John Curl's Blowtorch preamplifier part II - Page 1876 - diyAudio
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Old 4th December 2011, 10:21 PM   #18751
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No doubts. Perfect idea.
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Old 4th December 2011, 10:42 PM   #18752
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Have you emulated it, yet, PMA?
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Old 4th December 2011, 10:52 PM   #18753
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PMA, I would be interested it how it compares to other input stages, especially self biased comp. jfets, especially in distortion order.
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Old 5th December 2011, 12:26 AM   #18754
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Default some ADA4898 numbers (from the datasheet) with a little calc

if my calculation from the patent's equation was correct then the gm of the input stage is 1 ppm linear for input differential V of up to +/-4 mV

I also estimate that the output could slew ~2V/us with |4 mV| input difference (the datasheet psrr curves give a good estimate of the audio frequency open loop gain if you know Sakinger's “A General Relationship Between Amplifier Parameters, And Its Application to PSRR Improvement”)

note that this isn't the slew rate limit, or onset of gross distortion - this is for the input stage gm variation contributing less than -120 dB nonlinearity

2 V/us seems like it would cover a lot of consumer line level audio - with essentially no "phase modulation" from input gm nonlinearity


the output stage doesn't look current limited for 600 Ohm load - but does have rather high dropout from the supply V such that 10 Vrms isn't practical

Quote:
OUTPUT CHARACTERISTICS

Output Voltage Swing
RL // (RF + RG) = 500 Ω, see Figure 41 −11.4 to +11.8 −11.7 to +12.1 V
RL // (RF + RG) = 1 kΩ, see Figure 41 −12.7 to +12.5 −12.8 to +12.7 V

Linear Output Current (RMS) f = 100 kHz, SFDR = −70 dBc, RL = 150 Ω 40 mA

Short-Circuit Current Sinking/sourcing 150 mA
that's 6 Vrms into 150 Ohms
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Old 5th December 2011, 05:40 AM   #18755
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Quote:
Originally Posted by jcx View Post
i
I also estimate that the output could slew ~2V/us with |4 mV| input difference (the datasheet psrr curves give a good estimate of the audio frequency open loop gain if you know Sakinger's “A General Relationship Between Amplifier Parameters, And Its Application to PSRR Improvement”)

note that this isn't the slew rate limit, or onset of gross distortion - this is for the input stage gm variation contributing less than -120 dB nonlinearity

2 V/us seems like it would cover a lot of consumer line level audio - with essentially no "phase modulation" from input gm nonlinearity
Do you mean 2V/us at the output of the input stage according to the patent?

Please explain, the opamp has much higher slew rate.
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Old 5th December 2011, 05:46 AM   #18756
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Pavel,

Quote:
Originally Posted by PMA View Post
Do you mean 2V/us at the output of the input stage according to to the patent?

Please explain, the opamp has much higher slew rate.
What is there to explain? The input stage is followed by gain stages, so if these stages are not slew rate limited at all the slew rate of the IC would be 2V/us * Gain. In reality the other gain stages will actually limit the slew rate...

So the OP's point was that the input stage does not introduce any slewrate limit.

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Old 5th December 2011, 06:06 AM   #18757
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Quote:
Originally Posted by PMA View Post
Do you mean 2V/us at the output of the input stage according to the patent?

Please explain, the opamp has much higher slew rate.
jcx is saying 2V/uS at the output within the bounds of not exceeding 4mV (for 1ppm linearity) at the input. Implication being for slew rates greater than this, up to the stated limit, the linearity of the input stage will gradually degrade.
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Old 5th December 2011, 06:09 AM   #18758
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Thanks for the explanation, abraxalito.
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Old 5th December 2011, 03:00 PM   #18759
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I repeat again another point, low open-loop BW is meaningless. You can use positive/negative feedback to the null pins on many op-amps to make the Aol and 1/gmC break point almost anything you want (even make Aol switch sign!). I have posted pictures here on doing this. Try it PMA, you can make the 797 have 10^5 gain rather than 10^7 and lower the OLB 2 orders of magnitude. Unfortunately null pins are deprecated these days or you could take the AD825 and tweek it up to 10^7 Aol and see for your self it makes no difference.
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Last edited by scott wurcer; 5th December 2011 at 03:03 PM.
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Old 5th December 2011, 05:03 PM   #18760
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Scott, you should have a talk with Dick Sequerra about this. He calls an op amp a 'reactance modulator'. Perhaps you can talk him out of it.
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