How to obtain low dc offset at the Aleph amp output?

[kevin2a3]

I am new to the Nelson Pass Aleph amp, please advise. What minimum dc value is considered achieveable? Thank you so much !

 

[Zen Mod]

practically zero (few mV)

consider that , depending of several details in your actual build , there can be cold-hot difference of , say , 150mV

though , that's practically nothing , even for Lowthers

 

[kevin2a3]

Please advise what I should be looking for in order to keep the dc offset to a minimum, Fets matching, precision pots or even tight tolerance resistors and capacitors, pcb design etc. ?

 

[Dennis Hui]

I would consider well-matched front-end transistors to start. Is there a particular version of the Aleph you're interested in?

 

[Zen Mod]

specify your needs , then we can narrow down area of expertise

 

[kevin2a3]

Would like to try a 2pair power fets Aleph as a start.

 

[Eric]

The "Aleph" is an entire family of products. Which specific model are you looking to build/modify/adjust? Models that have seen particular interest among builders here include the Aleph-2, Aleph-4, Mini-Aleph, Aleph-X, Aleph-J...

 

[Zen Mod]

as we wrote - specify your needs - for which speakers/drivers you need it and why low offset is of primary importance

 

[kevin2a3]

Would like to try an Aleph 3 (2-pairs of IRF240 per channel). I am using a pair AV type Klipsch RF3 of 100dB sensitivity. Silly me, I just don't feel comfortable with dc at the speaker terminals.

 

[Zen Mod]

it behaves ; read ...

 

[kevin2a3]

Thanks guys! You have helped me to be confident enough to order the parts for an Aleph A3 tomorrow. Will use a 18Vac x2@12A transformer, 4kg heatsink for each channel, 800mA quiecent current on each IRFP244, 40,000uF on each 25Vdc rail. Will add a speaker protector as well. Any further advise before I take the plunge ?

 

[Dennis Hui]

You can use IRFP240. Don't forget you need to match them if you are using them
in parallel.

Also check out this thread:

http://www.diyaudio.com/forums/pass-labs/267857-aleph-design-reloaded.html#post4184920

 

[kevin2a3]

Hi Dennis, is IRFP250 ok too?

 

[Dennis Hui]

Input capacitance is about 2x that of IRFP240 or IRFP244. On the other hand,
it is a bigger part...

Looks like you want to run 1.6A current, so about 40W dissipation. I think
ZenMod uses a single IRFP150 in his Babelfish-J (in the thread he linked above)
so you should be able to do the same for the Aleph-3.

If you're going to just use IRF9160, an alternative part to consider is FQP3P20.

 

[kevin2a3]

I am getting ambitious, am switching to Aleph A5 instead. Please tell me the power transformer power rating, filter capacitance and DC voltage and current for each rail. Thank you !

 

[KatieandDad]

Do bear in mind that the MOS-FETs need to be matched. Not all together, but each CCS and Output set need to be carefully matched. This might take 5 x or more of the final quantities required to achieve. Even devices from the same batch can vary wildly.

 

[Dennis Hui]

Kevin, I sent you a PM.

 

[Eric]

Here are some notes that I recorded a few years ago. Some of the numbers might be a little off, but they provide a pretty good guideline.

Power transformer's VA rating needs to be 7.5x channel output power for each channel

per channel:
a40 24v 300va (~30v rails 40wpc output, 100w heat dissipation
Aleph 2 35v 750va (~45v rails) 100wpc output, 300w heat dissipation
Aleph 4 35v 625va (~48v rails) 100wpc output, 250w heat dissipation
Aleph 5 26v 300va (~34v rails) 60wpc output, 150w heat dissipation
Aleph 30 20v 250va (~25v rails) 30wpc output, 100w heat dissipation

 

[kevin2a3]

Valuable reference Eric ! What a great site.