I'll call the Open Air one, the one you saw in my last post, "WHA-97". Perhaps, one far day, I will assemble it in a cabinet which I must get from Italy (HiFi2000). Its schematic is in the picture here enclosed, very close to the original circuit. All components were bought by me but they are not precisely high quality (but far better than original Chinese ones); all small capacitors are silver mica. The other amplifier is the "WHA-217 Cu Edition" with 11 final pairs, which you can see at post #1817, whose components list you can see at post #1820. The boards of the WHA-97 were used in the WHA217 first edition and now, updated, in this WHA-97, its servo is the one you find on eBay, almost untouched, only the OpAmp was substituted with an OPA227. The servo on WHA217 has all its components upgraded with polypropylene capacitors low tolerance, audio-grade electrolytic capacitors, and Vishay Pro resistors.
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you used 2sc4883 in a pair of baxandal, but I think these transistors are not suitable there. They need low-power transitors with the smallest capacity Сob, for example 2n5401 showed themselves quite well there. Otherwise, you will get a large overshoot in the phase-frequency region.
I think there is a mix up. The 2sc4883 is NPN and the 2N5401 is PNP. I guess you meant to say 2N5551.
First: 
Happy New Year,
Felice Anno Nuovo
Godt Nytår,
С Новым Годом!
Feliz Ano Novo
........
I have managed to find a perfect match for 5401 and 5551 (both pairs Vbe=668, Hfe=196). The result is voltage steady within +/-10 mV. My initial intention was to match Vbe, but I was lucky enough to find many exact pairs between 5551 (CDIL) and 5401 (Hottech)
Results are: larger bandwidth starting with almost DC and nearly perfect square wave response at 5Hz, 500Hz, 20kHz.
Despite the tolerable voltage drift , I intend to use servo too. From previous discussion it appears that the use of servo is beneficial both for sound quality and safety of speakers. Well matched pairs would additionally prevent excessive servo interventions.
For servos I intend to consistently use components from reputable sources and to replace LF353 with superior OPA2134. The anticipated benefits should be: "Superior Sound Quality", "Ultra Low Distortion" and "Low Noise".
Currently there are two alternative servos, separate ad-on and built-in on-board. I have ordered 3 pairs of each to experiment and compare these alternatives. The main difference among these alternative servos is just the way they feed the op-amp with +-15V. The separate ad-on servo employs positive and negative voltage regulators whilst the on-board solution uses zeners. Otherwise, the both use the same configuration and similar components.
Board with built-in ad-on servo is shown on figure below.
I will report results as soon as I complete measurements.
Happy New Year,
Felice Anno Nuovo
Godt Nytår,
С Новым Годом!
Feliz Ano Novo
........
I have managed to find a perfect match for 5401 and 5551 (both pairs Vbe=668, Hfe=196). The result is voltage steady within +/-10 mV. My initial intention was to match Vbe, but I was lucky enough to find many exact pairs between 5551 (CDIL) and 5401 (Hottech)
Results are: larger bandwidth starting with almost DC and nearly perfect square wave response at 5Hz, 500Hz, 20kHz.
Despite the tolerable voltage drift , I intend to use servo too. From previous discussion it appears that the use of servo is beneficial both for sound quality and safety of speakers. Well matched pairs would additionally prevent excessive servo interventions.
For servos I intend to consistently use components from reputable sources and to replace LF353 with superior OPA2134. The anticipated benefits should be: "Superior Sound Quality", "Ultra Low Distortion" and "Low Noise".
Currently there are two alternative servos, separate ad-on and built-in on-board. I have ordered 3 pairs of each to experiment and compare these alternatives. The main difference among these alternative servos is just the way they feed the op-amp with +-15V. The separate ad-on servo employs positive and negative voltage regulators whilst the on-board solution uses zeners. Otherwise, the both use the same configuration and similar components.
Board with built-in ad-on servo is shown on figure below.
I will report results as soon as I complete measurements.
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I do not finish mine because of the many negative feedbacks about the sound.
I built amplifiers without feedback and I did not like them as much (I dismantle them after a couple of weeks of listening).
I was hoping this amp will sound better based on the price of the original DartZel.
One of the amp without feedback and Class A I built the SEWA.
It was not too bad but not enough control over the low frequency with a random preamplifier and to build a special (tube) preamp is just too much for the only 7-8 Watt power plus the tremendous amount of heat and Hydro bills.
There is the choke loaded version with much less heat, more power but I did not found any company to produce me 100mH min. 3A choke.
To order it from Europe the shipping cost even at that time was out of reach to me.
So I gave up just as I am thinking about this amp unless someone will come up with some improved version even if that not the exact clone of the original.
I built amplifiers without feedback and I did not like them as much (I dismantle them after a couple of weeks of listening).
I was hoping this amp will sound better based on the price of the original DartZel.
One of the amp without feedback and Class A I built the SEWA.
It was not too bad but not enough control over the low frequency with a random preamplifier and to build a special (tube) preamp is just too much for the only 7-8 Watt power plus the tremendous amount of heat and Hydro bills.
There is the choke loaded version with much less heat, more power but I did not found any company to produce me 100mH min. 3A choke.
To order it from Europe the shipping cost even at that time was out of reach to me.
So I gave up just as I am thinking about this amp unless someone will come up with some improved version even if that not the exact clone of the original.
Any system is a synergy of parts. And the same applies to an audio system.
If you combine a speaker with an attenuated treble with a tube amplifier it is probably going to sound lifeless.
It's the art -if you wish- of combining all the right elements like in a recipe to obtain the desired sound.
I think that once I finish the modified Dartzeel, it will sound great with my speakers (Linn) with their sparkling treble.
If you combine a speaker with an attenuated treble with a tube amplifier it is probably going to sound lifeless.
It's the art -if you wish- of combining all the right elements like in a recipe to obtain the desired sound.
I think that once I finish the modified Dartzeel, it will sound great with my speakers (Linn) with their sparkling treble.
My 458 "clone" is much better ( concernig the perceived sound ) than I expected.....but I only use the original circuit, original poor parts mainly, original internal synergy without any "improvement", etc....
This amp already needs the really high quality loudspeakers ( and others ) having the possibility to show the unbelievable deepness in its sound.
"...high-frequency cleanness and transparency combined with as perfect a high-frequency transient response as I've heard from any amplifier, all emerging from velvety-black backdrops...."
( darTZeel NHB-458 monoblock amplifier Page 2 | Stereophile.com )
This amp already needs the really high quality loudspeakers ( and others ) having the possibility to show the unbelievable deepness in its sound.
"...high-frequency cleanness and transparency combined with as perfect a high-frequency transient response as I've heard from any amplifier, all emerging from velvety-black backdrops...."
( darTZeel NHB-458 monoblock amplifier Page 2 | Stereophile.com )
Finally, the WHA-97 (scheme at post #1863) is assembled. I used a baking sheet where I could accommodate everything. The only things I bought brand new are the soft-start and the speaker protection boards, all the rest is recycled.
Only one transformer, about 60mF total capacitors, 110Vdc, no hum, no offset voltage, everything is perfect; the sound is very clear but seems to be poor in dynamic (not because of the power supply), and I can feel some listening fatigue (bad distortion ?); it is far different from the WHA-217, not so impressive, and totally refuses to drive my Acoustat 1+1, so I couldn't make any comparison.
I'm glad I didn't finalize the NHB-108 in this WHA-97 mod. I'm enjoying the WHA-217 a lot.
Only one transformer, about 60mF total capacitors, 110Vdc, no hum, no offset voltage, everything is perfect; the sound is very clear but seems to be poor in dynamic (not because of the power supply), and I can feel some listening fatigue (bad distortion ?); it is far different from the WHA-217, not so impressive, and totally refuses to drive my Acoustat 1+1, so I couldn't make any comparison.
I'm glad I didn't finalize the NHB-108 in this WHA-97 mod. I'm enjoying the WHA-217 a lot.
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Such a dartzeel plays very well, why be wise, change the scheme? , with selected input transistors without an integrator (which spoils the sound, many people talk about it) at the output of 2mV. In the form that is offered, this is a different amplifier, and you can open a branch for assembling based on dartzeel. These are not rudeness, just thoughts.
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