The Basic Difference
Solid-state amplifiers (SST), such as transistors and FET are driven by current and vacuum tubes are voltage driven. We need current to make a loudspeaker produce sound, so SST devices can be connected directly from their output to a loudspeaker. Vacuum tube amplifiers (VT) require a transformer between the device and the loudspeaker. Without going into the details of how transformers work, they basically exchange voltage for current (lowering the former and raising up the latter in this case). The increased current is then sent to the loudspeaker.
This is why you see three (typically large) transformers on every stereo VT amp – one for input voltage and one output transformer for each channel. It’s important that the output transformers don’t introduce any unwanted noise or distortion, so as always, a quality-designed and manufactured device will save you aggravation in the long run.
Beyond that, SST and VT amps do the same thing – they take a small input signal and amplify it so a loudspeaker can return the electrical energy to sound energy.
Do They Sound Different?
Generally, yes, there are some subtle differences. Of course, gear manufacturers may design their SST amps to sound like vacuum tubes and vice-versa, and every audio device has a unique voicing (or “color”). From brand-to-brand, SST and VT amps will sound different, but there are some very distinct differences between SST and VT technology regardless of brand we need to be aware of.
Because of the harmonics that are naturally produced in a vacuum tube, VT amps tend to sound “warmer” with a richer mid-range and a more natural reproduction of the human voice and acoustic-based instruments. Many people find this to be a more pleasing sound (you wouldn’t be out of line to compare the distinction to the differences between vinyl and digital).
Solid-state amplifiers tend to give you more bang for the buck (watt per dollar) with a more detailed soundstage and faster response, especially in the lower range.
That’s not to say that some SST manufacturers produce amps with sweet, warm output and that some VT manufacturers don’t produce amps with tight, controlled bass – with today’s technologies they do, but those are the technical differences translated into what we end up hearing.
Which One Is Best for Me?
If you find your music and movies are sounding harsh or strident and that your ears fatigue after only a few minutes of listening, a VT amp may be best for you.
If you like your music tight with a fast response, you may find VT amps to be too sluggish or soft, especially in the lower ranges, and an SST amp would be your best choice.
What About Matching My Speakers?
There are still some newer high-end speakers that require a lot of power, but typically modern high-end models are fairly efficient. If your speakers are power hungry and the impedance dips low, a solid-state amp is probably your best choice. But if your speakers have a flat impedance curve and are efficient, a vacuum tube amp could potentially be a great choice.
As always, when making high-end decisions, a qualified salesperson and the chance to demo your products is the most direct path to putting together a system you will love for years to come.
Maintenance and Upkeep
At some point during the life of your VT amp you’ll have to replace the tubes. It’s not hard to do and tubes are easy to find, so it’s really no big deal, but you should be aware that there is some upkeep and maintenance required with VT amps.
Why Are the Output Specs So Different Between the Two Types of Amps?
This all has to do with those output transformers we spoke about earlier, but mostly you’ll find that VT amp manufacturers tend to spec their amps differently than SST amp manufacturers.
But there’s another difference between the two that’s important to understand…
Do Vacuum Tube Amps Sound Louder Than Solid-State Amps with Higher Output Specs?
They sound louder but are not actually louder. It all has to do with harmonics. When a vacuum tube is driven into overdrive (outside its linear region), it goes into clipping at a much slower rate than a solid-state device. Clipping is unpleasant to listen to, so the longer it takes an amp to go into clipping the longer it takes for the amp to sound bad – even though there is still distortion.
The main reason VT amps appear to sound “louder” is because they develop even-order harmonics which we find pleasant to listen to. So even though the amp is being driven into distortion as far as to the order of 12dB (10% THD!), we really don’t mind it because it sounds good and natural to us. Solid-state amps develop odd-order harmonics that we find to be extremely unpleasant to listen to, so we reach our tolerance for the distortion at a much lower volume.
Basically, you need less power from a VT amp because of the slope of clipping and the type of harmonics a VT amp develops as compared to an SST amp. SST amps go into clipping and break-up way earlier and MUCH faster, plus they add odd-order harmonics which we translate as unpleasant noise, while the VT amp will sound good way beyond the same point.
Watts are Watts, but if you need less of them to do the same job comparing the specs won’t really help much.
It’s A Matter of Choice
It all boils down to what sounds better to you, and what fits your listening habits and budget better. Some folks have even bi-amped their systems using an SST amp for the low end and a VT amp for the mids and highs. And yes, those systems, when set up properly, can sound amazing. But you can get an amazing sounding system simply by putting together a system with components that sound amazing to you, whichever way you choose to go.