balanced diff. input stage - Page 2 - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

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 18th July 2002, 05:40 AM   #11
diyAudio Member
 
Join Date: Jun 2002
Cool, thanks for all the help. It's a fun learning experience.

Hmm... I'm having trouble determining the cap values for the inverted input. For instance, to make the leach amp accept balanced inputs, one would use the grounded part of the feedback network (below C6 and C7) as the input. Not sure about the ground below C8...

It uses a strange scheme which takes feedback from different areas of the amp (driver and output), though, so I'm not sure how to calculate the correct values on the caps. Why are those caps necessary, anyway?

Do the resistors have to be changed, too? The current gain setting (R19/(R17 + R18 + R19)) should work with balanced inputs, right?

Or should I just leave all as is, and replace the ground below C6 and C7 (leave C8 alone?) with the inverted input?

I hope I'm not asking too many questions. Thanks again, guys.
Attached Images
File Type: gif leachfdbknet.gif (3.3 KB, 436 views)
  Reply With Quote
Old 18th July 2002, 10:12 PM   #12
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
Maybe we can get Joe Berry to simulate this for
a follower output stage.
  Reply With Quote
Old 18th July 2002, 11:03 PM   #13
diyAudio Member
 
Join Date: Mar 2001
Location: USA
Glad to help out if I can. Which of the circuits described in this thread (or which general approach) would you like simulated?
  Reply With Quote
Old 19th July 2002, 01:07 AM   #14
The one and only
 
Nelson Pass's Avatar
 
Join Date: Mar 2001
That one ^ right above me.
  Reply With Quote
Old 19th July 2002, 03:51 AM   #15
diyAudio Member
 
Join Date: Mar 2001
Location: USA
Okay, the simulation is now working. I had to cheat as I don't have a very good selection of power BJT models at my disposal. Instead, I've used IR power MOSFETs in place of the driver and output devices (Q16-Q21) in the Leach 4.5 schematic.

This works pretty well in simulation land, so maybe I can make a preliminary comment or two. First, the dual feedback comes into play above the audio range (about 150 kHz, I believe), and even then, C8 is isolated by the 11 k ohms of R18 so I don't think it will matter to yout input signal.

For a tip on how to wire in your negative input phase without it shorting to ground through C6/C7 and without removing this 100% DC feedback, I suggest checking out the schematic for the Aleph 30 or 60 available from www.passlabs.com. (I think you go to Products, then click the Aleph link at the bottom of that page.)

That said, and with all due respect for Prof. Leach, I'm not convinced of the need for this fancy feedback arrangement. If I leave the lag compensation caps (C10/11) in place, I can stabilize the amp easily enough by shorting out R20 and removing C8.

While my simulations can generally identify a tractable design, they don't always (or even often) nail the optimum compensation cap value. Be prepared to experiment with C9 in the working amp if you decide to simplify the NFB. It's nice to have an assortment of mica caps on hand for just this purpose, say from 10-100 pF.

I'll report back if any of this needs updating in light of further progress with the model.
  Reply With Quote
Old 21st July 2002, 11:07 PM   #16
diyAudio Member
 
Join Date: Mar 2001
Location: USA
One thing I overlooked in questioning the complexity of the v4.5 feedback network is that it would offer greater rejection of RFI pickup via the speaker leads. This point in its favor could argue against simplifying the network if RFI is a problem in your area. It's conceivable that Dr. Leach encountered it in his GA Tech lab, which I know to be close to several broadcast facilities, both on and off campus.
  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
Input xfmr, diff stage, CCS questions zigzagflux Tubes / Valves 14 13th November 2008 12:31 PM
Balanced input stage beneficial for unbalanced turntables? leadbelly Analogue Source 37 28th April 2008 02:34 PM
balanced input stage sonix Tubes / Valves 54 5th June 2004 03:47 PM
Balanced Input Stage Bukitoo Solid State 5 11th March 2004 04:03 AM
Converintg analog stage to balanced on dual diff dacs? check pic. kasra Digital Source 14 31st August 2002 11:11 PM


New To Site? Need Help?

All times are GMT. The time now is 02:12 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