myhrrhleine said:Wouldn't the OPA827 be a better choice?
No, it would certainly be an overkill. The new OPA827 is amazing in the signal path, it's the best JFET opamp I have ever tested (as a trade between gain, noise, DC performance, etc...). I've tried OPA827 in YAP with excellent results. But it's not in production yet and can't be so far purchased. You have to apply for a couple of samples from TI.
john curl said:Do you mean that they just don't STEAL your name? I am missing out on something.
They have certainly tried, but usually without having paid for the design, either.
And there are a couple who have paid for the design but want their own name
on it.
You get all kinds.
SY said:The JC-1 input stage can be found here.
I am dissapointed. Ultra low noise 2SK170 input stage, then 100k/3k83 feedback resistors?
John, what is the measured noise figure of the JC1? I don't dare to tell what my estimate is, based on calculations http://focus.ti.com/lit/ml/sloa082/sloa082.pdf
Only the noise in a 100k resistor at 25C (298K) over the audio frequency range of 20 Hz to 20 kHz is Eth = 5.73 uV or –104.8 dBV. I'm looking at the JC-1 spec and I get completely different numbers:
Signal-to-Noise Ratio, input shorted
> 122 dB, IHF A-weighted, bias set to Low
> 120 dB, IHF A-weighted, bias set to High
> 113 dB, Unweighted, bias set to Low
> 111 dB, Unweighted, bias set to High
There is a 5pF across the feedback resistor that I didn't consider, transforming the white noise in pink-ish 1/f noise (the corner frequency is on top of my head around 300kHz) but still... Please, tell me I'm missing here something!
Wavebourn said:Can't you explain why you calculate for 100K, if it forms a divider with 3K83?
That would apply if you specify the input shorted unweighted equivalent input noise.
You guys worry too much. THIS IS A POWER AMP. Not a phono stage, or even a line stage. I purposely scaled UP the resistor array in order to NOT load the fragile open loop fet follower. Lower distortion vs slightly lower noise. Wavebourn's right, the 100K resistor doesn't add much at all.
john curl said:I give up. Tell me?
I have my guesses John. But I'm sure you've tried them all.
My first shots, if let's say I had opportunity to change/swap only ONE part, it would be...at first glance.. R36. But then again, I'm not designing them, I'm only finding myself doing this exercise for personal amusement on any design I come across, for the past 20 years. Not designing them puts me behind the 8 ball to a large extent, but the constant mental (and then actual single change) exercise helps out a bit. Except I rarely have the luxury of doing it with a schematic.
I'm talking strictly the quality of the resistor. To a ultra low ppm unit. If that was to be swapped out, then do R43, R45 to R48 and R35, all to the same brand and wattage as R36. PRP's best, or similar. This should increase the perception of speed, slam, stability in the bass, drive, a slight opening of the highs and softening of the treble/mids (increased warmth) into slightly more delicacy and liquidity. Soundstage and depth (detail in complex passages) should also improve in step with the others. All a function of the matching and the tempcos of the resistors as a set-as an affect on the flow of the circuit. Once again, just my guess. The effects will be subtle, but definitely there. That's seven resistors and now I'm in debt. I've overspent my $1.
john curl said:No, actually I really do wish that I could use polystyrene 5pf caps
I would twist a couple of wires instead.
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