Aleph Mini riddle makes no sense to me... - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Pass Labs

Pass Labs This forum is dedicated to Pass Labs discussion.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 4th August 2003, 02:24 PM   #11
grataku is offline grataku  United States
diyAudio Member
 
Join Date: Dec 2000
Location: -
N S,

I played with feedback extensively, and although I am not keeping a research notebook (it's not work) I seem to recall a feedback so large was bad news.
  Reply With Quote
Old 4th August 2003, 02:36 PM   #12
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Quote:
Originally posted by greyhorse
BMK, R17/18 in your schematic are drawn incorrectly, but is that the way you have your Aleph wired-up? If so, that would explain at least part of your results.

Re-check the schematic here: http://www.diyaudio.com/forums/attac...p?postid=15054

As you can see, R17/18 are supposed to go between the drain of the lower FET and the output. Current to the load is sensed across these, and this signal is fed back to the current source circuit.

If this isn't what's causing your problems, I can't say what it could be.

Greyhorse,

sorry - I drew the schematic after my notebook and after the fact. The wiring is correct, schematic was wrong. Here the correct version.

Grataku: In version 1.00 I had the normal 10k (input and feedback) and no 100 pF cap. The LF behaviour was the same. Can't comment on general sound because after the first listening and headscratching I raised the gain bec. my other amps are at 26dB, for easier swapping in and out. At that point I put the 100 pF in.

The two 1 nF (I used 1.5 nF) comp caps have been recommended elsewhere here, but I must say I had a helluva time trying to adjust AC gain, a lot of funny behavior, which i took for defective construction techniques - anyway after a lot of fiddling and swapping all of a sudden it fell into place and I got it where I wanted it, and stable. (I adjusted my R16 until the AC over each of the .39 Ohm resistors matched). But I DO suspect especially the 1.5 nF cap on the current source to create trouble.
Attached Images
File Type: jpg mini aleph 1.01.jpg (26.9 KB, 1048 views)
  Reply With Quote
Old 4th August 2003, 02:44 PM   #13
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Kristijan,

thanks, I had looked at yours too before I cooked up my circuit!

  Reply With Quote
Old 4th August 2003, 03:17 PM   #14
moamps is offline moamps  Croatia
diyAudio Member
 
Join Date: Oct 2002
Location: Croatia
[QUOTE]Originally posted by MBK
sorry - I drew the schematic after my notebook and after the fact. The wiring is correct, schematic was wrong. Here the correct version.[QUOTE]


Are you sure?
Attached Images
File Type: jpg minialeph.jpg (13.7 KB, 1012 views)
  Reply With Quote
Old 4th August 2003, 03:35 PM   #15
Netlist is offline Netlist  Belgium
diyAudio Moderator Emeritus
 
Netlist's Avatar
 
Join Date: Jan 2003
Quote:
Originally posted by MBK
Kristijan,

thanks, I had looked at yours too before I cooked up my circuit!

Hi
The Gate of Q7 is wired between the two 1.8K resistors.
In Kristijan's and other (for example Aleph2) the Gate is wired directly to the collector of Q6.

My 2 (Euro)cents

/Hugo - sorry Moamps, your were too fast...
  Reply With Quote
Old 4th August 2003, 04:34 PM   #16
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Default Arrgh, again!

Sorry, yet again, wrong on schematic, right in the actual circuit. Corrected below, along with R10 that's actually 330R (closer to the 392 it should have been).

I should look now for the only parts that are correct in my schematics, bec. they're probably wrong in circuit haha

And for once I wanted to try these fancy circuit maker programs, to have a nice schematic, unlike all the other ones drawn by hand on a messed up piece of scrap paper. I actually tried to scan my notes but they become unreadable.

Sorry to all, didn't mean to get you guys to do my editing
Attached Images
File Type: jpg mini aleph 1.01.jpg (27.7 KB, 912 views)
  Reply With Quote
Old 4th August 2003, 05:20 PM   #17
grataku is offline grataku  United States
diyAudio Member
 
Join Date: Dec 2000
Location: -
MBK,
after looking at your circuit some more I noticed that at 24 V rails your amp doesn't really qualify as a mini A but more like a aleph3 deprived of bias.
  Reply With Quote
Old 4th August 2003, 05:49 PM   #18
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Quote:
Originally posted by grataku
MBK,
after looking at your circuit some more I noticed that at 24 V rails your amp doesn't really qualify as a mini A but more like a aleph3 deprived of bias.
Grataku,

well, sort of in the middle: an Aleph 3 accepting a very high dissipation per device. The voltage drop over the source resistors (0.39R) turns out at slightly over 0.6 V, for a bias of a bit more than1.5 A. So dissipation should be around 22*1.5=33W, with 50% AC current gain giving 3 A into 8 Ohms. That would require about 24 V . With 24 V rails voltage just runs out slightly before current. My current chip amps have a 3A capability (5A peak) and run from the same 24 V, this was enough power wise for the system.

So this was my reasoning - I thought the numbers added up nicely if built in this way, plus in getting the same transconductance in one IRF140 as in 2 parallel IRF240's the only drawback should be the dissipation per device. The higher bias/device should be an advantage...

So it's say an Aleph 3 with 3/4 of the bias, or a Mini-A with a bit more voltage.
  Reply With Quote
Old 4th August 2003, 06:38 PM   #19
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Actually. Thinking that something is starving in the process and likely not current, but still the feedback is not fulfilling its purpose of lowering output impedance, I think I am getting somewhere.

I read in some thread that Mr. Pass noted that the first input transitor if left alone has a gain of 20 with the 392R resistor.

I put an 330R there so the voltage gain should be <20. Now if by virtue of my feedback choices I ask the second transistor to get up to a gain of 20, it can't actually do that: even completely closed so that the input Q gets all the current, there still will be no change since input gain is never up to 20. In other words, likely I could snip the feedback completely and it wouldn't change.

Since there is no feedback I get the full device resisitve parameters in my ouput Z: 2x transconductance + source R + output R. That can come close to the 2 Ohms that I measured! (I can't remember if I measured the resistive Zout before or after I changed the feedback to 20k).

So, back to 10k, put 392R in input tail and check. Or, put say, 511R in input tail and check

Now the other riddle with the woofer connection making the Aleph's bass output stronger ... I will test that some more by changing sides etc.
  Reply With Quote
Old 5th August 2003, 02:35 PM   #20
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Default progress update

So, I found out about the strong bass... Turns out the grounding connection was at fault. Since EQ and power amps are powered from the same PSU and tapped centrally (provisionally), some ground potential differences make trouble - problem solved by tapping the power of the power amp near the EQ and not centrally after a long line. Fascinating, a 2 dB low frequency boost in an unrelated amplifier operating in a separate band, just by wiring layout!!

For the gain / output Z and DC offset problem, swapping the 330R in the input Q tail got the DC offset to 9 mV. The output Z is lower, but still not spectacular, at about 0.75 Ohms now.
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Boulder's Jeff Nelson makes sense! mikeks Solid State 9 27th November 2011 06:16 PM
whats makes sense to have class A amp? space2000 Solid State 64 7th July 2008 03:46 PM
So, you still not sure if building a GC makes sense? Peter Daniel Chip Amps 65 28th January 2004 11:23 AM
Balanced DAC stage makes no sense to me jwb Digital Source 16 26th March 2002 03:20 AM


New To Site? Need Help?

All times are GMT. The time now is 08:48 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2