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Super Triode Connected (STC) chip amp
Super Triode Connected (STC) chip amp
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Old 11th July 2016, 12:12 PM   #1
FYC is offline FYC  Hong Kong
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Default Super Triode Connected (STC) chip amp

Hi all

based on the info from TubeCAD, I built a super triode connected chip amp. The "turbocharger" is a OPA541 power amp. Originally I wanted to connect a 6922 as a super triode, but later changed to a 6SN7. The major reason for the change is the low gain-bandwidth product of OPA541. With a high mu (33) of 6922, the bandwidth will be very limited. 6SN7 having a mu of 20 will give a more reasonable bandwidth. Of course the main attraction of this concept is to have the sound signature of 6SN7 while having a decent output power to drive the loudspeaker.

I basically follow the concept in TubeCAD with some adaptation. The 6SN7 is biased at 8mA, using a TL431-npn transistor CCS. The OPA541 works from a +/-20V, giving at least 30W output power.

Just completed building today, the circuit works! Definitely it sounds like a tube amp with quite a amazing bass (compared to my current Nelson Pass- JLH MOSFET amp). Still need to spend more time in listening but would like to say that the concept is a practical one using chip amp.
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Old 11th July 2016, 01:55 PM   #2
Ketje is offline Ketje  Belgium
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A cathode follower with the anode as an input, will be a difficult one for "DualTriode" :lol:
Why 200V ? Probably 120V and a current of 4mA will give the same result.
Mona
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Old 11th July 2016, 02:02 PM   #3
crazyfrog is offline crazyfrog  Canada
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Hi. I'm not quite skilled to appreciate your project but that hybrid version looks interesting. Do you use one 6SN7 for both channels?
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Old 11th July 2016, 02:40 PM   #4
FYC is offline FYC  Hong Kong
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Yes, I use 1 6SN7 for 2 channels. For detailed theory about this STC concept, you can get from TubeCAD and also a number of Japanese DIYers.

For the bias setting, I am in the impression that at 8mA I will be able to get better linearity?
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Old 11th July 2016, 02:57 PM   #5
Ketje is offline Ketje  Belgium
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Well at 4mA we are stil above the rounding of the lines.And if you are after linearity, why use tubes ?
Mona
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Old 11th July 2016, 03:07 PM   #6
hpeter is offline hpeter  Slovakia
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i like this idea, low mu will work best.
try <10
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Old 12th July 2016, 12:40 AM   #7
ballpencil is offline ballpencil  Indonesia
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@FYC: where do you source your OPA541? I tried this but i stubbornly used LM1875. Unsurprisingly, i end up with an oscillator.
I found OPA541 in Ebay but i don't fully trust them when it comes to semiconductors. Actually i wouldn't mind running fake chips with lowered voltage rails to be gentle to them but i'm not sure if they're unity-gain stable.
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Old 12th July 2016, 04:05 AM   #8
FYC is offline FYC  Hong Kong
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I bought these from Digikey, each one cost around US$24
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Old 12th July 2016, 09:12 AM   #9
DF96 is offline DF96  England
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Quote:
Originally Posted by Ketje
And if you are after linearity, why use tubes ?
A good question. I can think of some good reasons to make a tube amplifier; I can't think of any good reason to use a triode as the feedback element for a chip amp when resistors are hugely more linear than any triode.

As you say, using the anode as an input may create issues for some people!
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Old 12th July 2016, 10:47 AM   #10
FYC is offline FYC  Hong Kong
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My thoughts are:

When selecting an operating point, I would try to select the one with better linearity within the operating limits like power dissipation etc.

Using the triode in the feedback loop is to impose the triode sound over the chip amp, this is the main theme behind this concept. The chip amp in theory only provide the horsepower but the driving "characteristic" comes from the triode.

I cannot explain more as I am no expert. TubeCAD provides a detailed description in several occasions.
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