ZenV4 with differential pair input - 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 20th February 2003, 12:48 PM   #1
miguel2 is offline miguel2  Portugal
diyAudio Member
 
Join Date: Jun 2002
Location: Portugal
Default ZenV4 with differential pair input

Hi,

In the middle of the process of making my ZenV4 I managed to blow a ZVP3310. I tried to buy one locally but they are not available. So now its running with a ZVP3310 in one channel and an IRF9610 in the other. Surprisingly or not, sound is good.

But I want to get this fixed. So I though of using a differential pair in the input, like an Aleph, with IRF9610s. I am a bit unsure of how to implement it. I simulated the following circuit but it clips at very low levels on the input, so something is wrong. I dont know also if the feedback is in the correct phase.

Please give me a hand here. What do you think about the sound will it improve or not comparing with the original ZenV.4?

Thanks

Miguel
Attached Images
File Type: jpg zendp.jpg (40.9 KB, 642 views)
  Reply With Quote
Old 20th February 2003, 03:42 PM   #2
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
In other words, an Aleph. At www.passlabs.com you can find the service manual to an Aleph 3, and noting the similarities and differences between it and your schematic should prove educational. Well done, BTW.

  Reply With Quote
Old 20th February 2003, 03:48 PM   #3
uli is offline uli  Austria
diyAudio Member
 
Join Date: Dec 2002
Location: Vienna, Austria
Default clipping

Hi,
the drain of m5 is about +4 volt. this is the value of the source too. this fet is nearly shortcut.
Uli
__________________

'Jazz is not dead, it just smells funny' F.Zappa
  Reply With Quote
Old 20th February 2003, 04:12 PM   #4
miguel2 is offline miguel2  Portugal
diyAudio Member
 
Join Date: Jun 2002
Location: Portugal
Hi,

Thanks for the answers. Nelson, I really took a look at the Aleph circuit first and saw some similarities. But I want to keep the Zen output stage and the PSU, so I am looking only at the input stage.
I have to say that I still don't catch exactly how the differential pair works (I am reading now the A75 article to get it -excellent to learn).
I did some more simulation and replaced the zen feedback circuit with the Aleph feedback network and got a workable circuit that is in the figure. Problem is that R64 has a maximum value of 250 ohms before clipping with 0.75 Vin. Is this related with that short you mention Uli?
Attached Images
File Type: jpg zendp.jpg (47.2 KB, 573 views)
  Reply With Quote
Old 20th February 2003, 04:25 PM   #5
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Well then, you want to reference your input stage to a +DC
value above ground. This means that the + input has to be cap coupled, and the Gate of the + input will have a resistor to this v+ value instead of ground.

V+ should be about 10 volts or so, not a critical thing, and can be made from a resistor added to the resistor which biases your current source Zener, and then to ground, cap coupling the junction to ground to clean it up and taking your reference off that.
  Reply With Quote
Old 20th February 2003, 04:25 PM   #6
uli is offline uli  Austria
diyAudio Member
 
Join Date: Dec 2002
Location: Vienna, Austria
problem is that the 9610 or similar has about -3.5 - 4 v between
gate and source when conducting. this means if you have a
grounded input (ref to gnd by input res) your source voltage is
about +3.5 to 4 v above ground. this is the amount you need to drive the output thus you have no headroom left between source and drain. as the drain is 180 phase you vcannot drive the fet.
example:
source to drain say 20mV dc. gate voltage 200mV ac gain = 10.
if the gate goes up (+200mV) the source goes up at +200mV rel.
ok this means drain goes down 2V. ok.
B U T :
gate down 200mV, source too, drain up 2V - not possible theres
no place (picturesque) to go up! -> distortion!

Uli
__________________

'Jazz is not dead, it just smells funny' F.Zappa
  Reply With Quote
Old 20th February 2003, 04:28 PM   #7
uli is offline uli  Austria
diyAudio Member
 
Join Date: Dec 2002
Location: Vienna, Austria
Talking aleph

having taken everything into account, best solution:
Build an Aleph!

U.
__________________

'Jazz is not dead, it just smells funny' F.Zappa
  Reply With Quote
Old 20th February 2003, 05:02 PM   #8
miguel2 is offline miguel2  Portugal
diyAudio Member
 
Join Date: Jun 2002
Location: Portugal
Ok, I think I got the idea. Just don't like to put another cap in the signal path .

So here is the schematic I turned up to. But this does not work. Where is the error?
Attached Images
File Type: jpg zendp.jpg (48.3 KB, 541 views)
  Reply With Quote
Old 20th February 2003, 09:08 PM   #9
uli is offline uli  Austria
diyAudio Member
 
Join Date: Dec 2002
Location: Vienna, Austria
Default feedbackloop

the feedback loop is wrong!
its necessary to reference both ends of the fb to the same dc
level as the input gate. In your circuit you reference it to gnd and
the input gate to some dc value, thats the fault.
Try putting the fb from BEFORE the output caps and make
the dc bias for the input trimmable.

Uli / -who hates caps in the signal path
__________________

'Jazz is not dead, it just smells funny' F.Zappa
  Reply With Quote
Old 21st February 2003, 08:52 AM   #10
miguel2 is offline miguel2  Portugal
diyAudio Member
 
Join Date: Jun 2002
Location: Portugal
Thanks Uli, I think I got the idea. I took the feedback loop before the output caps, put a 10 uF cap in series with it and put the feedback resistors. Then I connected a 47.5K resistor from the the reference voltage to the minus input of the differential pair. Now everything is balanced and giving good smulation results. But there is one more cap there.

But then I realized that we are talking about 23 mA of current for the differential pair. So a small transformer with a nice regulation can do the trick. I can make a different PSU for the input pair (if I find a transformer for the right price: < 0 euro), with plus and minus voltages, and then feed the Zen. This circuit gives fine simulation results. But there is still the cap on the input of the IRF044 that does the amplification - M3. I saw that the voltage between the drain of M5 and R64 is -20. At the gate of M3 we have 4.6V. So it still needs that cap .
Attached Images
File Type: jpg z.jpg (45.6 KB, 462 views)
  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
Adding a third input to a differential input stage? maudio Solid State 15 11th October 2006 02:16 AM
Very basic -- and maybe silly -- differential-pair question Onvinyl Solid State 9 1st February 2006 05:46 PM
Is 12B4/A good for differential pair? leadbelly Tubes / Valves 3 19th January 2006 08:09 PM
Dual FETs for differential pair jwb Pass Labs 4 29th May 2003 07:39 PM


New To Site? Need Help?

All times are GMT. The time now is 03:25 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