need help on x converting to class B

[elviukai]

Hi there I was wondering if anybody has tried to lover Bias an up the gain provided by current source? From my prevous project I still have 2 enclosures they all stuffed and ready to connect driver boards( heatsinks have 24fets and 0.75ohm source resistors. ) I also have 2 toroids each 1200W 2x 21V secondaries which I want to use.

When playing with excel sheet I get this values-

Voltage after the rectifier 25.0
Voltage at the Fets 25.0
Bias 4.0
Ac current gain 90%
Number of fets 24
Peak current 20.00
Power 8 Ohms (RMS) 132
Power 4 Ohms (RMS) 242
Power 2,3 Ohms (RMS) 331
Power 2 Ohms (RMS) 361
Dissipation for one channel (complete) 214
Dissipation for one channel (fets) 200
Dissipation per fet 8.3
Bias per fet 0.33 A
Resistor to bias fet 1.50 Ohm
Chosen value of source resistor 0.75 Ohm value you´ve chosen
Current through source resistor 0.33 A 250mV



I want this amp to drive 2ohm load( 2x 4ohms subwoofers) I need 300-400Wats in 2ohm load. I heard few times saying that x will not provide more power in low impedances its just can provide the same power in 2 ohms as in 8ohms.. is it true?

As I am not very familiar with modifying circuits I would like to ask-


1)Can I leave 0,75ohm resitors and change other resistor(which one?) to keep 250mV voltage across them ?

2)how much I should lower resistor R12 value to achieve 90-95% gain provided by current source? I need very basic quess( like 200ohms ??)

I found this thread to calculate


http://www.diyaudio.com/forums/showthread.php?s=&threadid=38033&perpage=10&highlight=&pagenumber=3


If anyone would put quick input I will start to build this amps.

Thanks in advance

 

[elviukai]

anyone? 😕

 

[juma]

Aleph-type of circuits are all Single-Ended class A topology, meaning that it only can work in class A - you cannot "move" it to class B because only one device (or one set of paralleled devices) is doing the amplifying job in the output stage. Increasing the bias is the only way to get more power into lower Z loads.
Mr. Pass' "modulated CCS" in output stage is what makes Alephs special. It's a clever workaround to raise efficiency of the output stage within certain limits. It has it's sweet spot - about 50% AC current gain. Go higher than that and you'll loose that nice Single-Ended sound.
In your case, I would lower the PS voltage, but not below +/-20VDC (Vds lower than that changes the working conditions for output MOSFETs in unpleasant way). Increase the bias for the same amount (to stay in your heat dissipation limits move it to 5A) and you'll get more power into 2 Ohms load - how much more? - About 30 % more...

Besides, IMHO it's a bit of a waste to use ALEPH-x to drive subwoofers - many decent AB, even D class amps will take you there cheaper and better. Not that such amps are better than Aleph-x, but lower bass region is not the field where A-x really shines.

 

[elviukai]

thanks Juma, i have one pair of monoblocks with 22V rails and 9A, they deliver nice power in low loads. however i need way more power for subs(subwoofers is only 80db/w/m)

thats probbaly something rong in ax excel calculation sheet because i am able to get folowing power charts from my previuosly mentioned data-

and amp will draw only 230VA idle

 

[juma]

Your power chart looks OK - for lower Z loads power is more current dependent and for higher Z loads voltage on load makes the difference.
Having +/-22 VDC rails and 9A total bias makes consumption of 360W (add loses in PS) - so where the 230VA figure comes from ?

 

[elviukai]

yes my curent apms have total 420w dissipation, but I am talking about new ones i want to design. if i change curent from 50% to 90% provided by current source(pure class a is aproximately50%) with only 4A bias i have ay more power at low loads with the same apm. at least data told so. where is my mistake? 😕

 

[juma]

No mistake, it's just that you might not like the sound when you change AC current gain of Aleph CCS to 90%

 

[elviukai]

but why? is it discortions rises to unactable level? I will use it only up to 150Hz. and i want aleph x because it has good bass(not qty but quality)

 

[juma]

If you are impressed with bass from A-X, go ahead ! 🙂
I understand that you already have a working A-X, so it's easy to change one resistor and see what you get.

Have fun ! 😉

 

[elviukai]

actualy not 1 but 50 resitors(48fets ) with are caddock mp830 0.75ohm screwed to heatsinks with its own small heatsinks. 😀 amps is fullt asembled and it took 3 month to complete.by no chances i will dismount it.

but i have new stuffed fets and resisotrs amps without drivers boards.

I have few amps to compare(MBL and krell) and while they gives very slamming and controled bass for my taste I like xa bass better because more organic.

so if I want 90% current from CS I need change 1.2kohms to 200-300 or so? I will also need to lower bias from 8 to 4, how much resitors values i need change?

 

[juma]

...
so if I want 90% current from CS I need change 1.2kohms to 200-300 or so? ...

Right, that's the one to be changed. You don't need to change the bias - you can experience change of sound without changing the bias. If you happen to like it, you'll have the basis for further action.