A75 front-end collapsed - 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 30th August 2005, 02:22 AM   #11
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
Maybe you can buy/borrow a voltmeter - It is possible that there is that much offset, but if everything else seems to be right try measuring witha different tool.

Also, check for DC on the input - your signal generator may have some DC that is getting amplified.

Try measuring offset with the + input shorted to ground.

Is the inverting (-) input shorted to ground if you are using a single ended source?

Do you have R30 in place? What value is it? check for cold solder joints here.
  Reply With Quote
Old 30th August 2005, 07:22 PM   #12
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
In case you were not aware -

In the PC artwork of the original A75 boards
in Audio Amateur, there are three resistors
mis-labeled. The schematic is correct.

R22 on the PCB is R24 on the schematic


R26 on the PCB is R81 on the schematic


R24 on the PCB is R26 on the schematic

In addition to this, it is worth noting that
a modification was made in improve the reliability
of the front end when the amp is overdriven. Two
Zener diodes are added as seen in the diagram:
Attached Images
File Type: gif a75mod.gif (5.4 KB, 331 views)
  Reply With Quote
Old 1st September 2005, 06:19 AM   #13
diyAudio Member
 
Join Date: May 2005
Location: HK
Thanks Nelson.

I just only follow the document from DIYPass provided. Not sure there will have same problem to Audio Amateur.
Indeed, I found the mistake label on PCB, but you must aware it when stuffing components. Anyway I will check it again with respect to your reminding.

I think my circuit still have some problem. Perhaps due to above wrong things was be ignored. An unusual matter report, I touch the FETs feel their temperature very high. ( about 45-50 degree). Especially which happen in Q3,Q6,Q9,Q10 the folded cascode part.

Add two zeners into the circuit is wroth to do.

Wait for my acknowledgement later , as hurry as I test it again on this week end.

CK
  Reply With Quote
Old 27th September 2005, 06:09 AM   #14
diyAudio Member
 
Join Date: May 2005
Location: HK
Hello there ,

Especially for BobEllis & Neslon :>

My A75 found an important mistake but actually I can't ensure this is a crucial to affect the output signal... (Now the output gain is satisfy but still the waveform is not much pretty and with phantom.)

I am inattention use the IRF 9610 instead of IRF 9510. By datasheet indicate that IRF 9610's RDS on is 3 ohm but 9510 is 1.2 ohm. Is there the reason that I never get satified output ?

CK
  Reply With Quote
Old 27th September 2005, 11:17 AM   #15
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
CK - I doubt that using IRF9610s is the cause of your dissatisfaction. RDS is pretty irrelevant for this application. More important is Ciss, which is similar for both devices. The 9610 is a little lower, which should extend the top end a hair.

It sounds like you may still have oscillation issues. Have you increased R21 and R22 to >75 ohms?

Are you testing a completed amp or just the front end? remember that the front end cannot drive the zoebel by itself. If the output stage is underbiased, you will get lots of crossover distortion. be sure you have at least 100mA per device. Wait for things to warm up a bit - my A75s take a couple minutes for the bias to come up.
  Reply With Quote
Old 27th September 2005, 12:22 PM   #16
diyAudio Member
 
Alain Dupont's Avatar
 
Join Date: Sep 2003
Location: Paris - France
ckwong99,


>> I am inattention use the IRF 9610 instead of IRF 9510. By datasheet indicate
>> that IRF 9610's RDS on is 3 ohm but 9510 is 1.2 ohm. Is there the reason
>> that I never get satified output ?

NO, mine is assembled with IRF 9610 and is working perfectly ; follow the hint's of Nelson and after add 4 * 100 microF on the Pcb, one for each +- powersupply connector.

Good luck.

Alain.
  Reply With Quote
Old 27th September 2005, 02:13 PM   #17
diyAudio Member
 
Join Date: May 2005
Location: HK
Hello Bob & Alain,

I only test the single channel of front-end and without other connection from its output. R21 & R22 now is using 100 ohm. Should these value need to increase a bit more ?

From your experiences 9610 not be a main harm, so only left the event is regarding the circuit stability. My temporary DC supply had already installed 100uF + .1uF capacitors on each + / - rails.

My PCB C9 and C10 are empty. Does it require to put it back ?

Thanks all of you.
CK
  Reply With Quote
Old 27th September 2005, 02:46 PM   #18
diyAudio Member
 
Alain Dupont's Avatar
 
Join Date: Sep 2003
Location: Paris - France
ckwong99,

From A75 part1:
Two new capacitors, C9 and C10, add some lag compensation in the
event that a particular choice of components and layout are not
adequately stable. We offer no values, since we did not use them in the
amplifiers we built, nor have we encountered the problem in a range of
prototypes. If necessary, your choice of capacitors will be for whatever
prevents high frequency oscillation, and may range from 5 to 39pF. If
oscillation occurs, consider raising the values of the gate resistors on the
MOSFETs first.

I didn't use any... but you could try 10 pf to start.

>> My temporary DC supply had already installed 100uF + .1uF capacitors on each + / - rails.

--> These MUST be on the A75 front end PCB !!! just near the -+ connections to the board.
The problems comes from the wiring, between power supply and front end Pcb.

Alain.
  Reply With Quote
Old 27th September 2005, 03:47 PM   #19
diyAudio Member
 
BobEllis's Avatar
 
Join Date: Sep 2003
Location: Upstate NY
Just to emphasize the point made by Alain, if your 100 uf and 100nf are not right at the power terminals of the board, you are very likely to have oscillation. you'll want a set on each power connection, so 4 of each value are required.

I didn't have a scope when I built my first A75, so I used 10pf micas for C9 and C10 because I found someone else saying that worked without hurting the sound. After blowing up a few sets of outputs on the first, I decided to play it safe on my second A75, and used them again.

If you don't have caps handy, a couple inches of hookup wire tightly twisted together should give you ~5pf to see if that helps. NP mentioned this a couple years ago, and even provided a picture.
  Reply With Quote
Old 28th September 2005, 06:16 AM   #20
diyAudio Member
 
Join Date: May 2005
Location: HK
Hello Bob & Alain,

Thanks for your comments.

For the sake of my A75 and speaker, I decide insert it .

A common answer from you too upon the PSU filtering caps, I was not place it right. There I would be possible correct it.

CK
  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
unregulated A75 front end MEGA_amp Pass Labs 1 23rd December 2006 05:31 AM
Suspension collapsed davidsrsb Analogue Source 4 26th January 2006 07:06 PM
Front end for sub amp mashaffer Solid State 9 22nd July 2005 03:35 AM
Dipole as Front L/R In HT ? wingman Multi-Way 10 30th July 2004 07:42 PM
searching software to switch front speakers to rear and rear to front!-dolby surround MCM Everything Else 6 6th February 2004 09:29 PM


New To Site? Need Help?

All times are GMT. The time now is 01:58 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