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 > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 13th Mar 2023, 3:41 pm   #1
Bobsound
Tetrode
 
Join Date: Jan 2015
Location: St Jean d'Angely, Charente-Maritime, France
Posts: 81
Default Unpolarised electrolytics

I submit this question to the combined knowledge of the forum which I know to be both deep and wide.

I was given a Rotel RA-930X stereo integrated amplifier by a friend, a fellow engineer, who had got it off an auction site. He said the channels were intermittent but had not investigated it himself. The channels are indeed intermittent, the sound comes and goes, loud sounds sometimes break through on either channel. Rotating the input selector and various other controls can re-establish proper operation for a while.
The question is, looking at the circuit diagram, there are a number of 10mfd 50 volt electrolytics in the audio path that appear to have no DC polarizing voltage across them. Is this something anyone has come across before and could this be the reason for the problem? In my some 50 years of electronics I have not come across this except in loudspeaker crossovers where bi-polar electrolytics were used.

Bob
Bobsound is offline  
Old 13th Mar 2023, 4:37 pm   #2
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Unpolarised electrolytics

Electrolytics are frequently used with no bias, or little bias. They are generally used in this way in opamp based circuits to remove offsets stacking up, particularly sections that have rotary controls (volume, balance, tone controls etc) or where there are switches that make a click unless they do not have any DC there.

Some designers use servos to achieve the same thing, but use smaller value capacitors, usually film and give the filtering time constant by feeding the capacitor by typically 100k. They are trickier to apply to get the right servo loop gain - which is why the vanilla solution is to use 10uF electrolytics. Most amps do precisely this, with no DC bias. The only time I have seen non-polar caps is on the very input of an amp. But that is because there is no guarantee that there will not be a DC component of either polarity connected to the input. A non-polar in that position is good practice for that reason. But elsewhere polar electrolytics are just fine.

Unless they look in bad condition, I would not worry.

It seems to me that your problem is dirty or intermittent switches.

Craig
__________________
Doomed for a certain term to walk the night
Craig Sawyers is offline  
Old 13th Mar 2023, 5:36 pm   #3
ajgriff
Nonode
 
ajgriff's Avatar
 
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
Default Re: Unpolarised electrolytics

For what it's worth I have an RA-920X which has very similar circuitry. Apart from a failed bridge rectifier the only issue I've had has been dirty switch contacts as suggested by Craig. I'd be very surprised if the electrolytics are the source of the fading problem.

Alan
ajgriff is online now  
Old 13th Mar 2023, 7:05 pm   #4
Bobsound
Tetrode
 
Join Date: Jan 2015
Location: St Jean d'Angely, Charente-Maritime, France
Posts: 81
Default Re: Unpolarised electrolytics

Thank you for the replies. You learn something new every day. My first thought was to try the switches but with no apparent success, I shall have to try harder to get the fluid onto the contacts.

Bob
Bobsound is offline  
Old 13th Mar 2023, 7:26 pm   #5
stevehertz
Dekatron
 
stevehertz's Avatar
 
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 8,809
Default Re: Unpolarised electrolytics

If you're struggling to reach the contacts using spray switch cleaner, extend the can's thin application tube with a length of silicon rubber tubing, and attach another thin tube at the other end. You can then place and operate the can upright on your bench while applying the tube at any chosen angle or into any difficult to reach places and apply cleaner. Many switches often have no discernible access points to apply fluid, but the pressurised fluid flows at such a rate that it will find its way through cracks and joints in the switch housing. You must then operate the switch many, many times and maybe even repeat the application of cleaner. I have had to do this in the past, it's surprising the level of effect that poor contacts can have on an amp or receiver's performance.
__________________
A digital radio is the latest thing, but a vintage wireless is forever..
stevehertz is offline  
Old 13th Mar 2023, 9:24 pm   #6
williamsunique
Tetrode
 
Join Date: Feb 2023
Location: Watford, Hertfordshire, UK.
Posts: 98
Default Re: Unpolarised electrolytics

Non polarised electrolytics are fairly common in audio circuits. They are for want of a better explanation two capacitors connected in series with either their positive connections together, or their negative connections connected together. They have arguably better quality sound characteristics. They are often found in better cassette decks immediately after the playback head.

Concerning the Rotel, the output transistors may have dry joints. Resolder their connections even if no drys seen.

Paul
williamsunique is offline  
Old 13th Mar 2023, 9:28 pm   #7
stevehertz
Dekatron
 
stevehertz's Avatar
 
Join Date: Sep 2011
Location: Rugeley, Staffordshire, UK.
Posts: 8,809
Default Re: Unpolarised electrolytics

Quote:
Originally Posted by williamsunique View Post
Non polarised electrolytics are fairly common in audio circuits. They are for want of a better explanation two capacitors connected in series with either their positive connections together, or their negative connections connected together. They have arguably better quality sound characteristics. They are often found in better cassette decks immediately after the playback head.

Paul
Interesting. Can you tell me or provide a link to information explaining how they have better sound characteristics? Cheers.
__________________
A digital radio is the latest thing, but a vintage wireless is forever..
stevehertz is offline  
Old 13th Mar 2023, 10:56 pm   #8
williamsunique
Tetrode
 
Join Date: Feb 2023
Location: Watford, Hertfordshire, UK.
Posts: 98
Default Re: Unpolarised electrolytics

Quote:
Originally Posted by stevehertz View Post
Quote:
Originally Posted by williamsunique View Post
Non polarised electrolytics are fairly common in audio circuits. They are for want of a better explanation two capacitors connected in series with either their positive connections together, or their negative connections connected together. They have arguably better quality sound characteristics. They are often found in better cassette decks immediately after the playback head.

Paul
Interesting. Can you tell me or provide a link to information explaining how they have better sound characteristics? Cheers.

Capacitors in audio are often the subject of much debate. As I said bipolar (non polarised) capacitors have arguably better sound characteristics. Arguably being the appropriate word. The reason why they are found in better cassette decks just after the playback head is that there is no polarising voltage here only the ac signal voltage. Where there is no to very little polarising voltage then a bipolar would act as a straighter audio path than a polarised capacitor which would be reverse biased 50% of the time by the signal. This causes distortion.

Paul
williamsunique is offline  
Old 14th Mar 2023, 12:54 pm   #9
ajgriff
Nonode
 
ajgriff's Avatar
 
Join Date: May 2004
Location: Halifax, West Yorkshire, UK.
Posts: 2,583
Default Re: Unpolarised electrolytics

As it happens the coupling capacitors in this Rotel are very ordinary 10uF/50V polarised electrolytics and rarely cause problems when used in this way. However if determined switch cleaning doesn't resolve the issue it might be worth looking at the PSU's DC voltages when intermittency occurs. The bridge rectifier and the two zeners were somewhat marginally rated in the 920AX & 930AX. I'd replace (upgrade) the rectifier as a matter of routine if it shows any signs of physical stress such as swelling.

Alan
ajgriff is online now  
Old 14th Mar 2023, 2:07 pm   #10
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Unpolarised electrolytics

Quote:
Originally Posted by williamsunique View Post
Quote:
Originally Posted by stevehertz View Post
Quote:
Originally Posted by williamsunique View Post
Non polarised electrolytics are fairly common in audio circuits. They are for want of a better explanation two capacitors connected in series with either their positive connections together, or their negative connections connected together. They have arguably better quality sound characteristics. They are often found in better cassette decks immediately after the playback head.

Paul
Interesting. Can you tell me or provide a link to information explaining how they have better sound characteristics? Cheers.

Capacitors in audio are often the subject of much debate. As I said bipolar (non polarised) capacitors have arguably better sound characteristics. Arguably being the appropriate word. The reason why they are found in better cassette decks just after the playback head is that there is no polarising voltage here only the ac signal voltage. Where there is no to very little polarising voltage then a bipolar would act as a straighter audio path than a polarised capacitor which would be reverse biased 50% of the time by the signal. This causes distortion.

Paul
Well no that is not the case. The late great Cyril Bateman in his working life was an electrolytic capacitor designer for (I think) Erie.

He subsequently wrote a seminal series of articles called Capsound. In the first articles he describes the development of an ultralow distortion 1kHz oscillator and a twin-T filter to get rid of the fundamental. He then uses that with a spectrum analyzer to look at distortion residuals.

You can download the series from here https://linearaudio.nl/cyril-bateman...sound-articles .

His tests on 10uF electrolytics are quite interesing. At 0V bias, these produce typically 0.001% distortion. And that increases with increasing bias voltage. So the zero bias voltage case is the lowest distortion condition.

At 6V bias Bateman found that distortion tripled - to around 0.003%. Per capacitor. So if there are ten of them in the circuit that is getting up to 0.03% harmonic distortion. That might not seem much, but it is likely to be many orders of magnitude of the distortion of active circuits.

The other strong correlation was distortion as a function of working voltage. Going to a 50V part (as compared with 25V in the above) reduced distortion to typically 0.0004%

But his lowest distortion capacitor was two non-polar capacitors back to back. His measurement of two 220uF 63V non-polars back to back and zero bias gave 0.00016%.

Going back to regular electrolytics though, Douglas Self, arch objectivist, presents surprising results in Small Signal Audio Design (ed2) pp71-73.

He feeds a 47uF capacitor into a 1k resistor. So a first order high pass filter with a -3dB point at 3.34Hz. So that should work OK, right? Wrong. Distortion starts to rise at 200Hz, from a low level of 0.0002% to 0.05% at 10Hz. This leads Self to conclude that you need to increase the capacitor value by at least ten over what you intitally calculate from a breakpoint perspective. So in this case what might seem like a nutty capacitor value of 470u into 1k to keep distortion low down to 20 Hz.

This is all measured on an Audio Precision test set.

So electrolytic capacitors in audio circuits are not as simple as they seem.

Craig
__________________
Doomed for a certain term to walk the night
Craig Sawyers is offline  
Old 14th Mar 2023, 3:50 pm   #11
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Unpolarised electrolytics

Going back to that there might be a problem with using electrolytic capacitors with zero bias because an audio signal will drive it in reverse polarity when the signal transitions push it that way.

That is too simple a way of looking at it. Let's us take the OP's 10uF electrolytic capacitor, and assume that the load is 10k, which is the sort of load resistance it might see (so an LF break point of 1.7Hz). Now lets look at a signal at 1kHz. The modulus of the capacitive reactance at that frequency is 17 ohms, and the voltage across it is around 1.7x10^-4 times the input voltage. If that is 1V p-p the magnitude of the voltage across the capacitor is +/-0.17mV. So a truly tiny ac voltage across the capacitor.

Self finds that provided the voltage across the capacitor is less than 80mV, the distortion will be low. If it is more than 80mV the capacitor is starting to truly see a negative signal, so we should not be too surprised that harmonic distortion starts to be measured as the magnitude of the AC signal across the capacitor starts to rise.

If the load is indeed 10k or more, 80mV will only be reached at 2Hz or lower, so all should be good.

It is much more of an issue at lower loads - Self's measurements mentioned above were with a 1k load.

Craig
__________________
Doomed for a certain term to walk the night
Craig Sawyers is offline  
Old 14th Mar 2023, 4:36 pm   #12
mhennessy
Dekatron
 
Join Date: Jul 2007
Location: Evesham, Worcestershire, UK.
Posts: 4,241
Default Re: Unpolarised electrolytics

Yes, there's no doubt that electrolytic capacitors can produce distortion that is readily measurable with half-decent test gear (and easily avoided in practice). But whether that can be identified by ear alone in controlled conditions in a statistically significant fashion is another question altogether. Unfortunately, all the talk I've seen about capacitor sound over the decades is sighted, so heavily/totally influenced by experimentation effect. Properly controlled testing is never done by enthusiastic DIY-ers, and only very rarely by commercial companies, who generally leave all that to the marketing departments...

I have one of these amps in the workshop. I bought it for the father-in-law back in 1992, so it's lasted pretty well, all things considered. It's only ever needed switch cleaner. Unfortunately, once these switches get to the point where they need switch cleaner, it is a temporary cure - the original plating has worn through and the base metal is making the contact. That said, I think mine go for some 3 to 5 years in-between cleaning, so I guess I can't really complain. I doubt the original switches could be found new today, but if they were available, it would be worth considering replacing them. These amps are basic-but-good, and are well worth keeping alive.

I even have the matching tuner somewhere. It features a very liberal sprinkling Black Gate capacitors, so presumably is able to undo the effects of all those pesky industrial capacitors and cheap op-amps found in broadcast centres, not to mention the effects of the Orban Optimods and the NICAM system that got the signal from BH to the transmitter site
mhennessy is offline  
Old 14th Mar 2023, 5:30 pm   #13
Craig Sawyers
Dekatron
 
Craig Sawyers's Avatar
 
Join Date: Nov 2010
Location: Oxford, UK.
Posts: 4,941
Default Re: Unpolarised electrolytics

There are many crazily expensive boutique capacitors - including electrolytics, which should really be discussed on the Audiophoolery thread.

Craig
__________________
Doomed for a certain term to walk the night
Craig Sawyers is offline  
Old 22nd Mar 2023, 8:03 pm   #14
Bobsound
Tetrode
 
Join Date: Jan 2015
Location: St Jean d'Angely, Charente-Maritime, France
Posts: 81
Default Re: Unpolarised electrolytics

My original post seems to have generated some interesting discussion. Other priorities have prevented me from getting the amp on the bench yet but it will work its way up the very long to-do list. Thankyou for all your comments.
Bob
Bobsound is offline  
Old 1st Apr 2023, 3:52 pm   #15
Bobsound
Tetrode
 
Join Date: Jan 2015
Location: St Jean d'Angely, Charente-Maritime, France
Posts: 81
Default Re: Unpolarised electrolytics

The discussion about electrolytics has been very interesting. To wind this up, a few days ago I got the amplifier on the bench and investigated the fault. It appears that the switches were the problem. I found an Irish firm on the internet who provide spares for various equipment but they were unable to supply the switches for this amplifier because of its age. I therefore took the plunge and replaced the slider switches with the usual 2-pole, six position rotary switches. This involved some metalwork but was successful and the amplifier is now in use. Thanks to everyone who contributed to this thread.
Bobsound is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 9:14 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.