Common Amplifier Myths

 

You Should Always Use (Some Brand) of Tubes

There is seldom any reason to pay premium prices for "special" VTs. I'm certain that Western Electric NOS 300Bs are quite decent tubes. I'm also certain that they are not worth the $High Four Figure prices these command. You can see certain types going for some ridiculous prices these days. That's nothing but pure audiophoolery. Put an over priced tube in a mediocre amp, and you still have a mediocre amp. "NOS" is also not necessarily the magic word either. Back in "the day" good, bad, and indifferent tubes rolled off the assembly lines by the tens of thousands every day. Some war surplus is positively hideous, which is why it's surplus -- military rejects. Truth is: some NOS tubes are horrible, and there are current production tubes that are every bit as good as those made in "the good ol' days".

This is not to say that the seller is necessarily trying to put one over (although that does happen all too often). The advertised tubes may work especially well with whatever equipment he's using. It doesn't mean equivalent performance in random gear with random speakers.

"Tube rolling" does have a basis in fact. Unlike transistors, VTs are low gain devices. Being low gain devices, circuit performance is more dependent on the characteristics of the active device. Transistors have so much gain that circuit performance is totally dependent on the circuit itself. Unless you need some special property (such as an unusually low noise figure, or that can operate at unusually high voltages) it doesn't matter what transistors you use so long as the transistor in question can process the signal frequencies. Not so with tubes, especially in designs that don't incorporate any NFB. You may get a different sound from different tubes of different construction, name brands, model years. Whether it's worth it or not is something else.

It is not necessary to pay audiphool silly prices for good sound.

Point-to-point is Good, Circuit Boards are Bad

This is definitely true for RF circuits. For P-2-P wiring, the insulating medium is air. The resulting stray capacitance is much lower, and performance definitely enhanced. This is a good reason to opt for "dead bug" or P-2-P construction of RF circuits. At audio frequencies, there isn't enough of a difference in stray capacitance to make a difference. P-2-P has the advantage of allowing the free use of "parts on hand". Circuit boards have the advantage of easily replicated construction, and much better quality control for mass production. To be sure, a badly laid out circuit board will ruin the sonic performance of an otherwise excellent design. So too will a poorly laid out P-2-P job.

If You Use Circuit Boards, These Should be Teflon

Again, a good idea for UHF circuits, but the savings in stray capacitance will make no difference at audio frequencies.

You Should Always Use Teflon Insulated Silver Wire

Another excellent idea for UHF construction. The tiny savings in resistance and stray capacitance will make no difference at audio frequencies. The added expense isn't worth it for frequencies as low as the audio band.

You Should Always Use Carbon Composition Resistors

Another idea held over from RF practice. C-comp resistors have the advantage of having a very low inductance. C-comps hold their DC resistance well into the VHF band. Otherwise, C-comps are very noisy. Save them for your RF projects. Metal film resistors are a good deal quieter, and better suited for sensitive applications, such as low level stages. Those few nanohenries that are a big deal at 400MHz are of no consequence at 400Hz.

Negative Feedback is Bad

This bit of nonsense does have a basis in fact. In all too many cases, NFB is used to cover up a poor open loop design. Pour on enough NFB and you can sweep your mistakes under the carpet, along with a good deal of the vitality of the music. This, however, is not an indictment of NFB. Used properly -- to improve an already good open loop design -- NFB can make a good amp even better. This is the key: the open loop design must be a good one. If you find that you need excessive amounts of NFB, then it's better to go back to the drawing board and correct your open loop design. All too frequently, this is not the case.

Furthermore, there is no escaping NFB. Every device, vacuum tubes or transistors of every sort, have inherent NFB. This is the cathode/source/emitter resistance: r~= 1/gm. This resistance will cause degeneration just as surely as if it were an unbypassed resistor soldered into the circuit. Just because you don't see it doesn't mean it's not there. Furthermore, triodes have an additional feedback mechanism. Plate current is dependent on the VPK. When a signal tries to pull the control grid positive, plate current rises, but the VPK drops and tries to pull the plate current down. Since the plate and grid are pulling in opposite directions, this is negative feedback by definition. Yet triodes are preferred to every active device that doesn't have this property (pentodes and transistors).

If anyone claims to have a "no feedback" design, they're either ignorant or trying to put one over -- or what's worse: both.

Solid State is the Spawn of the Devil

Another partial truth here. The first solid state amps were truly awful: noisy, unreliable, and with hideous sonics. There has never been a technology with no down side, and there never will be. There are some things that SS does very well, and some things it does very poorly. The key is balancing the strengths with the weaknesses. The only reason to avoid all solid state is if you are replicating 1930s-era equipment for the nostalgia appeal. Otherwise, do you also say "No" to modern line, interstage, and output transformers and chokes that are made from exotic materials that didn't exist in the 1930s? All these devices can perform much better these days, and make those old circuits work much better now than they ever could then.

With zero impedance and perfect regulation, plate-circuit distortion does not exceed 2.0%... The driver stage should be capable of supplying the grids of the Class AB2 stage with the specified peak grid voltage at low distortion. The effective resistance per grid should not exceed 500 ohms...

-- 1624 Spec Sheet

Back in "the day", this was asking the nearly impossible. Getting Zo below 500 ohms is very difficult even for a cathode follower, unless it uses some of the big power triodes. These days, any power MOSFET source follower can come very close to the ideal of zero drive impedance, certainly below 10 ohms. Use solid state where it gets the job done right. The fact that it was a considerable challenge is testified to by the lack of Hi-Fi designs left over from "the day" that operate Class AB2. Most of the designs that did are for AM plate modulators and PA systems where the premiums were lotsawatts and efficiency, not sonic performance. These days, there is no reason not to take advantage of going Class AB2.

Another bit of mythology is the supposed noise that solid state diodes cause. Except for audiophile circles, I have never heard such a claim. Furthermore, I haven't seen it on the o'scope screen and I have never heard it from any receiver operated right next to any audio amp that used a solid state power supply. Gas and mercury vapor diodes, however, are notorious for producing all sorts of hash and incidental RF. Otherwise, solid state diodes offer much lower forward drops, don't get anywhere near as hot, have significantly higher Isurge ratings, and require neither heater power nor another hole in the chassis. They better lend themselves to voltage multiplier circuits, and bridged circuits. If you are that worried about high frequency noise from solid state diodes, the "fix" is a two or three pole LPF made from air coils and mica capacitors. Won't do any harm, but it sure doesn't look necessary either.

If there is any down side to solid state, it's that the high voltage comes up within a couple of seconds or so -- long before cathodes have a chance to warm up. This can overvolt direct coupled stages, possibly exceeding VHK ratings. Since silicon diodes can source much higher currents, there is the temptation to use them with huge reservoir capacitors. The increased current demand can stress vintage power transformers that weren't designed to handle currents in excess of those expected from vacuum diodes.

A solid state power supply may or may not lead to a different sound. If there is a difference, it's due to the better voltage regulation of the solid state power supply. For some applications, good regulation isn't desirable. This would apply to over driven guitar amps where the voltage drop at max current demand lends to compression, and "sustain" as the voltage comes back up. The same effect can color the sound of a Hi-Fi system. If you're used to that sound, or actually prefer it, then, of course, solid state will be considered inferior.

It is questionable whether it makes a difference in a PP amp since the AC sums to zero at the primary center tap, making it a virtual AC ground. It might make more of a difference in a SE design, since the power supply is in the signal path. Even more reason to use solid state, as the AC impedance is considerably lower.

Tube Watts are Bigger than Transistor Watts

A watt is a watt: one newton * meter per second. Transistors clip hard and fast. This is what makes them superior digital devices, but horrible analog devices. Add NFB, and clip behavior becomes even worse. Tubes clip much more gently, first simply rounding off the peaks, as opposed to just flattening them. This gradual rounding generates mainly low order harmonics that roll off quickly with increasing frequency. The sharp discontinuities of peak flattening generates high order harmonics that roll off gradually with increasing frequency. Since low order harmonics are much less detrimental, one doesn't hear the occasional clip on fast transients. A solid state amp needs to stay out of clipping. Therefore, a tube amp will sound louder than an equivalent solid state amp, since you don't hear the occasional clip, you can turn up the volume higher before obvious distortion is heard.

Tube Amps Have Poorer Bass

This may have been true at one point. One of the advantages to NFB is that it reduces the output impedance. A low output impedance is needed to damp the woofers, otherwise they will tend to produce their own resonance note, as opposed to the notes the musician actually played. Back in "the day", the main problem here was inadequate open loop bandwidth. You can have a billiard table flat frequency response from 30Hz to 20KHz, however, if it's NFB that's causing it, the bass will not be as good as you'd expect. If the open loop response prematurely rolls off the low end, the NFB decreases, thus driving up the closed loop gain. Less NFB is less effective NFB, and won't dampen the woofer(s) properly. This problem was due to interstage coupling capacitors that were too small. At one time, your choice for capacitors was limited to either PiO, wax paper, or ceramic. PiO and wax tend to be leaky (not the oil or wax, but of current) and deteriorated with age. Ceramic wasn't leaky, but also didn't come in the sizes and voltage ratings you'd like to see. Consequently, there wasn't enough open loop gain at the low end.

These days, with new polymers, it is possible to get capacitors in the sizes and voltage ratings you need for good open loop performance at the low end. Vacuum tube amps can have good bass, with adequate speaker damping. (About the only exceptions are sub-woofers with ceramic or carbon fiber cones and heavy voice coils. These sub-woofers are difficult for even a solid state amp to keep under control.)

"Magic" Resistors

Yet another out growth of a real phenomenon. During the early 1970s, it was discovered that capacitors really did have an effect on sonics. Up till this point, no one really paid any attention to passive components. They knew about such things as ESR, DF, Loss Tangents -- however it was only VHF designers who needed to worry about this. Capacitors introduce distortions, due to D/E field nonlinearities, piezoelectric effects, and even self-excited resonances. So far as dielectrics go, the worst offenders tend to be metal oxides (ceramics, electrolytics) and other high permittivity materials. The more benign being the polypropylenes and teflons. Unfortunately, these are also materials with low permittivities, and so do not lend themselves to packing a lot of capacitance in small packages.

While dielectrics have a proved effect on sonic performance, this has also occasioned a lot of audiophoolery. From legitimate claims about the effects of various dielectrics with data to back the claims up, to utter nonsense regarding the "sound" of resistors. There are lots of claims with zero data about the superior attributes (available at superior $Prices) of these things. Don't be conned into paying dollars for some part that would normally cost pennies.

If you see extravagant claims with nothing more than vague generalities backing them up, better get a tighter grip on your wallet.

Component "Burn-In"

You see these claims about a variety of components: they require X number of hours of "burn-in" before they start to sound good. This is complete nonsense. Capacitors polarize within seconds of the voltage application. I've seen claims of 100 -- 500 hours for Teflon capacitors, and with AuriCaps. The first time I replaced an electrolytic coupling capacitor with an AuriCap, I noticed the improvement at once. Of course, electrolytics make lousy signal capacitors, and AuriCaps are pretty good. There was no "burn-in" required at all.

Resistors sometimes burn out, but they never "burn-in". As for cables, well, you figure that one out. The only components that require a certain amount of "burn-in" are vacuum tubes. It may take minutes to hours for the bias to settle down when new tubes are put in service. This is completely normal, but does not apply to any sort of passive component, nor does it apply to solid state devices at all. Such claims are largely disengenuous, in that the time it takes for the new component to "burn-in" is equal to the time it takes for you to get used to the new (and most likely) inferior sound of whatever over priced, whiz-bang, gizmo you just likely spent way $Too Much to acquire.

Next time you see this particular claim, ask yourself one question: why didn't they burn the thing in at the factory before sending it out the door?


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