A Low Distortion Amplifier Based on a "Tandem" Correction Scheme Powered by a Modifie - diyAudio
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Old 12th May 2014, 04:01 PM   #1
TIAEK is offline TIAEK  Japan
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Default A Low Distortion Amplifier Based on a "Tandem" Correction Scheme Powered by a Modifie

Hi there, I just needed tiny but clear sound audio power amp,
the desire led me to the schemes Fig.1 and Fig.2.
Questions, comments and suggestions for improvements are welcome.

---------------------------------------------------------
This is a low distortion simple power amplifier which works with +12V power.
The output push-pull stage of a KURODA buffer (a boot-strapped low distortion "diamond" buffer)
was modified to an inverted Dalington configuration to obtain high hFE and thus high current conduction (Fig.1).
The output voltage of the buffer was followed and regulated in tandem
by two loops in one dual operational amplifier RC4558 (Fig.2).
The output distortion component of a 1kHz sinusoidal wave was essentially
of second harmonic alone, assuring good quality sound.
The THD less than 0.001% was easily achievable with 5ohm load (Fig.3).

# R 1.2k in Fig.2 is mistyped. Correct value is 12k.
# C3522 in Fig1 should be read as (2S)C3422. I apologize for my mistype.
Attached Images
File Type: jpg Fig1 KURODA buffer for 12V.JPG (36.9 KB, 454 views)
File Type: jpg Fig2 Basic Configulation.JPG (24.8 KB, 447 views)
File Type: jpg Fig3.JPG (82.0 KB, 439 views)

Last edited by TIAEK; 14th May 2014 at 07:22 AM.
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Old 12th May 2014, 04:20 PM   #2
TIAEK is offline TIAEK  Japan
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Default Note on a Modification of a KURODA buffer

# the thread title should be read as
A Low Distortion Amplifier Based on a "Tandem" Correction Scheme Powered by a Modified KURODA Buffer.


Toru Kuroda explains a boot strapped emitter-follower consists of 4 Trs and 2 diodes in page 136 of his book
"Introduction to Designing Transistor Circuits" (1999, CQ Shuppan, in Japanese).
Fig.4 depicts the basic design. Collectors of Q1 and Q2 are boot-strapped via C1 and C2 (together with R4 and R6),
keeping their VCB constant, against input signal voltage.
This technique virtually cancels the Cob of Q1 and Q2 because there is no AC in their VCB and thereby reduces
possible harmonic distortion due to voltage dependent nature of Cob.
The concept is also used in the output stage of OP amps such as AD846, as Mr.Kuroda describes.

The Kuroda buffer shows very good performance.
With 6k ohm load, NOISE+THD reaches as low as 0.0004% in audio frequency region.

With heavy load, hFE of final Trs, which limit affordable output current, must be increased.
This is achievable by adapting an inverted Darlington configuration.

Another point is the elimination of emitter resistance.
Resistance 47 ohm is used in the original Kuroda buffer (Fig.4) to restrict idling current (some 10mA) corresponding to diode voltage.
By simply removing the two diodes, surprisingly, the emitter resistance was found to be minimized, even to zero ohm (Fig.1).

Be careful here that tight thermal coupling between Q1-Q5 and Q2-Q6 in Fig.1 is essential.
Attached Images
File Type: jpg Fig4 original KURODA buffer.JPG (27.1 KB, 433 views)

Last edited by TIAEK; 12th May 2014 at 04:25 PM.
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Old 12th May 2014, 06:03 PM   #3
Bigun is offline Bigun  Canada
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Did you look at PSRR - I fear your modified version has a problem. When I look at this new design it almost like you have a Sziklai arrangement. Anyhow, what I see is that Q5 has an input signal defined by the voltage between it's base and emitter which encompasses the rail RC filter (3k3 resistor with cap to ground) and this will mean that the power supply ripple developed across the 'R' will be part of the signal to Q5.
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Old 13th May 2014, 04:24 AM   #4
TIAEK is offline TIAEK  Japan
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Thank you Bigun-san,

cut-off f of the RC is set to be low (0.5Hz) and PSRR doesn't seem to be so bad
--- actually the power is regulated by LM317 or LM350 to 9.6V on my test board
and ripple is small on the scope screen.

I didn't know the name Sziklai ----- the pair is called as inverted/reversed Darlington
or DarlingNOT or simply a kind of compound Tr pair here in Japan.
Well, it must be called as Sziklai pair --- "S may be not pronounced? correct?

I found this page
Compound vs Darlington
Big surprise of the Fig1 circuit is that it never run away thermally.
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Old 13th May 2014, 02:52 PM   #5
Bigun is offline Bigun  Canada
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OK good - with a regulated supply a low PSRR is a small problem, but the sound quality will depend on quality of the regulator

The Sziklai is also known by other names, often on this forum we call it 'CFP' which means Complementary Feedback Pair. In this arrangement, the thermal stability depends on the driver device not the power device. The driver device in your circuit would be the input devices - assuming I am correct even to think of this circuit as a CFP.
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Old 13th May 2014, 05:14 PM   #6
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Looks like a diamond buffer with the input transistors bootstrapped to keep a constant VCE. I wouldn't call it a Sziklai because there's no feedback between the input and output transistors and they are both working as emitter followers.
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Old 14th May 2014, 06:41 AM   #7
RNMarsh is offline RNMarsh  United States
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Quote:
Originally Posted by TIAEK View Post
Hi there, I just needed tiny but clear sound audio power amp,
the desire led me to the schemes Fig.1 and Fig.2.
Questions, comments and suggestions for improvements are welcome.

---------------------------------------------------------
This is a low distortion simple power amplifier which works with +12V power.
The output push-pull stage of a KURODA buffer (a boot-strapped low distortion "diamond" buffer)
was modified to an inverted Dalington configuration to obtain high hFE and thus high current conduction (Fig.1).
The output voltage of the buffer was followed and regulated in tandem
by two loops in one dual operational amplifier RC4558 (Fig.2).
The output distortion component of a 1kHz sinusoidal wave was essentially
of second harmonic alone, promising good sound quality.
The THD less than 0.001% was easily achievable with 5ohm load (Fig.3).

# R 1.2k in Fig.2 is mistyped. Correct value is 12k.
# C3522 in Fig1 should be read as (2S)C3422. I apologize for my mistype.
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Old 14th May 2014, 08:25 AM   #8
ontoaba is offline ontoaba  Indonesia
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There are different languages in this whole world. Diamond (it is english), sziklai (??), kuroda (japanese), intun (java).

May be tandem needs something good correcting buffer.

Last edited by ontoaba; 14th May 2014 at 08:28 AM.
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Old 17th May 2014, 10:21 AM   #9
Bonsai is offline Bonsai  Taiwan
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Very nice circuit TIAEK-san.

(BTW, I lived in Meguro and then Komazawa for almost 5 years - Japan was one of the best work/life experiences I ever had. Fantastic country.
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Old 17th May 2014, 10:29 AM   #10
PMA is offline PMA  Europe
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I had built a similar circuit, 12 years ago, as a class A power amplifier. I liked it very much.

Class A Push-Pull Follower Czech

Click the image to open in full size.
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