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Super Triode Connected (STC) chip amp
Super Triode Connected (STC) chip amp
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Old 14th July 2016, 05:14 AM   #21
FYC is offline FYC  Hong Kong
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It is quite interesting to read the comments and noted that with this concept, many see the chip amp as the master and the triode as the servant - where this servant adds distortion to the master chip amp. In reality, this is a valid viewpoint. And if we see it this way, then the concept is a daft one.

But this is a tube forum and I am surprised to see people do not treat the triode as the master and the chip amp as the servant. The master wants to drive the loudspeaker but do not have the muscle to do so, hence he asks the servant to do the donkey job. But in doing so, the servant must follow the master's way exactly and do not contribute to anything else other than power.

Nevertheless, this finally boils down to your own taste. After listening to this for 2 days, I believe this is a very good sounding amp. More listening to come.
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Old 14th July 2016, 05:37 AM   #22
aardvarkash10 is offline aardvarkash10  New Zealand
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the issue isn't master or slave in these two pieces.

The master should be the source.

The amplification chain should the the apocryphal "wire with gain". Thats where your amp falls down - it (perhaps intentionally) injects distortion to match an expectation of "tube-yness".

Some feedvback wrapped around the entire thing might correct that, but would likely remove all "tube-yness" in the output - except for that warm glow, no doubt located high and up front on the chassis.
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Old 14th July 2016, 06:49 AM   #23
Wavebourn is offline Wavebourn  United States
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Super Triode Connected (STC) chip amp
No matter how you see it, the result is multiplication of complex transfer functions. Period.

The only good thing that I see, input impedance of the triode that is high and more linear, less frequency dependent than of the chip amp, so the result of it's load of the source could be beneficial.
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Last edited by Wavebourn; 14th July 2016 at 06:53 AM.
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Old 14th July 2016, 10:11 AM   #24
DF96 is offline DF96  England
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I find it amusing that some people worry about resistor linearity (and choose particular brands) while others deliberately use a much more nonlinear element, such as a triode (as used here) or an LDR (as used in some volume controls).

Quote:
Originally Posted by FYC
In reality, this is a valid viewpoint.
More like the valid viewpoint. In this circuit the triode has a minor role as a cathode follower, and a major role as part of the feedback network. It is definitely a chip amp with added triode distortion. If you wanted to make a triode amp with added output power you would add a solid-state follower to a grounded cathode stage.
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Old 14th July 2016, 10:44 AM   #25
CFT is offline CFT  United States
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Nice to see a successful implementation of Tubecad's STC amp in action.

I am more familiar with Kamijo san's STC designs:
www2u.biglobe.ne.jp/~hu_amp/amput3e.htm

and his VFET- LM3886 amp is very intriguing and I hope to finish building it someday soon!

https://translate.googleusercontent....kbtjSeYEADIorg
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Old 14th July 2016, 04:32 PM   #26
smoking-amp is offline smoking-amp  United States
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Funny to see that TubeCad finally embraced the STC circuit.

Back in 2003 he commented negatively (on TubeCad) on a similar variant:
http://www.diyaudio.com/forums/tubes...amplifier.html

In any case, it is a good thought experiment for understanding N Fdbk. And has a functional utility for SET purposes.

Another way of looking at it: is as a V amplifier (tube) and an I amplifier (SS) in parallel, called a composite amplifier by some, where the feedback seeks to minimize the loading on the V amplifier by minimizing current variation in it (by controlling the I amplifier). Nothing wrong with such an approach.

One could get even more carried away, by using V AND I feedback to control the I amplifier. To set a specific output Z.

Or placing the I amplifier output after (at the secondary) of an OT for the V (tube) amplifier. Then the tube can do some heavy lifting as well.

And if you use a "perfect triode" for the feedback (or V amp), no reason it has to be distorting:

and (a tube veneer amp) just puts the final finish on the sound. (requires a very low Z secondary OT if the tube amp is smaller than the SS amp; has to be able to drive the same final current level):
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Last edited by smoking-amp; 14th July 2016 at 05:01 PM.
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Old 14th July 2016, 06:30 PM   #27
CFT is offline CFT  United States
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Nice work you did!
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Old 15th July 2016, 12:27 AM   #28
smoking-amp is offline smoking-amp  United States
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Huh. Kamijo san seems to have come up with the STC type ideas first for tubes, as far as I know. An independant re-invention here from thinking alike. The four basic composite amplifier approaches were published a long time ago in IEEE. Many variations around.

Last edited by smoking-amp; 15th July 2016 at 12:31 AM.
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Old 15th July 2016, 01:24 AM   #29
Wavebourn is offline Wavebourn  United States
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Super Triode Connected (STC) chip amp
Quote:
Originally Posted by smoking-amp View Post
and (a tube veneer amp) just puts the final finish on the sound. (requires a very low Z secondary OT if the tube amp is smaller than the SS amp; has to be able to drive the same final current level):
Also it has to be EXTREMELY FAST since solid state distortions are of quite high order. ;-)
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Old 15th July 2016, 03:34 AM   #30
FYC is offline FYC  Hong Kong
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Quote:
Originally Posted by smoking-amp View Post
Back in 2003 he commented negatively (on TubeCad) on a similar variant:
http://www.diyaudio.com/forums/tubes...amplifier.html

:
Thanks for the link and the info. Very interesting.

I did add a delay relay to avoid the power up issue that can damage the loudspeaker. The 6SN7 spec says warm up time ~ 10s, and I actually put in 20s delay. Speaker is connected quietly as I observed. Only when I power off, there is a very small "click" sound. Very small and should not be a big problem.

On DC offset, the TubeCAD version use 2 1M resistors to control. I measure about 30mV DC in both channels.

The only thing left now for me to consider to to add a DC speaker protection in conjunction to the time delay.
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