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Old 6th February 2004, 04:52 AM   #1
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Location: Bangkok, Thailand
Default Aleph's Topology Questions

I am new to the would of audio diy and to this forum. I am embarking on an Aleph 60 project. Having gone through a lot of readings (all Zens including Zv4, a lot of threads in this forum, etc.), I came across some questions. If they have been previously discussed somewhere before, I am sorry. It would be greatly appreciated if they can be answered. Here goes:

1. Regarding the output stage of the Aleph (cf. Q12-Q17 in the Volksamp 60 schematic), why is a common-source preferred to a source follower? As I understand, CS quite suffers from Miller capacitance. While its true that Miller capacitance is quite proportional to the load (which is low, 8 ohms, in this case), we need to consider the fact that this load (the speaker plus the Aleph current source) varies with the frequency. This should make up for a nonlinearity as looked into the gate-source capacitance in addition to the Cgs that's already been there.

2. Should the input (from line stage) ground go to the Star point or to the main PCB?

3. As I plan to do a few relays, I would like to know whether they have any conceivable effects on the quality of the sound?

4. I plan to wind the (air-core) 2.2 mH inductors in the (pi) PSU myself. Is there anywhere I can find the formula?

If my English sounds strange (or if some of the questions offend anyone in anyway), I'm so sorry.

Thank you guys. Thank you Mr.Pass for his generosity towards the diy community.

-Sean
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Old 6th February 2004, 03:20 PM   #2
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Join Date: Sep 2002
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Maybe I can give you some help with number 4.
The inductance in a long straight or toroidal solenoid of length l can be written as follows:
L = u0 * N * N * A / l
where
u0 = permeability in empty space (4*pi*10^-7)
N = number of turns of the wire
A = area
l = lenght

Remember to use the same units on both A and l!

I'm not sure how good an appoximation this formula is for a coil with a short lenght (l) in relation to its area (A)... Maybe you will need to do some integration!


Does anybody know a better approximation of the inductance of a coil?

/Niclas
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Old 6th February 2004, 03:47 PM   #3
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A formula that gives us the resistance of the coil is also usefull to have:
R = P * l / A
Please obseve that l and A are not the same as those used in the post above!
l = length of the wire
A = area of the wires cross section
P = (rho) resistivity of copper = 1,67*10^-8 [Ohm*meter]

Once you start calculating R above, you will quickly start to appreciate thick wire gauge!

Hope this helps!
/Niclas
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Old 6th February 2004, 06:13 PM   #4
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Default Re: Aleph's Topology Questions

Quote:
Originally posted by SeanPool
1. Regarding the output stage of the Aleph (cf. Q12-Q17 in the Volksamp 60 schematic), why is a common-source preferred to a source follower? As I understand, CS quite suffers from Miller capacitance. While its true that Miller capacitance is quite proportional to the load (which is low, 8 ohms, in this case), we need to consider the fact that this load (the speaker plus the Aleph current source) varies with the frequency. This should make up for a nonlinearity as looked into the gate-source capacitance in addition to the Cgs that's already been there.
The Common Source is used to develop additional voltage gain,
as the input differential pair is not enough to develop the full
voltage gain or rail-to-rail voltage swing, and I did not want to
add additional stages.

The Miller capacitance is the same for either CS or CD usage,
the transistor itself has no way of knowing which mode it's
used in, and experiences the same voltages and currents over
the operating cycle (although in reverse phase).
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Old 7th February 2004, 03:16 PM   #5
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Join Date: Jan 2004
Location: Bangkok, Thailand
Mr. Pass

I really appreciate your answer Mr.Pass. I want you to know that I enjoyed and learned a great deal reading your articles. Thank you very much for your time and efforts put into the diy community. Textbooks teach me what other people find out. DIY teaches me what I will find out. Much like doing reaseach, but way a lot more fun.

As for the Miller cap, I'm still not quite sure, however. From the text, its value as looked from the gate really depends on the voltage gain, which is more than 1 (although with negative sign) with CS, but almost 1 with CD. But this is just from the text. I've never had any lab experiences.


stappvargen

Thank you so much for the formula!!! I just came across the following url:

http://www.lalena.com/audio/calculator/inductor/

which may be useful (but is it correct?) !!

Guess what, just today, I went out and could find a 2.2 mH inductor (in Bangkok), but only on a 18 guage. Mr.Pass once pointed out that the wire should be at least 14 ga. Thus, I plan to buy two 3.9 mHs and put them in parallel (as this should make them an equivalent of a 12 ga wire). So, I will not have to wind it myself anymore. Actually this morning I thought about hand-wind this thing and found that it was actually very difficult. After all, unless the formula is very very accurate and with a professional winding tool, how would I know how much Henry it has gotten so far? Can't use meters as I would have to break the wire to put the meter on, which is not right.

-Sean
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