High Distortion on Aleph 5

[Jaac]

Hello,

i have a question about the distortion of mine aleph 5. I've been working on the amp for a couple of years now slow but i finely am in the final stage,....i hope.
Last night i was at the lab of a colleague of mine and we've tested one of my mono aleph 5. And came to the conclusion that there is a nasty distortion when we are putting more like 21,1 Watt (13 V in 8 Ohm ) and that looks like this:



It's about 0,025 % THD at 400Hz (nakamichi t-100)

At this point the distortion is building up to te point that the output is visable clipping at around 78 Watt ( 25 V in 8 Ohm ) then it looks like this:



And another picture at 15 volts into 8 ohm where you can see the sharp edge of the distortion and that flank is at almost the bottom of the sine.

We tried finding out the problem here and i've searched the forum for a solution. i find that there is maybe a problem with R17 the 4,75K resistor in a thread for the aleph 30

aleph 30 thread

because i'm using a 2x30 volt tranny that gives about 38 á 40 Volt DC. is that maybe the problem that i have to change the R17 to a higer resistance ???

Here are some more pictures about my amp

[Netlist]

Very nice amps.
If I understand you correctly, you have about 0.025% distortion at 21.1Watt and the amp clips grossly at 78Watt.
Looking at the specs of the stock unit, the amp is only capable of giving 60 Watt and a distortion of 0.025% is about the lowest I've seen in the Pass Labs forum.
If this is correct, your amps are fine.
What's the distortion at say 55Watt?

/Hugo

[Jaac]

hhhmm oke butt the kind of distortion is not normal i think the sharp edges on it are in my eyes some what hard. for THD at 55 Watt i most make a visit to my friend whit the right measure equipment.

or maybe i wrote the wrong numbers down while my friend was calling them ?? I was late last night so that could happen but the nasty distortion remains

[steenoe]

Sorry to tell, but I made an Aleph5 with stock components. The amp just played sweet music!! I think you made a mistake, some where?? Please, check out all your components!

Steen

[Nelson Pass]

I would say that your bias is running low and/or the current gain
of the current source is too high. The distortion you see is the
current source shutting off on negative current swing.

For optimal performance you want to confirm that the positive
current source doesn't contribute more than 50% of the
current (this need not be an exact figure) and then that the
bias current is greater than 1/2 the current expected into the
load with any particular waveform. For a 50 watt amp, I expect
this to be about 1.8 amps.

[Jaac]

Oke so i think that R21 the 453 ohm comes quite precieze ? i think i put an other value for that becuase i hadden't had the correct one around

i'll try it and for now thank u !!!

[wuffwaff]

Hi,

the 453R are quite precise but only for one special amp.

The best way to get it right is to measure ac-current-gain with a pot replacing R21. If you have set it to 50% you can measure the pot and replace it with a resistor (or leave it in).

In my Aleph5 klone the following R´s where measured (at 2,1A bias):

left channel:

66,2% 390R
55,6% 470R
47,0% 560R

right channel:

67,1% 390R
56,3% 470R
47,6% 560R

I drew a graph with excel and found 525R / 540R for the left and right channel.
Eventually I put in 500R in the left and right channel......

William

[Jaac]

Oké

yesterday i tried a 453R resistor at R21. It liked that the level at witch the distortion began is raised to 16V into 8 ohm. So we can say its a AC Current Gain issue.

But to avoid any confusion about AC current gain you must explain me how i can measure that. maybe just explain in clear words where AC current gain stands for.
Most i measure/calculate the input current and measure/calculate the output current ??
In my eye's gain is a number not a percentage ???? because of the English I'm a bit confused.
When i know how to get the ac current gain then I can do the same as you William and calculate the exact value for R21 and place a nice dale resistor there and enjoy a bit of music..... or start building me another Pass amp !!

[Nelson Pass]

Somewhere on this forum I have posted a detailed procedure
for measuring and setting the ratios, maybe some good
Samaritan will point you to it.

But you have a distortion analyzer! I suggest you put a pot
there and trim the value until you're happy.

[Jaac]

Oke i was bad on the searching part.

Now i know it and here is a link to the correct thread

Proper Current Source Adjustment

and as attachment the qoute of mr Pass.

i will do the measurement later this week maybe after i complete the other channel