Another "ancient" amp - 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 November 2012, 04:07 PM   #1
diyAudio Member
 
Join Date: Oct 2012
Default Another "ancient" amp

Welcome.
My first thread here.
About me: live in Poland, love vintage audio.

A power amplifier based on polish projects from 70ties. Simple, reliable, sounds very good, chip parts. In my project I used really "rubbish" elements, mostly from early 80ties but it works fine.
One can say it is nothing specjal, but this amp produce characteristic sound, something like old german Grundig amps.
It's RMS power is just about 25W sin for +-25V Ucc. THD about 0.1%.
Attached Images
File Type: jpg ancient schematic.jpg (233.3 KB, 347 views)
Attached Files
File Type: pdf ancient power amp.pdf (74.6 KB, 57 views)
  Reply With Quote
Old 18th November 2012, 04:43 PM   #2
lineup is offline lineup  Sweden
diyAudio Member
 
lineup's Avatar
 
Join Date: Dec 2005
Location: the north
Yes, it is not a very special amplifier.
It is like many other.
But if you say it works good, I believe you
__________________
lineup
  Reply With Quote
Old 18th November 2012, 05:12 PM   #3
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
It has one feature that isn't too common. R15, the biasing resistor, is boot-strapped from the feedback node (bottom of C3). This provides high input impedance while matching the DC values of the base resistors (R14=R15=3.3K).
That minimizes offset at the output due to bias current. That could otherwise be quite high on this design given the 3 mA tail current.

It seems a bit odd to have so large a value for C8...that dramatically decreases the miller feedback capacitance goodness. Perhaps it was needed for stability, but there might have been better ways to achieve that.

Akitika GT-101

Update My Dynaco
  Reply With Quote
Old 18th November 2012, 06:29 PM   #4
diyAudio Member
 
Join Date: Oct 2012
djoffe, you are right. Using such type of biasing was not common. The reason was a dramatic lack of good semiconductors in Poland. Originally polish electronic industry used obsolete BC157.

C8 too large? Ha! My prototype was dramaticly unstable, I tried to solve the problem. The problem is lack of the rc filter in the input stage, but R5 solve it. Actually there is no reason for such large capacity, works even without it.

Of course one can simplify this circuit removing T3, T4, D1, D2. Olso works fine without R1 and R2 ( more power).

Got enough time - try it, just for curiosity.

Sorry for my english!

Last edited by magui1; 18th November 2012 at 06:50 PM.
  Reply With Quote
Old 18th November 2012, 08:37 PM   #5
djoffe is offline djoffe  United States
diyAudio Member
 
Join Date: Oct 2008
Actually, R5 may have another mechanism to stability...If the input is open circuit, then the bootstrap connection provides positive feedback almost equal to the negative feedback, which would be quite bad for stability. The presence of R5 divides down the positive feedback thru the bootstrap path a bit, adding to the stability.

I wonder...should this amp also have a little bit of RF filtering on the front end? maybe 1k series and 100 pF shunt?
  Reply With Quote
Old 18th November 2012, 08:47 PM   #6
Bigun is offline Bigun  Canada
diyAudio Member
 
Bigun's Avatar
 
Join Date: Jan 2009
Blog Entries: 2
Quote:
Originally Posted by djoffe View Post
It has one feature that isn't too common. R15, the biasing resistor, is boot-strapped from the feedback node (bottom of C3). This provides high input impedance while matching the DC values of the base resistors (R14=R15=3.3K).
The input impedance is set by the shunt resistor R5. And an LTP usually has high enough input impedance without any bootstrapping. I don't see any need for the bootstrap except if the whole scheme was to allow low impedance feedback - afterall R14 is only 3k3 and without the bootstrap it should be sized to match R5.
__________________
"The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.
  Reply With Quote
Old 19th November 2012, 06:47 AM   #7
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
Default Bias

Do you recall the bias current setting for the output stage or have a recommended value?
__________________
Ian
  Reply With Quote
Old 19th November 2012, 11:41 AM   #8
diyAudio Member
 
Join Date: Oct 2012
djoffe:
This amp works properly and stable without RF filtering if it is conected by a pot 47k to a preamp. R5 is instead a pot, just for tests. But - RF filteringis should be used anyway.

Bigun :
See my prior post. This is a really ancient amp, construction from early 70ties, when polish production transistors were really bad, especially bad to use in low noise stages or differentail pairs. Such BC157 got the gain about 100 and were so noisy.... That times Poland was trully a poor country....
Amplifier works with no problems without R5 wen conected to a preamp via volume pot.
Using good transistors there is no need tu use such a bootstrap, but it is an "ancient" amp.
One can built more modern one, I love such oldies. Once I built modernized by Pavel Macura Sinclair's Z-30. Simply a "dinosaur", but sounds so sweet.....

Ian Finch :
I don't remember; about 30mA. I 'll measure it.
  Reply With Quote
Old 19th November 2012, 05:35 PM   #9
diyAudio Member
 
Join Date: Oct 2012
Here are photos of first "ancient" built for tests and a final wersion, still during tests.

There is a current source in the test version instead of a bootstrap.
Can't hear any difference in compartison to bootstrap.

Ian Finch : bias should be set from 20 to 30mA. I set 30mA for R21/R22 value 0.5 ohm. Works fine.
Attached Images
File Type: jpg ancient prototype.jpg (82.8 KB, 101 views)
File Type: jpg ancient.jpg (75.2 KB, 88 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 "home style" I/O to a "Pro" Amp Soldermizer Class D 2 13th March 2012 04:17 AM
"TECHNO"Amp building in progress-using "Iron Method" zox2003 Solid State 23 30th March 2010 09:42 PM
sim for ancient Iskra 8" FR (HELP!!) Zen Mod Full Range 8 22nd May 2009 02:07 PM
WTB: Focal & Audiom & Volts 11", 12", 13", 15" woofer gengis Swap Meet 0 17th March 2005 03:29 AM


New To Site? Need Help?

All times are GMT. The time now is 02:49 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