Aleph J - Transformer choice confusion (18 or 20 V)

I' d be absolutely happy, if anyone could help:

Which secondary tranny voltage would you prefer 18 or 20v when going the standard psu route?

Many thanks


It depends. I am assuming you already have the heatsinks, which determine the max. idle power (=heat) with your amp. If you want to get as much output power (before clipping):
  • If you have high-impedance speakers, you need more voltage capability (with lower idle current).
  • If you have low-impedance speakers, you need more current capability, i.e., higher idle current (with lower voltage).
That said, I usually don't listen at full output, so I don't care too much about clipping. Instead I tend to prefer high bias current (and hence low voltage), because the output transistors tend to be more linear at high current.
Typical bridge rectifiers have a forward voltage drop from 1.5 to 3 Volts at the bias current of the Aleph J. So it is not correct to simply multiply the secondary voltage by 1.414, or even 1.4. Power transformers are not perfect either. A better estimate is to use a factor of 1.3.

I run my Aleph J with dual 300VA, 20V transformers. The bridge rectifiers are LVB2560, which have a low voltage drop. Rail voltage comes out right at 26V, with 1.8 Amps bias current. The amp sings beautifully.
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To make use of the extra voltage you will potentially also need to bias the amp higher.
This amp will not go into Class B but will current limit.
If you have 16 Ohm speakers the extra voltage would be worthwhile.

It's not that straight forward to answer that question. It depends.

I guess if you want some additional overhead for future build options then yes 20V is probably worthwhile getting.
The amp is going to run hotter so as long as you have got the extra heat covered it won't hurt.

One possible draw back of going to 20V secondaries is that you will be forced to use 35V caps. For me personally I don't see that as a problem, but maybe someone else might.
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Production First Watt Aleph J used a 300VA/18v transformer, right?

Also, general advice on the illustrated build guide thread seems to be either 18v or 20v is fine. In choosing between 300VA, 400VA, or 500VA, also consider the size of your chassis, your internal layout plans, and heat.
I want to build the J, dual mono power, 5U chassis. Antek only has 500VA transformers in 18V. Everything smaller is sold out. Am I correct that dual 500VA is too much and I just need to wait? I intend to use the soft start board regardless.
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