John Curl's Blowtorch preamplifier part II - Page 1601 - diyAudio
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Old 14th September 2011, 08:08 PM   #16001
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Quote:
Originally Posted by jcx View Post
I guess it must be the models - I used fet models from looking for spice models

Cordell's Q models, maybe Bob's (skeptical) touch "spoils" the sim?

all the part models are on the schem

the result is, again, No difference in distortion or distortion "profile" with the the output in Class A with or without the inner feedback 1 M resistors - down to -170 dB below the fundamental

when bias is cut back a little, edging into AB output operation, removing the inner feedback Rs reduces some higher harmonics

I am getting tired of this - my impatience may mean there are errors in the sim - find them, put up PMA's models or work the excuse list a little harder...

I found a small error in your circuit: the value of Rlocal1, and Rlocal2 are slightly different:
-Rlocal1 = {1e6/(a+1u)}
-Rlocal2 = {1e6/(a+1m)}
I changed the second one to be the same as the first one - see the attached files.
Of course, this doesn't change the results: there are practically no differences in the output distortions with/without the feed back resistors.
Attached Files
File Type: txt jc3_full_SMALL_CORRECTION.asc.txt (9.5 KB, 9 views)
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Old 14th September 2011, 08:13 PM   #16002
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thanx, I changed one on the fly when I could see a little I(Rlocal) in @1, @2 steps doing a "sanity check" plot just to be sure the R was really in/out of the circuit when I thought

both the same is a little more logical - but being "out of the circuit" by 1000x or 1e6 x shouldn't matter


just to clarify - my position isn't that the JC-3 inner feedack R can never have any "benefical" effect whatsovever on the circuit distotion performance - if you look at my simplified sim you will see that I only claimed the "lowering drive impedance" at the internal node to reduce (PIM) distortion heuristic was flawed, since the inner loop feedback doesn't "perform" in that respect I see no reason to avoid the (large) added output stage Verror reduction by using the loop gain in the global loop

I think there is also going to be little improvement in the input stage performance at audio frequencies when you consider that you can have the same feedback, less the ~10% loading drop of the output stage by using just global feedback

excepting possible (single harmonic?) cancellation interactions which may arise with all stages distortions modeled - clearly more output stage distortion does give more inteaction possibilities

Last edited by jcx; 14th September 2011 at 08:36 PM.
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Old 14th September 2011, 09:47 PM   #16003
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Quote:
Originally Posted by scott wurcer View Post
See, it is the Krill. Hint, start swapping models and then change the magnitude and phase of the load like a REAL speaker and watch the fuzz at HF move around. At least the Krill tweak actually removed the dominant thirds. Just watching now.
I thought I would have been forgotten by now. I try to keep a low profile these days.
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Old 14th September 2011, 09:58 PM   #16004
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I don't really have a 'dog' in this fight. I find the models only approximate, especially because they do not emulate the original parts that I first used in 1974, when I made the closest version to this specific design as a horn tweeter amplifier with 108 or so SPL output at 1W. The amp only worked in this project at 4KHz and above.
Looking at the QUALITY of the substitutions, I find that the output devices are not necessarily more linear than what I originally used, and the greatest improvement of substituting the Toshiba devices for the Siliconix devices that were originally used was slightly more open loop gain.
The REASON for adding the secondary loop was to make an amp design based on the principles used in the earlier (1973) Lohstroh-Otala design originally developed at Philips Research Labs. So instead of using resistors to 'throw away' the excess gain, I simply fed it back to the input, in order to LOWER THE DRIVE IMPEDANCE to the complementary darlington output stage, that ideally runs class A, and at the BETA PEAK of the output devices. This cannot be a 'bad thing' to do, as the alternative was to throw away the extra gain. ONLY with this early prototype, did I use a single pair of power output transistors, the 2N5884-6, that are more linear than higher voltage versions of essentially the same devices. The actual JC-3 used 2 complementary pairs of output devices running at 1A per output pair. How on earth, any significant higher order terms could be generated in this design is a mystery to me, as it is virtually class A in every stage, at any working voltage or current (for the most part). Where is the 'kink' that will create the higher harmonics?
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Old 14th September 2011, 10:48 PM   #16005
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Originally Posted by Steve Dunlap View Post
I thought I would have been forgotten by now. I try to keep a low profile these days.
Hi Steve,
It's nice to hear from you.
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Old 14th September 2011, 11:35 PM   #16006
Pano is offline Pano  United States
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Old 15th September 2011, 01:42 AM   #16007
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Hi Steve,

I hope your health is holding up. I was refering more to the way the thread played out with all the sims than the circuit.
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Old 15th September 2011, 01:49 AM   #16008
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Originally Posted by john curl View Post
Where is the 'kink' that will create the higher harmonics?

The resistors cause almost 50dB more sensitivity to what is left of the crossover distortion. Even if you reduce it a lot some might come through. It's not rocket science the circuit has 80dB of gain out to a few kHz, but the resistors limit this to 50 (~34dB). What's left is a simple voltage loop. Secondly the simple thirds of the non-linear C multiply by being "in the loop".
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Last edited by scott wurcer; 15th September 2011 at 02:01 AM.
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Old 15th September 2011, 02:34 AM   #16009
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What crossover distortion?
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Old 15th September 2011, 02:53 AM   #16010
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I would like to point out to everyone that we did not always design amps in exactly the same way that IC's are made today.
Back in the 1960's we had somewhat limited devices, BUT we got BETTER spec sheets on the ones that we could get, so we knew more about the devices, and we cared more about optimizing the devices for lowest distortion.
Also, in the 1960's, we normally used the 'current drive' model of circuit amplification, rather than rely on Vbe deviations. This made us more sensitive to an output stage's distortion from beta nonlinearities, AND this happens when the output stage is CURRENT DRIVEN, which it essentially is, in the JC-3 without the additional feedback resistors. This is one of the reasons that we were so generous with the quiescent currents of each stage, often using a TO-5 transistor where today a TO-92 package might be used. This meant that the current change was less and the beta remained close to optimum. Today, especially in IC's, I can easily believe that higher order distortion artifacts can be easily generated, and the only reason they are not apparent is that they are suppressed by the negative feedback, and the noise floor of the the test equipment, but this was not the case with the Levinson JC-2 or the JC-3.
Today, I do not use the multi-loop feedback approach, because I use complementary mosfet drivers where the driver transistors for the output once were. This allows (to a greater extent) the allowing of a high impedance drive impedance from the previous stage. Instead of trying to force the open loop frequency by reducing loop feedback, I just make faster circuits, with output devices that are about 10 times faster than what I had in 1974. I do not achieve 20KHz open loop bandwidth, but I do have a higher open loop bandwidth than many amplifier designs, and I like it that way.
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