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"Light Touch" Class A Amp
"Light Touch" Class A Amp
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Old 2nd May 2017, 07:27 PM   #1
wrenchone is offline wrenchone  United States
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This thread will chronicle development of an amp that blatantly rips off the optocoupler-mediated current source used in the GeneSic amp investigated by Papa in another thread. Zen with GA10JT12


Not liking amps with only a few ohms of input impedance, I substituted a buffered "Fake SIT" for the GeneSic device. The resulting amp should be much easier to drive. Sound? I'll find out, sooner or later. This is a placeholder until I get around to attaching supplemental material. Layout is pretty much done, so I will need to pencil in some appropriate component values in order to proceed.

Last edited by wrenchone; 2nd May 2017 at 07:30 PM. Reason: More info
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Old 2nd May 2017, 09:06 PM   #2
wrenchone is offline wrenchone  United States
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For convenience, the "rippee", Nelson's original GeneSiC design (with one added resistor), and the proposed "ripoff" version replacing the GeneSiC device with a buffered fake SIT. Again, placeholders, as the design has progressed slightly since.

Edit - In fine ripoff tradition, I borrowed the GenSiC schematic from Lazybutt, who posted it in the thread cited previously. As mentioned above- I stole what I liked about it, and changed what I didn't like....
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File Type: png GA10JT12_dchematic_modified.png (79.1 KB, 1282 views)
File Type: gif Light_Touch.GIF (110.1 KB, 1261 views)

Last edited by wrenchone; 2nd May 2017 at 09:13 PM.
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Old 2nd May 2017, 10:57 PM   #3
Nelson Pass is offline Nelson Pass  United States
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"Light Touch" Class A Amp
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Originally Posted by wrenchone View Post
This thread will chronicle development of an amp that blatantly rips off the optocoupler-mediated current source...
Ah, you mean "pays flattering tribute to"
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Old 2nd May 2017, 11:18 PM   #4
wrenchone is offline wrenchone  United States
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If I remember correctly, you used an opto-based bias scheme for an amp in your early days...

Edit - Ah, yes, steal from the best, leave the rest...

Last edited by wrenchone; 2nd May 2017 at 11:20 PM.
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Old 5th May 2017, 04:55 AM   #5
wrenchone is offline wrenchone  United States
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Here's a prelim schematic, with two resistors left up for grabs - the unimportant one will likely be a jumper for Rev 1, the other is TBD - guess which is which? Tell me how my circuit works, and learn in the process...
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Old 5th May 2017, 06:42 AM   #6
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"Light Touch" Class A Amp
R6 is dumber than me

go figure ........

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Old 5th May 2017, 06:50 AM   #7
wrenchone is offline wrenchone  United States
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Not really - call it an ultimate current limit if the input really gets slammed around for some reason. I used it on my first SIT-ish amp, the "Half-Nelson", and could always replace it with a jumper if I so choose...
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Old 5th May 2017, 08:54 PM   #8
LBHajdu is offline LBHajdu  United States
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I am not sure what the function of R6 is, however if it gets too large there may be a turn-on thump. The time constant of R9 and C2 slows down the turn on of the top CCS so it slows down the time it takes the + side of C3 to rise (charge) to half the rail voltage. I do not by the way see how it can be assured it will rise to half the rail voltage. The gate-Source threshold voltage of Q1 will vary by a few volts from lot to lot (datasheet says -1.5v to -3.5), if you try to adjust R5 to compensate for variation from amp to amp the feedback value will be different and so the gain from amp to amp will be different. You could try doing two loops one AC loop blocked by a cap. and a high resistance pot for the DC control loop. I am not sure if the DC blocking would keep the feedback loop a "Fake SIT".

I give you a lot of credit for building though, I really need to build more.

What I really want to ask is the subjective question, does this topology sound like a true First Watt SIT? Srajan from 6moons attributes a magical "big" sound to the true SIT-1. I suspect SET amps may sound so good with full range drivers because the square law response may cancel distortion as the voice coil starts to move outside of the speakers magnetic poles.
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Old 7th May 2017, 08:11 PM   #9
wrenchone is offline wrenchone  United States
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One way to make both sides of a stereo amp the same gain would be to match the bottom side fets, both p and n channel, allowing R6 to be the same for each.

One could also break the ground side of R1 loose and apply a bias voltage to adjust output centering independent of gain. A 9V battery and a pot would be a useful exploration tool in that case...

Taking the design a bit further, one could add a bipolar supply and p-channel differential pair at the input and feed back to it, allowing one to ditch the output caps. This would make the design similar to the "Aleph McSIT", which uses a largish Ixys depletion mode fet for the topside current source.

As for R5, if I add it at all, my inclination would be to set its break point with C2 to 5Hz or below 100 ohms would do nicely.

As for sound, I have not neither heard nor seen a real SIT (even Papa's), but I have been messing with fake SITS (knowingly or not) since 2007, with listenable results.

Last edited by wrenchone; 7th May 2017 at 08:26 PM.
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Old 7th May 2017, 08:33 PM   #10
wrenchone is offline wrenchone  United States
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I find that I can avoid turn-0n thump be using a capacitor multiplier for ripple filtering. This adds a slow-rise characteristic that is useful for preventing thump.
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