Low-distortion Audio-range Oscillator - Page 76 - diyAudio
Go Back   Home > Forums > Design & Build > Equipment & Tools

Equipment & Tools From test equipment to hand tools

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 20th October 2012, 12:22 AM   #751
richiem is offline richiem  United States
diyAudio Member
 
richiem's Avatar
 
Join Date: Sep 2010
Location: Grapeview, WA
@Samuel Groner -- I think that the lamp R was too small in any case to see the kind of issue that happened with the old HP200CD et al, which had lamps with *much* higher operating point resistances. Even so, I'm now not sure that any voltage coefficient effects could possibly be large enough, even at 10 or 20VRMS output to actually make a difference in stability. But it's an open question for now.
__________________
...................
Dick Moore
  Reply With Quote
Old 20th October 2012, 08:43 AM   #752
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
I've built several lamp-based "thermal" compressors for musical instrument use, there I noticed the mentioned slope of distortion, too. In fact I was exactly after that as a welcomed side-effect, the smaller the lamp's thermal inertia the higher the distortion (I used the smallest lamps comercially availabe, 1.2V/10mA, as well as big ones with several watts). I biased the lamps with DC right at their resistance knee, just below a visible red glow, this gave raise to strong second harmonic at very low frequencies and low levels which is what I wanted.

I also noted that a hot lamp must be well shielded from vibration (notably with DC bias it literally is a microphone) and I've seen this done in commercial lamp-stabilized oscillators by Grundig.

Last edited by KSTR; 20th October 2012 at 08:46 AM.
  Reply With Quote
Old 20th October 2012, 06:45 PM   #753
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by KSTR View Post
I've built several lamp-based "thermal" compressors for musical instrument use, there I noticed the mentioned slope of distortion, too. In fact I was exactly after that as a welcomed side-effect, the smaller the lamp's thermal inertia the higher the distortion (I used the smallest lamps comercially availabe, 1.2V/10mA, as well as big ones with several watts). I biased the lamps with DC right at their resistance knee, just below a visible red glow, this gave raise to strong second harmonic at very low frequencies and low levels which is what I wanted.

I also noted that a hot lamp must be well shielded from vibration (notably with DC bias it literally is a microphone) and I've seen this done in commercial lamp-stabilized oscillators by Grundig.
Hi KSTR,

Was this an optical coupling of the lamp or is the lamp in the amplifier circuit.
How did you isolated the DC bias from the amplifier? I would expect a strong dc offset in the amplifier from the bias.
__________________
David.
  Reply With Quote
Old 20th October 2012, 07:29 PM   #754
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
The lamp was the actual passive "gain-cell", lamp as series resistor, another R to GND to form a divider. The bias current was injected with a floating current source accross the lamp, and the whole thing was AC-coupled at in and out driven by small pwr stages, also there were two of these cells in series to handle the dynamic range.

I'm aware this introduced additional effects from the biasing alone working back on the capacitors. An alternative would have been to use HF bias but that seemed unneccesary for the application (bass guitar amplifier) because a little "thumping" was not a problem there.

Using no bias didn't sound as good because the time constants and dynamic response was not optimum.
EDIT: and no 2nd harmonics, of course.

Last edited by KSTR; 20th October 2012 at 07:35 PM.
  Reply With Quote
Old 20th October 2012, 07:39 PM   #755
KSTR is offline KSTR  Germany
diyAudio Member
 
KSTR's Avatar
 
Join Date: Jul 2007
Location: Central Berlin, Germany
Quote:
Originally Posted by coluke View Post
Yeah - I was wondering about Victor's phase noise and THD+N. A full spectrum comparison betweem AP's and Victor's would be interesting.
I might do this measurement. With full spectrum you mean DC to xxkHz, linear view? With or without fundamental notch?

I might also scope the gate drive to see how much it fluctuates.
  Reply With Quote
Old 20th October 2012, 08:27 PM   #756
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by KSTR View Post
The lamp was the actual passive "gain-cell", lamp as series resistor, another R to GND to form a divider. The bias current was injected with a floating current source accross the lamp, and the whole thing was AC-coupled at in and out driven by small pwr stages, also there were two of these cells in series to handle the dynamic range.

I'm aware this introduced additional effects from the biasing alone working back on the capacitors. An alternative would have been to use HF bias but that seemed unneccesary for the application (bass guitar amplifier) because a little "thumping" was not a problem there.

Using no bias didn't sound as good because the time constants and dynamic response was not optimum.
EDIT: and no 2nd harmonics, of course.
So the idea was to add some 2nd H to the base. I presume this fattened up the base sound.
__________________
David.
  Reply With Quote
Old 21st October 2012, 12:04 PM   #757
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Quote:
Is the AP oscillator stabilized (disciplined)? It shows less close in phase noise than Victors.
No. The sidebands are also affected from the amplitude leveling loop and the plain noise floor of the filter and multiplier (close to the fundamental frequency, noise gain get's very high).

Quote:
Our AP is about two years old, is that late enough?
No idea, I don't have any details.

Quote:
I might do this measurement. With full spectrum you mean DC to xxkHz, linear view? With or without fundamental notch?
To evaluate sideband performance I'm using these settings:

* Direct FFT (without notch--it would cut most of the sideband).
* Linear frequency scale, range about +-10% (or even just +-5%) of the fundamental (e.g. 900-1100 Hz).
* Amplitude range set such that most of the fundamental is chopped off, and the sidebands good visible.
* A FFT window with very good side lobe behaviour is important; IIRC the "equiripple" design is the best for the SYS-2722.
* Use the highest possible FFT resolution (32k) and lowest possible sampling rate to get the most narrow main lobe.
* Experiment with different average numbers--I've seen that for some oscillators the sideband noise drops with higher averages. I'm not sure what the exact mechanism is, but there is surely more going on than just plain "phase noise" as we know it from HF oscillators (e.g. there's also amplitude noise).

Samuel
  Reply With Quote
Old 21st October 2012, 03:13 PM   #758
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by Samuel Groner View Post
I'm not sure what the exact mechanism is, but there is surely more going on than just plain "phase noise" as we know it from HF oscillators (e.g. there's also amplitude noise).

Samuel
Another observation on my impractical thermally stabilized oscillator experiment is that when it was stable the side bands virtually dissapeared.
__________________
"This logos holds always but humans always prove unable to understand it, both before hearing it and when they have first heard it."
  Reply With Quote
Old 21st October 2012, 04:41 PM   #759
diyAudio Member
 
Join Date: Jun 2005
Location: Zürich
Yes, I've observed too that low phase margin in the leveling loop leads to pronounced sidebands (also funny stuff like peaking a few Hz from the fundamental). On the other hand, I've found that loop compensation for optimum settling time is, most unfortunately, not the one for best sideband performance.

Samuel
  Reply With Quote
Old 21st October 2012, 05:41 PM   #760
coluke is offline coluke  Italy
diyAudio Member
 
Join Date: Mar 2009
Quote:
Originally Posted by Samuel Groner View Post
Yes, I've observed too that low phase margin in the leveling loop leads to pronounced sidebands (also funny stuff like peaking a few Hz from the fundamental).
A kind of very low level (and very low freq) amplitude modulation? I've seen it too, and spent quite a few hours before figuring out what the hell was going on

L.
  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
Radford Low Distortion Oscillator Series 2 audiomik Equipment & Tools 21 19th February 2014 10:46 AM
ultra-low distortion audio oscillator geekysuavo Analog Line Level 16 26th March 2013 03:04 PM
Low distortion oscillator? rjm Equipment & Tools 30 4th May 2011 10:45 PM
Can we improve this low distortion sine oscillator ? gaetan8888 Solid State 22 29th March 2009 12:30 PM
Simple, low distortion 1kHz oscillator jackinnj Solid State 4 6th October 2003 03:58 AM


New To Site? Need Help?

All times are GMT. The time now is 11:17 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