PREAMP technology shift? - Class A/ balanced/ dual-mono/ zero feedback vs. op-amp preamps

My apologies Johnny, your automotive metaphores are quite impenetrable.

　

Per the following page:

　

http://www.aragonav.com/8008-bb-mondial

　

The Aragon 8008BB has a balanced input impedance of 44KOhms, which is commensurate with the 40KOhms of three of my amps. It does not appear to have the 100KOhms input impedance that you asserted.

　

Interestingly, the input/output impedance ratio of my 40KOhms amps vs Ref 3 -- as well as versus ARC LS28 -- is

　

40,000Ohms / 600Ohms = 66.66 (periodic),

　

which exceeds the minimum safe threshold of 20 by a factor greater than 3. This strongly suggests that my Rowland amps are comfortably compatible with REF3. Similarly, the corresponding ratio on the Aragon 8008BB is

　

44,000Ohms / 600Ohms = 73.33 (periodic),

　

which is not staggeringly different.

　

The hybrid ARC LS28 is doubtless a lovely linestage. I have heard it at RMAF, and liked it. From my admittedly non-tuboholic point of view, with its SS power supply, it has the advantage over the REF series of limiting retubing to every couple of thousand hours or more for the 6H30s, as there is no fast-burner 6550 in the power supply which in old REF3 required a replacement every 1K hours or so.

　

It is worth pointing out that at $7500 new, LS28 is almost twice as expensive as the Rowland Capri S2 at $3995. From sheer memory, and not having heard the two linestages in the same system, it was not apparent to me if LS28 yielded any audible advantage over Capri S2. On the other hand, a side by side comparison of a well broken in pair in the same system might shed some light about any relative audible advantages of one over the other. However, preferability is hardly an absolute: personal musical taste and sonic priorities of the end user play a major role in every evaluation.

　

Saluti, G.

Enter your text ...

If you like soundstaging abilities, so far I've yet to see a solid state preamp that can keep up with a good tube preamp in that regard. It always seems that low level detail suffers on the solid state preamps.

I have noticed that my tube amp has a much deeper and wider sound stage than my AB amps. When I tried a Class D amp, the sound stage was incredibly shallow. Is this due to the damping factor (output impedance) or something else? 

^^ Something else :)

 

This is a great discussion, and I am grateful for all of the advice and opinions. Concerning impedance matching: I know about the 20x rule of thumb for input impedance of the amp over the output impedance of the preamp. If the dividend is in the ballpark, does the impedance ratio really matter to the sound? What does a low or high ratio roughly within the "20 times ballpark" sound like? 

I shouldn't rule out tubes. I've never A/Bed good tubes vs. good solid state in my system, and I probably should. I just know I would be neurotic about swapping out tubes, worrying about when to do so. It's bad enough worrying about needles! ha ha

 

Actually that rule of thumb is 10:1. So if your output impedance is 500 ohms you should be able to drive 5000 ohms.

There is a complication with many tube preamps, which is the size of the coupling capacitor at the output. The issue is that to drive 10K ohms (which most of the time is the minimum you will see in a consumer amplifier) you need a pretty big coupling cap at the output of the preamp if you want it to play bass (which, if the cutoff is 20Hz, will be 10uf; but to avoid phase shift in the audio passband 100uf is recommended. You're not going to find very many preamps with a value that large- they usually split the difference in some way with 20uf being about as big as you're going to see). The bigger the cap, the more its going to color the sound so every manufacturer has a different hedge on this. For example, ARC noticed that many solid state amps have a higher input impedance and so limited their preamps to about 30K as a minimum in order to play bass right. Tube amps are generally 100K and some tube preamps have commensurately smaller coupling caps as a result, ignoring the solid state market altogether.

If there are tests available that plot the output impedance, you can see this by looking at the 20Hz output impedance as opposed to 1000Hz which is usually the frequency at which this spec is usually stated. The frequency plot will tell the story- often the 20Hz value will be multiples of the 1000Hz value. If its still 1/10th of the input impedance of the amp you're OK.

We avoided this by developing a direct-coupled output circuit for which we got a patent. As a result the output impedance vs frequency curve is a flat line- the same at 10 or 20Hz as it is at 1000Hz or 10KHz.