Thanks Colin. That's advanced my understanding. Although I don't understand what output impedence is. Never mind. More importantly, how much stock do you put in damping factor Colin? Opinion appears to be divided. To what extent is it a consideration when you're designing your noise makers...?
It depends on the sound I want from a product and the budget for said product.
I like amps that control the speakers, and since a amplifier is a power supply and the load is a motor or generator depending on the signal applied or not, a good power supply should follow the reference signal or voltage and drive or damp the load according to that, so to do that well you do need a amp with a very low output impedance, but the trick is to also make it fast.
And without overshoot or undershoot so no ringing were the load is uncontrolled. At that point all sorts of distortion are produced and phase errors. Not Good.
So I would go for high damping factor but fast, and some amps do not meet those criteria at all.
The early Crown Amps had fantastic damping factor but were so slow it was crap to listen to, it was like the music was slugged and not allowed to breath.
Too much damping factor and the treble and mids will be too dry and unnatural, too little damping factor and you get looser and warmer, sometimes false sense of deeper, bass. It is how well an amp responds to and controls the speaker "kicking back" into it, tricky balancing act with having just the right amount of damping factor...
Too much damping factor and the treble and mids will be too dry and unnatural, too little damping factor and you get looser and warmer, sometimes false sense of deeper, bass. It is how well an amp responds to and controls the speaker "kicking back" into it, tricky balancing act with having just the right amount of damping factor...
The treble and mid will be bad only if the amp slew rate is rubbish.
And one note bass is often caused by the front edge of the notes are to slow.
I have heard amps with 'Damperings' in excess of 4000, aven a pre-amp with more than 1000, and the treble is as sweet and full bodied as... anyway, was really good. Really, really good. The bass is both thunderous and frightening too.
Colin's reply above really helped me understand why it's not always seen as desirable, which has been puzzling me somewhat.
The degree of damping factor required is really dependant upon the speakers being driven. Some loudspeakers can get away with a very low damping factor (eg horn speakers driven by single ended amps) and its not the be all and end all. What matters is that you understand your speakers impedance characteristics. Unfortunately most manufacturers only ever supply a thing called "nominal impedance" for their speakers which tells you next to nothing. On its own, its not enough to select a matching amp.
For example, take Tannoy's HPD drivers. The 12 inch versions have a nominal impedance somewhere close to 7 Ohms, and are 92dB sensitive (note I didn't say "efficient" which is a different measure).
That looks like an easy load and you'd be forgiven for thinking so, until you see the actual impedance plot from a speaker using this driver. let me enlighten further.
Here's an actual impedance plot from HPD equipped speakers:
That 7 ohm nominal load and high sensitivity doesn't look as benign now does it?
So what happens if you drive speakers like these which have an impedance plot resembling the Alps with an amp having a very low damping factor?
Well first you need to consider total load resistance which is also made up of the reactive impedance and DCR of the speaker cables to get the total speaker load. This at any point on the impedance curve is divided by the output impedance of the amp to give you a damping factor.
What is immediately apparent is that if your impedance curve isn't linear (for example, impedance correction circuits haven't been employed in the crossover) is that the damping factor will VARY with frequency.
At low frequencies, generally speaking, the damping factor lowers and the amp loses grip on the speakers, so the critical thing is to find the lowest impedance response (usually in the bass area somewhere slightly above driver resonance) and calculate the required damping factor on that so you can be sure that at higher impedance values, there'll be no problem. As Col says, slew rate is also important, but at the moment, we're just looking at generalisms to help understand the importance of damping factor.
In reality, it doesn't have to be that high. Even for the Tannoys shown, a damping factor of say 10 is more than adequate, so amps boasting of a factor of 100 or more, well imho, that can be misleading in terms of they don't control the speakers any better than one with ten times less damping.
If you use speakers which exhibit a very linear impedance variation across the frequency bandwidth (and by linear I mean within a few Ohms) then you probably can get away with very low damping factors (and here, single ended valve amps with enough grunt should be sufficient).
What matters more in many cases is power output because the other important thing to note is that you can destroy tweeters by driving speakers hard with an under powered amp, so generally speaking, whilst you cannot afford to under - drive speakers (using solid state amps) you rarely have to worry about using high powered amps (you still have a volume control!).
The exception is valve amps where you can drive them into even order distortion without square wave clipping damaging speakers...they just sound more euphonic and long run it wont do the valve life much good but you're less likely to damage speakers.
As Col can probably attest, there's much more to it than my simple explanation but hopefully this helps...a bit!
Another point worth making here is that when looking at the figures for damping factors quoted, initially high damping factors look to be the way to go, so faced with say a valve amp with an output impedance of 1 Ohm or a SS amp with an output impedance of say 0.05 Ohms, and you have a set of speakers which are a nominal 8 Ohm load, you'd be forgiven for thinking that the comparative damping factors are 8 for the valve amp and 160 for the SS amp.
That would be wrong as an assumption though as it negates reactive impedance and the combined resistance the amp sees in the crossover circuit and speaker voice coil itself.
Take the above example and lets assume a simple 1st or 2nd order crossover. the first thing the amp sees when driving the speaker is an inductor and typically, the bass inductor would have a DCR of about 0.25 Ohms. The speaker voice coil is the next point of significant resistance and this typically for an 8 Ohm unit might be say 6 or 7 Ohms DCR, so lets use 6.5 Ohms.
Take the original SS output impedance and then add 0.25 plus 6.5 to it and we have a total of 6.3 Ohms, so the corrected damping factor now becomes 1.27. Do the same for the valve amp and the corrected factor becomes 1.03, so not a million miles adrift from the SS amp.
This is often forgotten about when comparing amplifiers.
For the Tannoys shown in the above example, a well designed valve amp, something like say a Radford (output impedance of just 0.18 Ohms) performs almost as well as many SS amps.
What you have to consider too are the power requirements which not only vary according to the speaker sensitivity but also with inertia and acoustic loading. So a sealed cabinet with a "Q" of around 0.7 using a 6 inch paper cone will be an easier load than say a 10 inch polypropylene driver in a bass reflex enclosure which will take a higher power and damping factor to control it, even if both have identical nominal impedance and sensitivity. Typically, bass reflex drivers have an electrical "Q" of around 0.3 to 0.5 as the lower value basically means a stronger magnet/drive mechanism, whereas sealed units can get away (and indeed need) lower strength magnets if they are not to become over damped.
So speaker inertia, damping factor, size and type must also be considered when looking at what constitutes an adequate damping factor.
Thanks Paul. As always I am very grateful for the time that you put into providing your detailed technical insights. I do feel frustrated when the understanding and modelling of reality is so complicated tho...!
What are you (or Col!) able to explain about the output impedence of an amplifier? I can understand what the impedence of a speaker is, but I haven't yet got my head around how an output can have resistance / impedence... :-?
Output impedance of any source or amplifier Ben is simply the summed resistance to passing an alternating current at a specific frequency made up of total resistive, capacitative and inductive elements of impedance. It varys according to the type of circuit employed.
Using generalisations again, when looking at say a preamp with an output impedance of 150 Ohms, this can be considered as a "perfect" source with 150Ohms resistance in series with the output looking at it simplistically.
Where a power amp is concerned, you don't want the amp behaving inefficiently, you want as much voltage and current that the circuit's designed to provide going straight to the speakers. The higher the output impedance, the less power is available to drive the loudspeakers.
Its a little more complex though (isn't it always) as looking at power amps there's two impedance definitions to consider: rated output impedance and actual output impedance. The former is the rated impedance that the amplifier can drive without becoming unstable or failing (so amps are rated usually into 2, 4 or 8 Ohms) and actual output impedance is usually calculated from the loudspeaker load and damping factor:
Z(actual) = Z(Loudspeaker total load)/(Damping factor). For many amplifiers (solid state) the value of Z(actual) is often less than 0.05 Ohms at 1KHz.
As to what makes up output impedance, well the type of circuit does rather than any one component.
If we look at a couple of examples, and take first a bog standard push pull valve amp using 6550 valves in the output stage, two valves per channel with a 5KOhm primary transformer winding then the sum plate impedance is 10KOhms. If the 2ndary winding has an impedance (typical) of 8 Ohms then the impedance ratio is 625 so total output impedance is 10KOhms/625 = 16 Ohms. This is WAY too high so most amps use what's referred to as a negative feedback loop to reduce the output impedance. In order to get it down to acceptable levels, around 20 to 30dB of feedback is employed. Some say that the use of negative feedback destroys the purity of the signal but it is essential to keep the amp stable into difficult loads.
This is where some single ended amps can actually score well, particularly if using a little negative feedback. If we look at a typical 300B valve amp, and say the primary winding was 4KOhms and the secondary 8, the impedance ratio is 500. If you then look at the tube plate impedance of a single 300B it's 680 to 700 Ohms.
The output impedance without feedback is then 700 divided by 500 = 1.4. A little negative feedback can get this figure appreciably lower so I simply don't understand why manufacturers insist on the rose tinted glasses which accompany the purist thought of zero negative feedback single ended amps, when clearly, a little feedback is a GOOD thing. EAR for example use a little negative feedback with the 869 to achieve a very low output impedance capable of driving surprisingly difficult loads with a modest 15wpc.
I can't speak for the design of a SS amp, this is more Cols expertise than mine, but hopefully it gives you at least an idea of what it's about. Col, please correct me if I've made any blundering errors as the memory's not as good as it once was!
Comments
I'll see if i can move in mein fuhrer.
Although I don't understand what output impedence is. Never mind.
More importantly, how much stock do you put in damping factor Colin? Opinion appears to be divided. To what extent is it a consideration when you're designing your noise makers...?
Colin's reply above really helped me understand why it's not always seen as desirable, which has been puzzling me somewhat.
As always I am very grateful for the time that you put into providing your detailed technical insights.
I do feel frustrated when the understanding and modelling of reality is so complicated tho...!
What are you (or Col!) able to explain about the output impedence of an amplifier? I can understand what the impedence of a speaker is, but I haven't yet got my head around how an output can have resistance / impedence...
:-?
This has been a really informative thread for me all round.
Thanks to all. :-)