Hybrid Amplifiers Revisited © 2001 Alan Kimmel

Alan Kimmel

From the orginal TubeLabs site

Copyright © 2001 Alan Kimmel. All Rights Reserved

Hybrid Amplifiers Revisited  

Some audiophiles want to know how the linearity of various mu-mode circuits compare against one another. A true mu-mode stage will allow the triode in the bottom half of the stage to reach its greatest linearity. That linearity is whatever linearity was built into that triode at the time of manufacture. And it will vary from tube to tube even though all tubes are the same tube number, and it can vary even among the same manufacturer. Generally the largest variations in THD will be from manufacturer to manufacturer.  

I have never tried to see just how low in THD my mu stages can go. To get the lowest THD in any [zero NFB] stage requires carefully hand-picking the tube (in the case of a mu-mode stage, carefully hand-picking the triode that will be used in the bottom half of the stage) . I have not published the lowest THD figure I have ever obtained. Someone could say that all we have to do is computer simulate the THD figures. Computer simulations can be useful for many things until we ask too much of them. They might say that a certain tube and circuit deliver a certain amount of THD, but before you can bank on it, you must measure it to be sure. Some simulations can be too optimistic, while others may not be optimistic enough. When it comes to computer simulations, a former President's advice can be helpful here, even essential: "Trust, but verify".  

If anyone wants to compare the linearities of various mu-mode circuits with any degree of accuracy the exact same triode must be used in the bottom half of every mu-mode circuit that is being compared. Otherwise the different THD figures you get will probably be due mainly to the variations among the triodes you're using in the bottom half of the circuits. You also have to ensure that exactly the same amount of cathode current is flowing through the triode in every circuit. Otherwise, this too can skew the results. And ideally all the test circuits would use the same B+. 

Hybrid amplifiers should have the warmth, air, and "presence" of tubes plus the superior bass performance of a solid-state output stage. This would be an ideal combination. But hybrid amps of the past did not deliver the full sonic potential that ought to exist in an amplifier that has a tube front end and a solid-state output stage. The fault was not that those amps were hybrid; the fault was that the topology used in the tube section of those amplifiers was run-of-the-mill, the-way-it's-always-been-done "plain-Jane" tube topology. Such topologies do not allow tubes to have Total Sonic Freedom (discussed in the Introduction). No wonder early hybrid amplifiers sounded less than wonderful. Contrary to what some people may have assumed, the fault was not in the output stage. The tubes were hampered by mediocre tube circuitry. Mediocre circuitry is notorious for delivering mediocre sound no matter what kind of amp uses it. Tube fans therefore decided that hybrid amps sounded "okay" but not great. So the hopes and expectations awaiting those hybrid amplifiers became somewhat of a disappointment. But it need not be that way!

My hybrid amplifiers eliminate the mediocrity that plagued some past hybrid amps. I use my proven mu stage tube technology for the front end of the amp circuit. And, every stage has lots of Power Supply Rejection. This design delivers the great combination of the musical midrange and high end of tubes plus the clean low end of solid-state: the marriage that was meant to be!  

As mentioned, some hybrid amps of the past earned a bland reputation but that wasn't the fault of the hybrid concept-- it was the EXECUTION of the hybrid concept that was faulty. Just as OTLs can be designed and built to work and sound great, likewise hybrid amps can be designed to sound great too. And be reliable. It's all in the execution. That goes for hybrid amplifiers and audio in general. (To see why hybrid amps can be designed to sound great read the High Gain Followers page.)    There are three basic design formats, or types, of hybrid amplifiers:

Type 1:
This type of hybrid amp has a tube driver circuit at the "front end" of the amplifier circuitry to provide all the voltage gain and transistors are used only for current gain. If done properly this type can make the most of the strengths of tubes and of transistors, avoiding or minimizing their weaknesses, resulting in an impressively musical amplifier!

Type 2:
This is the reverse approach - It uses transistors for voltage gain and tubes for current and power gain. A major reason 100 percent solid-state amps don't sound as good as we'd like is because solid-state amps use transistors for voltage gain. Conventional transistor voltage gain circuitry guarantees mediocre solid-state sound. As explained in my High Gain Followers page, transistors are naturally musical and linear when used for current gain but not voltage gain. To put it simply, Type 2 hybrid amps don't use their tubes to do what tubes do most musically and linearly (which is voltage gain) and they don't use their transistors in the best way either (current gain); instead, this kind of hybrid reverses the ideal roles of tubes and transistors-- which is why I call such an amp a "reverse hybrid". 

Type 3:
This type would be any variation of the above-- such as transistors in the middle of the amp circuitry, or tubes in the middle.
Just as there are many different ways to design tube amps and transistor amps there are many different ways to design hybrid amps too. All three of the above formats have been and are being tried by various audio adventurers. If anyone can make Type 2 or Type 3 hybrids sound good, more power to them. In my experience, a proper execution of the Type 1 format delivers maximum music!  

And some hybrid amps of the past had reliability problems. I design conservatively; reliability is one of my main design ingredients. I like to think of my hybrid amps as "low maintenance OTLs" with improved bass over OTLs.  As a matter of fact the purpose of my hybrid amps is to provide the superb mids and highs of a very good OTL and extending this all the way down into the bass. Plus solid-state reliability. In other words an OTL without OTL shortcomings! It would also be correct to say that it has the advantages of a very good SE amp without the shortcomings. I've designed and tested almost every kind of tube amp and they sound very good but my hybrid amps have - at least as excited as anything I've heard or done in audio.

In many ways these amps let you have your cake and eat it too. You get the convenience and reliability of transistors -- no retubing to spend money on. My hybrid amps use only one or two driver tubes in the amp circuit-- no power tubes which is obviously a major savings. The tubes operate conservatively and will last a very long time. And there's the issue of room heat -- the driver tubes in my hybrid amps produce very little heat which makes for better summers. So in virtually all ways you have the advantages of tubes without the disadvantages, and the advantages of transistors without the disadvantages. 

Today people are thrilled when they hear what a properly conceived and well designed hybrid amp really can do. If you audition one of these new hybrids you'll understand the excitement. A totally new breed of hybrid amp has been created! I humbly state that the hybrid promise is actually delivered in my hybrid amplifier designs which unleash the maximum musical magic of triodes. And the transistors are used in the best way too so that they faithfully interface the triode's magic to the real world. This combination succeeds in putting music and listener together!  I'm not saying my hybrid amps are the only good ones. I'm simply saying that my hybrid amp designs achieve my goals of what an amplifier should do and be