Mullard Mustard capacitors

[Mark Hennessy]

Hi Rob t, what is the critical part of the capacitors construction that makes the difference, not doubting anything so far but would like to learn a bit more about how the difference is achieved so long as there are no heavy mathematics involved.

I'm interested in what Rob will say, but my take is along these lines:

Basically, manufacturers aren't content to give you just a capacitor. Try as they might, they just can't help themselves. They feel utterly compelled to throw in a couple of resistors and an inductor, and maybe a few other things for good measure. Who said manufacturers were in it for the profit???

Of all the various "parasitic" components, one resistance is in series with the "pure" capacitor buried inside, and this is called ESR. For a large electrolytic, this is sub 0.1 ohms. For a signal capacitor, perhaps an ohm or two, depending on the value and physical construction. How much difference this extra resistance makes depends entirely on the circuit; look at the surrounding conditions. It matters hugely in a switched mode power supply, but does the odd ohm or two make a difference in a circuit where the source impedance is 10s of k-ohm, feeding into a grid resistance of 1M? No, this is not the source...

Another parasitic resistance is a large resistance in parallel with the capacitor. Should be infinity ohms, in practice it's hard to measure without specialist equipment if the capacitor in question is a good non-electrolytic example. So, this isn't a problem either. Until the capacitor starts to leak with age - you've no-doubt heard people refer to "that" capacitor when talking about servicing valve radios?

Yet another component is a low value inductor in series with the pure capacitance. This is called ESL. As with ESR, its effect depends on the surrounding circuitry and also the frequencies of operation. With many types of capacitor, the inductance of the leads are the dominant factor.

There are some other secondary effects - some capacitors can be microphonic, suffer from piezoelectric effects, have voltage-coefficient effects, suffer from "soakage" (AKA dielectric absorption) - but these are all well-documented, and it's easy to take care of these at the design stage.

So, it's pretty hard to objectively see where all the "white heat" around capacitor sound comes from. However, spend just a short while on audio forums and you'll see no shortage of discussion about them (and just about any other passive and active component - not to mention wire, construction techniques and cabinet materials - even the type of feet!). Surely there must be something in this?

About 10 years, respected engineer Cyril Bateman published a series of articles in Electronics World about capacitor performance, and they are well worth seeking out. There were some useful conclusions, especially if you are designing filters (like an RIAA phono stage), but for straightforward coupling capacitors, there is very little objective evidence to suggest reliable subject differences that can be recognised in blind tests.

Personally, I've gone from a position of really wanting to believe - after all, all these people surely can't be wrong - to accepting that there is an awful lot of "emperor's new clothes" out there. Perhaps my system and/or my ears lack the "resolution" or whatever to hear these differences, and perhaps therefore I'm happily living in ignorance of what I'm missing out on, but given that proper audio is my day job, I'm comfortable with that. But I remain willing to experiment. Just recently, I was given an old Rotel tuner, and it turns out that it's a "tuned" model that has Black Gate output capacitors. These capacitors are considered to be the holy grail of electrolytic capacitors, especially now they are no longer manufactured, so one day I'll try some double-blind tests with them. You never know

When I restore an old item, looks are just as important as electrical performance. So I'll use Philips/BC axial capacitors rather than modern radials, as these were used by many manufacturers back in the 1970s. I would humbly suggest that this is a factor in the decisions. In the overall scheme, the brain plays a large part in how audio equipment sounds, whether we can admit it or not

Happy new year!

Mark

[AlanBeckett]

An A-B test behind thin curtains is very easy to do, but most don't.
It's very easy to say A is different to B, but a lot less easy to say how.
It's easy to say 'I prefer A to B', but a lot less easy to define why.
If you can't measure it, it ain't science.
It's best to stick to saying 'I like this one' than pontificating about guitars, pickups and amplifiers.

Alan

[Bushbaby]

Hi Alan,
I agree at the end of the day its whats pleasing to your ears that counts and not the components that the item is made up from. Is it not the case that sometimes things that are simple are often the best? its seems strange that after many years of producing more and more complex instrument/hifi amps that a state of simplicity has returned. Even Ipods seem to have small valve amps to drive them to get 'that' sound.

Regards

Rob

[AlanBeckett]

Well said Rob. The fundamental sentence that springs to mind is 'I like the sound of that one, regardless of what's in it'.
Allan

[pwdrive]

I am not a hi fi expert or anything like that but will await to hear Rob T reply to my question.
On many occasions when I have first got an old valve receiver working my wife comments that that's a nice tone. In most cases the ouptut stage of these receivers has simply been a standard single ended output driven by a single triode from the detector and the HF audio was set via the tone control for a reasonably sharp roll off into a pretty bog standard speaker. I have over many years had the same comments from many people that know nothing of the workings of these receivers/amps etc.

Lawrence

[Skywave]

In the early '70s there was a lot of enthusiasm for transistor power amplifiers. I seem to recall that the phrase 'the transistor sound' was often banded about: typically - and vaguely - described as a certain 'sharpness and harshness', especially at higher volume levels. Obviously, this subjective assessment was made in relation to valve power amplifiers, but few seemed to appreciate (at the time) that this 'characteristic' was indeed relative to such valved equipment. (What the ear had got used to hearing: all physiological sensations are relative). Now if my understanding of what I have read about the active devices in power amplifiers is correct, valves have a transfer characteristic that tends towards a square law, whereas transistors follow a curve that is far more 'curved'. As a consequence, valve amplifiers produce significant amounts of third harmonic distortion (especially when driven hard), whereas transistor amplifiers tend to produce even-order harmonics, predominately second-order. Since third harmonic products are a lot more pleasing to the ear than even-order products, this may go quite some way to explaining why valve amplifiers have a more 'mellow sound' compared to transistor amps.

Its also worth noting that transistor amplifiers generally dispense with an output transformer - and this helps to produce a higher power-bandwidth compared to valve amps. Consequently, a given design of negative feedback circuit will respond differently to high-frequency transients developed in a transistor power amplifier compared with that of a valve amp. This adds to the alleged 'harshness' of transistor amps. This difference in transient response in amplifiers with N.F.B. is difficult to measure and quantify in the laboratory.

Anyway, that little lot is what I have read about over the years, and to some extent, agrees with my experiences.

Al.

[Mark Hennessy]

Hi Al,

By coincidence, this is being discussed on UKVRRR at the moment. Allow me to quote a couple of my posts here:

There are many differences between valve and transistors amplifiers that can all be objectively measured and discussed. It's too simplistic to dismiss it as "audiofoolery".

I had typed a decent reply which got lost when I clicked on Preview, so in sum, here are the main points:

Differences in harmonic content: we like 2nd order harmonic distortion, which is why we like valves, vinyl and tape. We don't like odd harmonics, nor do we like high order harmonics. Transistor amplifiers produce these for many reasons, not least crossover distortion.

Differences in the amount of negative feedback that can be safely applied: valve amplifiers have much less - transistor amplifiers need rather more to counteract crossover distortion. This changes the amplifier-loudspeaker interface, and as loudspeaker designers assume a zero ohm voltage source, it's not surprising this affects the sound.

Differences in the clipping behaviour: in part, caused by the differences in NFB. When a solid state amplifier clips, it does so with "sharp" edges, that generate lots of high-order harmonics. We don't necessarily interpret mild clipping as obvious distortion, but we do notice the sound becomes "harder", especially in the midrange. By comparison, a valve amplifier rounds off much more gently - gently compressing the peaks in the same way an over-modulated tape does, and it sounds quite reasonable to most people. This is where the notion of "valve watts" comes from, BTW. There is no such thing; it's just that it's a nicer form of distortion compared to the solid-state equivalent.

There are numerous other factors too, including the output transformer distortion and power supply rejection/modulation to name but two. But it's certainly possible to make very nice and very poor amplifiers using either valves or transistors

There are reasons outside of the amplifier too. Because "valve watts" don't come cheap, loudspeakers had to be efficient, which means lightweight paper cones. But paper cones are known to cause colouration, and are also known to be be inconsistent from sample to sample, which is why the BBC started making its own drive units in-house back in the 1950s. In general, the most tonally accurate loudspeakers will be inefficient (I could cite countless examples), and won't respond well to a typical low-powered valve amplifier with a high output impedance. Efficient paper-coned loudspeakers are certainly "exciting" to listen to, but are not the last word in accuracy. Interestingly, paper is making a comeback as a cone material, and I can only assume that modern production methods yield more consistent results today...

And...

I'm not sure that the transfer function of a valve is the primary cause of "valve sound". The relationship between voltage in and current out is broadly exponential, and the same can be said of bipolar transistors (see the Ebers-Moll equation) and similarly JFETs (which are more square law than exponential, but still rather non-linear).

It probably has more to do with factors like the need for simplicity (in a transistor circuit, it's no hardship to throw in a few small-signal transistors to linearise parts of an amplifier - perhaps current sources or current mirrors or EF buffers). The high voltage operation of valves mean that output transformers are essential (although I did once see a direct-coupled valve amp - it was an exotic and expensive beast). The high impedances seen in a valve amplifier contribute to the operating noise, and the lack of current sources and supply regulation in typical valve design compromise the power supply rejection ratio. Going to the effort of building a regulated power supply for a typical valve amplifier is well worth it - I know a few people who have done this.

Some years back, famous designer Tim de Paravicini put forward the argument that it's possible to build a valve amplifier and a transistor amplifier that both sound the same. Hi-Fi News reviewed them - they were commercially available at the time - and concurred with this. It was an eye-opening article that showed that valves and transistors are not as different as people assume; both are voltage-in, current out devices after all. I'm fairly convinced that the differences have more to do with the typical circuits used than the actual devices, some of these difference I mentioned in my previous post.

Hope some of that is of interest,

Mark

[tiv48]

let me explain how i first came to be interested in this subject.
about 15 years ago a guy came to the shop with a beam echo advantic integrated amp that was very distorted on one chanel with one el84 glowing red.this was caused by one of the hunts 0.1mf coupling caps going very leaky, this was changed along with the one from the other half of the output stage useing grey radio spares ones, as the glowing valve had suffered it was changed for a pinical el84. the amp was checked using a pair of old music centre speakers.no distortion so amp was collected only to come back a week later with the complaint that the left and right chanels sounded different.useing the same workshop setup i could hear no difference.so i went up to the guys house to hear for my self ,the difference was suttle but there,after swapping the speakers and the inputs over to make sure it was the amp i agreed to have another look. back in the workshop i hooked the amp up to an audio gen and a scope across a 10 ohm dummy load,monertering both channels and reeding both sine and squar waves at 1kc/10ks and16kc i could see no difference, so i phoned my father who has built valve amps as a hobby for years,he told me the obvious in a push pull amp output valves should allway be a matched pair and in a stereo amp in matched quads so out came the pinical and in went a pair of mullards.back to the coustomer sounded better but could still hear a difference so back to shop as the only other thing i had done was the coupling caps so in went four 0.1mf mullarad mustards,amp returned .about 10 days latter was askes to go up there again this time he was happy and said both channels now sounded the sameand if anything he was hearing more in the music than befor he was happy to pay the £30 bill and i learned that valves and caps have there own sound. i may start a thread about hifi myths and facts
rob t
ps i dont know why caps sound different but they do

[tiv48]

al i think you have it the wrong way round vavle amps produse even-order harmonics and solid state amp odd order.mark explains it well.some people who have bought valve amps have been dissapointed with the sound mainly because they are trying to use them modern inefficient speakers
rob t

[pwdrive]

Hello Rob t, an interesting account of that customer and the amp. but as the reply is worded, then for me it is just here say, and thus could be interperated as a case of the Emporor's New Clothes. I still have to be convinced on the capacitor issue, I can understand that the different characteristics between different manufactures of electrolytics could make a difference in power supply regulation and other effects in very low impedance coupling circuits, it is the humble paper/foil/film capacitor that i would like a credible explanation for.

Lawrence.