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Old 23rd September 2006, 09:57 PM   #1
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Default Help with Aleph-X

Hi All. I need help with my ax. First here is the schematic I am using. I changed some of the values. I changed them because looking over Grey's and the hifizen schematic some of the values seemed way off. Values I changed are,

vr1 and vr3=100k,

vr2= 200 ohms.

r13 and r63= 511 ohms.

r101 = 1.5k,

r103=330 ohms.

r104=475 ohms.

r11 and r61=47.5k,

r12 and r62=1k

I am using 8 fets per channel, +/- 21.5 rails. All fets are matched to .01v. the 9610's are matched .001v, r1 and 51 are matched to .001v. All the source resistors are matched to .001v. all other resistors are 1% or better. I would like to use about 5 amps per side. I miscalculated my source resistor values, I am using .3 ohms which works out to be .287 ohms. At the value I should have about .350v across them for 5 amps per side? Now my problem, at Q1 I have .406 and .676, Q2 I get .422 and .600. Q8 .412 and .394 Q9.479 and .306. this is after 2 or 3 minutes. I know this will all change but I don't want to let it keep warming up with that much current. Now I have played with VR1 and VR2 to get values that low. They are as low as they can get. Value at r1 is 4.14v r51 is 4.47v. DC between outputs is about .05v, out to ground is about .3 volts. Now I have no idea how to get this thing to bias up correctly. I would love some help. Thanks for reading this jumbled mess. I hope I can get it working.
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Old 24th September 2006, 04:16 AM   #2
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Let me get this straight...
The values looked way off. You changed them. Now the circuit doesn't work.
Sigh...
Okay, first things first. We'll need a schematic. I have no earthly idea what R101, R103, and R104 are. My schematic didn't have resistor labels like that, and I'm way too far into sleep deprivation to try to guess what they might refer to. Since I first posted, others have come up with about a thousand minor variations on the Aleph-X and I haven't made any attempt to keep up with them all.
Second, I've got this funny feeling that you didn't read the original thread. I'm not sure how you arrived at any of the values for the MOSFET Source resistors: voltage, current, or resistance. The basic idea is that you should expect something on the order of .5V across the Source resistor. The value of the Source resistor in conjunction with the (roughly) .5V value will give you the approximate idle current through the MOSFET. I'm not clear on whether you're using .3 Ohms or .287 Ohms, but let's use .3 Ohms for ease of calculation.

.5V/.3 Ohms = 1.667A per output device

Your power dissipation should be somewhere on the order of 36W/device at idle. A little high, but doable if you have sufficient heatsink capability. Needless to say, you're going to have to have a pretty robust power supply.
You give two voltages per output device. I admit to being confused. Assuming that you're reading the voltage across the Source resistor, why do you have two readings?
If you're looking for 5A quiescent bias per channel then I would suggest something like:

Four pairs of output devices per channel (eight total)
Source resistors on the order of .39 Ohms or so

.5V/.39 Ohms = 1.28A per device (or per pair, since they're in series)
1.28A * 4 pairs = 5.12A total quiescent bias
21.5V * 1.28A = 27.5W power dissipation per device

That should give you about 88W into 8 Ohms. If you want to run lower impedances, you'll need to increase the bias current. Remember that many nominal 8 Ohm speakers are actually less and that there are frequently impedance dips that can suck a prodigious amount of current out of an amp.

Grey
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Old 24th September 2006, 12:16 PM   #3
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Hi Grey, here it is. I thought I added it. I have read lots of the ax thread but by no means all of it! Nor do I remember it all. I have the wiki. I am using four pairs of output devices per channel. So I have four pairs of source resistors. I am reading each source resistor.
Attached Files
File Type: pdf alephxcircuit.pdf (24.7 KB, 178 views)
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Old 26th September 2006, 02:11 AM   #4
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I've got four different disasters going here at work, so this will be brief.
There's no VR2 on the schematic you linked to. For the moment, I'm going to assume that it's the front end CCS adjustment.
The values for the resistors under the tail of the front end CCS are critical. I published two sets of values for these resistors: one in the original schematic, the other later on in a separate post. When in doubt, use the values in the schematic. They will fit a wider set of circumstances.
If you fiddle those values without having a pretty clear idea of what you're trying to accomplish the amp won't bias properly.
That's a polite way of reminding you not to change resistor values just because they look 'way off.' I didn't just pull those values out of thin air. I posted them because they will (for most of the people, most of the time, assuming IRF-type vertical MOSFETs at the output, etc.) apply the right DC offset across the front end load resistors to bias the outputs.
I don't have my original schematic values for those parts with me. Quoting from memory, the 200 Ohm pot seems right, but I'm not so sure about the 330 and 475 Ohm. They may be right. They may be wrong. I'd have to dig to find alternate sets of values. Unfortunately, I don't have time to calculate all this by hand, as IBM just walked in to service our 3494.

Grey
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Old 26th September 2006, 09:27 PM   #5
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Thanks for the help Grey. I am in the same boat as you. I changed values to match your schematic. The reason I thought they needed to be changed is because in reference to your schematic things did not seem to be right. You are correct about the 330 and 475, they are wrong and I fixed them. I have new source resistors on order. The front end seems to be working. Once I get new resistors in I may be back for help.
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Old 12th October 2006, 05:31 PM   #6
Eric is offline Eric  United States
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I am still in the process of building my Aleph-Xs and am by no means an expert on this circuit, I have been building a web page that tries to compile much of the discussions and variations that have been discussed here over the past few years.

If you are interested, you can find it here

As I began working on this project, I found that the information in the Wiki was presented at too high of a level for my "beginner" status, so my goal was to make my web page as basic as possible for others like me.

As my work progresses (ever so slowly - too many project up in the air right now), I will made additions to the page.

Eric
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Old 12th October 2006, 08:57 PM   #7
Netlist is offline Netlist  Belgium
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I didn't read the whole page but it looks like the most complete info available on one page. I'm sure you put a lot of effort into it.
Thanks for that, congratulations.

/Hugo
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Old 13th October 2006, 12:04 AM   #8
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Nice page. I have used that as my main resource. I have new source resistors to swap in. I just need to find the time to change them out.
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Old 13th October 2006, 12:04 PM   #9
Eric is offline Eric  United States
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Thanks for the complements! Here's hoping that I don't come across any significant problems as I progress.

Eric
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Old 28th October 2006, 10:15 PM   #10
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Ok so I got the new resistors in and the amp now seems to bias ok with .48v on the source resistors. The front end seems to be working, I have 4.6v on r1 and r51. dc offset is low. When I feed it a signal I get a very low siginal at the output. If I hook a speaker up to it I can hear music but very faint. I thought q3 and q53 might be on but I checked them and they are not. Any ideas why my output devices are not working? They are pretty warm.

Thanks.

-eric
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