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9th Aug 2021, 7:27 am | #1 |
Nonode
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
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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 Last edited by Electronpusher0; 9th Aug 2021 at 7:43 am. |
9th Aug 2021, 8:58 am | #2 |
Octode
Join Date: Jul 2009
Location: Carmel, Llannerchymedd, Anglesey, UK.
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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.
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9th Aug 2021, 8:58 am | #3 |
Moderator
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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
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9th Aug 2021, 9:06 am | #4 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
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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 |
9th Aug 2021, 9:09 am | #5 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
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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 |
9th Aug 2021, 9:26 am | #6 |
Moderator
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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
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9th Aug 2021, 9:59 am | #7 |
Dekatron
Join Date: Jul 2009
Location: Sleaford, Lincs. UK.
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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.
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9th Aug 2021, 1:26 pm | #8 |
Nonode
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
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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 |
9th Aug 2021, 2:35 pm | #9 |
Octode
Join Date: Aug 2003
Location: Welwyn Garden City, Herts. UK.
Posts: 1,906
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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.
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9th Aug 2021, 3:24 pm | #10 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Low Distortion Oscillator output puzzle
Looking at the schematic and the board layout I would say the same.
Lawrence. |
9th Aug 2021, 3:30 pm | #11 | |
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Re: Low Distortion Oscillator output puzzle
Quote:
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9th Aug 2021, 3:58 pm | #12 |
Dekatron
Join Date: Apr 2011
Location: Cornwall, UK.
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Re: Low Distortion Oscillator output puzzle
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9th Aug 2021, 4:44 pm | #13 | |
Nonode
Join Date: Jun 2016
Location: Bognor Regis, West Sussex, UK.
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Re: Low Distortion Oscillator output puzzle
Quote:
Peter |
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9th Aug 2021, 4:45 pm | #14 |
Nonode
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Location: Bognor Regis, West Sussex, UK.
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Re: Low Distortion Oscillator output puzzle
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9th Aug 2021, 5:17 pm | #15 |
Heptode
Join Date: Dec 2020
Location: Swaffham, Norfolk, UK.
Posts: 582
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Re: Low Distortion Oscillator output puzzle
So the AC signal can be applied across the capacitors without there being any polarising pd with no problem? Seems very bad practice
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9th Aug 2021, 11:40 pm | #16 | |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
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Re: Low Distortion Oscillator output puzzle
Quote:
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 |
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10th Aug 2021, 7:31 am | #17 |
Hexode
Join Date: Oct 2019
Location: Weymouth, Dorset, UK.
Posts: 474
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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.
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10th Aug 2021, 8:12 am | #18 |
Dekatron
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,942
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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 |
10th Aug 2021, 12:50 pm | #19 |
Dekatron
Join Date: Feb 2007
Location: Lynton, N. Devon, UK.
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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!
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10th Aug 2021, 3:00 pm | #20 | |
Octode
Join Date: May 2009
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Re: Low Distortion Oscillator output puzzle
Quote:
/Martin
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