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|30th December 2012, 02:48 PM||#61|
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
small resistors in Jfet gates are there to prevent any possibility of oscillations ; use them , or not , but it's always nice to have place for them
resistor in series with pot , in bjt emiter (input LTP CCS) is to allow usage of smaller value pot , so you have broader range (more turns) for fine setting .... in same time securing that you'll always have minimum (fixed) resistance there ;
resistor in collector side of same bjt - to burn some dissipation on it , not everything on transistor ; if voltage is Ub-9V , then it's safe to burn some 6-7V on it
bypass caps ( small red rectangulars) - obvious reasons
pot in series with fixed resistor in Aleph CCS - almost same reasons as for pot in LTP CCS
in your boots - I'll use small BF245C CCS , in series with R5
in fact , I already was in your boots - made Babelfish J ........ even if (today ) it would be somewhat even more complicated - more LEDs , LTP cascodes
|30th December 2012, 05:49 PM||#62|
So what about my questions:
1. Is it possible to build Aleph J 60 with j74 in the front and 6 outputs?
2. If yes, why not make convertible boards like Tea-Bag's F5T with the same front end topology but optional number of outputs? Personally I think this is the way to go for DIYers.
|30th December 2012, 06:25 PM||#63|
Join Date: Jan 2003
Location: ancient Batsch , behind Iron Curtain
1. yes , preferably 2+2 2SJ74 in LTP
|30th December 2012, 11:39 PM||#65|
The other thing about lots of outputs is that you will need to start paralleling the long tail pair, to give more oomph. So now we need a matched quad of J74 per channel? That's a tall order. If Linear Systems ever makes their J74 replacement, then maybe, but as for now, it's simpler to keep it a set design.
The inclusion of the pots on the PCB and some other tweaks is still a matter of serious thought for me. I really wanted to keep it as simple as possible...
One thing that we, as DIYers don't have is access to a zillion transistors like Nelson does when building his circuits -- he was able to match his transistors much more tightly than most of us can, because he had access to so many. We will have to tweak the circuit with resistors and variable resistors to get everything tuned up and humming.
There is nothing wrong with that, and arguably it's a cheaper and more precise way to do it -- But it adds complexity to a circuit that is going to be built by enthusiasts of many different skill levels, and I'm hoping for many completions.
I guess when it's all done I'm trying to make the best board (and project) for the "Aleph J", instead of a flexible board for many versions of Aleph amps.
This following quote is by Nelson from the Aleph J manual - it's quite interesting and very intriguing, what he says about it makes me want to try it just as designed, no more, no less.
"First Watt Aleph J Power Amplifier
Itís been about 9 months since the introduction of the First Watt
F1, and a few weeks since the introduction of the F2. Normally I
would wait a while before introducing another amplifier, but it
seems that the majority of potential customers request a more
regular kind of amplifier, something that can drive an ordinary
loudspeaker - ďJust a few more watts and some dampingĒ.
Meanwhile, itís been about 13 years since I designed the Aleph 3,
and I have such affection for that series that I revisited the design.
I wanted to better address the customer who owns high efficiency
loudspeakers and maybe a tube preamp. From my experience
with small amplifiers I know that this person values the
naturalness and musicality of the amplifierís sound over
The logical alternative is a high quality tube amp. Am I trying to
emulate a tube amplifier? No, but I make use of some of the
principles that make tubes sound good - simplicity and linearity.
Toward that end, I wanted to simplify the signal path and reduce
negative feedback without giving up on the measured
The Aleph J has about 15 dB less negative feedback than the
original and achieves comparable distortion, bandwidth, and
damping figures. It also has about 15 dB less noise. Those of
you with delicate tube preamps will be happy to see a 242 K ohm
input impedance with vanishing capacitance.
I consider it the best of the Aleph series.
The Aleph J carries on in the tradition of the Pass Labs Aleph
series, combining those elements that were particularly right
about the Aleph 3 and 30, and re-thinking those areas open to
improvement. Unlike its First Watt predecessors the F1 and F2,
the Aleph J is a voltage source amplifier Ė a regular sort of
amplifier like the ones you already know and love.
There are differences between the Aleph J and its predecessors:
1) Improved power supply filtration with about 20 dB less
2) Reduced gain on the active current source, giving better
overall performance into 8-16 ohm loads
3) Input stage using high quality matched JFETs
4) Much higher input impedance and vanishing capacitance
5) 15 dB less negative feedback.
6) Even greater stability, operating without lag compensation
7) 15 dB less noise
8) No electrolytic capacitor in the signal path
Some things have remained the same Ė the Aleph J has the same
basic 2-stage topology and uses output MOSFETs operating in
single-ended Class A mode. Itís distortion character is still 2nd harmonic, and itís sound is still natural and liquid. It is still very
reliable. I donít know of a load that can damage it."
|31st December 2012, 12:41 AM||#67|
I think he did another run recently. Use the contact form on Audiosector.com or PM him. They are nice boards, I have a few sets.
|1st January 2013, 10:25 AM||#70|
Late suggestion. How about multiple LED spots to replace Zener reference in LTP CCS. IT would allow adjustment abd little twekablity. Lower noise as well. Also, place for second set of input jfets.
...Shape the sound , Man!
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