Go Back   Home > Forums > >
Home Forums Rules Articles diyAudio Store Blogs Gallery Wiki Register Donations FAQ Calendar Search Today's Posts Mark Forums Read

Solid State Talk all about solid state amplification.

Bob Cordell's Power amplifier book
Bob Cordell's Power amplifier book
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 3rd January 2018, 03:05 PM   #8471
Bob Cordell is offline Bob Cordell  United States
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally Posted by anatech View Post
Hi Bob,
I wasn't taking a shot at you or the practice of simulation. It was more an observation that some people take the simulation as gospel without acknowledging that a simulation is an approximation of the truth. All those decimal points it displays doesn't help the situation at all either.

For the record, I own and have read books from both you (Bob) and from Douglas. I find all of them are valuable references, even when the two of you disagree. I find that I greatly prefer J-Fet inputs over BJT's. I think that distortion of the input signal before it enters an amplifier is a real effect that is reduced when a low impedance output preamplifier is used in the combination. This is just an unmeasured impression that some of my friends also accept as "a thing". Certainly an input buffer can mitigate most of this issue. The first major consumer amplifier I know of that uses an input buffer would be the Marantz 500 and others in the line. More recently, Nakamichi resurrected the input buffer calling it "HTA". I forget what that stands for, but their listening panel found value in it enough so that they included it in receivers and gave the effect a marketing name. I'm pretty sure that there are many examples of this in the market. It does warrant further investigation, that's for sure. I'm going to bet that a J-Fet diff pair has more advantages than just lower audio rectification of RF signals. The only BJT input that comes close to this are the complementary differential pairs that John Curl popularized. Only when that design is executed properly though.

So, simulate away, but never forget that a physical circuit is required to be tested before you can draw any solid conclusions. Simulation is a valuable tool. I wish I knew how to use these programs, so I design on paper instead.

Best, Chris
BTW, speaking of input buffers, the use of emitter followers in front of the input LTP was a distinguishing feature of Tom Holman's APT-1 amplifier. It was cheap and simple, and added very little to the noise. It greatly reduced input bias current and input signal current (and thus the amplitude of any nonlinear input currents).

Cheers,
Bob
  Reply With Quote
Old 3rd January 2018, 03:22 PM   #8472
matze is offline matze  Europe
diyAudio Member
 
Join Date: Sep 2012
Quote:
Originally Posted by jan.didden View Post
It's a bit like masturbating. If you do it often enough you start to think it's the real thing ;-)
Cheers, Mr. Didden!
  Reply With Quote
Old 3rd January 2018, 05:27 PM   #8473
Mark Johnson is offline Mark Johnson  United States
diyAudio Member
 
Mark Johnson's Avatar
 
Join Date: May 2011
Location: Silicon Valley
Bob Cordell's Power amplifier book
Quote:
Originally Posted by Bob Cordell View Post
... the use of emitter followers in front of the input LTP was a distinguishing feature of Tom Holman's APT-1 amplifier. It was cheap and simple, and added very little to the noise. It greatly reduced input bias current and input signal current (and thus the amplitude of any nonlinear input currents).
Here it is. I wonder whether R6 and R13 add very little to the noise.

_
Attached Images
File Type: png apt1.png (32.6 KB, 236 views)
  Reply With Quote
Old 3rd January 2018, 05:28 PM   #8474
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by Mooly View Post
Not really If you read the thread you will see those that took part really enjoyed it.

And don't forget it was you that seemed thrilled at the idea only a few posts back You should try it.

We did a simulation of the old Texan as well, somewhere buried in this lot:

Test your ears in my new ABX test

I was joking in response to Jan's comment.
__________________
David.
  Reply With Quote
Old 3rd January 2018, 05:29 PM   #8475
jan.didden is offline jan.didden  Europe
diyAudio Member
 
jan.didden's Avatar
 
Join Date: May 2002
Location: The great city of Turnhout, BE
Quote:
Originally Posted by matze View Post
Cheers, Mr. Didden!
Prosit! Happy New Year!
__________________
Cable: a potential source of trouble interconnecting two other potential sources of trouble - Erik Margan
Linear Audio pubs and articles . The SilentSwitcher now at diyaudio store SilentSwitcher. Keeping in touch with SY.
  Reply With Quote
Old 3rd January 2018, 05:36 PM   #8476
mlloyd1 is offline mlloyd1  United States
diyAudio Member
 
mlloyd1's Avatar
 
Join Date: Feb 2001
Location: Northern Iliinois
Bob Cordell's Power amplifier book
Hi Bob,

i "think" i also read he would have preferred to use a dual JFET as the input LTP if he could have found one at the the time that met his constraints, one of which was he wanted a US part.

mlloyd1

Quote:
Originally Posted by Bob Cordell View Post
BTW, speaking of input buffers, the use of emitter followers in front of the input LTP was a distinguishing feature of Tom Holman's APT-1 amplifier. It was cheap and simple, and added very little to the noise. It greatly reduced input bias current and input signal current (and thus the amplitude of any nonlinear input currents).

Cheers,
Bob
  Reply With Quote
Old 3rd January 2018, 08:00 PM   #8477
Bob Cordell is offline Bob Cordell  United States
diyAudio Member
 
Join Date: Sep 2006
Quote:
Originally Posted by mlloyd1 View Post
Hi Bob,

i "think" i also read he would have preferred to use a dual JFET as the input LTP if he could have found one at the the time that met his constraints, one of which was he wanted a US part.

mlloyd1
Probably so. I used the National NPD5564 in my MOSFET power amp with error correction circa 1983. I don't recall how expensive it was at the time. I'm guessing it was available when Tom was doing the his design, but cost and availability might have been issues. The NPD5564 was a wonderful low-noise part, with good transconductance and available offset of less than 10mV. In terms of characteristics, I think it was much like the Toshiba '389 and the LSK389. Not sure if the Toshiba part was out before or after the 5564. These days I like the LSK489 for power amp front ends.

Cheers,
Bob
  Reply With Quote
Old 4th January 2018, 05:49 AM   #8478
PB2 is offline PB2  United States
diyAudio Member
 
PB2's Avatar
 
Join Date: Sep 2004
Location: North East
I wonder if "Distortion in low-noise amplifiers" by Eric F. Taylor, Wireless World August, 1977
might shed any light on the input current distortion question. We've discussed the article back in
2005 but I've not looked at the article since then. I remember that someone had it in .pdf format.

Found it in .pdf format, in case there is interest: http://www.keith-snook.info/wireless...amplifiers.pdf

Last edited by PB2; 4th January 2018 at 06:18 AM.
  Reply With Quote
Old 4th January 2018, 06:46 AM   #8479
RNMarsh is offline RNMarsh  United States
diyAudio Member
 
RNMarsh's Avatar
 
Join Date: May 2012
Location: 2457 Cascade Trail; Cool, CA. 95614
Bob Cordell's Power amplifier book
uh. I found neutralization on input pair to help, also. Sometimes easier and/or cheaper than cascoding ips.

Just sayin'



THx-RNMarsh
  Reply With Quote
Old 4th January 2018, 08:21 AM   #8480
keantoken is online now keantoken  United States
diyAudio Member
 
keantoken's Avatar
 
Join Date: Aug 2006
Location: Texas
We can calculate the nonlinear base current due to Vaf to see if it matches the distortion figures.

Cordell's BC550C model tells me Vaf is 162 and from experience Hfe is about 500. Let's say 20Vce, 5mA Ic, and 1V input.

Vaf is the voltage at which Hfe doubles (Ever heard that before? I hadn't, until I figured it out). So from 0 to 20Vce Hfe rises by 500Hfe/162Vaf*20Vce=+61.7Hfe at quiescent. 561.7Hfe/162Vaf*1Vppce=3.47Hfepp variation across a 1V input sine wave. 5mA/561.7-5mA/565.17=54.7nApp total error base current, which we can assume is mostly 2nd harmonic.

The Vbe modulation due to this current is canceled by the symmetry of the LTP so we can set it aside.

So let's assume 1k total unbalanced source impedance. 57nA*1k means 57uV error per 1k imbalance. 57uV error in 1V is 0.0057%. Most of this is still the fundamental though so it doesn't contribute to THD.

I've compared simulated Vaf directly with Vaf measurements and the real Vaf is nonlinear whereas the simulated Vaf is not. I would say that outside quasi-saturation, Vaf is probably better than 5% linear within a 1Vce range. This would make our final THD figure somewhere under .0057%*5%=.000285% (the data is noisy so it could be much lower).

Note that even if all of the Early effect error were harmonics, THD could not be more than 0.0057% with 1k unbalanced source resistance. For instance if you said that Early effect could cause 0.005% distortion (I don't have Self's book so I don't know what he claimed), then you are saying that the base current is 88% harmonics.

But if that were due to Early effect over a 1V change in Vce, then over 20V or so those errors would compound and this would easily be visible on Ic/Vce charts in the datasheet. In reality those lines are usually ruler-flat outside of quasi-saturation. So in a way the datasheet actually disproves that a large amount of harmonics are coming from Early effect.

So while simulation might not catch all the nonlinearities of Early effect, the discrepancy is not necessarily very significant unless you are at very very low distortion.

Note that this in no way addresses distortion coming from Cob nonlinearity.
__________________
The Kmultiplier rail filter! -=- The Simple Kuartlotron Superbuffer!
Need something built, repaired or modded? PM me and ask what I can do!

Last edited by keantoken; 4th January 2018 at 08:50 AM.
  Reply With Quote

Reply


Bob Cordell's Power amplifier bookHide 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

Forum Jump

Similar Threads
Thread Thread Starter Forum Replies Last Post
Project 11.1 from Slone "High-Power Amplifier" Book Karl71 Solid State 47 12th April 2018 02:47 AM
book-audio power amplifier by Douglas Self mikee12345 Solid State 12 6th January 2018 09:57 AM
Another realization of Bob Cordell's THD Analyzer giulianodes Equipment & Tools 136 6th June 2016 08:06 AM
best audio amplifier book?? Bouvett Everything Else 30 13th August 2012 02:43 AM
Amplifier Design Book pixie Everything Else 27 11th June 2010 08:36 PM


New To Site? Need Help?

All times are GMT. The time now is 08:59 AM.


Search Engine Optimisation provided by DragonByte SEO (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Resources saved on this page: MySQL 15.00%
vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2018 DragonByte Technologies Ltd.
Copyright ©1999-2018 diyAudio
Wiki