Aleph-X builder's thread.

[JBL4435]

Originally Posted by Zen Mod

now seriously - there is some excessive capacitance in system ( now when input CCS current issue is confirmed as non existent ) ..... that's why decrease in gain in frequency domain

Excessive C? So all unneeded C's out, right?

I removed C12 and C13 (220uF Panasonic FC)
I also removed C9 and C10 (3n3) lag cap
I don't use these caps in my bigger amp also.

Now it is much better.
Power out with 20kHz and 1% THD is 40W/8Ohms now.
Before it was 20W/8Ohms.

Power out with 1kHz and 1% THD is 65W/8Ohms now.
Before it was also 65W/8Ohms.

So I still have problems with higher frequencies but much better than before.

[JBL4435]

Originally Posted by Eric

Your scope and wave-form testing should take place with measured OUTPUT levels of 1.0vRMS and 10.0vRMS. Both of these will correspond to input voltage levels far below 2.8v. Output sine and square waves measuring 1.0v and 10.0v RMS should look very clean on your scope.
Eric

With these values they do look perfect.
I wanted to check the behavior in full power and to get full power I need 2.8V unbalanced in.

Didn't really nobody look into THD at full power with frequencies above 1kHz?

[Eric]

I have no way to measure THD with my (borrowed) scope, so no, I haven't looked at THD before. I have noticed that high frequency output tends to distort more quickly than lower-frequency output for the same input level, though I haven't made a formal study of it yet. Perhaps I'll have a look at this the next time I have the scope hooked up...

Funny, I have measured maximum output into 4ohms and 8ohms lots of times, but I never actually measured the magnitude of the input signal that provides my maximum undistorted input signal.

Guess its time to get the scope out again soon...

I still think you'll find benefit to use fewer output transistors and drive them with greater power (20w or so power dissipation).

[JBL4435]

Originally Posted by Eric

I have noticed that high frequency output tends to distort more quickly than lower-frequency output for the same input level, though I haven't made a formal study of it yet. Perhaps I'll have a look at this the next time I have the scope hooked up...

Seems like my problem is more normality than a problem and I realized it only because I tried my new toy (HP8903B Audio Analyzer)

Originally Posted by Eric

Funny, I have measured maximum output into 4ohms and 8ohms lots of times, but I never actually measured the magnitude of the input signal that provides my maximum undistorted input signal.

Try it. You'll be surprised

Many thanks to all that tried to help here!!!!

I'll try to experiment with fewer FET's

[Eric]

That's quite the nice toy that you've acquired! I've been trying (to the extent possible) do things "on the cheap," so I've been borrowing equipment from the EE lab on campus.

Looks like I need to drag out the scope tonight or tomorrow to do some more learning. In reference to your removal of caps, I found square wave distortion when c9 and c10 were removed (detailed on my web page that you've been reading). I have never run the amp without C12 or C13...

[JBL4435]

Originally Posted by Eric

In reference to your removal of caps, I found square wave distortion when c9 and c10 were removed (detailed on my web page that you've been reading). I have never run the amp without C12 or C13...

I have a pair of 140W Aleph-X build with Karis boards. No C9/C10 and no C12/C13 and NO Problems

[Eric]

Hmmm... I'll have to give it a go without C12 and C13 and see what differences show up in the measurements and audio performance.

Ok, after carrying the 70lb beast upstairs and waiting for it to warm up, I made some measurements of input and output voltages at the visible onset of clipping in the scope. Here is what I found:

1 kHz clip onset, 3.66v peak-to-peak input, 150w output into 4 ohms
5 kHz clip onset, 3.64v p-p input, 139w into 4 ohms
10 kHz clip onset, 3.05v p-p input, 96w into 4 ohms
15 kHz clip onset, 2.80v p-p input, 80w into 4 ohms
20 kHz clip onset, 2.56v p-p input, 65w into 4 ohms

These measurements were made with 22.4v rails and 9.4A bias current. Measured voltage drop across source resistors averages 0.515v. Final heat sink temp is about 55c.

I stand corrected, 2.8v input is not too high of a signal level. I thought for sure this was part of your problem. I did, however, duplicate your situation of reduced power output as input frequency increases.

[JBL4435]

Originally Posted by Eric

Here is what I found:

1 kHz clip onset, 3.66v peak-to-peak input, 150w output into 4 ohms
5 kHz clip onset, 3.64v p-p input, 139w into 4 ohms
10 kHz clip onset, 3.05v p-p input, 96w into 4 ohms
15 kHz clip onset, 2.80v p-p input, 80w into 4 ohms
20 kHz clip onset, 2.56v p-p input, 65w into 4 ohms

Similar to my findings. App. half power with 20kHz than with 1kHz. From 20Hz to 1kHz nearly no change.
Without C9/C10, C12/C13 it got significantly better.
As I removed all 4 caps at one time I can't they which ones are "bad".

We will have to wait what you find when removing C12/C13 only. This way we will know.

[Luke]

Originally Posted by JBL4435

Similar to my findings. App. half power with 20kHz than with 1kHz. From 20Hz to 1kHz nearly no change.
Without C9/C10, C12/C13 it got significantly better.
As I removed all 4 caps at one time I can't they which ones are "bad".

We will have to wait what you find when removing C12/C13 only. This way we will know.

I can understang gain changeing with frequency but why the input voltage, shouldnt this be constant? Input impeadance doesnt change so it doesnt make sense.

[JBL4435]

Originally Posted by Luke

I can understang gain changeing with frequency but why the input voltage, shouldnt this be constant? Input impeadance doesnt change so it doesnt make sense.

Input voltage does not change!
Distortion occurs earlier with high frequencys, thus lower input voltage.