So you want to buy a tube preamp? Good move- a tube preamp can be one of the most rewarding upgrades you can make to a stereo system.
Apart from a new set of speakers , a good tube preamp will give you the most audible change to your system.
But like speakers, there are good and not so good preamps , so what should you look for when previewing preamps?
1. TYPE OF TUBE PREAMP
There are many ways to build a tube preamp. It can be very simple with one tube per channel , and even simpler with one double-triode tube for both channels.
And it can be very complex with lots of tubes and balanced operation.
The main advantage of tubes over solid state transistors is that zero feedback can be utilised in a tube preamp, whereas its very difficult , and usually unsuccessful, to build a transistor pre with zero feedback.
Now feedback isn’t necessarily a bad thing , but the amount of feedback needed to linearise a transistor preamp is.
The beauty of a tube preamp is it can be kept as simple as possible , and with this simplicity comes a purity of tone , and a more immediate access to the heart and soul of the music.
But you don’t want to get too simple. There were (and still are) some really nice simple and elegant tube preamp designs done back in the middle of the last century.
Maybe a pair of triode tubes with a tube rectifier in the power supply and not much more.
Driving a pair of tube power amps with a nice easy 100Kohm load they can sound quite nice , albeit usually with a “warm, romantic” tone.
Which can be initially beguiling , but any soft flavour can become annoying after the initial amusement wears off.
And there is no way these simple preamps will drive a solid state amplifier accurately .
And you don’t want to get too complex. Mega tubed preamplifiers , some with a dozen tubes in them were common at the turn of the century.
They were capable of extra-ordinary technical specifications – response from 0- 500KHz +,- 0.5dB and output impedance below 100ohm, so they could drive any load.
They were often fully balanced (hence all the tubes – to do a true balanced tube preamp you must have double the circuit, double the tubes, double the components.)
These super complex preamps were just plain horrible to listen to, not much better than a mediocre solid state preamp.
A good modern tube design will be single ended Class A design with enough tubes and components to do the job and achieve that tube purity we all love.
With a fully balanced preamp, doubling the tube count and doubling the circuitry leads to a kind of double tracking of the sound, there is a smearing and veiling of the sound that robs the tube preamp of its inherent purity of tone.
Yes balanced operation is everywhere now , but there is absolutely no need for it in a home stereo environment . Its sole purpose is to eliminate noise in very long (>100 feet) cable runs.
It comes from the stage world and that is where it belongs.
If you want to hear the best that tube pre-amplification can deliver than preferably no balanced inputs.
You want a pure single ended source into a single ended tube preamp that is designed to do absolutely zero degradation to the signal.
(Balanced outputs is a different story . Here we can use a centre tapped transformer to elegantly and simply generate a positive and negative phase for each channel, at the output of the single ended circuit. And a simple switch will convert it harmlessly to unbalanced or balanced operation. Its one of the few win-win situations in audio, although my ears still always prefer unbalanced operation. But its handy if you have balanced power amps. )
So our ideal tube preamp will have a single ended circuit, designed to achieve an appropriate amplification of the signal with zero degradation, perhaps a touch of “good” harmonic distortion (or euphonics) with suitable specifications.
2. SPECIFICATIONS
There’s four main specifications you need to worry about with a tube preamp.
The first is frequency response. The limits of human hearing are 20Hz to 20Khz , at best.
But even though I can only hear sine tones to 12Khz I can “hear” if a preamp rolls off under 20Khz, so obviously we can hear the harmonic overtones , and so a preamp should be able to extend out to beyond 20Khz at least.
If you are going to connect a subwoofer than you want to get down to 10Hz for infrasonic sound that you feel rather than hear.
The second spec and closely related to the first is output impedance. The lower the output impedance of a preamp the lower input of a power amp it can drive, linearly .
Linearly, means flat frequency response with no deviation from 20Hz to 20Khz .
The ideal ratio is at least 1:10. So if a preamp has an output impedance of 300 Ohms it will easily drive a power amp with an input impedance of 3000 ohms (3Kohms)
Going back to our example above of a simple triode preamp – it will have an output impedance (Z) of around 6K, so you wouldn’t want to use it with a power amp with input Z of less than 50K .
Most solid state power amps are around 10K so the ideal output Z of a tube preamp is <1K.
How to derive that impedance with maximum fidelity is continued below.
The third spec to consider is gain. In theory a preamp is required to supply enough gain for the following power amp to reach maximum output or power. How much depends on the sensitivity of the power amp, ie how much voltage it needs to reach its maximum power .
That can vary, but these days power amp sensitivity is usually quite low , sometimes as low as, or less than 1 volt. Really, these are not power amps, but integrated amps.
However, a high gain preamp always sounds better than a low gain preamp, they have a sense of greater dynamic range , and have a bigger soundstage and imaging properties. They are more capable of “magnifying” the sound, with more realism and texture.
But if the following power amp has very high sensitivity there may be a problem with the volume control being used at low levels and there is a possibility of noise issues.
The ideal solution is for the preamp to have adjustable gain , so as to adjust for the ideal synergy between the two units.
(Well actually the ideal solution is to have a high gain preamp and a low gain power amp- the preamp provides voltage and the power amp turns that voltage into current to drive the speakers. )
Lastly, the noise (and hum) of the pre should be inaudible . Tubes are noisier devices than transistors, but good engineering of the power supply will ensure the pre is quieter than any noise you can hear.
Its particularly critical with the phono section, as the gain will be as high as 10,000 for a moving coil cartridge. Most manufacturers cant get this much gain with tubes without a lot of noise, so they drop the gain and use a step up transformer for MC cartridges.
It can be done without the step up , but it requires a lot of experience to get the earthing right , and ensure quiet operation. Its difficult but worth it to get the more dynamic sound of an all active phono stage.
There are other specs to consider, but these are the 4 main important ones, get them right in the context of your system and you cant go wrong.
3. OUTPUT IMPEDANCE
We said that we want an output Z of <1K ohms. Most preamp tubes have to have some sort of conversion to get a Z under 1K.
There’s a number of ways to do that.
The easiest , and most common way is to use a double triode and use one half of the tube as a cathode follower circuit. Technically, it works very well – it needs very few components and gives a low impedance that will drive just about anything.
Trouble is , it uses 100% generative feedback , and it sounds like it. Its quite veiled sounding and tends to shrink the soundstage.
Very common in Chinese preamps, and low cost preamps , and manufacturers with little tube design experience.
Another way is to use negative feedback, either local or global. But to get a typical preamp tube’s output Z low with feedback, you need a lot of it. Again the same problems as with cathode follower.
Low plate resistance tubes give low output Z, and while not many can get much below 1K ohm they can do a good job , with a little or no feedback .
Tubes like E810F , D3a, 6E6P, 4P1L etc in triode mode, or even power tubes like the 2A3, 300B etc can be used as preamp tubes with the appropriate design.
But these tubes can be rare and need some serious engineering to get the best out of them.
The optimal solution to getting low Z with a suitable preamp tube is to use a step down output transformer.
With a tube like the 6SN7 coupled to a suitable output transformer its possible to get a Z of around 600 ohms- perfect for just about any power amp.
And as discussed above we can easily put a centre tap in the transformer and get a true balanced output , which can be switched in and out. It’s elegant , and if its a high quality transformer will give excellent response and phase performance.
Its my preferred method of design, it can use a little feedback or none at all , and has a harmonic overtone that is very accurate, transparent and musical.
4. TUBES
Common preamps tubes are the 12AU7,12AX7, 12AT7, 12AY7, ECC88 ,6922, 6H30, 6SN7 and some rarer NOS tubes.
The most used are the 12AX7,12AU7, 12AT7 tubes. These are cheap, made in a variety of factories are are just about standard in all the low to medium cost preamps.
They were designed to be used in televisions , or as cheap replacements for the older audio only preamp tubes.
They are an inferior audio tube and have no place in a good audio preamplifier.
The 9 pin ECC88, 6922, 6H23, 6H30 have their place in preamp design, but for line-stage use its hard to beat an octal 6SN7 tube transformer coupled to the output.
The Directly Heated Triode (DHT) tubes can be very good , and suit those after a slightly more euphonic sound, but do need some serious engineering to get the best out of them.
5. RECTIFICATION
Rectification is the process of converting the power transformers AC voltage to DC voltage.
It can be done using rectifier tubes or solid state rectifiers.
There’s a belief that tube rectifiers have a “warmer” sound but the design of the power supply has more effect on the preamp than using a tube rectifier.
Tube and solid state rectifiers and a hybrid design using both solid state and tubes works quite well.
Tubes have the advantage of a “softer” start than solid state.
The more common tube rectifiers are the 5AR4, GZ34, 5U4G, 5Y3G , 274B, EZ81 and 6CA4.
A common mistake in many preamp designs is to use too much capacitance after the tube rectifier.
Tubes like the 5U4G/274B don’t like more than 4uF after them, and I’ve seen many expensive preamps with far too much capacitance , leading to premature rectifier failure.
6. POWER SUPPLIES
Simply put, bigger is better . Manufacturers have to choose a fine line between performance and cost, but the bigger and better engineered a power supply is , the better performance and reliability will result. Its one of the most important factors of preamp design , the faster, cleaner, stronger the power supply is, the better the sound.
Power transformers, chokes , good solid vibrationless cabinets are heavy, and the heavier a preamp, the better indication that it is a quality unit.
Seperate power supplies , connected to the preamp by a umbilical cord, ensure quiet hum free operation , and are essential for phono preamps .
Voltage regulators are a sign of good, knowledgable design , and give very worthwhile improvements in sound quality.
The king of the regulator is the tube shunt regulator. This keeps the voltage of the preamp circuit stable and constant, no matter the conditions .
7. OPTIONS
Audiophile capacitors, NOS tubes, “super” volume controls.
When a manufacturer makes a bigger deal out of available options than he does on the actual design of the preamp, be worried.
The design and construction of the preamplifier is far more critical and important than having the option of putting profit driven components in to a preamp.
The quality of the components used is important- paper in oil capacitors do sound different to polypropylene capacitors , but the difference is far less than the difference between a good circuit design and a poor outdated design.
A tired and unimaginative design from 1950 fitted with expensive capacitors will not come close to a modern, innovative design with good quality capacitors.
Similarly , you can spend a lot of money on volume controls, but you will never get a “better” sound than from a good quality Alps Blue pot. Different maybe, but certainly not better.
Circuit design and implementation is the beginning and end of quality sound.
Digital volume readouts might look nice, but in a tube preamp? No thanks.
8. WARRANTY
The warranty given by a manufacturer is a statement of how much confidence they have in the engineering of a preamp.
If the warranty is for a year , then that implies they don’t want to take the risk that it might break down after one year.
If a manufacturer gives a lifetime warranty , then the preamp has to be engineered to last a lifetime.
That takes some pretty serious engineering. But it can be done.
I have a couple of sets of Australian Trimax EL 34 power amps that were built in 1956. They’ve had a few capacitors replaced, but they are performing as good as new. That is because of the quality of the power transformers, the output transformers and the quality of the original components , and the construction.
As far as I know, Supratek is the only preamp manufacturer that gives a lifetime transferable warranty with its products. That is enough said.