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Thread: A tale of two amplifiers

  1. #61
    Join Date: Aug 2009

    Location: Staffordshire, England

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    I'm Martin.

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    Not knocking Leben, the only one I have heard was very impressive, but you should really consider a Radford too if it is within your budget. They don't sound 'tubey' but they have a rightness to the harmonics that can make other amps sound broken by comparison.
    Current Lash Up:

    TEAC VRDS 701T > Sony TAE1000ESD > Krell KSA50S > JM Labs Focal Electra 926.

  2. #62
    montesquieu Guest

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    Indeed. In my view having owned both the sta25 sounds better than the 660p. The 1000p otoh is a different beast entirely


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  3. #63
    Join Date: Dec 2015

    Location: Alicante. Spain.

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    I'm Adrian.

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    If the dickhead on eBay would post the 1000 it would be in my system!
    Technics SP10 mk2
    Jan Allaerts MC 1 Boron mk1 cart
    Miyajima Shilabe cart
    Hashimoto HM-X SUT
    Siggwan (gimballed not unipivot) Cocobola 12"
    Aurorasound Vida LCR Phonostage
    The Truth linestage
    Dave Slagle Autoformer Volume Controller
    Cary 805c SET amps
    Audio Note ANe-SPX speakers
    Townshend Isolda speaker cables
    Cardas Golden Presence interconnects

  4. #64
    Join Date: Aug 2017

    Location: Derbyshire

    Posts: 19
    I'm Malcolm.

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    Quote Originally Posted by montesquieu View Post
    Well my experience with many different sets of Tannoys and thirty-odd amps tried with them is that for the most part (with a few very rare exceptions) single ended designs don't work terribly well. I used HPDs myself currently. The output impedance figures for the Bernings bear out my concern - Tannoys need a decent damping factor to perform properly (which is exactly the issue with most SET amps).

    A properly functioning Radford BTW is the opposite of 'typical tubey' - not sure where you got that? Have you ever heard one?

    I agree that watts often have little impact on things - I have a Leak Stereo 20 whose 12 watts drives HPDs surprisingly well, though it is better suited to Monitor Golds. But most Tannoys do respond well to having a few watts up them. Again speaking from (lots of) experience.
    Yes I would agree a typical SET is unlikely to be the best match, especially in view of those large cones But then we are not talking about typical amplifiers here.
    The Berning aims to maintain a constant output impedance across the entire audio spectrum and has some of the best controlled and textured bass I have heard form a valve amplifier. The Berning is Push Pull - Zotl not single ended but does have many sonic attributes of a SET but with much better extension at the frequency extremes.
    Many valve amplifiers, including those you mention have very low damping factors I believe so on the face of it they would not be an ideal match. On the other hand solid states tend to have very high damping factors so should be ideal. However its not as simplistic as matching a nominal 8 ohm speaker with a quoted amplifier damping factor at a certain frequency. I have copied here an excellent explanation from Paul Coupe of this parish (who i'm sure you are very familiar with) from another forum which discusses the issue of Damping factor and the Tannoy HPD Driver

    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.

    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.

    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.

  5. #65
    montesquieu Guest

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    Quote Originally Posted by Eaton Mess View Post
    On the other hand solid states tend to have very high damping factors so should be ideal. [/COLOR]
    Long experience with Tannoy DCs suggest the opposite. Tannoys are best with a damping factor somewhere the (conceptual) middle - I would say between 10 and maybe 30 or 40 with an optimum of 20 or so. So single ended amps are out (for the most part) as they have insufficient damping factor - while they will drive Tannoy DCs and there will often be plenty of bass, it won't be the nice, taught, well defined kind. (This may not matter so much in a small cabinet where there isn't much low bass to speak of anyway).

    High power solid state on the other hand typically (though again, not always) has too high a damping factor resulting in the life being sucked out of DCs - a perfect example would be the Krell KSA100 I tried out a while back. Truly awful with Tannoy MGs, though I'm sure it has a place driving multi-driver speakers with complex crossovers, where its vice-like characteristics are far more welcome.

    I know Paul very well, he designed my speakers (taking into account my room measurements and nodes), built my crossovers and has spent quite a bit of time in my listening room. I think he'd concur with the above.

    Also like Paul I think the superstitious aversion to modest amounts of negative feedback used intelligently is retrograde thinking. I believe amplifiers should be designed by a judicious combination of measurement and ear. Not by religious dogma. (It's not a bad approach to choosing one either).

    I would love to hear the Bernings with my Canterburys, they have a good reputation with Quad ESLs (of which I'm also a former owner) and intuitively amps that are good with the original ESLs also tend to work well with Tannoys.

  6. #66
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    For what it's worth an STA25's output impedance is approximately 0.2 ohms (Occasionally it dips even lower depending on the valves) in the mid-band, which for a valve amplifier is very good indeed - low enough so that it is hard to differentiate the subjective effect of damping from a "0 ohm" source solid state design

    There are certainly unconventional (ie not your standard high feedback class A/AB type) solid state designs out there with a much higher output impedance, as well as higher distortion!
    Radford Revival
    www.radfordrevival.co.uk

  7. #67
    montesquieu Guest

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    Quote Originally Posted by Radford Revival View Post
    For what it's worth an STA25's output impedance is approximately 0.2 ohms (Occasionally it dips even lower depending on the valves) in the mid-band, which for a valve amplifier is very good indeed - low enough so that it is hard to differentiate the subjective effect of damping from a "0 ohm" source solid state design

    There are certainly unconventional (ie not your standard high feedback class A/AB type) solid state designs out there with a much higher output impedance, as well as higher distortion!
    I was astonished at the STA25 I'm running at the moment (I'm using it while the Radford STA100 is away being restored). It had 90% of the big Radford's bass authority. Strikingly, it replaced a KT120 PP amp with a massively bigger footprint - I would never have expected an EL34 amp to outperform in bass authority and grip a KT120 amp with notionally twice the output power, but better it it did. Design really matters.

  8. #68
    Join Date: Oct 2012

    Location: The Black Country

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    I'm Alan.

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    Quote Originally Posted by montesquieu View Post
    Design really matters.
    Output iron, i.e. the output transformer design (and size) is the key.
    I love Hendrix for so many reasons. He was so much more than just a blues guitarist - he played damn well any kind of guitar he wanted. In fact I'm not sure if he even played the guitar - he played music. - Stevie Ray Vaughan

  9. #69
    Join Date: Apr 2012

    Location: South West of England

    Posts: 263
    I'm Will.

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    Quote Originally Posted by montesquieu View Post
    I was astonished at the STA25 I'm running at the moment (I'm using it while the Radford STA100 is away being restored). It had 90% of the big Radford's bass authority. Strikingly, it replaced a KT120 PP amp with a massively bigger footprint - I would never have expected an EL34 amp to outperform in bass authority and grip a KT120 amp with notionally twice the output power, but better it it did. Design really matters.
    It does indeed, it's (conceptually) easy to build an amplifier that delivers gobs of power, but to do the same thing, especially with valves, is another task. The Radford can be thought of as a fancy Mullard 5-20 with good output transformers.

    Quote Originally Posted by Firebottle View Post
    Output iron, i.e. the output transformer design (and size) is the key.
    Indeed, and the conflicting requirements between the low end and high end frequency response design constraints! You have to juggle efficiency, low frequency response, high frequency response, as well as other esoteric parameters that are not immediately obvious
    Radford Revival
    www.radfordrevival.co.uk

  10. #70
    Join Date: Mar 2011

    Location: Preston

    Posts: 197
    I'm Paul.

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    Quote Originally Posted by Radford Revival View Post
    It does indeed, it's (conceptually) easy to build an amplifier that delivers gobs of power, but to do the same thing, especially with valves, is another task. The Radford can be thought of as a fancy Mullard 5-20 with good output transformers.



    Indeed, and the conflicting requirements between the low end and high end frequency response design constraints! You have to juggle efficiency, low frequency response, high frequency response, as well as other esoteric parameters that are not immediately obvious
    As the owner of a pair of Mullard 5-20s with ‘middling’ transformers I’d love to try one of your amps... unfortunately I would then have to give it back.... so I’ll stick with mine for now. They do work very well though
    SBT/Chromecast/TEAC T1 into TEAC D-T1 DAC or olde-worlde Rega Planar 2 controlled by Rotel RC850 into HH S500D or DACT Passive Pre into Mullard 5-20 monos. Then into Tannoy Cheviots and a Wharfdale sub. All connected by colourful cables.

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