Engineer, help please - Neg. feedback/infilters - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State
Home Forums Rules Articles diyAudio Store Gallery Wiki Blogs Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

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 7th July 2011, 11:32 PM   #1
diyAudio Member
 
Join Date: Nov 2010
Location: Devonport, Tasmania
Default Engineer, help please - Neg. feedback/infilters

Hello again from down south - Tasmania.

I have a very complicated problem concerning negative feedback for a circuit that I have borrowed from one of the British Universities. I don't have a lot of money to spend on expensive Zetex transistors or special transformers but have two 200va 80V centretapped toroids and a heap of salvaged trannies as shown in the diagram.

I am not a lazy man, - I have tried to understand pol and db and all the rest - but it is wearing me down and without help I will continue to struggle, or, even worse, GIVE UP.

I don't actually need an explanation of how to calculate these values because It has been explained to me before. I just dont have the time to research such a complicated issue, nor the capacity (at this stage) to understand it.

Can someone calculate the values for me?

Values for the components that are highlighted in blue. (think its blue, I'm colour-blind too!)

I am hoping that someone out there with a bit of practical experience (tradesman or engineer perhaps?) might be able to do it 'easier' than I can't do. If that makes sense.
Attached Images
File Type: gif Phil's-new-Amp.gif (34.6 KB, 459 views)

Last edited by farmerjack61; 7th July 2011 at 11:45 PM.
  Reply With Quote
Old 7th July 2011, 11:49 PM   #2
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Looks mostly ok to me.
You will need a little more gain so change 19k to 47k.
__________________
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 8th July 2011, 01:30 PM   #3
diyAudio Member
 
Join Date: Nov 2010
Location: Devonport, Tasmania
Default neg fb - gain control

Thanks Nigel. That circuit was developed by The university of Sheffield back in 2003. I chose it because it was simple and well explained in the accompanying circuit description. Now I know what a differential amplifier is and where the voltage amplification stage is. My next task is to look up some information (www) on input filter design, I have built a few speaker x'overs, but I use coils & caps, not resistors & caps.
Thanks again. I was on the verge of putting 4 dip switches or a rotary sw. and four resistors in that gap! Is it worth trying 39, 43 & 47 in this manner? A nagging voice is telling me I'm over thinking this problem. Would the circuit below be unnecessary clutter? Not to mention the risk of an accidental open feedback loop - all contacts open.

The following circuit might work;

P.S. I will look up 'PCBCAD' on google and see if I can get a screen shot of the application - it's priced right for a peasant like me!!! The one I use is very easy to use but limited in it's function.
Attached Images
File Type: gif F.B.-Gain-control.gif (26.4 KB, 409 views)

Last edited by farmerjack61; 8th July 2011 at 01:42 PM.
  Reply With Quote
Old 8th July 2011, 01:39 PM   #4
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: The great city of Turnhout, BE
Blog Entries: 7
I agree with Nigel, it looks pretty good.
Though, I believe 47k is a bit high. The gain of the amp is the ratio of that R to the 1k, so if you use 47k, you get 47 x gain.
Most power amps have around 30 x gain, and a 27k or 33k resistor is better IMHO.
With the 47k you might have to turn your volume control all the way down and that doesn't have a comfortable 'feel'.

My two eurocents worth.

jan didden
__________________
Whether we like to think of it this way or not, an audio engineer shares the professional goal of a magician - Richard Heyser
Linear Audio Vol 11 is out!
  Reply With Quote
Old 8th July 2011, 02:54 PM   #5
diyAudio Member
 
Join Date: Nov 2010
Location: Devonport, Tasmania
Hi there Jan, nice to meet you. It's a very good suggestion. I know very little about this kind of thing, but a little voice told me to put a mini 1P3T-on-on-on switch with 39, 43 & 47 but with Nigel's permission I could make it 27, 30 & 33 then I can try each setting and see how it goes. It really won't take up too much room on the board & I haven't even drawn the pcb yet.

I too like to be able to turn the volume pot up to at-least the 10 o'clock position.

You can take a look at the .gif above if you like, it's pretty boring really but it shows how I would wire the resistors with the pcb switch.

The following may be unnecessary and Idle chit-chat Jan, but I am very excited about getting some oscillation happening.

I scored a giant db meter from a fellow on ebay (200mm Dia.) it is very 'off-road' or 'robust' with a lovely bakelite housing and 150mm pointer. I built a little integrater/amp for the movement and the pointer bounces around really nicely and seems to correspond to the musical peaks or voltage units (switchable) but for me it's just an aesthetic that helps to show how silly I really am! I am going to backlight the dial with a warm LED glow from a circular pcb mounted behind the dial and surrounded with some smd emitters. The whole project will be enclosed in a whoppin great aluminium box - I want lots of room inside there in case this particular design gets warm & I need room for the two transformers 200VA each side. I will put my salvaged yamaha HTR-5840 aluminium heatsink inside the box. it is a really big slab of aluminium with beautifully riveted thin alloy fins. So now you know where I got my output transistors from too!

Once again I would like to thank every one involved for solving my quandry.

Most sincerely, Phil Elliott
  Reply With Quote
Old 8th July 2011, 03:29 PM   #6
diyAudio Member
 
Join Date: Apr 2011
Location: Pretoria
Farmer Jack, I note that you use the 2sc4468 and 2sa1695. Do you happen to have SPICE models for them?

Google draws a blank on this. About the best result is a very old thread on this very forum. It only contains one post asking the same question:

2SC4468 - 2SA1695 Spice models
  Reply With Quote
Old 8th July 2011, 08:19 PM   #7
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
The input filter breakpoint is 1/2 pi r c which is around 50,000Hz.
__________________
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 8th July 2011, 08:32 PM   #8
Banned
 
Join Date: Jan 2005
Input resistor between B and GND and feedback resistor needs to be the same value, otherwise the amp will have big dc offset.
  Reply With Quote
Old 8th July 2011, 08:36 PM   #9
diyAudio Member
 
nigelwright7557's Avatar
 
Join Date: Apr 2008
Location: Carlisle, England
Quote:
Originally Posted by Tekko View Post
Input resistor between B and GND and feedback resistor needs to be the same value, otherwise the amp will have big dc offset.
I should have spotted that but I use a resistor in each leg of the LTP.
i.e. one 560r and the other a 1k pot to balance out the LTP and so fix the dc offset.
__________________
Murton-Pike Systems PCBCAD51 pcb design software. http://www.murtonpikesystems.co.uk
  Reply With Quote
Old 9th July 2011, 01:31 PM   #10
diyAudio Member
 
Join Date: Apr 2011
Location: Pretoria
Default LTspice simulation

I had a go at this circuit with LTspice IV. Spice models for the 2SC4468 and 2SA1695 are not available. The BC612 is another unknown - I can't even google up a datasheet for it. I therefore made a few substitutions. For the BC212 I used the 2SA970 simply because they are already used. For the output I took the tried and trusted 2SA1302 and 2SC3281.

To get the output to swing anywhere near the substantial 55V rails on only 200mV of input, the feedback resistor had to be increased by an order of magnitude. The DC offset is significant at -540mV.

The use of diodes for biasing might have suited the original output devices, but for my substitutes there is not enough control to keep them out of class AB (with all the problems that arises from that). The THD at 1kHz is 0.27% - not bad but not inspiring either.

I will substitute a Vbe multiplier for the diodes and report back. It might also be worth the effort to try a few other complementary pairs.
Attached Images
File Type: jpeg PhilNew03.jpeg (136.0 KB, 338 views)
File Type: jpg PhilNew02.jpg (141.5 KB, 326 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
Neg-Feedback into Grid of gain stage instead of cathode? Alastair E Tubes / Valves 3 25th January 2011 10:19 AM
6550a question for engineer? KJ2005 Tubes / Valves 59 24th November 2009 03:33 PM
help me reverse engineer this sub epalla Subwoofers 21 21st July 2006 08:44 PM
Capacitans-multiplier with neg out imp? Circlomanen Solid State 0 8th April 2005 10:18 PM
re engineer Bookshelve - is this possible dbargna Multi-Way 5 7th November 2004 03:03 PM


New To Site? Need Help?

All times are GMT. The time now is 06:15 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2016 DragonByte Technologies Ltd.
Copyright 1999-2016 diyAudio

Content Relevant URLs by vBSEO 3.3.2
Wiki