High-End Tube preamp with ECC88 - Page 12 - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 1st November 2012, 04:46 PM   #111
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
There are thousands who disagree with me about alien abductions and the existence of fairies. So what? Data are everything. No data, you're just waving your hands.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 1st November 2012, 04:57 PM   #112
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
I've built several amps that would not tolerate greater than 8dB of gnfb without degrading the sound quality noticeably, with a measurable increase in high order harmonics (out to the 20th). Adjusting the GNFB while watching the spectral response allowed for accurate prediction of when the amp would sound good and when it would not. The levels of the harmonics when measured in percent were so low as to be effectively unmeasurable in that they were in the noise. The harmonics were in the -100dB range(ref 1Vrms=0dB). When measured in dB, it was a different matter.

Measure it with the right tool and use the proper units of measurement and it correlates well.

It is no wonder that %thd is a useless measurement. It is based on the wrong units of measurement, but it is easy to banter around and sounds good.
  Reply With Quote
Old 1st November 2012, 05:08 PM   #113
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Quote:
Originally Posted by Shoog
The feedback has changed the quality of the sound in some significant way which is not revealed by looking at the THD figures.
Why do people keep dragging out THD, as though they think 'we' believe in it as strongly as they ignore it? A discussion can't get very far when one side criticises views the other side has not said and does not believe.

Quote:
Needless to say, Nelson Pass strives to use the minimum amount of Global Negative Feedback possible. Put that down to fashion if you like, but I think there are thousands who would disagree with you.
Since when has truth had anything to do with gurus or democracy?

Quote:
Originally Posted by TheGimp
I've built several amps that would not tolerate greater than 8dB of gnfb without degrading the sound quality noticeably, with a measurable increase in high order harmonics (out to the 20th).
Thank you for demonstrating a well-known problem with insufficient feedback. People often seem to think that if a little feedback is bad then more must be worse. Not so.

Quote:
It is no wonder that %thd is a useless measurement. It is based on the wrong units of measurement, but it is easy to banter around and sounds good.
On here THD seems to be mainly bantered around by people who ignore it, but think others swear by it.

Last edited by DF96; 1st November 2012 at 05:12 PM. Reason: extend
  Reply With Quote
Old 1st November 2012, 05:19 PM   #114
diyAudio Member
 
Miles Prower's Avatar
 
Join Date: May 2005
Location: USA
Blog Entries: 6
Quote:
Originally Posted by artosalo View Post
This is not the case with tubes. GNFB will reduce all harmonic components almost linerly. I have once studied this, but unfortunately I could not find my files at the moment.
It should make no difference. After all, Norman Crowhurst's theoretical and practical research concerning the effect of NFB on low level, high order harmonics was done with VTs.

Quote:
I think that those people who can hear the difference with local and global NFB should make more deeper analysis about the possible reasons. We, who do not hear it are not right persons to study this.
Believe me, I would like to. One of my projects included variable gNFB, from none at all, to about 13db of gNFB. While playing with that twiddle knob, the difference was obvious. Running open loop produced the expected "edginess" or overly "aggressive" sound. Dialing in more first took the edge off, then by the time you hit the full 13db of gNFB, the sound was definitely developing that "solid state" sound. It might not have been a bad compromise for a production amp, and it wasn't bad with some forms of music (soft rock, classical) but left others (metal, techno) sounding "subdued". Reducing the gNFB brought metal and techno "back to life".

You may not "hear it" only because aural memory is very short lived, and highly unreliable. You need to be there, and listen as different levels of NFB are selected in real time.

Then there is the phenomenon of "getting used to". You can listen to some truly horrid sounding designs until you "forget" how appalling they really are. You can even come to prefer it, given enough exposure. This doesn't happen in other areas of design. A poor video amp never starts looking good no matter how long you look at a video display with all the wrong colors, smeared large objects, missing low level detail, broken vertical lines, and all the other artifacts of phase shift and high frequency attenuation.

On one forum for music of the 1960s, one contributor said he didn't want to listen to the good remasters of old songs on decent equipment precisely because he didn't want his memories of listening to these songs on a pocket transistor AM radio (and you know what sonic abominations those things were) messed with. Yeah, I can understand how being able to actually understand the lyrics of Walk Away Rene (Left Banke -- a song whose lyrics are notoriously difficult to hear clearly unless played through premium sound systems. Even the 4 Tops got the lyrics wrong on their cover version) just might do that to youthful memories.

This is what audio grifters mean by "burn in" after they con you into smearing their "magical" goop on the cones of your expensive speeks. (And, yes, this was a real product -- C-37 I believe it was called -- and a real claim. At least the "magic stones" just sat on top of your speeks, not destroy them.) Or why they tell you their $1000/foot "magic cables", or their lousy beeswax capacitors rolled by 20 y/o virgins on their bare thighs in a Tibetan Buddhist monastery by the light of the full moon on Beltane eve cost $Hundreds whereas coupling capacitors from Mouser cost pennies, need "burn in" time. Again, time to convince yourself you wasn't had, and that it made a real difference when it didn't really make any difference at all, or made the sound worse, not better. Or why some audiophoolz spend unnecessary BUX on silver or silver plated copper wire. It makes a big difference at 400MHz, but none at all at 400Hz. I save the silver wire for the UHF projects, not waste it on low frequency projects.

If you see a claim that a component needs "burn in", ask yourself: why didn't they burn it in at the factory before it went out the door?
__________________
There are no foxes in atheistholes
www.dolphin-hsl.com
  Reply With Quote
Old 1st November 2012, 05:25 PM   #115
TheGimp is offline TheGimp  United States
diyAudio Member
 
TheGimp's Avatar
 
Join Date: Aug 2009
Location: Johnson City, TN
The sound quality improved from no GNFB to around -6 to -8dB GNFB. From there sound quality continued to degrade until I ran out of available gain to continue increasing the feedback.

So two conclusions can be drawn (1) increasing feedback beyond -8dB degrades sound quality, or (2) I had insufficient gain available to allow me to increase feedback to the point where it again improved sound quality.

IIRC, I could only go to about -15dB GNFB before output dropped to the point where listening tests were useless due to the decrease in volume.
  Reply With Quote
Old 1st November 2012, 05:29 PM   #116
diyAudio Member
 
Wavebourn's Avatar
 
Join Date: Sep 2006
Location: Pleasant Hill, CA
Send a message via Skype™ to Wavebourn
Data is available, Stuart. Take for example typical Williamson, 3-stage capacitive - coupled, with global feedback by voltage in series with input signal.
When OL gain goes down due to non-linearities and losses in output transformer and output tubes the driver pushes harder. It causes grid currents on peaks that charge coupling capacitors shifting bias point.
Also it causes increase of voltage swing between grid and cathode of the 1'st stage (highlighted very well in Jan Didden's presentation on the BAF!), increasing distortions of the stage that corrects errors.

I.e. global feedback causes distortions modulated by envelope. Edit: sorry, more strict would be to say that it changes this modulation, and can make it more pronounced.

In order to maintain stability we need to add compensation that means feedback on higher frequencies is lower, and this lack of feedback on highs even highlights this "dynamic modulation of distortions".

The problem is, when we measure data using steady signals we measure what we hear less rather than what we hear more but that we don't measure: dynamic change of distortions modified by feedback itself. When you hear change in spectrum the faster it happens the more it is audible. It is the main contradiction between hearing and measurements. But the data is available. You can push your amp harder than in real life, in frequency, speed of changes, levels, then extrapolate and predict distortions in the real life. And you can analyze processes that happen in each stage when they are surrounded by common feedback loop. Just pay more attention on dynamics that is more audible than static measurements reveal, and we all will be happy.

Global feedback is not bad. What is bad, it's understanding.
__________________
"Our youth [...] have bad manners, contempt for authority; they show disrespect for their elders, contradict their parents, [...] and tyrannize their teachers. -- Plato, 447-367 BCE

Last edited by Wavebourn; 1st November 2012 at 05:32 PM.
  Reply With Quote
Old 1st November 2012, 05:37 PM   #117
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by Wavebourn View Post
Data is available, Stuart.
We're talking about preamps here. Overload in power amps driven to clipping is a different story, thus the rather elaborate measures I used in my power amp to minimize recovery time, or your soft clipping circuits.

Quote:
Global feedback is not bad. What is bad, it's understanding.
Precisely. As with any other aspect of circuit design, if you don't understand it, you're likely to get a bad result. Some attribute it to the design element rather than their own inability to understand and use the tool properly.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  Reply With Quote
Old 1st November 2012, 05:47 PM   #118
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Quote:
Originally Posted by DF96 View Post
Since when has truth had anything to do with gurus or democracy?
At this point you lose all credibility. The F5 amp is probably the best amp I have ever heard - which ultimately is what matters here.

He is not a Guru from selling snake oil, he sells solidly engineered products based on sound understanding of his chosen components. I would take his word over almost anyone on these boards.

Shoog
  Reply With Quote
Old 1st November 2012, 05:52 PM   #119
diyAudio Member
 
Shoog's Avatar
 
Join Date: Aug 2002
Location: Eire
Quote:
Originally Posted by SY View Post
Precisely. As with any other aspect of circuit design, if you don't understand it, you're likely to get a bad result. Some attribute it to the design element rather than their own inability to understand and use the tool properly.
The practical experience of people like The Gimp and Miles Prower give the lie to this statement.

Shoog
  Reply With Quote
Old 1st November 2012, 05:56 PM   #120
SY is offline SY  United States
diyAudio Moderator
 
SY's Avatar
 
Join Date: Oct 2002
Location: Chicagoland
Blog Entries: 1
Quote:
Originally Posted by Shoog View Post
At this point you lose all credibility.
Quote:
Originally Posted by Shoog View Post
The practical experience of people like The Gimp and Miles Prower give the lie to this statement.
I suppose this is easier than providing actual, you know, data.
__________________
You might be screaming "No, no, no" and all they hear is "Who wants cake?" Let me tell you something: They all do. They all want cake.- Wilford Brimley
  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
high end sound laptop to tube amp (no preamp) pforeman PC Based 11 31st December 2011 12:22 PM
High-end preamp for my GC carlosfm Chip Amps 477 2nd May 2006 03:05 PM


New To Site? Need Help?

All times are GMT. The time now is 06:31 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