Aleph J illustrated build guide

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#2,841
I'm about to begin stuffing my Aleph-J boards and have a question about the input differential. I have two pairs of 74's set aside for this build: one pair matched at 8.45mA and the other pair is matched at 9.73mA.

I presume that by adjusting the R8 pots, I will be able to adjust the bias through the Long-Tailed-Pairs so they match across boards and I won't get channel imbalances. Is this correct, or should I use a quad matched set of 74's?
 
#2,843
Hi kstagger,

You wrote: "Do I need to change the 1K resistor in parallel to a different value or ?"

Are you saying that your 'R7' location has both a 2K pot and a 1K resistor?

I don't think you need both. Can you please confirm what parts you have there and their values?

Thanks,
Dennis
 
#2,845
Dennis Hui said:
Hi kstagger,

You wrote: "Do I need to change the 1K resistor in parallel to a different value or ?"

Are you saying that your 'R7' location has both a 2K pot and a 1K resistor?

I don't think you need both. Can you please confirm what parts you have there and their values?

Thanks,
Dennis
Click to expand...

Dennis - looking at a blank PCB and a populated one, there is no 1K resistor in parallel with the 2K pot in the R7 position.

ie - only the 2K pot is used for dialing out the output offset.
 
#2,846
Hi kstagger,

So a single pot at R7 and you've bottomed it out and have 4.65V across it?

Can you please confirm the value of this pot her?. It almost sounds if you may
have 1K here. (You may want to check the model number of the pot to verify its
value.)

Also, what do you have at R8? Resistor or pot? (And what value?)


Thanks,
Dennis
 
#2,849
You shouldn't need super tight matching to get decent offset figure.

Since you've bottomed out the 2k pot, can you perhaps try this? Let the amp warm
up, and then adjust R8 in the opposite direction slowly while monitoring the offset.
(Make a small adjustment and let the offset catch up.)
 
#2,850
Dennis Hui said:
You shouldn't need super tight matching to get decent offset figure.

Since you've bottomed out the 2k pot, can you perhaps try this? Let the amp warm
up, and then adjust R8 in the opposite direction slowly while monitoring the offset.
(Make a small adjustment and let the offset catch up.)
Click to expand...

R8 is a fixed 1k resistor right now - but I do have a 1K pot in my stash. So I'll try this over the weekend.

thanks!
 
#2,851
The input stage front end CCS (the thing that would be adjusted by R8 if it were a pot) has a very small amount of adjustment from the pot as it's operating point is set mainly by the zener. Which is to say, don't bother installing the pot.

What is your 'bias' set to on the output? Are you measuring 0.35-0.40v across either R18 or R19?

If everything is seeming to be operating normally and you are just running out of pot travel you can add series resistor to the pot, or a bigger pot, but that requires unsoldering and lifting legs

I'd still like to see some photos to see if there's an issue somewhere else...
 
#2,854
shoot for desired value of Iq (voltage across source resistors) , fiddling with R27 group ohmic value

then , if output offset is positive (black robe on GND , red on output) you need to either slightly increase ohmic value of R7 group , or slightly decrease ohmic value of R8 group

AJ_Sch.png
 
#2,859
Really dumb question here - but I bought the Aleph J transistor kit from DIY Audio and noticed there are two types of supplied zener diodes here. Two 5.1V, and a pair of 6.1Vs the latter listed as optional. Which is the correct one to use and why are they different than the schematic given 9.1V?

I believe when I built the second PCB I grabbed one of the 6.1V or 5.1V zener. Would that cause my offset issue?

I do have some 9.1V that I have from Digikey - and I can't remember for sure but I think I used this for my working side.
 
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