John Curl's Blowtorch preamplifier part II - Page 1883 - diyAudio
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Old 7th December 2011, 05:30 PM   #18821
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Hi,

Quote:
Originally Posted by Bob Cordell View Post
Also, see my PIM paper on my web site (CordellAudio.com - Home) where it is shown that it is modulation of the CLOSED LOOP BANDWIDTH that results in PIM, and, as such, the generation of PIM has little to do with open-loop bandwidth. The math does not lie.
Yet the closed loop bandwidth is a function of the open loop one (and gain linearity)... You cannot divorce the two...

Ciao T
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Old 7th December 2011, 05:30 PM   #18822
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Quote:
Originally Posted by PMA View Post
We speak about VAS load by a resistor, and the VAS is inside feedback loop.
Ok, I thought that the discussion was about "VAS" loading to make a volume controll.
This is what I'm getting with a VGT (Variable Gain Transimpedance) volume controll, by stepping the volume controll resistors (I'm not even using all the "tricks" to get a low THD). It is ofcourse an open loop design.
BTW: It's an all BJT design, no obsolete parts.
Simulation is done by factory models.

Cheers
Stein

Sorry the file is to large, I'll reduce it.
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Last edited by stinius; 7th December 2011 at 05:41 PM.
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Old 7th December 2011, 06:24 PM   #18823
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Quote:
Originally Posted by ThorstenL View Post
Hi,



Yet the closed loop bandwidth is a function of the open loop one (and gain linearity)... You cannot divorce the two...

Ciao T
The confusion still remains, take an amplfier with a 100MHz GBWP and make it have an Aol of 10^7 so open loop BW is 10Hz with some super process ultra-high Early voltage whatever and then make the >SAME< 100MHz GBWP amp have an Aol of 10k with an ordinary/bad process and an open loop BW of 10kHz and make a closed loop 20dB amp out of them they both are 10MHZ closed loop BW (within a very small error).

What John means by open loop BW is the 10Hz or whatever corner where the DC open loop gain breaks not the GBWP.

You could almost do the experiment with the AD829, it has an Aol of 100dB but I bet it's linear enough to feedback to the null pins from the output to push it up to 140dB or so. This part has lots of pins available, unfortunately the cap load helper R/C loads the VAS and is horrible for distortion even with no cap load (not my idea).

BTW is the 10H choke really needed in that RIAA preamp?
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Last edited by scott wurcer; 7th December 2011 at 06:34 PM.
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Old 7th December 2011, 06:31 PM   #18824
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Scott, I think it is cheating, since everything is optimized for low currents and high dynamic load resistance. Loading VAS only screws down input stage as well. Also, an output stage expects deep feedback around. High OL bandwidth opamp has to be designed, it is not enough to take low OL bandwidth opamp and load VAS on plain dumb resistance. It is a complete different story.
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Old 7th December 2011, 07:08 PM   #18825
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Darn, a guy can't get a few hours of sleep, without controversy breaking out. '-)
Now, I tried to be very diplomatic about what I think is a misunderstanding of what Charles Hansen is doing to change the open loop gain of his preamp.
I suspect that instead of decreasing the LOAD resistance, he is increasing the SOURCE resistance. This is obviously impossible with a typical op amp, but Charles and I use it all the time. I am using it in my newer phono stages that are discrete, for example. I also used it more than 35 years ago for microphone amps. (again discrete) I can't be absolutely sure of this, as I do not have access to his schematics, but I am fairly sure that I am right. IF I am right, then Scott has formed a false conclusion about Charles Hansen's most exotic preamp design.
There is also something 'half baked' about Scott's opinion of what I did with the HA911, then the best op amp for audio that either Dick Burwen or I could find (independently).
There, in the quest for modifying the open loop bandwidth, I added a 1 meg resistor from pin 8 to ground or the (-) supply, in order to open it from 100 Hz to perhaps 10KHz, 38 years ago. I EXPECTED the distortion to increase, especially since I had thrown away 40db (100x) global loop feedback, but with my trusty IM analyzer I got the OPPOSITE effect! I got LOWER distortion, instead! Why? Isn't this an interesting mystery? Of course, you have to be open to the concept, that it is even possible. I could repeat the test today, but I have other things to do, especially alone, without any technical support. Enough said.

Last edited by john curl; 7th December 2011 at 07:26 PM.
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Old 7th December 2011, 07:12 PM   #18826
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829 is difficult to be kept stable even as it is.
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Old 7th December 2011, 07:27 PM   #18827
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pushing up the low frequency gain should have next to no stability consequence - you're moving the open loop gain pole, already > ~ 100x below the loop gain intercept for moderate gains - it is only contribuitng a fraction of a degree phase shift

while it would have to be really fast, especially at low gains, a unity buffer in the loop at the output would avoid loading, keep the added R,C largely bootstrapped
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Old 7th December 2011, 07:30 PM   #18828
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Maybe - in case the real world is not more complex.
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Old 7th December 2011, 07:55 PM   #18829
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Quote:
Originally Posted by john curl View Post
IF I am right, then Scott has formed a false conclusion about Charles Hansen's most exotic preamp design.
There is also something 'half baked' about Scott's opinion of what I did with the HA911, then the best op amp for audio that either Dick Burwen or I could find (independently).

I added a 1 meg resistor from pin 8 to ground or the (-) supply, in order to open it from 100 Hz to perhaps 10KHz, 38 years ago. I EXPECTED the distortion to increase, especially since I had thrown away 40db (100x) global loop feedback, but with my trusty IM analyzer I got the OPPOSITE effect! I got LOWER distortion, instead! Why? Isn't this an interesting mystery? Of course, you have to be open to the concept, that it is even possible. I could repeat the test today, but I have other things to do, especially alone, without any technical support. Enough said.
I was never talking about his preamp, in a private conversation he made the same claims you make and BTW thank you for verifying, resistor loading VAS. Without all the data there is no way to determine what you saw and putting it to a supply might change the circuit bias.
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Last edited by scott wurcer; 7th December 2011 at 08:00 PM.
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Old 7th December 2011, 08:42 PM   #18830
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I tried AC coupling the resistor as well. Attaching the 1meg to the (-) supply gave the most symmetrical slew rate, but created a slight input voltage offset. DUH! '-)
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