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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 18th February 2021, 08:12 PM   #981
keantoken is offline keantoken  United States
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Yep, that would be it.
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Old 19th February 2021, 12:12 PM   #982
rookiesparks is offline rookiesparks  Guernsey
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Default dc offset

hi i cant seem to get any adjustment on the dc offset , multimeter reads .745v even when i remove 5m trimpot altogether it seems to be the same reading any ideas ?? thanks
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Old 6th March 2021, 06:58 AM   #983
Zoran is offline Zoran  Serbia
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Hi
With "little" delay I tried Germanium version of this buffer.
And it was OMG sound last evening...
Circuit working with Ge devices AC187 and AC188.
For the test I not solder unmark parts, RC input filter, choke 2.2uH.
Capacitors at the PS are ommited too because I power device from the 2x9V battery pack.
Rest 2 capacitors was 10uF (against 470uF in the original sch.)
.
-It was possibile to set 1/2 +V at the test point.
-I just set another pot to 56ohms didnt measure and eventualy correct dist yet.
-off set is about -40mV with (I think) pot of 2M full value. Initialy was -52mV and with inreasig value droped.
-Transistors randomly picked.
-No overheating, 1st pair are consume about 4mA second pair is about 8.5mA.
-Second pair just slightly warm. First pair are cold.
-Circuit not showing any hum or his. Very silent.
I tried with Quad II power amplifier and with transformer input Western Electric Germanium amplifier.
.
The sound was vey good. Natural and with "body". Digital media are somehow moved to analog sound and made ballans with start-finish of the tone. Not to emhesize the finishing HF end. Circuit with Ge somehow "reduce" digital "signature". Instruments are neutral and transpraent, echoes are clear and also with fundament. Bass is very good.
But the voices are beautiful
.
I just listen one channel last evening until I the batteries are almost drained
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Old 6th March 2021, 07:55 AM   #984
keantoken is offline keantoken  United States
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Ha, cool. I wonder what the harmonic spectrum looks like.
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Old 6th March 2021, 11:05 AM   #985
Zoran is offline Zoran  Serbia
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I didnt measure
was involved to listen as much as batteries hold
one channel only
I just set the 100ohm trimmer to 56ohm value lsame ike fixed resistor (not measured, I should...) and not touch that part.
But I measure total distortion with buffer in system (experimantal dac-filter-buffer and it was delta only in 0.01-0.015 to naked dac output. That is with passive 5ord filter and 10K@input buffer. That is not huge but dac module have 2.5K of output Req. So this 10K input res. probably "damped" more than it should.
.
The transfer is pretty flat even with 10uF of C inside the circuit used against 470uF. (i didnt have when soldering...). No any cut of low end
.
I MUST prepare one IV converter circuit from Ge transistors, after even small listeninig of this Ge version buffer...
.
I have some Q about the using Ge:
- What is higher value of Rin that could be used?
- Can I use higher supply voltage?
- Is that currents of 4mA(input branch) and 8.5mA good?
- Can I use other types for output NPN stage, like AD161?
- Can I ommit output serial reistor (when loading output with transformer)
.
cheer and thanks
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Last edited by Zoran; 6th March 2021 at 11:23 AM.
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Old 6th March 2021, 11:20 AM   #986
Zoran is offline Zoran  Serbia
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I will try to reduce more -40mV offset. By increasing BE 2meg resistance to 2.7meg in the schematic. And make thermal coupling. But even without themal coupling this circuit with Geramnium devices not showing any drift of test points.
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Old 6th March 2021, 07:04 PM   #987
padamiecki is offline padamiecki  Poland
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extremely interesting!
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Old 7th March 2021, 01:23 AM   #988
keantoken is offline keantoken  United States
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Yes, thermal coupling doesn't matter much here because the tranistors all heat up equally. Where it might be useful is if the board is mounted with a hot heatsink on one side.

The specified transistors have fairly consistent Vbe, but some of your offset could be coming from your Ge transistors. You would measure this by measuring the voltage between input and output. If so you might be able to reduce it by swapping the PNP or NPN transistors.

If you can reduce offset to zero with the trimmer, then what's left is thermal drift. At some point as you increase source resistance (including the input resistor) the thermal drift will become tunmanageable. I don't know what that would be for Ge transistors and I hear the AC127/128 can be highly variable between batches and manufacturer. But if it will be permanently used with your DAC with 2.5k output resistance, you could actually remove the input resistor as it is only there in case the source is disconnected.

4mA and 8mA are fine and might actually be better for Ge transistors if the input transistor is slow. On the other hand by reducing the input transistor bias you may reduce distortion by reducing the harmonic currents from the input transistor base. You can do this by changing the 56R resistors.
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Last edited by keantoken; 7th March 2021 at 01:35 AM.
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Old 7th March 2021, 07:12 AM   #989
Zoran is offline Zoran  Serbia
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Thanks for useful informations.
Can I rise a bit power supply voltages?
To say +-12V or +-15V?
Then transistors will opperate at 6 or 7.5V supply individualy?
I supose that currents will be higher but not sugnificantly if I rise proportionally 1K R from -V and 1K R from +V?
Now transistors are cca about 4-4.3V from B to C...
.
What maximum signal P-P the buffer can transfer without clipping?
.
Will be some benefits to use JFET Isource insted 3.6K R?
.
I am far from "expert" in transistors field, please explain more about
"If so you might be able to reduce it by swapping the PNP or NPN transistors."
or make some sketch?
.
cheers.
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Last edited by Zoran; 7th March 2021 at 07:16 AM.
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Old 7th March 2021, 07:25 AM   #990
keantoken is offline keantoken  United States
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Swap Q1 and Q3, or Q2 and Q4. You may get better offset if the Vbes are mismatched and you find the right combination.

You can increase the voltage but you will need to reset TP to half the supply voltage. There is no reason to use a Jfet CCS.
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