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The Kuartlotron - keantoken's simple error-correction superbuffer
The Kuartlotron - keantoken's simple error-correction superbuffer
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Old 25th December 2018, 01:55 PM   #951
Zoran is offline Zoran  Serbia
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Quote:
Originally Posted by keantoken View Post
Try it with the AC187/188. The 2G339 is fast but the Vce limit is not quite where I would like it to be.
OK thanks.
But is there some resistor value to correct?
Should R1=R2 be lower values?
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Old 26th December 2018, 12:44 AM   #952
keantoken is offline keantoken  United States
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I think it will actually work fine with the circuit as it is. Try it and report the voltages at each transistor pin, then we can make some adjustments if necessary.
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Old 27th December 2018, 03:38 PM   #953
Zoran is offline Zoran  Serbia
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I think it will actually work fine with the circuit as it is. Try it and report the voltages at each transistor pin, then we can make some adjustments if necessary.
OK
I will do it this way
thanks
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Old 6th June 2019, 07:22 AM   #954
keantoken is offline keantoken  United States
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I am about to order a new batch of PCBs. The new PCBs will have curved traces but other than that will not be any different.
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Old 7th June 2019, 07:10 PM   #955
Bigun is offline Bigun  Canada
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The Kuartlotron - keantoken's simple error-correction superbuffer
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The new PCBs will have curved traces
I like that idea
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Old 17th June 2019, 06:57 PM   #956
keantoken is offline keantoken  United States
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The new PCBs are in.
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Old 23rd August 2019, 10:56 PM   #957
JasonB299 is offline JasonB299
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Default Push-Pull Kuartlotron

Hi Kean,

A young starting DIYer here. I am intrigued by your circuit. Have you ever tried a push-pull buffer like in the attachment? I guess referring to the enumeration from your site, the different Vce of Q1 and Q3 in this arrangement could be suboptimal for the error correction, because for transistors with limited VA the current through R1 and R2 and hence the Vbe of Q2 and Q4 would be different and signal dependant. But on the other hand the input capacitance of Q1 is now bootstrapped, which might help with high frequency linearity, when a high impedance source (read pot) is used. In any case, the buffer can now source and sink high currents by transitioning in class AB and - if I see it correctly - when Q2 and Q4 are thermally coupled as intentioned, the bias can be set by the current through the new input resistors and the ratio between input and output resistors plus, importantly, the bias is temperature stable. The only downside I can see in this regard, is that we will likely suffer from gm-doubling in the class A region due to the new output resistors, since the output impedance of each kuartlotron half is probably near zero from the error correction scheme.
What do you think about it?

Many greetings,
Jason
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Old 1st January 2020, 04:08 PM   #958
silasmellor is offline silasmellor  Denmark
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The Kuartlotron - keantoken's simple error-correction superbuffer
Default Another Kuartlotron lives

Hey there, just finished my second Kuartlotron using Project16s boards. The first one is living with a friend who is reportedly enjoying it alot! I unfortunately forgot to take pictures of that one on completion, but the one i've just built is basically following the same overall layout.


I used a fixed 56 ohm resistor instead of the pot. Offset was 2 and 3 mV without any adjustments of the last pot, so i left it at that. 100 K input impedance, with a 10K pot in front (this was mentioned in a blog post by Nisbeth for this).



The case is a modushop galaxy maggiorato 2U (the largest size). The PT is an old 2x13V from a now-defunct Danish company. I did have some hum from it while testing but am happy to say that in the case it makes no sound. The PSU is a Whammy PSU (circuit from wayne and adapted to single board by Nisbeth) using CRCRC in front of LM7x10 regs.



I used another ebay find for the input relay selector. I bought the bare PCBs and used 5V omron relays. The pot is an ebay DACT type stepped potentiometer.


I built it to serve as preamp to an LM3886 amp also in the works. The power amp will take a while to finish, but i don't mind, because it sounds great with my F6 powering Pensil 10ps! I am using it instead of a BA-3 preamp at the moment but really not missing gain at all. Very nice! Big thanks for Keantoken and also to Project 16 for sharing his layout.


Silas
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File Type: jpg kuartlotron1.jpg (659.2 KB, 155 views)
File Type: jpg kuartlotron2.jpg (699.0 KB, 152 views)
File Type: jpg kuartlotron3.jpg (522.8 KB, 144 views)
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Old 1st January 2020, 10:42 PM   #959
keantoken is offline keantoken  United States
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Nice, thanks for sharing!
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Old 1st January 2020, 11:25 PM   #960
keantoken is offline keantoken  United States
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Quote:
Originally Posted by JasonB299 View Post
Hi Kean,

A young starting DIYer here. I am intrigued by your circuit. Have you ever tried a push-pull buffer like in the attachment? I guess referring to the enumeration from your site, the different Vce of Q1 and Q3 in this arrangement could be suboptimal for the error correction, because for transistors with limited VA the current through R1 and R2 and hence the Vbe of Q2 and Q4 would be different and signal dependant. But on the other hand the input capacitance of Q1 is now bootstrapped, which might help with high frequency linearity, when a high impedance source (read pot) is used. In any case, the buffer can now source and sink high currents by transitioning in class AB and - if I see it correctly - when Q2 and Q4 are thermally coupled as intentioned, the bias can be set by the current through the new input resistors and the ratio between input and output resistors plus, importantly, the bias is temperature stable. The only downside I can see in this regard, is that we will likely suffer from gm-doubling in the class A region due to the new output resistors, since the output impedance of each kuartlotron half is probably near zero from the error correction scheme.
What do you think about it?

Many greetings,
Jason
Sorry I missed this.

I've tried this. As you say you will have Gm doubling, except unlike a real EF, the Kuatrlotron has no antilog characteristic and so the transition between class A and AB will be a discontinuity that generates a spray of high order harmonics. This is much like many past active AB circuits which are rarely used. You also have each transistor turning on and off at the same time which creates a lot of uncertainty in potential glitching behavior. So it is only interesting really as a class A buffer. You can add keep-alive current sources in various ways but this always disturbs the log-antilog error correction.

So if we settle for it as a class A buffer what we find is that the output bias resistors increase the output impedance, THD might or might not still be vanishingly low, and the parts count has more than doubled. I didn't go this route because it was a lot more work and cost for what may actually be a loss in performance. Keeping the Kuartlotron simple makes it accessible to more builders and gives it a smaller footprint that can fit in more places.

As for the input transisor bootstrap, I considered that too. Notice that the input transistor still sees the output transistor's antilog voltage at it's collector voltage. It's Vce is now also mismatched with the upper PNP transistor, which might not necessarily be that much of an issue but it will run cooler in addition to it's Vbe changing with Vce as a side effect of Early effect. I weighed the effects and decided to err on the side of matching. What you can do if you want some bootstrapping here is to connect the collector's PSU capacitor to the output point instead. Then what you get is matched Vce, but unmatched voltage swing, along with the desired bootstrapping. I wanted to avoid any potential stability or latching issues this might cause as a multitude of builders are likely to uncover any such issue if it exists. There was no clear benefit so I preferred to keep it simple and try to err on the side of reliability as I didn't think the hypothetical performance advantage would matter in the long run. Sacrificing reliability for fractional performance gains is a sure way to lose your mind, and doesn't necessarily even sound better.

Edmund Stuart's ABII output stage is the closest I have seen to a perfect active class AB circuit but even it still has a mathematically imperfect transfer curve with a slight discontinuity, and it still uses feedback to correct the antilog characteristics rather than error correction. But then it gets lenience because it is a power output stage and much harder to linearize in the same way as the Kuartlotron.

Auto Bias part II
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