The Mini-A

[clm811]

the aleph's are about 10-20 watts depending on rail voltage.

If one were to use a 300VA Traf with dual 18vac secondaries(vs 15vac), and using a CRC supply (30mF/0.1ohms/30mF) what would be the power output into 8 or 4 ohms?

Any idea what the approximate supply ripple would be?

THX,

-Chas

[pchw]

There is an incomplete question.
What is the bias point? How many devices? These affect the end result.

Once you have decided these. You can use the attached spreadsheet to get the answer. Just watch out the SOA of the devices.

[clm811]

Cool spreadsheet! Is it for Aleph in particular, or all class-A amps in general?

Thanks, Fred.

This is my first Class-A amp, so I'm getting my feet wet, so to speak.

Happy New Year!

-Chas

P.S. Anyone have info on the amount of ripple for 30mF/0.1ohm/30mF CRC with 18VAC traf?

[AndrewT]

download Psud11 and insert your data.
Allow about 1000ms after start up for the PSU to settle, then state for a time for the analysis period.

[pchw]

Hi Chas,

Happy New Year to you, too.
This spreadsheet is specific to the Aleph.

A sturdy 300VA 18VAC x 2 transformer is good for the Aleph 3 and Aleph 30 which are 30W into 8R and 40W into 4R.

[slomatt]

Recently I build a Mini Aleph (details at Mini Aleph Build) and I'm now in the testing and tuning phase of the project.

For the initial build I used the values specified in the BrianGT schematic, these resulted in a bias current of ~0.9A which is a bit low for the Mini Aleph. I substituted a 200k ohm potentiometer for R13 (R19 on the Pass schematic) and was able to adjust the bias up to 1.17A, but cannot get it above that point. My target value is 1.2-1.3A based on heat sink capacity.

After analyzing the schematic I'm starting to suspect that the values of R14 and R16 (R17/R18 on the Pass schematic) are incorrect. The spreadsheet specified 1.5k ohm resistors for both the Aleph30 and the Mini Aleph, but there are a handful of posts that say that these should be 750 ohm for the Mini Aleph.

The Mini-A
The Mini-A

I could not find any definitive resolution on this, so I'm hoping the community can help out. All part numbers below are based on the "miniasch.pdf" schematic from 1/20/2009 which is linked at other places in this thread.

Based on my understanding of the circuit, Q7 is used as a current source to bias Q6 at ~1A. Q7 is controlled by Q5 which uses the voltage across the sense resistor R28 to set the bias current. The collector of Q7 is connected to the + rail through R14 and R16, and R13 is used to adjust the voltage at the base of Q7 and therefore the bias current.

In the original Pass Aleph 30 schematic it shows the following target voltages:
+26v positive rail
+15v at the bottom of R14 (11v drop across R14)
+4v at the bottom of R16 (11v drop across R16)
+0.8v at the base of Q5.

Based on the assumption that the Aleph30 and the Mini Aleph bias Q5 at the same current then is it possible that the voltage drop across the 1.5k ohm R14/16 resistors is too high to allow enough adjustment to drive Q7 above ~1.17A, assuming ~15v rails? If this is the case then it is strong argument for changing R14/R16 to 750ohm.

I'm itching to finish this build and any input on the above is greatly appreciated. Thank you.

- Matt

[Zen Mod]

decrease all big resistors for same amount and re-set bias an offset

shave them to next lower value or at least -30% , either replacing them or soldering some in parallel

[boywonder]

Originally Posted by Zen Mod

decrease all big resistors.......

The mosfet source R's?

Will bad things happen if R14/R16 are reduced to 750R?

[Zen Mod]

I told you what to do

change , enjoy , think later

[slomatt]

Originally Posted by Zen Mod

decrease all big resistors for same amount and re-set bias an offset

shave them to next lower value or at least -30% , either replacing them or soldering some in parallel

To clarify, are you suggesting that we drop the "big resistor" sense/source resistors from 0.47 ohm to somewhere around 0.33 ohm? That would definitely give a higher bias current for a given voltage across the sense resistor, but I still wonder if changing R14/R16 would have the same effect with less de-soldering and re-purchasing required.

Any input on if R14/R16 should be 1.5k ohm or 750ohm for the Mini Aleph?

- Matt