyone else use emitter follower or source follower as output......For very good reason.


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PostPosted: 23 Sep 2018, 03:32 
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No worries about drift using single jeft, simliar drifts to bipolar.
The main reduction on dc drift I tend to drop more emitter voltage on voltage gain stages, also use some emitter resistor on diffrential input stage for both mosfet and transitors.

However this means less votlage swing avlible, gain reduction and lower frequency response due to higher output imedance of gain stages driving output power stage of op amp.
I tend to drop about 100mV on emitter resistor with op amps that I give high precision temperature independant biasing and drop about 200-600mV for more sensitive designs.

My 4 stage op amp has very high gain, this allows me to drop much less emitter voltage while mainting high dc stabliilty a extra to get higher frequency response.

You got me thinking about using faster transitors for OPS since I rember using small transitors as class b ops and seeing the frequency response go crazy high was nice.


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PostPosted: 23 Sep 2018, 04:10 
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Location: Bayarea
The faster the transistor, the better, then you can extend the BW and get more loop gain up to 20KHz. If you look at my schematic, those are the fastest transistors I can find when I designed the amp. They are all still quite slow in my book, the medium power transistors are discontinued by Toshiba, I just bought like 200 each to give me enough supply for my whole life time. I have no intention of marketing it, only for my own build, that will be enough for 100 amps!!!


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PostPosted: 23 Sep 2018, 15:03 
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I think using transistors of 30mhz should be enough for what I need unless I am cascoding.

Past exprience showed any 2sc 2sa (Japanese) transistor to have totally inferior sound compared to non Japanese ones, so I had been staying away from them. Talking about direct replacements and no frequency response increase due to using same old slow driver.


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PostPosted: 23 Sep 2018, 17:18 
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Location: Bayarea
I don't think BJT makes a difference in the sound. You look at the spec. and determine what fits your need. I don't believe for a moment 2AS/2SC give worst sound. Even jFET just make very minor difference, it's the design. I have been using them and all kicked the butt of the PA-7 and Acurus 3X200. Those are no dogs particular PA-7 was the Stasis design by Nelson Pass that put him on the map of high end audio.


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PostPosted: 23 Sep 2018, 19:17 
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I'm not supprised that you are beating those amps because looking at pa7 and 3x200 they have lesser op amp gain stage and lesser output stage compared to your schematic.


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PostPosted: 24 Sep 2018, 01:00 
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Location: Bayarea
ILoveHiFi wrote:
I'm not supprised that you are beating those amps because looking at pa7 and 3x200 they have lesser op amp gain stage and lesser output stage compared to your schematic.


I don't know how you count the gain stage, I only have two gain stages, the other two amps also have two gain stages. Just I have higher gain per stage using current mirror as load on the IPS and I use darlington VAS to have very high input impedance to avoid loading the IPS stage. I use 3EF OPS to present high input impedance to avoid loading the VAS stage. OPS is not a gain stage as it doesn't have gain.

I Just optimize the the circuit compare to the other two amps. Most amps don't have as high gain as my amp, it's much harder to tame with high gain.

This amp is not the lowest THD, the one I am working on right now has THD of 0.0024% at 20KHz. Slew rate is 40V/uS at large signal of 60Vpp swing.

Only thing I did not do is cascode VAS. That was a hard decision. Putting in cascode eat up at least 2V from the rail. I am using low enough supply voltage already, I can't afford to lose another 2V.


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PostPosted: 24 Sep 2018, 01:37 
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Joined: 23 Feb 2017, 02:02
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Cause current mirror load on input stage greatly increases your openloop gain thats why the distortion is lower and making yours sound better than with out.

No current mirror load gain is less than 100, mostly less than 50.
Current mirror load and darington buffed vas easily go past 200x gain on input stage.


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