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Aleph-30

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The Aleph 30 is the successor to the Aleph 3 and brings some nice improvements: higher input impedance, increased number of output mosfets to decrease distortion and to increase the damping factor. The original commercial product also featured better matched FETs. Discontinued in 2002.

Topology


All later Alephs are 2 stage designs consisting of in input differential and an outputstage. The outputstage is fed by the Aleph-current source which feeds at DC a certain bias current through the output fets. The Aleph-current source however reacts to an input signal by increasing bias current; how much it is increased depends on the AC current gain, set by the resistor going from the output via a cap to the base of the npn-transistor controlling the current source.

The fearless DIYers can calculate the AC-gain according to the formular described [here].

the mysterious resistors without values on the schematic


There are a couple of resistors without specified value on the schematic in the service manual, according to [Nelson Pass] they serve following purposes:

"R1 is used to trim Power Supply Rejection

R8 is used to trim DC Offset

R0 is used to reduce ground loops.

The input stages of the Aleph 30 and 60 get
devices which are matched in the amp itself
by inserting devices until we get what we want.
R1 and R8 are then optionally applied at varying
values to further trim the performance."

panos29 did a nice Aleph30 pcb, his [file] contains also a mask for the component placing.

Peter Daniel's [Aleph 30 pcb] is also good looking, [here] you find his pcb as a pdf-file in better quality, and the [component placement] or [alternatively here] as well.

He is currently offering [a limited number of boards] - so you might get them while his stock lasts.

Beware that Peter Daniel's boards use miniature resistors, like Phoenix MRS16; you can get them at digi-key or at Farnell.

Input protection


If you want to use protection diodes at the inputs, on PD's pcbs they're located [underneath].

However, Nelson Pass [doesn't use them for his home gear], but they certainly wouldn't hurt. See also in the previous link for hints how to connect the zeners properly to avoid oscillations, that is to connect them outside of the gate resistors.

Grounding


The pcb offers various possibilities for ground connections, see for more info [here] and for tying signal ground to psu ground [here]; the original Aleph 30 configuration is also [possible].

[Here] are some hints for avoiding rectifier noise.

A photo of his pcb can be seen [here] and [that's] what it looks like when populated and like [this] when the amp is finished.

[Here] you find a parts list.

Beware Peter Daniel is using IRFP140 in the TO247 package whereas the original design was using IRF244 in TO3. [Here] are some alternative mosfet suggestions, though the probably simplest would be what [Nelson Pass himself] says, to use the IRFP240.

Heatsinks


Mosfet temperature should be around 50-55C, so roughly 25-30C above ambient, [a bit higher values] are also possible though at a certain risc.

To calculate the necessary C/W rating, you can find [in this post] various links to necessary formulas and to a spreadsheet calculator that makes things easy.

See here for example [Tarasque's Aleph 30] for a nice and very compact build with the heatsink inside the enclosure.

Bias & DC offset


It is important not to deviate from the 50% figure for current gain on the current source, since [otherwise distortion will occur at highest output]. Read [here] how you can adjust the current gain. Alternatively one can use more bias.

[The input pair is responsible for the DC offset], so take care to get well matched ones. Alternatively use R8 for compensation (see above info about the mysterious resistors).

PSU


Of course one can build the psu according to the original schematic, however [adding an inductor between the capacitors] - like [in the schematic here] - would further improve noise (see also [here]).

Quality of components


Given the minimalist approach used in the design of Aleph amps, some DIYers consider it is worth spending some extra money on the best possible components. Often, this is based on subjective tests, but some quality criteria are justified by the design itself, e.g. high temperature capacitors (105C) are better suited in a very hot class A environment, resistors with low manufacturing tolerances (1% of the nominal value, or better) and thermal drift (ppm factor) are justified (and needed) given the absence of adjustments to be made and to preserve the matching of active components etc.

Small value capacitors used by DIYers are usually siver mica, polysterene, and so-called film capacitors: MKP (metalised polypropylene) or KP (tin-plate polypropylene), MKT (metalised polyester), MKC (polycarbonate) etc. which exhibit the lowest losses and the highest speed (needed to perform well at high frequencies). These are generally available only up to 50-100uF and the size increases rapidly together with their capacitance. The three 220uF caps used in the Aleph 30 are thus, in principle, electrolytic caps. Special audio-grade capacitors, with reasonably low losses etc. and which are small enough to be mounted on a PCB, have been developped: Black Gate, Rubycon, Panasonic for instance, are considered to be among the best choices. Each brand has its "aficionados".

1/4W or 1/2W resistors: are usually metal foil/layer type. Some are non magnetic (Vishay-Dale RC series for instance), which are considered to cause the lowest distorsion to the signal; for the same reason, some Valve gear builders also like carbon resistors, although these have the reputation of being noisier. The Aleph 30 schematic refers to all 1/4W resistors as metal ("MF") type, with 1% tolerance. The "big" 0,47 and 3 Ohms resistors (3W) are also "MF" type.

Nelson Pass recommends [Schottky diodes] - BTW [here] you find nice oscilloscope pics of the rectified output of Schottky and standard bridge rectifiers, however with a real load [it doesn't look so different].

As regards zener diodes used as voltage reference, some DIYers have done [noise measurements], including of LED Diodes.

Passive Preamp


Thanks to the high input impedance und low input capacitance of the A30 it is possible to use a passive preamp. [Use a potentiometer with either 25k or 10k or in between] in front of the A30 to adjust volume and together with a selector switch one has a nice passive preamp.

Some DIYers have come up with interesting ideas to build a passive volume control. See [this example] which uses a resistor+potentiometer combination.

The gain of Alephs in general and ways to increase it up to a reasonable level was discussed [here]. Where the increase is achieved by modifying the value of feedback resistors, Nelson Pass recommends not to exceed 30dB (input signal voltage x 33); he also suggests to experiment with a potentiometer in the feedback loop as an alternative volume adjustment method. Earlier Aleph designs had more gain than the A30, especially the high power versions when operated in unbalanced input mode (and jumper in place on the XLR connector to have the "minus" input connected to ground) ; for instance 29dB in the case of Aleph 1.2 and 26dB in the case of Aleph 2 and 4. The gain adjustment via the feedback network of the Aleph 30
was discussed [here] for instance.

Improving the Aleph


DIYer's often ask how can this excellent design be improved even further. The PSU section above covers the use of Schottky rectifiers. The thread on [Top ten ways to a better Aleph] has several recommendations.

Further improvements has come from First Watt in the form of the Aleph J, with the use of jfets in the front end.


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