John Curl's Blowtorch preamplifier part II - Page 3600 - diyAudio
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Old 28th February 2013, 12:01 AM   #35991
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Quote:
Originally Posted by scott wurcer View Post
The use of transient in that term is inaccurate, the input transfer function is there for contiuous time signals. The tanh shape for an undegenerated bipolar is directly related to the total current i.e. when at max there is the onset of hard slew. R.D. Thornton and J.K. Roberge at M.I.T were teaching it this way in 1969, Barrie Gilbert was already preaching from this pulpit in 1968 and before.
Scott

There actually may be a bit we agree on. I don't like the TIM label and quite a few of the other audio labels (PIM etc).

But the point is drifting. Otalla et al identified a mechanism that ran counter to most folks intuition about global feedback amplifiers at that time.

I haven't looked at all of the possible other earlier approaches to see who was the first to correctly publish on the issue.

Now why there is heat about old issues makes little sense. However there are still a lot of folks who seem confused about what is going on here.
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Old 28th February 2013, 03:27 AM   #35992
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Knock, knock, knock…
Hello, is there anybody here?
Is this thread about John Curl's Blowtorch preamplifier?
Well, I went through the first pages of "John Curl's Blowtorch preamplifier" (part I) and I have a question concerning the Blowtorch topology.
The topology is, as described by SY: "All FET, no feedback, servoed. First stage is 4 quadrant comp diff, second is folded cascode using MOSFETs run at high current. Servo with roughly 1 sec time constant." JC added: "The CTC preamp is a transconductance amp that is current output."

Now, my questions to JC (and to all others):
John, in one place you wrote: "Yes, more current is better than less current in the output devices.", while elsewhere you wrote: "I can only use the highest Idss (JFETs) devices in this design, because the output follows the input, and this is what gives me some current drive capability."
So, it seems that it's only the Delta-I, or the input JFETs current changes (following the input signal voltage) that need to be high enough. The MOSFETs idle current is set much higher than the JFETs idle current.
Is that correct?
Also:
Would 2 BL grade JFET devices in parallel (in each leg of the quadrant) be as good as 1 V grade one for supplying enough output current drive?
What will you consider a good idle current for the MOSFETs in that topology?
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Old 28th February 2013, 04:08 AM   #35993
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Mr. Curl can (and hopefully will) answer for himself, but I can say that your thoughts about the cascodes' operation are correct. The second ("folded") half has no current gain. I'd guess that it's driving a resistor of between 100 and 600 Ohms, in parallel with some presumably large cable capacitance. Elegance costs idling current here, but it's one thing that all DIYers can duplicate.

All good fortune,
Chris
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Old 28th February 2013, 04:14 AM   #35994
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Well Joshua, you are perceptive. Yes to all your questions, mostly. The second stage uses 1A mosfets that become more linear (slightly) with more idle current. 50 ma is not too much.
Paralleling input jfets, WILL work, but at the cost of twice the input capacitance.
The load is 1K per quadrant.
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Old 28th February 2013, 05:13 AM   #35995
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Quote:
Originally Posted by jan.didden View Post
"Very big exaggeration of possible real situations" means, of course, that it is not a possible real situation.

We had another member here years ago who invented the term 'first cycle distortion', something similar as what you showed.
Several people suggested he limit the signal to a reasonable bandwidth, say 30kHz. Of course he refused, because it would destroy his carefully set up argument. I can prove anything when allowed very big exaggeration.

As to the second part of your post, unfortunately I have no idea what you are talking about, or what you are guessing, apologies.

jan
Thanks, Jan, but maybe even in exaggerated example some small part of truth could be present.
First, my estimate for typical settling time from 10ns till 1000ns was too optimistic. I proceeded from my NoGNFB designs, for which I measured something like 20-30ns.
For wide spreaded deep GNFB designs, it will be better to speak about 1-5us settling time. This is already close to 10us of front duration of low-level high-frequency audio signals. Since even tiny cable effects are listenable, one could expect, that "settling errors" caused by higher level signals, even being smaller than the low-level high-frequency signals, can nevertheless affect them, causing different perception of sound from GNFB amps compared to No GNFB amps.
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Old 28th February 2013, 07:20 AM   #35996
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Quote:
Originally Posted by Joshua_G View Post
Is this thread about John Curl's Blowtorch preamplifier?
Well, I went through the first pages of "John Curl's Blowtorch preamplifier" (part I) and I have a question concerning the Blowtorch topology.
The topology is, as described by SY: "All FET, no feedback, servoed. First stage is 4 quadrant comp diff, second is folded cascode using MOSFETs run at high current. Servo with roughly 1 sec time constant." JC added: "The CTC preamp is a transconductance amp that is current output."
Look, the topology was not only been described, but also shown as a schematics (approved by JC to be very close to reality) at least 5 times here. You MUST be aware of this fact. For potential questioners, please use SEARCH engine to find the appropriate posts with schematics. Circles, circles, circles.
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Old 28th February 2013, 07:29 AM   #35997
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Quote:
Originally Posted by VladimirK View Post
Thanks, Jan, but maybe even in exaggerated example some small part of truth could be present.
First, my estimate for typical settling time from 10ns till 1000ns was too optimistic. I proceeded from my NoGNFB designs, for which I measured something like 20-30ns.
For wide spreaded deep GNFB designs, it will be better to speak about 1-5us settling time. This is already close to 10us of front duration of low-level high-frequency audio signals. Since even tiny cable effects are listenable, one could expect, that "settling errors" caused by higher level signals, even being smaller than the low-level high-frequency signals, can nevertheless affect them, causing different perception of sound from GNFB amps compared to No GNFB amps.
Vladimir, how does this story look if you limit the input signal to say 20kHz or 25kHz? See attached. Start with a 'exaggerated 10uS rise time input, 1st order roll off at 25kHz to simulate audio signal.
Put that in your amp, and look at the ringing.

I won't even get started why you think a well-designed audio amp should have 5uS settling time...

jan
Attached Files
File Type: pdf vladimir.pdf (14.1 KB, 37 views)
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Old 28th February 2013, 07:39 AM   #35998
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Quote:
Originally Posted by Joshua_G View Post
Knock, knock, knock…
Hello, is there anybody here?
Is this thread about John Curl's Blowtorch preamplifier?
There is an original thread about the Blowtorch. The original topic had been closed and moved here, to be transformed in a boxing theater. The ring is used by several audio designers and other electronic engineers, but not reserved to professionals.
Feel free to enter in the ring any time. Nobody had been killed till now, but several seriously injured, including John, the titleholder.
Notice, it is free entrance, opened 24/24 and the show turn circles non-stop.
In red, the 'objectivists', in black, the 'audiophiles', in the middle, J.C.
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Last edited by Esperado; 28th February 2013 at 07:42 AM.
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Old 28th February 2013, 07:52 AM   #35999
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Yes Jan, these stories here are funny, funny and nothing but funny. Please let me attach a step response of the 250W/4ohm power amplifier, with the evil Global Negative Feedback. 106Vp-p voltage swing.
Attached Images
File Type: png Step response 2 channel.PNG (20.5 KB, 143 views)
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Last edited by PMA; 28th February 2013 at 07:58 AM.
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Old 28th February 2013, 08:13 AM   #36000
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I'm stuck with a little stupid problem, you know how it is.
it is absolutly out of topic, but there is the place where i have more luck to find clever people :-)
U3 sum double full wave rectifiers and amplify the rectified signal. It works fine, but the + & - signal balance is affected by he RV value, so R need to be finely tuned for each value of RV. (giving R the same value than RV, between 10K and 1Mohm)
Any idea to get rid of this without adding an other OPA ?
Attached Images
File Type: gif quest1.gif (4.6 KB, 131 views)
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Last edited by Esperado; 28th February 2013 at 08:26 AM.
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