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Old 2nd April 2003, 10:37 AM   #11
diyAudio Member
Join Date: Sep 2002
Location: Sweden
Nice to see that someone has built the Zenquito and like it. I have
been tempted to try that amp myself for some time. Although
one cannot judge sound quality from a schematic I like the
simple straightforwardness of the design.

I do have a few questions for you, if you don't mind:

Do you use the transistors suggested by the schematic? When
searching the web for sources of the input JFETs, they seem
to be very hard to find and are listed as obsolete.

Which MOS-FETs and how many, do you use? The suggestions
given on the website seems way too conservative. The Hitachi
devices can dissipate 100W each so there seems hardly any
reason to doubling them even for the 80W amp even if
running in class A. Using double BUZ905DP seems extreme
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Old 2nd April 2003, 10:58 AM   #12
Electrons are yellow and more is better!
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peranders's Avatar
Join Date: Apr 2002
Location: Göteborg, Sweden
Blog Entries: 4
Originally posted by Da5id4Vz
Those chokes on the outputs are wonderful, how did you calculate and fabricate them?
It's real easy to make those chokes, just take something round and wind! The value isn't very critical. 0,5-2 µH is a normal value. 15-25 turns, diam 10-15 mm. Wire diam 1-2 mm.

I hate AWG

Nice amp BTW.
/Per-Anders (my first name) or P-A as my friends call me
PA03 LM4780 amplifier group buy, SIGN UP HERE for the group buy 0 boards left. 118 paid.
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Old 2nd April 2003, 11:56 AM   #13
SDF is offline SDF  France
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Join Date: Jun 2002
Location: FRANCE

I use two pairs of Hitachi transistor (per chanel), the couple 2SK1058 and 2sj162, but with my (too) high voltage I must use them in class AB. The first stage transistors are JFET 2SJ103 2SK246, there are available in France, but anywhere else, I don't know.

About sound, even if I don't know your taste, I'm sure you will be enjoyed. Sound really accurate and powerfull seriously and to compare with commercial product I won't exchange my amp by a Krell KAV300i.

I give you an advice to do one with a 2*30Vac transformer and you will listen lot, lot, lot of music
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Old 2nd April 2003, 04:37 PM   #14
Jay is offline Jay  Indonesia
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Join Date: Feb 2003
Location: Jakarta

Compared to Crescendo (2SK135/J50), how much better is this amp?
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Old 3rd April 2003, 09:27 AM   #15
SDF is offline SDF  France
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comparing with crescendo, more details and the smooth of medium treble is better too. Do it and you will compare yourself
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Old 9th April 2003, 06:12 PM   #16
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Join Date: Sep 2002
Location: Sweden

have you measured the idle current of the amp? Although i don't
really speak french, I understand most of the text about the amp,
so I see that it should have 1.5A idle current in class A. however,
I cannot find any info on the idle current in class AB. If I don't
misunderstand, the bias resistors should be 68 Ohms for class AB
and 150 Ohms for class A, so the idle current would be rather
high also for class AB. I have done some Spice simulations today,
but I used different transistors in the VAS, since I don't have
any models for the ones in the schematic. Hence, I may get
values which are quite off from what they should be.
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Old 10th April 2003, 08:54 AM   #17
SDF is offline SDF  France
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Hello Christer,

The idle current in my amp is 235 mA, the first stage is about 2 mA, the second made with 2NXXXX, is about 10 mA, and the final stage should be biased at 110 mA by device (I use two pairs), so the Idle current in class AB, is 2+10+110+110=232 mA,

Approximately 235 mA.

Concerning the bias resistor, you said true
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Old 10th April 2003, 11:27 AM   #18
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Join Date: Sep 2002
Location: Sweden
Hm, that's interesting. If you have 2mA in the input stage, and
10mA in the VAS, the Vbe of the VAS would be 2V. That seems
unreasonable for any BJT.

Anyway, It was foremost the OPS bias I was interested in and
110mA seems a reasonable value. Is that the value you
happened to get or have you found some info specifying that
it should be 110mA? The design is somewhat unusual in that
the OPS bias is set indirectly by adjusting the current in the
input stage. As far as I understand, the text says the input
stage current should be 1mA, rather than 2mA as you have.
Did I misunderstand that or otherwise is there a reason you
set it to a higher value?
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Old 10th April 2003, 12:29 PM   #19
SDF is offline SDF  France
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Join Date: Jun 2002
Location: FRANCE
I never mesure how current flow in the first and second stage, 2 and 10 mA was given to have an 'order of idea'.

The 110 mA I choose, give the thermal indefference, same stand off point for 25 and 50°C.

For have a good bias (near the linearity of class A), the typical curves Id=f(Vgs) give 150 mA bias by device.

So I try with this bias and the result was not really different (I don't listen, I mean same sound ), so I decide to get down until 110 mA in the target to reduce the dissipate power.

I didn't take over the two first stage, I juste set in gross the current which flows in output device. 110mA for me

Make this amp and bias it with 150 mA or 110mA, in this two cases you won't be disappointed....
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