Aleph 30 - Page 14 - 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 January 2003, 07:05 PM   #131
diyAudio Member
 
Algar_emi's Avatar
 
Join Date: Oct 2002
Location: Canada, Qc
Default Aleph30 Distorsion

The sound is very detailled, with a lot of air, but lean bass.
But all this only at low level. If I tried to crank to volume, the sound change and become congested.
I have a pair of acclamed Axiom 3Ti.
My preamp is a Bride of Zen.
The distorsion I mentionned is a higher listening volume, that why I think it explained th change in sound.
As described by others users, the aleph used the active Current source to increase efficiency. So, I think that the change in sound occurs when the active current source pull the amp in class A-B mode rather that pure Class-A. The Capacitor in the current feedback path is the on passing the AC and contributing, maybe, to the distorsion. By the way, both my channels are behaving the same.
Is a better FET matching in the Current Source could eliminate this distorsion?
I will tried to open the AC feedback an listen to it...
I'm waiting for any suggestions.
  Reply With Quote
Old 4th January 2003, 09:00 PM   #132
diyAudio Member
 
wuffwaff's Avatar
 
Join Date: Aug 2001
Location: Ingolstadt Germany
Algar,

I never heard such behaviour even with a 75% ac-current gain setting. Can it be that your amp is clipping?

This could be the case if your ac-current gain is chosen too low (under 50%) and your speakers demand the current but don´t get it.

william
__________________
een ooievaar is geen konijn want zijn oren zijn te klein!
  Reply With Quote
Old 4th January 2003, 09:54 PM   #133
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
When you deviate from the 50% figure for current gain
on the current source, you get more distortion at the
highest output, either because you are current-starved,
or because the current source is shutting off on negative
peaks. The cure in both cases is more bias.

Come to think of it, more bias is almost always the cure
for any limitations of Alephs.
  Reply With Quote
Old 5th January 2003, 12:08 AM   #134
Coulomb is offline Coulomb  England
diyAudio Member
 
Coulomb's Avatar
 
Join Date: Jul 2002
Location: Ancaster, Ontario
Quote:
Originally posted by Nelson Pass
When you deviate from the 50% figure for current gain
on the current source, you get more distortion at the
highest output, either because you are current-starved,
or because the current source is shutting off on negative
peaks. The cure in both cases is more bias.

Come to think of it, more bias is almost always the cure
for any limitations of Alephs.
Well we all have our Biases, some more than others.
  Reply With Quote
Old 5th January 2003, 05:17 AM   #135
diyAudio Member
 
Algar_emi's Avatar
 
Join Date: Oct 2002
Location: Canada, Qc
Default Aleph30 Distorsion

Thanks Mr. Pass for participating to the discussion.
My current biais is 0.5A, as per Aleph30 service manual.
I adjusted the AC gain to 50% one week ago using the procedure described in one of the thread. I changed R21 from 820 ohms to 1K and got a perfect 50% gain.
My supply is using a 18+18V, 500VA transformer and the same bank of cap as the original Aleph30.

I tried using 0.8A biais but my output DC offset increased from the actual 120mv to more than 300mv! So I put it back to 0.5A.
This DC offset is causing me also a lot of problem. My input FETS are well matched. The Current and Active FET are almost identical also, except for one of the Current Source One.

Can I try to balance my Current FET by tuning one of the 221 ohms gate resistor on the FET that is different?

Mr. Pass you mentionned that R8 can be used to adjust the offset. What are the typical values to use? Do I need to used both R8 at the same time, or they are indicated at two different places?

Thanks for any comments.
  Reply With Quote
Old 5th January 2003, 10:37 AM   #136
diyAudio Member
 
wuffwaff's Avatar
 
Join Date: Aug 2001
Location: Ingolstadt Germany
Algar,

the 0,5A bias you mention, is this per fet or in total? The Aleph 30 has ca. 1.5-2A total bias and a 25 volt supply

Normally only the input diff pair is responsible for dc offset. If this changes so dramatically when changing bias I think something else is wrong.

william
__________________
een ooievaar is geen konijn want zijn oren zijn te klein!
  Reply With Quote
Old 5th January 2003, 03:16 PM   #137
diyAudio Member
 
Algar_emi's Avatar
 
Join Date: Oct 2002
Location: Canada, Qc
Default Aleph30 Distorsion and Traces routing

The Biais is 0.5A each, total 1.5A per rail, 3A total.
What else can be wrong? Do you have any test that I can do to start finding the problem with the DC offset?

Both my channels have the same behavior. So I exclude bad soldering and bad parts. Possible causes then are:
bad schematic to start with, but the amp is working so...
Bad traces routing. The more I think, my problem is maybe cause by the way I connected the FET using point to point wire. They are connected like this (See picture):

+V +V +V
| | |red wire (all AWG18 wires)
Q6 Q7 Q8 FET Current Source (ref Aleph30 Schematic)
|___|____|0.47 ohms and org wire
|
| wht wire
|______________________
6 resistors 0.47 ohms |
| yel wire | wht wire
Speaker out + |
|-------|---------| org wire
Q9 Q10 Q11 Active FETs
| | | org wire and 0.47 ohms
--------------------
| yel wire
-V

I suspect That the single wht wire connecting both FET banks and the single wire connectiing the Active FETs to the negative rail are choking the current and cause current limitation in the circuit.

Any suggestion for the power traces and the speaker connection point
Attached Images
File Type: jpg fet trace routing_small.jpg (57.3 KB, 1526 views)
  Reply With Quote
Old 5th January 2003, 03:32 PM   #138
diyAudio Member
 
Algar_emi's Avatar
 
Join Date: Oct 2002
Location: Canada, Qc
Default Aleph30 Distorsion and Traces routing

Here a better schematic
Attached Images
File Type: jpg trace routing schematic.jpg (15.2 KB, 1592 views)
  Reply With Quote
Old 5th January 2003, 08:03 PM   #139
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
I don't think the negative supply leads are the problem
assuming the front end is also sharing exactly the same
negative supply, which is necessary.

As to offset, I assume you are using a cap in the feedback
loop. As your rail voltages are a bit low, I suggest that you
reduce the 4.75K resistor feeding the NPN positive bias
circuit to 3.3 K or so to compensate. This may in fact
cure your high power distortion.

Also you can adjust the Source resistance on the current
source which feeds the input diff pair to adjust the offset.
  Reply With Quote
Old 6th January 2003, 12:32 AM   #140
diyAudio Member
 
Algar_emi's Avatar
 
Join Date: Oct 2002
Location: Canada, Qc
Default Aleph Measurements

Thanks Mr. Pass. Your help is very appreciated.
I will tried these tonight.
You don't think my wiring can affect the sound?
I will rewire one of the channel so each current source feed one Active Fet using its own wire.

Just to be sure the rest of my circuit is Ok. Here the complete measurements:

Aleph30 Test Results

Supply Rails: +/-23.5V

Left Channel Right Channel
Diff Pair FETs Diff Pair FETs
3.751 3.745V 3.644 3.636V
offset 71mv offset 132mv

Current FETs Currents FETs
FET Vres Vgs FET Vres Vgs
Q6 .254 3.832 Q6 .245 3.735
Q7 .254 3.834 Q7 .248 3.727
Q8 .231 3.856 Q8 .241 3.729

Active FETs Active FETs
FET Vres Vgs FET Vres Vgs
Q9 .238 3.860 Q9 .220 3.947
Q10 .251 3.847 Q10 .246 3.920
Q11 .259 3.840 Q10 .242 3.923

Vres is the voltage accros the 0.47ohms on each FET.

Are the FET correctly MATCHED?

Thanks in advance.
  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
Aleph (Single Ended) + Larvadin (no Memory Distortion) = Ultimate Aleph? tiefbassuebertr Pass Labs 19 25th January 2014 02:15 AM
First version Aleph 0 (Null, ZERO, O, Os, 0s) versus later Aleph versions tiefbassuebertr Pass Labs 13 27th April 2012 02:13 AM
Mini Aleph Board & Aleph 2 Parts List gpsmithii Pass Labs 3 16th January 2010 03:58 AM
First version Aleph 0 (Null, ZERO, O, Os, 0s) versus later Aleph versions tiefbassuebertr Pass Labs 0 6th August 2009 11:11 AM
PCBs for Aleph 2, Aleph 3, Aleph 4 and Aleph 5 amplifiers kristijan-k Swap Meet 64 12th September 2002 07:55 AM


New To Site? Need Help?

All times are GMT. The time now is 04:56 PM.


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