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Old 9th September 2013, 09:36 PM   #131
lhquam is offline lhquam  United States
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In post #103 I made some observations about the Valorous design, in particular about the effectiveness of the capacitors across the source resistors. I revisited some simulations I did a while back of a circlotron and an F5-like push-pull amplifier. I was looking for ways to reduce H3 (and other odd harmonics) by eliminating source degeneration. With ordinary push-pull output stages, reducing degeneration also reduces odd harmonics, but not with the circlotron output stage. I have not seen an explanation for this, but I believe it is because of the difference in global feedback between to two topologies. With push-pull, H2 is mostly cancelled in the output stage itself, before being sent to the global feedback network. In the circlotron, uncancelled single-ended outputs are sent to two global feedback networks, which convert H2 into higher harmonics via feedback.

What I conclude from this is that in a circlotron minimizing degeneration only serves to increase open-loop gain (OLG) but will not significantly improve distortion. The most effective ways to reduce distortion in the circlotron is by using lots of DC bias current and lots of feedback.

If others are interested, I could put together some simple simulations that illustrate these points.
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Old 10th September 2013, 04:24 AM   #132
The Dastardly Dad of Three
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Hi lhquam - interesting stuff! I'm a bit preoccupied putting the amp together right now on Buzz's beautiful PCBs. They just got here - I'm stuffing them and also intend to try a few interesting parts (some SMDs for the ip, SICs for ops) to see what we get. Multiple output pairs are also on the to-do list. Along with cap/no-cap. I'm not wedded to anything in particular....and my takeaway on this is there's probably no inherently superior topology - most of these things seem to revolve around fb amount (whether thru degeneration or loop), bias/heat levels, FB type (CFB vs VFB), # of devices that you're willing to accept, measured distortion targets & then the consequent stability issues arising. Then you have PSU complexity vs PSRR, noise, CMRR, need for balanced vs unbalanced inputs. Practical issues of parts matching/availability and most importantly - how it all sounds to you.

What you end up with depends on the choices you make for the above. At least that's what it seems like to me.

As regards the sims also check out the post i made on the distortion and nfb thread. There will also be many many folks there interested in the results! I will also try to contribute when/if I can.

Last edited by kasey197; 10th September 2013 at 04:28 AM. Reason: rearranged for readability.
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Old 10th September 2013, 06:07 AM   #133
lhquam is offline lhquam  United States
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I forgot to state that the simulations were without cascoding. When I did simulations with cascoded output FETs, the capacitors make a BIG difference. I need to do a thorough analysis using simplified output stages to understand why eliminating degeneration doesn't always help to reduce odd harmonics.
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Old 10th September 2013, 09:46 AM   #134
The Dastardly Dad of Three
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i took a look at the basic case (not cascodes) - my results do not agree with yours.

Whether complementary push-pull or circlotron,
whether with feedback or without

Getting rid of source degeneration reduces H3.

I'll get round to measuring this in the real world at some point.
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Old 11th September 2013, 02:08 AM   #135
lhquam is offline lhquam  United States
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Quote:
Originally Posted by kasey197 View Post
i took a look at the basic case (not cascodes) - my results do not agree with yours.

Whether complementary push-pull or circlotron,
whether with feedback or without

Getting rid of source degeneration reduces H3.

I'll get round to measuring this in the real world at some point.
I do not claim to fully understand what is going on, but the following might explain some of the differences in our results. My non-cascode results were for two pairs of FQA19N20C FETs rather than one pair, increasing the gate capacitances, and reducing by 1/2 the magnitude of the degeneration seen by each FET. I will put together some idealized simulations in an attempt to illustrate how the output stage topology influences distortion. Real devices have additional effects (e.g. capacitances and Early effect) the complicate the analysis and lead to more complex circuits. One step at a time.
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Old 11th September 2013, 04:03 AM   #136
The Dastardly Dad of Three
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some sim results here for those so inclined:
Distortion and Negative Feedback
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Old 11th March 2014, 07:44 AM   #137
The Dastardly Dad of Three
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So... been busy playing with these amps on the nice pcbs that buzz made. I have made just one change to the scheme - adding tempco so that biasing is quicker and easier.

Without some form of temp compensation, i ended up having to wait for more than an hour each time i fired up the amp from cold. Now it gets within 90% of final bias in like 10 mins or so. yeah

I also asked the talented Alex-MM to have another go at the layout on a compact footprint - these are off to the pcb-fab today.. take a look: He's got serious layout mojo flowing in his veins!
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Old 11th March 2014, 03:28 PM   #138
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One of the more liberating things I did when developing the amplifier was to poke around with the feedback loop opened. So open loop.

I was struck particularly by the wide the open loop response and I think goes some way to explaining it's rather excellent and open sound.... This is with a pretty decent 60db open loop gain mid band....I think this is better for audio that the 110db OLG with 10 or 100 hz rolloff points like you get with many opamps.
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Last edited by kasey197; 11th March 2014 at 03:34 PM. Reason: Removed unwarranted diatribe
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Old 11th March 2014, 03:54 PM   #139
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Alex did a great job on the layout. Hope you enjoy.
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Old 12th March 2014, 12:25 AM   #140
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Wouldn't have come this far without your help Buzz so thank you !

Plan is to proof the changes and then I'll publish everything in one go with build guide and BOM for the two pcb options depending on what chassis people have. One is your long board for the store chassis and another the square one above.

Maybe a very small GB to include the exotic parts version.

Last edited by kasey197; 12th March 2014 at 12:39 AM.
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