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Old 15th November 2010, 06:34 PM   #2021
JBL4435 is offline JBL4435  Germany
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OK

I did change R17 to 2.2k. That makes the current through zener
(18.4V-9.1V)/2.2k=4.2mA

Result: the same behavior. No improvement.

Interesting is that the sine wave and distortion get's better again at 80kHz and up. Around 20kHz seems to be worst.
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Old 15th November 2010, 06:43 PM   #2022
JBL4435 is offline JBL4435  Germany
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Quote:
Originally Posted by Eric View Post
Looks like I'm a few days late to the party here, though you've received some great advice already!

A few things that stand out to me as potential things to check:

1) Obvious question, but I have to ask: are the fets well matched?

2) Another obvious thing (already asked by Zen Mod) is the measured value of resistors in the circuit vs. values indicated in the schematic. I learned long ago to measure, measure, measure each and every part. This stemmed from ordering 220ohm resistors and receive 220kohm resistors for a past project.

3) How do things behave if you re-set AC Current Gain back to "normal" (50%) ?

4) Bias current sounds very low for using 12 mosfets per mono. Your bias and rail voltage appear a bit low for that many fets. What is the actual power dissipation for EACH fet? Measure voltage drop across each source resistor, divide this by the value of the source resistor, and multiply by rail voltage. Ideally, each fet should be running in the 20-30w range. If your dissipation is much lower than this, perhaps you want to parallel fewer fets for the output stage?
Eric, thanks for joining

1. The Fets are very well matched. I have no problems with DC offset at all
2. I measure every part too
3. I'll try to set AC current gain back. But I've another higher power X-Aleph and it is also set to 69% and makes no problems
4. URail is +/- 18.4V
Bias is 3.3 Amps
Power dissipation per FET is around 10W
The stock XA60 uses 12 Fets per channel also, right?
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Old 15th November 2010, 07:09 PM   #2023
Eric is offline Eric  United States
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I just plugged some of the data you've provided into William's handy-dandy Aleph-X spreadsheet and some strange results pop up.

First, your input data for one monoblock: 19v rails, 3.1A bias, AC gain at 69%, 12 fets

Output data from the spreadsheet: 72w into 8ohms, 50w into 4 ohms, 9w dissipation per fet, and with the same level of heatsinking on my monoblocks (0.08c/w), a total thermal rise of 9c with a final chassis temperature of 32c (given 23c ambient). Not even body temperature.

My conclusion (though I may be wrong), is that you're not running the bias point high enough (currently less than 0.5A per fet) for the transistors to properly conduct, hence the distortion. This is a Class A design, it should run HOT! Yours is still well below body temperature (or it would be with my heat sinks). My guess is that your current configuration indicates a voltage drop somewhere near 0.17v across the source resistor for each fet. Essentially, your amp is current hungry (bias point is too low).

Here is a quick thing to try: crank up the bias point until you are measuring somewhere closer to 0.35v drop across your source resistors (assuming their value is 0.33 ohms) to your output mosfets. You want each mosfet to be dissipating closer to 1.0A. This should raise your bias point to about 6A and would be pushing about 20w through each output fet. At this point, your amp should begin getting warm and be on its way to getting hot.


If raising the bias point works (amp gets warm/hot, output waveforms look more clean), then I would recommend the following changes to have your amp still produce 70w into an 8ohm load:

1) Clip the power supply to the third mosfet on each transistor bank. This means your monoblock is now running with 8 instead of 12 fets.
2) Adjust the bias (v1 and v3) until you are measuring a voltage drop of 350mV across the source resistors to your output fets. This should provide a total bias setting of about 4.3A. Each fet should be dissipating about 20w (about 1.0A each) now.
3) Reset AC Current Gain back to 50%
4) You should be able to measure about 70w into 8 ohms and about 35w into 4 ohms.

Hope this helps!
Eric
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Last edited by Eric; 15th November 2010 at 07:19 PM.
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Old 15th November 2010, 07:19 PM   #2024
Zen Mod is offline Zen Mod  Serbia
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good points Eric ;

but - I think that temperature isn't possible culprit , but current through each output mosfet

you need to get them in "linear" area ...... it doesn't matter are you cooling them with heatsinks , water , or keeping them in fridge
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Old 15th November 2010, 07:29 PM   #2025
JBL4435 is offline JBL4435  Germany
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Quote:
Originally Posted by Eric View Post
Output data from the spreadsheet: 72w into 8ohms, 50w into 4 ohms, 9w dissipation per fet, and with the same level of heatsinking on my monoblocks (0.08c/w), a total thermal rise of 9c with a final chassis temperature of 32c (given 23c ambient). Not even body temperature.
My heatsinks are only 0.175 k/W for each monoblock. It's a small case.
21C above ambient.

I could crank up the bias to 5A. Means 32C above ambient. Dissipation per FET then 15W.

I'll set the bias to 0.57V at the source resistor (it's 0.69 ohms). That will give app 5 amps bias.
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Old 15th November 2010, 08:22 PM   #2026
JBL4435 is offline JBL4435  Germany
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5 amp Bias
52% ac current gain

NO CHANGE

This drives me mad

Maybe I really need to go down to 8 Fets / Channel
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Old 15th November 2010, 08:36 PM   #2027
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try this :

remove source resistors of all mosfets except one pair per side ; that way you'll have just one pair per side active

re-set offsets if needed

then test have you better linearity
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Old 15th November 2010, 09:02 PM   #2028
JBL4435 is offline JBL4435  Germany
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Can one overdrive the input of the Aleph-X?
I'm measuring with single ended input and we are talking about 2.8V input when the distortion at higher frequency occurs.
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Old 15th November 2010, 09:30 PM   #2029
Zen Mod is offline Zen Mod  Serbia
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read this pdf ;

two things :

1. try adding these 5V6 zeners

2. relax with volume ( if you need bigger amp , try with bigger one )

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

my habit is to use one IRFP150 instead two smaller IRFPs ... ( 240 , 244)

twice power and current , but capacitance pretty much lower than with 2 smaller ones
Attached Files
File Type: pdf Aleph CCS modification.pdf (52.2 KB, 92 views)
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Old 15th November 2010, 10:07 PM   #2030
Eric is offline Eric  United States
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Quote:
Originally Posted by JBL4435 View Post
Can one overdrive the input of the Aleph-X?
I'm measuring with single ended input and we are talking about 2.8V input when the distortion at higher frequency occurs.
This is your problem! 2.8V input signal is waaaaay too much. Most every amp will clip with a 2.0v RMS input.

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.

Regardless of the outcome, I would still run the amp with 50% AC gain and set up for only 8 output mosfets. Paralleling extra mosfets and increasing AC current gain are usually "tricks" for getting more output of your amp without overheating the mosfets. Since the power output target you've specified is not terribly demanding (70w into 8 ohms), I see no need to use 12 mosfets or increases in AC current gain.

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