What sort of output coupling Capacitor on a Ravensbrook
 

Rogers Ravensbrook series2.
The output is coupled to the speakers via 1500uF 30V DC wkg electrolytics.
I'm refurbishing this amp as most of the electrolytics are original and it seems sensible to do a full recap in stages, testing the amp as I replace each pair of electrolytics. Including a tidy up of my original PSU bodge done many years ago as an inexperienced youth................ It still works so I think i did ok it just looks scruffy. (I blew the pass transistor with an electric guitar cos the 1A quick blow fuse wasnt quick enough and Dad was not pleased! But I did get it going again)
I'm ok with the rest of the electrolytics, I'll use decent branded 63V devices even where some are rated at 6V as the physical size of modern stuff is much smaller. It does look to me like Rogers specified voltage ratings right on the limit as I guess back in the 60s the costs would have been very tight. The PSU voltage is regulated at 43V by a zener and pass transistor.
Anyway The output caps I am a bit unsure about, I figure they are rated right on the limit anyway but along with the working Voltage are there types that should be fitted here given that they are passing a sizeable current into the speakers? I'm thinking stuff like ESR etc. Or will a decent branded general purpose Capacitor be as good as it gets? These were pretty fine amplifiers back in the day and I do remember it being very good indeed and Dad said he chose the Rogers over the Armstrong/Leak/japanese competition based on its very neat and tidy construction on a single fibreglass pcb with components well spaced which to his non electronic eyes suggested a high quality construction. It certainly is nicer to work on than a lot of contemporary gear.
Thanks,

Andy.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

For electrolytics it's reckoned to be better to stick within a reasonable proximity between operating and rated volts. A factor of two max, say. Go for known brand, 105C rated types for best reliability, they'll also be a bit closer to the original size than modern 85C (almost certainly the rating of the originals) types would.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Just my opinion, but it depends on what you want. The better sounding output caps for me are the Nichicon Fine Gold types. I also liked the now obsolete Panasonic HA series.
I would go for 50 volt 2200uF (or 3300uF). Then if you ever have an output transistor fail, the caps will survive.

Not sure if spending lots on the caps will get you better performance. Dumb as it seems, I find the larger the physical size, the better they sound?

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Electrolytic capacitors are made out of two strips of aluminium foil and two strips of paper separator. The paper is soaked in conductive goo (electrolyte) the layers are alternated on top of each other. the long side of one foil protruding a little at one side, the other at the other then it's swiss roll time. Connections added and a case with an extra squirt of electrolyte.

Note that it's symmetrical. Why is ther a need for a positive or negative marked terminal?

But it's also not much of a capacitor.

Next comes 'forming' stick a good high voltage on it and keep it on for a long while. Electrolytic action strips all the oxide off of one foil and builds it up on the other. The naked foil is our connection to the goo, the built up oxide is our dielectric. Oh, and we textured those foils to boost their surface area.

NOW we have a capacitor!

If we leave it alone for a very long time, that electrochemical forming we did undoes itself. But we could re-form it.

We don't want to have to keep re-forming capacitors in equipment in use, so we need to run them with a minimum amount of DC bias. With modern capacitors, we want a minimal amount of DC across them when the gear is turned on. >10% of rating is a decent choice. S look at your circuit, look at the running DC voltage across a capacitor. Choose one of a higher voltage rating by all means, but best stay below ten times what it will run at.

63 v rated replacing something rated 6v might be pushing it a bit. Not asking for failure soon, but then not giving as long a life as it could.

Similarly, life starts dropping if you run them close to rating. Try not to exceed 80%

Output capacitors for audio amps have to work fairly hard. Think of your power rating, think of what AC current this is into the speaker and this is your ripple current. Good capacitor makers give ripple current rating specs, often at different frequencies in this world of SMPS. A good way to spot better quality capacitors is to look to see if they even have a ripple current spec. Again you can push them a bit, but probable life suffers.

That goo is water based and slowly dries out as water vapour escapes leaking past seals, through rubber etc. As this happens ESR gets bigger, heating gets bigger and the capacitor dies in a runaway process. So you want plenty of goo to start with.

Bigger capacitors have a bigger volume to surface area ratio. Seals are bigger, So leakage may be a bit more, but it's disproportionately less significant because of the bigger volume of goo inside.

So for your output capacitors I'd choose 56v or 63v rating I might put the capacitance up a size to say 2200uF (1500u is a bit parsimonious for 8 Ohm speakers) I might even put the voltage up a bit more to find something in a can close to the original size

105 centigrade rated parts means that their sealing is extra good to keep that precious water in. Therefore generally desirable.

David

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Thanks Al.
Would I get away with 4700uF? I ask because I have dozens (literally) of good quality Samwha 63V jobs that are a good physical size match.
The output is a fairly standard quasi complementary pair, transformer driven. The types in my amp are Solitron SDT9201 which I think are very similar to the ubiquitous 2N3055. I think Rogers would buy in batches of semiconductors from whoever they could get the best deal off as the types do seem to vary according to date of manufacturer and model issue.
I was also scratching around for 6uF electrolytic interstage couplers but I figure using my stock of Nichicon FG 10uF would be close enough?
A.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

David

Thanks for that excellent and thorough explanation of electrolytics. A revelation to me, and one to file away carefully!

Mike

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Thanks for the revision course David, although I am au fait with the various construction methods of capacitors it does no harm to be reminded and others will be reading this too :thumbsup:.
I feel better that you explained things again. I've got a lot of very low voltage electrolytics that would be ok in less critical areas like decoupling so I might be ok there. If I can sub 10uF for the original 6uF coupling caps as the amp is AC coupled all the way through then again I have stuff that will work. I did seriously consider using 5,6uF Plastic film caps as couplers but I doubt I'd get any measurable or audible advantages.
One things for sure, it looks like Rogers went very close to the limit on all the electrolytics voltage ratings.
It makes sense to me to use high ripple caps in output coupling as I guess they pass a fairly high AC current which is a similar effect to AC ripple on a psu?
Thanks.
Andy

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Generally, replacing an electrolytic capacitor with a plastic film one sounds like a good idea.

But there are exceptions to look out for. Some decoupling circuits only work because of the ESR in the electrolytic which acts to kill the Q of spurious resonances created by the inductance of a wound capacitor resonating with another, smaller capacitor bridged across it. Swap this electrolytic and some wild things can happen.

Some three-terminal linear power supply regulator chips assume electrolytic decouplers and will burst into oscillation with anything 'better'. Once bitten you check datasheets VERY carefully for any hint of this. It is rarely phrased overtly.

David

Re: What sort of output coupling Capacitor on a Ravensbrook
 

It is generally considered not good practice to have the value of the output capacitors larger than the power supply reservoir capacitors. The original output capacitor is 1500uF and the reservoirs 4000uF.


So by putting in 4700uF's in the output stage, you'd need at least 10000uF or even 15000uF as the reservoirs. That would put a strain on the bridge rectifier which was not designed to feed into such large capacitances. It might even affect the mains transformer and the mains fuse.


If I were you I'd put in 2200uF's for the output stages and 4700uF or 5600uF or maybe 6800uF for the reservoirs.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

point noted.
I've just ordered up a batch of electrolytics from RS as replacements. I went for Nichicon as a known brand and bog standard stuff mostly rated at 105 degC where I could get them. Using 6.8uF electrolytics as the nearest I could get to 6uF which struck me as an odd value even then. They are all interstage couplers so I figure it isn't really that critical.
I figured I'd get no advantage using more expensive plastic film caps here and standard Electrolytics will likely out live me now!
I figure also that nowadays decent electrolytics will be good enough and although it was a good amp in its day, its hardly cutting edge now. I will be very happy if it functions as designed as a reminder of a very enjoyable childhood and youth when I lived at home and it was our main source of music. My names on the original Wharfedale Denton mk1 that came with it. Still in amazing cosmetic nick serving as kitchen speakers at Fathers and working perfectly. They knew how to build em then. Sometimes I wonder if speaker design really has moved on that much.
Thanks David I think I will have a pleasant day replacing the caps on the board. I'll probably replace them a pair at a time or at least a stage worth at a timeand measure things before going to the next. That way if I cock anything up the fault should be easier to trace than if I'd replaced 30 capacitors all at once.
A.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

A long time ago, I was told that increasing the capacitance value of speaker coupling capacitors was not a good idea. The reason given was that it increased the load to the amplifier, as in lowering the load resistance / impedance. So a load of say 8 ohms could become perhaps 5 ohms, as seen by the amplifier output stage. This ohms value would change depending on the coupling capacitor value. Personally, I would try to keep the values as close as possible, thus would look for 1500uf or 1600uf or 1800uf at a pinch. Don't think that I would go for 2200uf as it's 50% greater in value. Thats just my take on it and I might be wrong.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

The output capacitor is in series with the output, not across it, so the only sense in which it is loading the output is whilst it charges up on switch-on. A larger value would reduce LF distortion - the criterion here is that not more than 80mV should be dropped across the capacitor at the lowest frequency of interest. Some exotic components (for example, a Serafine Supercap as measured by Self) have lower measured distortion at higher frequencies as well, but this is probably irrelevant to the present case.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Ted you referenced the 80mV voltage drop figure across the output capacitor for my Ferrograph F307, out of technical interest how is this figure derived/calculated, for example empirical or theoretical ? you referenced a blow by blow account but I was not able to find it.

David

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Cyril Bateman, Electronics World July 2002 - March 2003 (6 parts). The figure was derived by experiment, as described in the articles. Self gives the essence in Small Signal Audio Design.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

I don't know where you got 4000uF for the reservoirs Michael as the original user manual shows it as 1000uF and that was what was fitted on the amp.
So I have ordered up new Nichicon caps in values as close as I can get, as per the owners manual. its a full wave rectifier using a pair of silicon diodes and a CT transformer on the ravensbrook series2 just for reference. Its a bit different to its big brother. The circuit is completely capacitor coupled right through with a transformer phase splitter/driver for the output pair.
I remember, going back to my younger days when I blew the original pass transistor in the regulator ( an early lesson in why hi fi amps do not make good instrument amplifiers, I was young and daft, now old and daft) I thought it might be a good idea to increase the value of the reservoir. All it did was to make the sound seem very bass heavy and anything above a very moderate volume caused the 1A PSU protection fuse to blow on transients, caused I assume by the extra reserves ability to deliver a high instantaneous current.
So I am leaving things as they are. In any case I merely want it to work much as it did when new, which was very well indeed.

Andy.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Thank you Ted, I now see that Self is the name of the book author, "Self" was referenced for the previous blow by blow account, but at the time I did not realise what the reference to Self meant.

David

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Sorry about that - there are several Douglas Self tomes, so I could have been clearer on that count alone. They cost a pretty penny, but are worth it, both for content and lucidity. The 80mV figure is surprisingly robust across different manufacturers and circuit topologies., and in general the method is simply to increase the capacitance until the condition is satisfied.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

I think I got mixed up between the Ravensbourne and the Ravensbrook. Sorry about that.

Re: What sort of output coupling Capacitor on a Ravensbrook
 

Having just had chance to check into my previous comments re increase of speaker coupling capacitor. The value increase could supply more bass to the loudspeaker, which in turn may or may not be able to handle it at medium to high drive levels. This could cause the loudspeaker cone to move excessivly and either get too hot or end up becoming distorted with the windings being damaged by the magnet metal work. And then subsequently damaging the amplifier, it all depends on what everything was rated at, as whether it would be safe to do.
This is all somewhat mutual, since the correct capacitors are in stock and available. Having checked on Farnell's web page, whilst placing an order, I found that they have 1500uf in 16, 25, 35 and 63v radial can types. If you really want to go for it, they also have 400v and 450v types, mind you, you would need deep pockets for those!

Re: What sort of output coupling Capacitor on a Ravensbrook
 

No problem Ted, I have been looking online at some of his books material, certainly looks very good but as you say not inexpensive.

David