Tubes vs. Transistors White Paper
- Wednesday, 09 March 2011 09:53
- Last Updated on Wednesday, 30 January 2013 11:24
Many audiophiles question my recommendation for using solid state amplification for driving ESLs since tube equipment is often used for this purpose. There is also a lot of confusion and controversy about tubes and transistors in general. The purpose of this discussion is to examine the pros and cons between the two, present some solid engineering and scientific evidence used to evaluate them, and then draw some reasonable conclusions you can use in deciding what to use.
To do so, I must first give you some history and discuss some technical issues in a way that I hope will be understandable. I have designed and built tube amps going all the way back into the 60's. My most memorable design was a Class A, high voltage, transformerless output, direct-coupled tube amp for driving electrostats. I published the design in "The Audio Amateur" magazine back in 1976. You can still read it on our website at:
I have also used many other tube amps over the years and have also helped design the iTube, which was a conventional tube amp that was optimized to drive ESLs. The point I'm trying to make is that I don't have any bias towards one or the other type of device. I've used, designed, built, and marketed both types.
So what I am about to say does not come from any particular partisan point of view. It is simply what I have learned over the last 38 years of research into producing the best sound I could.
I have been in the unusual position (for an audiophile) of having a fully-equipped test bench, including a spectrum analyzer. This has made it possible for me to carefully do both measurements and listening tests to correlate the two and to find out the reasons we hear the things we do.
This research has been fascinating and very educational. It has also made it possible for me to develop truly high-performance electronics.
There is no doubt that we all hear differences between tube and transistor amplifiers. The big question is what is causing the differences we hear between them. After all, well-designed examples of both types measure well enough that we should not hear any differences between them. So what gives?
I spent a lot of time looking for the reasons. It was an extremely interesting and entertaining search. I don't have time to explain all the work I did over the years in these studies in this message, but will be happy to discuss them over the phone (303 838 8130) if any reader wants to know. I'll just have to summarize here.
To begin, you need to understand how much power is required to play musical peaks cleanly and without clipping an amplifier. It takes a surprising amount.
To see what is going on with an amp when playing music only requires an oscilloscope. These are very fast (the slowest ones will show 20 MHz) and will clearly show amplifier peak clipping when music is playing. A meter is too slow to do so. A 'scope is cheap (you can get them for $100 on eBay all day long). So you don't have to take my word for what I am about to explain. Feel free to get your own 'scope and examine your system's performance.
You simply connect the 'scope across your speaker or amplifier terminals (which are electrically the same), adjust the horizontal sweep as slow as you can while still seeing a horizontal line on the screen. Don't go so slowly that you see a moving dot.
Now play dynamic music at the normally loud levels you enjoy. Adjust the vertical gain on the 'scope so that the trace stays on the screen.
As music plays, you will clearly see if clipping occurs. The trace (which will just be a jumble of squiggly lines) will appear to hit an invisible brick wall. It will appear as though somebody took a pair of scissors and clipped off the top of the trace. That's where the term "clipping" comes from.
If you see clipping at the levels you like to listen, then you are not using a sufficiently powerful amplifier to play your music cleanly. Your system is compromised because your amplifier will have compressed dynamics, sound strained, lose its detail, and have high levels of distortion.
The 'scope will be calibrated so that you will know the voltage at which clipping occurs by observing the grid lines. If you know the voltage and the impedance of your speakers, you can easily calculate the power.
Power is the voltage squared, divided by the impedance. So if the 'scope measures 40 volts at clipping, and you are driving 8 ohm speakers, you know that 200 watts are being produced at clipping -- and this is insufficient power for your particular system because it is clipping.
You will find that conventional, direct-radiator (not horn-loaded), magnetic speaker systems of around 90 dB sensitivity, require around 500 watts/channel to avoid clipping. More power is needed in larger rooms or if you like to play your music more loudly than most.
The key point I'm trying to make is that audiophiles usually are using underpowered amplifiers and are therefore listening to clipping amplifiers most of the time. When an amplifier is clipping, it is behaving (and sounding) grossly differently than its measured performance would suggest. This is because we always measure amplifiers when they are operating within their design parameters -- never when clipping. A clipping amp has horrible performance, so attempting to measure it is a waste of time.
In other words, we usually listen to an amplifier when it is clipping and we measure it when it is not. This is why amplifiers sound so different than their measurements would imply. It is not that measurements are wrong, it is simply that we are listening and measuring different conditions.
It is essential to understand that when an amp is clipping, it will sound quite different than when it is not clipping. It is also important to realize that different types of output devices (tubes vs. transistors) clip in very different ways, so sound quite different when they are clipping.
Finally, it is important to realize that an amp does not instantly recover from clipping. It takes several milliseconds for its power supply voltage to recover, for it to recharge its power supply capacitors, and for its internal circuitry to settle down and operate properly again. Therefore, even though an amp may only be clipping on the musical peaks, it will not immediately operate properly at average music levels where it is not clipping.
It should now be obvious why objective measurements don't seem to give much insight into the performance of amplifiers. It is not that objective measurements aren't accurate (they are superb), but simply that we don't usually operate amplifiers within their design parameters. So we aren't listening to them at the power levels where they operate properly and where their measurements are meaningful.
Now let's analyze tube and transistor equipment with regards to clipping, since that is the condition to which we usually listen. There is "hard" and "soft" clipping. If you go back to the oscilloscope investigations, you will see that solid state amps clip "hard" in that there is an absolute, rock-solid, limit to how loudly they will play. As soon as you reach that point, they immediately clip. This point is their power supply rail voltage.
A tube amp clips "softly." This is because tubes produce a cloud of electrons around their cathodes. This cloud has surplus electrons available so that for sudden current surges (such as musical peaks), a tube can deliver more current (electrons) and voltage for a few milliseconds before they clip. So their clipping threshold is not rigidly fixed as it is in a transistor amp. It varies depending on the dynamics of the music played.
The age of the tube matters a lot in this situation. As a tube ages, its emissions decrease and it cannot develop as many electrons in the cloud. So old tubes will tend to hard clip while new tubes will tend to soft clip.
Transistor amps usually must employ protective circuitry. Tubes do not need any. Protective circuitry will trigger anytime a transistor amp "sees" an excessive or dangerous load. Generally, this means that most transistor amps will trigger their protective circuitry at or about the time of clipping. They will also go into their protection modes at very low power levels if they see difficult loads (like electrostatic speakers).
Protective circuitry works by switching off the power to the output transistors for very brief periods of time. Well-designed protective circuitry will trigger on and off hundreds or even thousands of times per second to limit the power that the output transistors must handle.
Protective circuitry sounds awful. It literally puts gaps in the music, which adds a type of grainy quality to the sound. But more importantly, anytime you flip a switch, whether it is a light switch or an output transistor, you will get a voltage spike. So protective circuitry will replace a smooth musical signal with a chopped up one that has voltage spikes on each side of the gaps in the music. Is it any wonder that transistor amps sound harsh when clipping?
In addition, when a tube amp clips, it produces a lot of lower harmonics in its distortion profile. Low harmonics are relatively benign and don't sound too badly. But distortion is still distortion and these harmonics don't belong there. Also, just because a tube amp makes a lot of lower harmonics, doesn't mean that it doesn't also make higher harmonics. It does. And high harmonics tend to sound dissonate and unpleasant.
This is easily seen on a spectrum analyzer, which shows each harmonic and the percentage of distortion it adds to the sound. It truly is an amazing tool.
Transistor amps tend to produce a lot of high harmonics. This is actually due more to the operation of their protective circuitry and all the spikes it produces. So generally, transistor amps will have more of the unpleasant higher harmonics than do tube amps.
It is important to note that if a transistor amp does not have any protective circuitry, its distortion profile will be much more similar to a tube amp than to a transistor amp with protective circuitry. The effect of protective circuitry is a very critical issue in the sound of solid state amps and should be more widely recognized for the problems it introduces to the sound.
What all this boils down to is that clipping tube amps sound rather soft and smooth. Clipping solid state amps sound harsh and edgy. I think it is safe to say that we would all agree that if you must listen to a clipping amp, a clipping tube amp is more pleasant than a clipping solid state amp.
It should now be apparent from where "tube sound" and where "transistor sound" comes. It comes from the sound of clipping amplifiers, which do indeed sound quite different.
Of course, when clipping, neither amplifier sounds good. They both lose their dynamics, sound "mushy", lose their detail, sound strained, tend to sound harsh (particularly transistor amps), and are somewhat distorted.
Note carefully that human hearing is rather insensitive to transient distortion, so even though both amps will produce several tens of percent distortion when clipping, we generally won't recognize the distortion for what it is, because it is too brief. Instead we will perceive and describe the sound as "harsh", "strained", "fatiguing", "muddy", etc.
To have a truly high-fidelity music system therefore requires very powerful amplifiers. Amplifier power is the single most important factor in choosing an amp. Without adequate power, all amplifiers sound badly. You can pick a clipping amplifier based on it not sounding as badly as another amplifier (tubes usually preferred over transistors), but if you really want clean, dynamic, effortless, and smooth sound, you simply must use adequate amplifier power.
In short, my take on amplifiers is to use a tube amp that clips gracefully if I must listen to a clipping amp. But I'd rather have an amplifier with so much power that it never clips! The sound from powerful amps is dramatically better than underpowered amps, even if they clip nicely.
There are three quality criteria that a good amp must meet. It must have inaudible noise, it must have flat frequency response, and it must have distortion of less than 1%.
Interestingly, tests conclusively show that humans cannot hear distortion of less than 1%. So even though one amp may have 1% distortion and another 0.001% distortion, they will both sound identical to us.
My spectrum analyzer will show distortion down to around one ten thousandth of one percent (0.0001%). It shows amazing differences between properly operating amplifiers. But as long as those distortion levels are below 1%, the amps will not sound any different to us.
It should now be clear that tubes only sound significantly different than transistors when you are listening to clipping amps. If the amps aren't clipping, or if you are using a component that doesn't clip (like a preamp), you won't hear any significant difference between well-designed tube and transistor equipment.
So a hybrid amp (tube front end and transistor output stage) that is not clipping will not sound any different than a pure tube or transistor amp. If it is clipping, it will sound like a transistor amp, not a tube amp, because it is the type of output stage that determines the sound of a clipping amp.
Now with the historical and general information covered, I can now turn directly to your question. So let's examine tube and transistor amplifiers with respect to their performance with ESLs (because I am a manufacturer of ESLs).
Recall that the basic quality performance criteria requires that an amplifier have flat frequency response. This is a huge problem for tube amps due to impedance variations in the load. Let me explain.
One of the laws of physics states that the source impedance must be lower than the load impedance or the load will be starved for current. What this translates to is that the amplifier's output impedance must be lower than the speaker's input impedance or the frequency response will be rolled off in those areas where there is this impedance mismatch.
Tubes are inherently high impedance devices. A large power tube like a 6550 or KT-88 has an output impedance of around 2,000 ohms. By comparison, a large power transistor has an output impedance of less than one ohm.
Tubes cannot drive loudspeakers directly due to their high impedance. To correct this problem, output transformers are used in most tube amps. These transformers have a specific turns ratios that will convert the tube's impedance from several thousand ohms to typically 4, 8, or 16 ohms.
Therefore, if you use the 8 ohm taps on the amplifier's output transformer with an 8 ohm loudspeaker, there should be no impedance mismatch, the frequency response should be linear, and the amp should deliver its maximum power. Unfortunately, this is never the case because loudspeakers do not have a constant impedance across their full frequency bandwidth.
Look at the impedance curve of any conventional loudspeaker and you will see that it varies from slightly below its "nominal" impedance to around 50 ohms. This will cause the frequency response from a tube amp to have errors. This is also another reason why tube amps sound different from transistor amps.
This impedance problem is relatively minor when dealing with conventional, magnetic speakers. But an electrostatic speaker is an entirely different animal. An ESL is a capacitor, not a resistor like a magnetic speaker. The impedance of a capacitor is inversely proportional to frequency. Therefore the impedance of an ESL typically varies from around 150 ohms in the midrange to about 1 ohm at 20 KHz.
A tube amp will be able to drive the high impedance frequency bandwidth (the midrange and lower highs) of an ESL with linear frequency response. However, at higher frequencies, the impedance of the ESL will drop below the impedance of the amplifier and the amp will then roll off the highs to some degree depending on the exact impedance mismatch and the frequencies involved.
This impedance mismatch problem can be minimized with both types of speakers by using a lower impedance tap on the tube amp's output transformer. For example if you use the 4 ohm tap with 8 ohm speakers, you will probably not encounter any impedance mismatch, so the system would then have linear frequency response.
Using the 4 ohm tap with ESLs will help, although it will still not eliminate all the high frequency impedance mismatch because the speaker's high frequency impedance will fall below 4 ohms. But probably only the top octave or two will be affected, which is hard to hear so the roll off may not be noticed subjectively.
But there is a problem when you use a lower impedance tap -- the drive voltage drops. Or to put it another way, the amplifier's output voltage is directly proportional to its output impedance.
Understand that the power available from an amplifier is a function of its output voltage. Ohm's Law is very simple and states that, "One volt will drive one amp through one ohm." With this simple concept, you can calculate virtually anything having to do with electronics as you will soon see.
Voltage is the pressure used to push current through an electrical circuit. Current is the flow of electrons in the circuit -- like water flowing though a hose. Current is measured in amperes, commonly called "amps." Power is measured in watts and is the product of volts times amps.
Resistance is measured in ohms. The term "resistance" is used in DC (direct current) circuits. "Impedance" is the same thing as resistance. But it is used when discussing AC (alternating current) circuits because the impedance often varies with the frequency of the AC.
Since power is the product of volts times amps, you can see that you must get current to flow through the speaker's impedance. This requires volts.
For example, if you have an 8 ohm speaker, how many volts must the amplifier produce to push enough current through the speaker to produce 100 watts? How many amps of current will be flowing through the speaker at 100 watts?
There are simple calculations for determining this. The volts can be calculated by taking the square root of the power times the impedance. So for the example above, the watts are 100, multiplied by 8 ohms, gives you 800. The square root of 800 is 28.2 volts (RMS).
The current can be calculated in several ways, but the most common is take the square root of the power divided by the impedance. So in this case, the current flow would be 3.5 amps.
If you have a 100 watt amplifier, you can see that its output voltage will be limited to about 28 volts. If it could produce more voltage, it could produce more power, so you know that its voltage will be limited to 28 volts or it would have a higher power rating. Of course, all this assumes that the amplifier's power supply and output impedance is such that it can deliver the 3.5 amps needed to produce 100 watts of power.
You can also calculate that if an amp can produce 28 volts into 4 ohms (half the impedance of the above example), that the current would double to 7 amps and the power would double to about 200 watts. Hence you see transistor amps with power ratings listed for both 8 and 4 ohms.
Tube amps are different in that if you reduce the impedance of the transformer from 8 ohm to 4 ohms to match the impedance of the speaker, the output voltage will drop as a function of the turns ratio of the transformer, and so will the power.
The turns ratio is the square root of the primary impedance divided by the square root of the secondary impedance. This always works out such that the voltage will drop to the point where the amplifier will put out the same power at either impedance when driving a matching load.
When driving an ESL, voltage is everything. So when you drop the impedance of the output transformer, you reduce the output that the amplifier can produce from the ESL. In short, you have to trade output for more linear frequency response. This is a huge problem. It's a battle that you just can't win.
Note that OTL tube amps don't solve this problem. They have no transformer, so must relay on putting many output tubes in parallel to lower the impedance. This quickly results in having an absurd number of tubes with all their heat and power requirements. So OTL amps do not get down to very low impedances.
Most get to just around 10 ohms and the best only get a bit lower. As a result, they have severe impedance mismatch issues and are really quite a poor choice for driving ESLs. They also measure really poorly on a spectrum analyzer compared to transformer-coupled tube amps.
By comparison, powerful solid state amps typically have output impedances of around 0.02 ohms. They therefore have no trouble driving any speaker impedance with perfectly linear frequency response.
Solid state amps have high output voltages compared to tube amps. So they will drive ESLs quite loudly before clipping (unless they have protective circuitry that trips them up).
Well designed solid state amps have much lower distortion than tube amps. The best conventional tube amp I've ever measured was a McIntosh 275 with new tubes. It had only 0.3% distortion at an output level of 75 watts/channel (it clipped at 90 w/c). Most tube amps have distortion of somewhat more than 1%, even at levels well below clipping.
The worst tube amp I measured was an Atma-sphere OTL amp. It was rated at 30 watts. It only produced 14 watts into an 8 ohms load and had 13% distortion. It was a new amp and both channels measured virtually identically. So I don't think it was defective.
My specially designed iTube amp measured only 0.1% distortion in the midrange frequencies and went up to 1% by 20 KHz. It would do so at 150 w/c. But this was a special-built device and is not typical of conventional tube amps.
By comparison, most quality, solid state amps have distortion levels down around 0.002%. This is magnitudes better than tube amps. However, it is also true that humans cannot hear the reduced distortion levels in solid state amps, even though a spectrum analyzer will show dramatic differences between them.
Still, distortion is distortion. Why have any more than you must?
I think you can now see why I prefer very high power, solid state amps without any protective circuitry for driving ESLs. This is because they can drive ESLs with linear frequency response, while tube amps roll off the highs.
Solid state amps are much more powerful than tube amps and can supply vastly higher output voltages. As a result, a good solid state amp can drive my ESLs to ear-bleeding levels without clipping. And remember, it is clipping that produces "tube" or "transistor" sound. If a solid state amp does not clip, it does not sound harsh. It sounds just as clear and soft as a tube amp that is not clipping.
Transistor amps can run cool and efficient. My ESL amp runs only warm, yet can deliver the equivalent of about 1000 watts into an electrostatic speaker. No tube amp can do so, and even a relatively low power tube amp will run very hot and waste a lot of expensive electricity.
Tube amps are expensive compared to good solid state amps of similar power. Tube amps require expensive tube replacements, while a quality solid state amp is a no-maintenance, lifetime item.
Tube amps require biasing. Traditionally this had to be done by the audiophile on at least a monthly basis. This was a hassle and rarely was done, so most tube amps were always running far from their ideal performance levels.
Some tube amps tried to get around this biasing issue by using a "self-biasing" system. But this cut their power by about 30%. Some of today's latest tube amps use servo biasing systems, which are great if they work reliably. Often they don't.
Due to their high internal voltages and high temperatures, tube amps are unreliable. They often fail and have to be returned to the manufacturer for expensive repairs.
In short, tube amps can't drive ESLs linearly, cleanly, without clipping, to high output levels. So why put up with all their problems of heat, cost, maintenance, and unreliability when a properly-designed, solid state amp solves all these problems?
I therefore no longer design, manufacture, or recommend tube amps. I only build very powerful solid state amps that have no "transistor sound" because they do not clip or have any protective circuitry to ruin the sound.
Using my ESL amp on the panels of my speakers, and my Magtech amp on the woofers, results in approximately 1,400 watts of power for the speakers. This makes it possible to reproduce something like a grand piano or drum set at live levels in your listening room without clipping. You can reproduce a full symphony orchestra at Row A concert hall levels.
If you have a particularly large room or play your music at ear-bleeding levels, you can use the monoblock versions of my amps. The ESL amp will deliver more than 2,000 watts to the panels and the Magtech will deliver about 1,800 watts to the woofers. Clipping simply isn't an issue and the speaker can take the power.
This performance simply cannot be obtained using conventional tube equipment. So I really have no choice but to use and recommend excellent solid state amps.