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Old 12th November 2012, 05:32 PM   #2061
RNMarsh is offline RNMarsh  United States
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Very good numbers and down from 20 to only 15 transistors per channel, too. And, drives low Z loads. Nice cmr number. :-)
S/N ? and at what gain where your numbers done with? x100? What is cmr with gain = x10?

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Old 12th November 2012, 07:46 PM   #2062
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Very good numbers and down from 20 to only 15 transistors per channel, too. And, drives low Z loads. Nice cmr number. :-)
S/N ? and at what gain where your numbers done with? x100? What is cmr with gain = x10?
Haven't really looked at noise with SPICE. I expect similar noise to SW-OPA if running the same input currents. Presently I'm only using 2x1.2mA while SW-OPA has 2x5mA

The gain structures are very similar if running the same currents & bandwidths (OK they aren't but sorta are for noise )

There has to be a reason for a new circuit and I figured this is the Golf GTi to Scotts Audi A3[*] so I'm going for less current and a hair shirt stone age approach

I'm also nervous about increasing the zillion V/us slews as I think both are already in OPA627 & AD797 layout & decoupling territory.

eg it's likely most existing circuits using JE990 will have clean & dirty earths joined at the OPA cos the on board 0u1. But this is unlikely to give optimum results with AD797/OPA627 or these circuits.

Distortion 20kHz 10Vp 600R @ 100x gain cos its difficult to get either design to show more than -120dB products on less gain.

.. except the 15R results which were Voltage Follower. This 3.162 Vp test is really small PA territory but I wanted some numbers to compare with NWavguy's O2 headphone amp. Also to see what happens if you run deep Class B

CMR at 10x gain is 107dB to 20kHz. The Texas/AD sim test circuits (which I assume have some similarity to how they test in real life are 1x. It's all roughly scaled to NFB. So 20dB less at 10x gain 40dB less at 100x etc as you'd expect. And don't forget its a sim with 0.0001% matched FETs & resistors so I don't really believe we could build something with similar CMR to AD797/OPA627 like the numbers claim.

[*] Of course SW-OPA is really Mercedes SLK rather than these boy racer examples.
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Old 12th November 2012, 09:53 PM   #2063
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Do I believe this SPICE sh*t?

As apart from the 15R results, the distortion products are mostly around -100dB, it is likely that this and SW-OPA will be as critical as AD797 and OPA627 for layout & decoupling. In many situations, layout & decoupling is the MOST important factor in noise & distortion.
Well, you can, if you include real parasitics everywhere! Normal test circuits are riddled with assumptions, like perfect, non-noisy power supplies, perfect non-inductive connections, every ground is a perfect ground, zero capacitive coupling between parts of the circuit, no aspect can act as antennae, and on and on ...

It's a wonder that Spice does as well as it does at times ...

Frank
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Old 12th November 2012, 10:08 PM   #2064
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BTW, Richard, very nice effort in doing that development! I'll give it a bit of a thrash, try not to wrap it around a tree ...

Frank
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Old 12th November 2012, 10:52 PM   #2065
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BTW, Richard, very nice effort in doing that development! I'll give it a bit of a thrash, try not to wrap it around a tree ...
Thanks Frank. No traction control at present. And thanks for your many tips and inspiration along the way too.

Much of the time its me staring at the results and asking, "Do I believe that?"

Trying to avoid having an inductor on the output like a power amp which would make life so much easier. But SMD inductors are nasty and IMHO only suitable for EMI/RFI.

LTspice 'AC Analysis' dun always make sense compared to 'Transient' for dis stability stuff. I'm leaning towards 'Transient' for real life. Or it may just be stupid me
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Old 12th November 2012, 11:35 PM   #2066
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No worries .. I'm impressed by the energy you're throwing into this .. !!

Frank
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Old 13th November 2012, 01:36 AM   #2067
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I'm posting what is very near a final[*] design for a JFET 990. Such beasts have appeared in the past so what's different?
[*] in as much as anything which hasn't been built can be called final
Lots of good stuff here, worth a build. We could simplify but let's try it first.
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Old 13th November 2012, 01:47 AM   #2068
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No worries .. I'm impressed by the energy you're throwing into this .. !!

Frank
Ditto.. so much in such a short time.
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Old 13th November 2012, 05:04 AM   #2069
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A great thread although a few designs were trampled underfoot. Too much over reliance on FFT. Music is complex.

The main focus of preamp s is low noise, due to extremely high amplification factors and extremely high input impedance due to fragile input currents.
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Old 13th November 2012, 09:11 AM   #2070
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The main focus of preamp s is low noise, due to extremely high amplification factors and extremely high input impedance due to fragile input currents.
Wanna suggest some other criteria? Preferably some can translate to design objectives.

Don't think we've even touched on 'low noise'. Nothing suggested so far has voltage noise within 3dB of AD797 though of course much better current noise cos FET i/p
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