Aleph Mini riddle makes no sense to me... - 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, 12:57 PM   #1
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Default Aleph Mini riddle makes no sense to me...

So I built a mini-Aleph to experiment with, and thought for use in my biamped system (fullrange 6.5" and woofers, both as dipoles on same panel, with line level active EQ and X/O).

For the circuit I mixed elements of the various iterations available. As usual after buying components I was missing some so I substituted some parts as well.

Implementation now is, one channel built, gain=20, comp caps 1.5nF, feedback R = 20k = input R, IRFP140N, +-24V from the same 375 VA 18-0-18 PSU that feeds the rest of the system, bias 1.6A, AC current gain adjusted at 100 Hz.

Now the riddles. First I listened to this one channel thrown into the system. It's biamped and the three other amps are OPA549, I used the mini Aleph for the left channel's fullrange only.

Result: sounds sweet, but strong/bloated bass. It is even more noticeable when touching the left fullrange's cone: it moves a lot more than the right's fullrange's one driven by the OPA548.

So I measured some simple parameters.

DC offset: high, 100-120 mV. Maybe my poor choice of input tail R of 330R instead of 392R or incometent matching (my best pair of 9610's, matched at 0.01V!!). OK, I accept.

Frequency range: I have a DIY fast peak meter, very precise but pot adjusted, so 1dB is the limit in resolution. Anyway I get flat over the range but -2dB down at 20kHz. Surprise but OK, I accept. (comp. cap 1.5 nF over 20k should give 80kHz cutoff no??).

Output Z: a 6.7V no load output becomes 6.62V into a 13.5 R load(that's what I had lying around...). So I calculate
Zout = R(L) * {V(0)/V(L) - 1} = 0.16 Ohms, OK.

So how come I get uncontrolled bass in a fullrange driver with Fs 80 Hz but cut off at 310 Hz by a 24 db/oct HP??.

So I measure V(out) with and without driver leads attached.

Riddle 1: now I get 2 dB down with this 8 Ohm load (no x/0, naked driver) vs. no load. I calculate output impedance at 2 Ohms, that's bad indeed.

Riddle 2: connecting or disconnecting the woofers on the same side as the mini-Aleph changes the electrical response of the mini Aleph. I get electrical outputs of +1dB at 125 Hz, +2 dB at 100 and at 60 Hz, when I connect the same side woofers, the ones on the same panel as the fullranges driven by the mini Aleph. There is no change if I connect or disconnect the other side's woofers. The bloated midbass output appears when I do connect the same side woofers.

All amps are fed from the same PSU, centrally connected, so it can't be an electrical feedback problem, else the other side woofers would have the same influence. The bloated bass only disappears when the same side woofers are disconnected so it can't be simply high Q resonance whatever.

I can only imagine that the mechanical excitation of the fullrange by the woofers operating on the same panel, creates an EMF and the mini Aleph tries to fight it by raising output. Due to the high Q driver and the high Z(out) of my mini Aleph it doesn't manage though, or even increases the resonance.

Right? Or is there something else? It sounds so outlandish to me.

Back to Riddle 1:

why is the measured output impedance so much higher with a real driver attached, than with a dummy R load? Not just much higher, but very high in the absolute? The IRFP140N should have double the transconductance of the 240's, so a single transistor pair should do as well as 2 pairs in the Aleph 3... or? And the voltage drop is the same if I disconnect everything else from that 375 VA PSU.

I could double up the output devices but I'd rather understand what's happening here first...

Sigh.

Circuit below.
  Reply With Quote
Old 4th August 2003, 01:00 PM   #2
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Default circuit

below
Attached Images
File Type: jpg mini aleph 1.01.jpg (26.8 KB, 1353 views)
  Reply With Quote
Old 4th August 2003, 01:07 PM   #3
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
setup
Attached Images
File Type: jpg setup.jpg (44.7 KB, 1305 views)
  Reply With Quote
Old 4th August 2003, 01:08 PM   #4
MBK is offline MBK  Singapore
diyAudio Member
 
Join Date: Apr 2002
Location: Singapore
Default looks

looks
Attached Images
File Type: jpg mini aleph 08-03.jpg (42.8 KB, 1306 views)
  Reply With Quote
Old 4th August 2003, 02:27 PM   #5
diyAudio Member
 
Join Date: Feb 2002
Location: Ontario
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.
  Reply With Quote
Old 4th August 2003, 02:39 PM   #6
diyAudio Member
 
nobody special's Avatar
 
Join Date: Oct 2002
Location: USA
Default I could be wrong, but...

I think that Q5 should connect directly to r19, and then r17 and r18 should be in series with the load from that point, with the feedback taken from the load side.
I could be wrong, but the simulator flips out when it is connected as you have it.
If you connect it the normal way, your offset will go up to about 2+ V, and you will need to increase your current through the front end quite a bit to compensate for it.
The bandwidth simulates -3.2db at 80Khz.
__________________
Steve
  Reply With Quote
Old 4th August 2003, 02:40 PM   #7
diyAudio Member
 
Join Date: Mar 2002
Location: CROATIA
Hello,

Try to rewire you circuit like the circuit diagram below.

You can even use it in balanced configuration with the Balanced
Line Stage preamplifier.

Best regards,
Kristijan Kljucaric
http://web.vip.hr/pcb-design.vip
Attached Images
File Type: gif mini-a.gif (13.7 KB, 1395 views)
  Reply With Quote
Old 4th August 2003, 02:43 PM   #8
diyAudio Member
 
Join Date: Jun 2002
Location: Italy
Also R10 is a bit low , usually it is 390 ohm.
  Reply With Quote
Old 4th August 2003, 03:04 PM   #9
grataku is offline grataku  United States
diyAudio Member
 
Join Date: Dec 2000
Location: -
MBK,
your 120mV dc offset can be cured by putting a 500ohm trimpot in place of r7 and adjusting the dc offste to 0.
From my experimentation with the circuit the 100pF cap in the feedback loop with a MUCH larger than normal NFB will twist the phase of high frequencies like there is no tomorrow, the amp will likely sound like crap. I also got very funny results playing with the 1nF compensation caps.
I don't know what a AC current gain will do but I imagine it also could cause the dc offset to misbehave and the distortion to raise.
  Reply With Quote
Old 4th August 2003, 03:18 PM   #10
diyAudio Member
 
nobody special's Avatar
 
Join Date: Oct 2002
Location: USA
Quote:
Originally posted by grataku
MBK,
your 120mV dc offset can be cured by putting a 500ohm trimpot in place of r7 and adjusting the dc offste to 0.
From my experimentation with the circuit the 100pF cap in the feedback loop will twist the phase of high frequencies like there is no tomorrow, the amp will likely sound like crap. I also got very funny results playing with the 1nF compensation caps.
I don't know what a AC current gain will do but I imagine it also could cause the dc offset to misbehave and the distortion to raise.
Is this true even with all of his impedances scaled down to where they are? It seems like that cap would be appropriate, considering the 1k to ground, the 20K feedback, etc.
__________________
Steve
  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 07:16 PM
whats makes sense to have class A amp? space2000 Solid State 64 7th July 2008 04:46 PM
So, you still not sure if building a GC makes sense? Peter Daniel Chip Amps 65 28th January 2004 12:23 PM
Balanced DAC stage makes no sense to me jwb Digital Source 16 26th March 2002 04:20 AM


New To Site? Need Help?

All times are GMT. The time now is 02:35 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