Low-distortion Audio-range Oscillator - Page 136 - 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 29th December 2012, 10:45 PM   #1351
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by RNMarsh View Post
This circuit will fit easily onto PCExpress size. Lamps off board, of course. Could use a quad IC as well... at least for proof of concept. Such as OPA1644. -RNM
Hi Rick,

Yes the multiplier will fit but I meant the oscillator and multiplier and that won't fit.
The AGC I will put on another board for now until it's all working and then it will all go on one board.

There is also another circuit which goes before the lamp drivers to correct for the non linear VR curve of the lamps.
I didn't include it here because it takes some explanation as to why and how it works and that is an essay of it's own.

Cheers,
__________________
David.

Last edited by davada; 29th December 2012 at 10:50 PM.
  Reply With Quote
Old 29th December 2012, 10:51 PM   #1352
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Blog Entries: 3
I'm sorry about not being clear, what I meant was to sense the dc offset at the main output where the lowest distortion signal is and remove DC offset from there correcting for offset buildup from the cascaded amplifiers.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 29th December 2012, 11:33 PM   #1353
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by 1audio View Post
I'm sorry about not being clear, what I meant was to sense the dc offset at the main output where the lowest distortion signal is and remove DC offset from there correcting for offset buildup from the cascaded amplifiers.
Hi Demian,

It's not the offset from the amplifier cascade that is a problem. If that's all it was we could just leave it there. The DC off set with no lamp drive is less than a mV and no more the 3mV with drive voltage and the servo correction. With no servo the offset is as much as 300mV. It's not to much to just leave it and block with capacitor coupling but the offset does seem to raise distortion some in the oscillator. I have not been able to measure the distortion of the multiplier at it's normal operating levels. I don't have an oscillator low enough in distortion to measure it. All I can say is the distortion of the multiplier appears to be less than the oscillator itself.

The offset is there because of mismatched lamp VR curve so something has to be done to change the DC voltage across the lamps to match the lamp currents. This can be done in two ways. By offsetting the control voltage to one of the lamps or by applying an offset to one of the multiplier nodes. By applying an offset to the non inverting node the inverting node is forced to match which changes the voltage across one or both of the lamps. Returning the servo to the non inverting node satisfies the phase requirement of the servo saving an inverting stage. We could add an inverting stage and return the servo to the inverting node but this drive the noise and distortion up and it's the same as using the inverting driver stage. The reason I say using the non inverting node is a better arrangement is because there is a bit less noise. We are not passing the servo through a second amplifier. But it does affect the attenuation at the non inverting node. There is some signal returned through the servo and this needs to be taken into account. It will effect the gain AC of the multiplier particularly at low frequency. But this did not seem to be a problem when I tested a 10Hz.

I did try lamps with heaver filaments but the SVO at least did not settle well at all frequencies. The 7220 works right across the bandwidth and within the multiplier gain range.

That's not to say other lamps won't work. It was not practical to try them all and I got tired of growing my stock with parts I'll never use.

By main output do you mean the multiplier output?

Cheers,
__________________
David.
  Reply With Quote
Old 29th December 2012, 11:50 PM   #1354
1audio is offline 1audio  United States
diyAudio Member
 
Join Date: Mar 2004
Location: SF Bay Area
Blog Entries: 3
I was thinking of removing any residual DC at the main output of the oscillator in the process. For some applications (like precision measurements) the DC at the output causes problems. The vacuum thermocouple, for example, can't tell DC from AC. If one servo could do both it would be nice.
__________________
Demian Martin
Product Design Services
  Reply With Quote
Old 29th December 2012, 11:51 PM   #1355
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by 1audio View Post
Its a great solution. Could you take the servo from the main output and cancel the offset through the system?

The time constant of the thermal device will limit the low frequency performance and harmonic generation. Something with a big filament would be better (heavier) but two full size light bulbs may be a little weird in a box. Thermocouples won't work since they generate DC from the heat instead of a resistance change. I looked at Thermistors which could be valid http://www.digikey.com/Web%20Export/...f?redirected=1 however the note indicates a voltage sensitivity which would be all wrong for this app. This may be the most obscure possible solution for stabilizing http://www.amperite.com/assets/Documents/Ballasts.pdf and they are still made. It looks like they could work pretty well. Now you can have tubes in a all solid state circuit. . .

These current ballasts look very interesting. I wonder what there distortion characteristic are like?
__________________
David.
  Reply With Quote
Old 30th December 2012, 12:10 AM   #1356
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by benb View Post
You can't really get $17 each unless what you really want is three copies of the same board 3.8" x 2.5" - the miniboard service is $51 for three boards, the "best" deal they offer for a board anywhere near that size. They've had the same services at about the same pricing for over 12 years now ... here's November 2000 website showing the miniboard service $59 postpaid (to USA only) verses current pricing of $51 + postage: How much ExpressPCB costs - PC board layout editor for Windows. They must be making money at what they're doing. With so many other services available I would have thought they would have dropped their prices.
Please point me to a cheaper service. They do add soldermask etc. for not that much more. Mind you I have no interest in buying a high end CAD program or managing drill layers etc. I tried EAGLE and it was not for me.
__________________
Silence is so accurate.

Last edited by scott wurcer; 30th December 2012 at 12:13 AM.
  Reply With Quote
Old 30th December 2012, 12:17 AM   #1357
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by 1audio View Post
I was thinking of removing any residual DC at the main output of the oscillator in the process. For some applications (like precision measurements) the DC at the output causes problems. The vacuum thermocouple, for example, can't tell DC from AC. If one servo could do both it would be nice.
I see. Well I'm concerned for the offset in the multiplier because I believe this is where the rise in distortion originated at. Not sure about using a servo on the oscillator it self. Servos introduce a TC and this might upset the oscillator. It could also upset the AGC. But we can investigate the effect of offset in an oscillator by introducing some and see what happens.

Some of the theories around oscillators are very old and where valid for the kinds of circuits being used at the time. I think we should retest these theories and not just accept them as valid. Offset in a single discrete amplifier oscillator may be different than ones build with op amps. How much offset?

"For some applications (like precision measurements) the DC at the output causes problems."

Maybe the best place to address this would be right at the output. Like if a buffer or output amplifier stage is used then deal with the offset there. I like to avoid series blocking capacitance if at all possible so servos are attractive. But then again a nice floating balance output eliminates the offset problem.
__________________
David.
  Reply With Quote
Old 30th December 2012, 12:20 AM   #1358
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by davada View Post
Hi Scott,

What doesn't work with one oscillator type might work with another.

I have a multiplier along the lines of what Demian is suggesting. It doesn't work with a Bridged T but it works perfectly with a SVO. It doesn't work with a fast ALC because the tau of the lamp is too long. It doesn't respond fast enough. It is capable of stabilizing the SVO on it's own. All it really needs is an auto set level sort of thing. Some thing to just bring the level to a dc reference slowly but not so slow it is annoying.

Can you share some part numbers for these plentiful thermal RMS detectors?

Cheers,
Why does it have to be thermal RMS why not log computing? I forgot to mention that that was what I made, I epoxied a power resistor to the 3300ppm Tellabs resistor and put it in a styrofoam isolation surround. When it finally locked in I got -130dB or so.
__________________
Silence is so accurate.
  Reply With Quote
Old 30th December 2012, 12:21 AM   #1359
davada is offline davada  Canada
diyAudio Member
 
Join Date: Jul 2004
Location: Fort St John, BC Canada
Quote:
Originally Posted by scott wurcer View Post
Please point me to a cheaper service. They do add soldermask etc. for not that much more. Mind you I have no interest in buying a high end CAD program or managing drill layers etc. I tried EAGLE and it was not for me.
I use Diptrace myself. A non profit license is not much. I didn't like Eagle either.
__________________
David.
  Reply With Quote
Old 30th December 2012, 12:27 AM   #1360
diyAudio Member
 
scott wurcer's Avatar
 
Join Date: Jan 2004
Location: cambridge ma
Quote:
Originally Posted by davada View Post
I use Diptrace myself. A non profit license is not much. I didn't like Eagle either.
Thank's

As for RMS in this application neither the BW or dynamic range is an issue so something like an AD637 should work just fine. I found a huge stash in my junk bin (nice gold DIP's) if you want a couple.
__________________
Silence is so accurate.
  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 04:21 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