UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Other Discussions > Homebrew Equipment

Notices

Homebrew Equipment A place to show, design and discuss the weird and wonderful electronic creations from the hands of individual members.

Closed Thread
 
Thread Tools
Old 9th Aug 2021, 7:27 am   #1
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,288
Default Low Distortion Oscillator output puzzle

I have recently obtained a Marconi TF2331 distortion analyser thanks to Dickie.
I need a low distortion AF signal generator to go with it and have decided to build the sig gen section of the Total Harmonic Distortion meter published in 3 parts in Audio Magazine.
I have laid out a pcb in KiCad and 5 boards are on their way.

I have a question about the output stage.
This uses two polarised electolytic capacitors to make a non polarised one.
The normal way of doing this is to connect two capacitors back to back in series. In fact this is what I did for the pcb.

However in this design they use use 2 capacitors back to back in parallel! I am assuming this is an error but would like to know what others think.

Peter
Attached Thumbnails
Click image for larger version

Name:	Low Distortion AF gen op.JPG
Views:	235
Size:	30.8 KB
ID:	239109   Click image for larger version

Name:	pcb.JPG
Views:	104
Size:	57.8 KB
ID:	239112  
Attached Files
File Type: pdf thd_analyzer-1&2.pdf (2.70 MB, 93 views)
File Type: pdf thd_analyzer-3.pdf (1.21 MB, 69 views)

Last edited by Electronpusher0; 9th Aug 2021 at 7:43 am.
Electronpusher0 is offline  
Old 9th Aug 2021, 8:58 am   #2
lesmw0sec
Octode
 
Join Date: Jul 2009
Location: Carmel, Llannerchymedd, Anglesey, UK.
Posts: 1,498
Default Re: Low Distortion Oscillator output puzzle

I agree that it is rather odd. If there was a question about residual offsets, I would have used the series arrangement. Can't think of a reason for the suggested arrangement.
lesmw0sec is offline  
Old 9th Aug 2021, 8:58 am   #3
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Low Distortion Oscillator output puzzle

Big big big error.

Consider one of the capacitors. It does not care whether or not there is another capacitor across it, but it does care about the polarity of DC applied to it.

Aluminium electrolytics normally need about 10% of rated voltage as a minimum DC across them in order to maintain their polarisation.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 9th Aug 2021, 9:06 am   #4
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
Default Re: Low Distortion Oscillator output puzzle

The two capacitors in that still well-thought-of design, make a total output capacitance of 200uF (2 x 100uf in parallel). To make the same value with two in series would need 400uF, or the nearest available value.

Alternative you could buy a Nichicon 220uF 16V non-polar electrolytic from https://www.hificollective.co.uk/cat...8_322_363.html for 66p and just use one part instead of two.

Or Farnell 2326022, 220u 10V non-polar for £1.13 each.

Craig
Craig Sawyers is offline  
Old 9th Aug 2021, 9:09 am   #5
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
Default Re: Low Distortion Oscillator output puzzle

In fact Cyril Bateman, the guy who wrote a series of articles on capacitor distortion, found that two non-polar capacitors in series gave the same distortion as a polypropylene.

Articles downloadable from here https://linearaudio.nl/cyril-bateman...sound-articles

The particular article is the one entitled "Test results for electrolytics 10uF to 100uF at 100Hz"
Craig
Craig Sawyers is offline  
Old 9th Aug 2021, 9:26 am   #6
Radio Wrangler
Moderator
 
Radio Wrangler's Avatar
 
Join Date: Mar 2012
Location: Fife, Scotland, UK.
Posts: 22,801
Default Re: Low Distortion Oscillator output puzzle

Cancellation of curvatures.

Bateman's articles also include a very low distortion audio oscillator, a very competently designed one as well.

Oh, there is also a plan B method: Use no output capacitor. Use an RC filter to take off the audio signal from the output, leaving just the DC offset value. Usa a low offset low bias opamp to compare this to ground and to steer a DC offset into the audio output amplifier.

So you get very low DC on the output (capacitors with leakage current don't give zero anyway) AND you can do it with no significant signal voltages or currents in capacitors. AND you can use modest capacitor values.

David
__________________
Can't afford the volcanic island yet, but the plans for my monorail and the goons' uniforms are done
Radio Wrangler is online now  
Old 9th Aug 2021, 9:59 am   #7
Diabolical Artificer
Dekatron
 
Diabolical Artificer's Avatar
 
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
Posts: 7,639
Default Re: Low Distortion Oscillator output puzzle

I have a schematic for a very low distortion wein bridge oscillator that can do 0.0025% THD,uses a TL072 and a 28v light bulb to regulate gain. It's very simple and though for one frequency it could be adapted for various frequencies, one only really needs 1khz though. I'm having trouble finding it at present,drop me a PM if interested.

Andy.
__________________
Curiosity hasn't killed this cat...so far.
Diabolical Artificer is offline  
Old 9th Aug 2021, 1:26 pm   #8
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,288
Default Re: Low Distortion Oscillator output puzzle

Thank you all for the replies.
I thought the electolytics in parallel must be an error, as I said I have put them in series on the pcb (and made the footprint large enough to increase the value).

Craig, thanks for the link to Batemans articles, some bedtime reading there.

I am pretty well committed to the design I posted at the top of this thread but would be interested in seeing the design you mention Andy, I will drop you a PM.

Peter
Electronpusher0 is offline  
Old 9th Aug 2021, 2:35 pm   #9
peter_sol
Octode
 
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
Default Re: Low Distortion Oscillator output puzzle

The power supply to the op amps is balanced so the voltage in the output pin will be zero within milivolts so that parallel arrangement will be quite OK.
peter_sol is offline  
Old 9th Aug 2021, 3:24 pm   #10
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Low Distortion Oscillator output puzzle

Looking at the schematic and the board layout I would say the same.

Lawrence.
ms660 is offline  
Old 9th Aug 2021, 3:30 pm   #11
Guest
Guest
 
Posts: n/a
Default Re: Low Distortion Oscillator output puzzle

Quote:
so that parallel arrangement will be quite OK.
And redundant, a wire link will give lower distortion!
 
Old 9th Aug 2021, 3:58 pm   #12
ms660
Dekatron
 
ms660's Avatar
 
Join Date: Apr 2011
Location: Cornwall, UK.
Posts: 13,454
Default Re: Low Distortion Oscillator output puzzle

If anyone's interested:

https://www.cordellaudio.com/

Lawrence.
ms660 is offline  
Old 9th Aug 2021, 4:44 pm   #13
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,288
Default Re: Low Distortion Oscillator output puzzle

Quote:
Originally Posted by ms660 View Post
If anyone's interested:

https://www.cordellaudio.com/

Lawrence.
This site has the THD Analyzer available to download (under Audio Instrumentation). This is the same article I posted in post 1 above but as a single file.

Peter
Electronpusher0 is offline  
Old 9th Aug 2021, 4:45 pm   #14
Electronpusher0
Nonode
 
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
Posts: 2,288
Default Re: Low Distortion Oscillator output puzzle

Quote:
Originally Posted by merlinmaxwell View Post
Quote:
so that parallel arrangement will be quite OK.
And redundant, a wire link will give lower distortion!
Good point, I can always fit a link to my pcb.

Peter
Electronpusher0 is offline  
Old 9th Aug 2021, 5:17 pm   #15
G.Castle
Heptode
 
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
Default Re: Low Distortion Oscillator output puzzle

Quote:
Originally Posted by peter_sol View Post
The power supply to the op amps is balanced so the voltage in the output pin will be zero within milivolts so that parallel arrangement will be quite OK.
So the AC signal can be applied across the capacitors without there being any polarising pd with no problem? Seems very bad practice
G.Castle is offline  
Old 9th Aug 2021, 11:40 pm   #16
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Low Distortion Oscillator output puzzle

Quote:
Originally Posted by G.Castle View Post
Quote:
Originally Posted by peter_sol View Post
The power supply to the op amps is balanced so the voltage in the output pin will be zero within milivolts so that parallel arrangement will be quite OK.
So the AC signal can be applied across the capacitors without there being any polarising pd with no problem? Seems very bad practice
It certainly violates good practice, as per David's post:

Quote:
Originally Posted by Radio Wrangler View Post
Aluminium electrolytics normally need about 10% of rated voltage as a minimum DC across them in order to maintain their polarisation.
In the OP's circuit, there's just about zero voltage DC and only a few mV of AC anyway.

Last edited by kalee20; 9th Aug 2021 at 11:43 pm. Reason: Last sentence added
kalee20 is offline  
Old 10th Aug 2021, 7:31 am   #17
JonSnell
Hexode
 
JonSnell's Avatar
 
Join Date: Oct 2019
Location: Weymouth, Dorset, UK.
Posts: 474
Default Re: Low Distortion Oscillator output puzzle

Surely the better way to cater for the few millivolts of DC offset would be to place the capacitor between R31 and ground like most designers do.
__________________
Valve equipment repairs since 1968 https://jonsnell.co.uk
JonSnell is offline  
Old 10th Aug 2021, 8:12 am   #18
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
Default Re: Low Distortion Oscillator output puzzle

Since Bob Cordell is very much alive and kicking, why not ask him what his design intent was regarding output capacitors?

https://www.cordellaudio.com/

bob@cordellaudio.com

Craig
Craig Sawyers is offline  
Old 10th Aug 2021, 12:50 pm   #19
kalee20
Dekatron
 
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
Posts: 7,061
Default Re: Low Distortion Oscillator output puzzle

And (seeing that this thread now exists and we're all very interested), post up many response for us to read!
kalee20 is offline  
Old 10th Aug 2021, 3:00 pm   #20
Dekatron
Octode
 
Dekatron's Avatar
 
Join Date: May 2009
Location: Linkoping, Sweden
Posts: 1,463
Default Re: Low Distortion Oscillator output puzzle

Quote:
Originally Posted by Craig Sawyers View Post
Since Bob Cordell is very much alive and kicking, why not ask him what his design intent was regarding output capacitors?

https://www.cordellaudio.com/

bob@cordellaudio.com

Craig
I did so yesterday but forgot to mention it here!

/Martin
__________________
Martin, Sweden
Dekatron is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 1:10 pm.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.