Current mode vs voltage mode

Hi Bob, -you got it backwards- the higher output impedance amplifiers behave as 'current sources' rather than 'voltage sources'.

Actually the two concepts are a bit misleading and counter-intuitive. A low impedance source is so because it can make the same voltage into any load. So it is a 'voltage source'. Its often said that this type of amplifier is 'load impervious'. Myself I think that there is a lot of mythology involved: for example such an amp if it does say 100 watts into 8 ohms, will do 200 into 4 ohms and so on. Conversely, it will do 50 watts into 16 ohms and 25 into 32. This is typical of a lot of transistor amplifiers, and you can see why it might not work on all speakers, ESLs for example (which often have higher impedances at low frequencies). The mythology comes in when it is said that the amp is thus 'load impervious'!

A 'current source' amplifier will *attempt* to make constant power, not constant voltage, into these same impedances. If a speaker is designed for a 'voltage source', it may not have flat frequency response with the 'current source' (even though it may sound better in other ways). OTOH, speakers that expect 'current source' characteristics (i'e. most but not all tube amps) will have flat frequency response despite being the exact same technology.

Its all in what the speaker designer expects of the amplifier. Since they are expecting different things. it can be a bit tricky (and expensive!) to make the right choice.

Sorry for the winded explanation.

Sean or anyone? I had thought this thread was a question about current vs. voltage based amps. I don't see the type of I/C used as relating to how the amplification is accomplished.
Sort of like Nelson Pass's amps are current driven whereas most are voltage driven, if I'm not mistaken.
Maybe now this is off-topic but since you are posting, any insights on advantages of current vs. voltage driven amps?
Are chip amps (National LM3875) also voltage driven? Can't figure out why they sound so good.

Thanks Bob

Your informative post is very helpful. I'm not sure if impedances are the entire story, however, as what I am reading in the Halcro manuals is very similar to Krell's description of their CAST/KCT technology with "infinite" output impedance on their preamp current outputs and minimal, under 100 ohm, input imedances on their amp current inputs. Or is the Krell current tunnel technology nothing but a choice of impedances to simulate a tube amplifier?......sorry if my electrical background is lacking.

Brian

Paul: I didn't say that the only reason why tubes "sound good" ( obviously, a subjective opinion ) was due to distortion. What i did say is that ( typical ) digital recording and playback circuitry and "typical" tubed circuitry ( if there are such things ) tend to form complimentary colourations that many people tend to prefer. That is, in comparison to "typical" SS gear with "typical" digital recordings and playback gear.

Tubes can have several different design advantages to them. On the other hand, they also have several different design limitations to them. Same goes for SS devices. As such, i can't understand why more manufacturers aren't combining the two i.e. not so much "hybrid" SS & tubed circuits, but actually using tubes that have quite a few "solid state" traits. That is, high voltage tubes that can also pass high levels of steady state current.

These are used in commercial RF devices all the time. How & why they haven't trickled down into audio in greater quantity is beyond me. Obviously, the reason isn't price, as there are companies willing to charge and people willing to pay massively high prices. Sean
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Sean,

Good post, but I think you're mistaken on a couple counts.

The first is that people who judge amps based largely or mostly on THD (and sometimes IMD) don't exist anymore. They do - some of them spend $40,000 on Halcro amplifiers, for example.

The second is that tubes sound good because of harmonic distortion. That's at best a small part of the reason and at worst - and what I think is accurate - completely mistaken.

Tubes sound better becuase their higher linearity results in simpler circuits with fewer parts and much less (or NO) negative feedback. That's it in a nutshell.

Paul

Yeah, I agree with Sean. The sound you get with XLRs is probably the closest to Halcro's true sound.

This has to do with the mating impedances of the equipment and cabling involved, the stability of the circuit(s) involved, etc...

While i'm not real familiar with the electrical characteristics of the Halcro designs ( other than using higher levels of negative feedback ), my "edumacated guess" is that you are actually hearing what the Halcro gear sounds like using the XLR connection with less outside interference from cable influences. By introducing an impedance mismatch via the RCA based connections, the cables themselves will have a higher level of influence on what you hear. Evidently, you prefer the influence of this specific cabling and the impedance mismatch that exists rather than the nominal "house sound" of Halcro.

Most designs using higher levels of negative feedback tend to sound lean and sterile. That's what i hear when listening to Halcro. It reminds me of very high quality SS gear during the "distortion wars". This is where THD, IMD, etc... measurements where how many people judged how good gear "could be". As such, GOBS of negative feedback were incorporated into the circuitry. The end result was SS gear that measured very well, but sounded lifeless.

Combining this type of "sterile" SS gear with modern "sterile" digital recordings and playback equipment is what has driven the huge tube resurgance. That is, the natural "richness" i.e. higher levels of pleasant sounding harmonic distortion of tubes helps to contribute some form of warmth and musicality back into what folks hear.

As a side note, it seems that those that love this type of sound are big fans of Nordost speaker cabling. That is, they prefer the very fast, highly detailed ( almost "etched" ) sound that, to me, lacks naturalness and musicality. This type of system / sound can be quite dramatic with a phenomenal "hi-fi" presentation. Whether or not one finds it ultimately satisfying in terms of actual music reproduction depends on personal preference.

Stick with what works for you. If you like the sound better one way over the other, and you've already got the gear ( components and cabling ) to make you happy, consider yourself lucky and ahead of the curve. Sean
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