Actually, let’s start with a slightly different question…
Are valve amps louder than solid state amps?
They often are. Take for instance a Marshall MG30 and a Vox AC30 and crank them to the max clean volume. There’s no question at all that the Vox will sound significantly louder. Or take a Fender Blues Junior (15W) and a Fender Frontman 15. The perceived volume difference is huge.
But hang that’s not really comparing apples with apples is it? Onwards.
Let’s talk about perecived loudness
I’ve heard many people say a valve amp sounds twice as loud as a solid state amp.
There’s a whole field of psychoacoustic study to delve into here. I’m just going to pick one thing out.
an increase of 10dB results in double the perceived volume.
So if we can account for 10dB of difference then we can begin to understand why some valve amps are louder than some solid state amps.
Why are valve amps louder than solid state amps?
- Player demographic.
- This is the biggie.
- Transistor technology is more affordable than the thermionic valve. There’s no doubting that.
- So the cheaper amps are often solid state, the more expensive are often valve.
- Cheaper amps are more likely to be sold to ‘bedroom’ players and children. Someone who’s gigging is more likely to spend more and be able to afford a valve amp.
- The amp development engineers know this, so they design ‘bedroom’ amps to give sensible volume control in a bedroom range. The user can turn the volume dial to about 12 o’clock before the parents complain. They design ‘gigging’ amps to give sensible rehearsal volumes from about 9 o’clock and full on ‘fight the drummer’ volumes at 12 o’clock
- This is just sensible design.
- Speaker sensitivity (efficiency) – for more info
- See point 1, solid state amps are often used at the price conscious end of the market
- Speakers have a sensitivity, measured in dB/Watt
- Let’s look at some real speakers, chosen at random from Celestion’s range:
- Celestion Vintage 30 – used in mid and high end gear. 100dB/W
- Celestion 70/80 – used in low mid range amps. £500ish. 98dB/W
- Celestion Rocket 50 – used in amps around the £200-350 mark. 95dB/W
- You can’t get efficiency data for the speakers used in even cheaper amps.
- The difference between a vintage 30 and a rocket 50 is 5dB. That means that the Vintage 30 will be perceived by the ear as 50% louder.
- Output load rating and power amplifier current/voltage control
- Valve amps use an output transformer. Although it’s primary purpose is just to match the high impedance valve output with the low impedance speakers, there’s a side benefit – many transformers have tappings to allow a valve amp to match a 4 ohm, 8 ohm or 16 ohm speaker. This means that the amp delivers roughly the same power into any load.
- Very very few solid state amps have an output transformer because they’re not needed – it’s easy to create a low impedance output with transistors.
- So this leads to a scenario in which a solid state amp outputing 15.5Vrms will deliver roughly 60W into 4 ohms, 30W into 8 ohms and 15W into 16 ohms. When the engineering department tell the marketing department this, guess which figure the marketing team put on the front of the amp?
- It is possible to build a voltage drive (with a switch) OR current driven solid state amp that will deliver the same power into different loads. But guess what? It’s more expensive due to the higher voltage rails required, see point 1.
- Output impedance
- Most solid state amps are voltage controlled (certain era Roland JC, Marshall 80XX series, Fender ultimate chorus and some Trace gear excepted). voltage control results in a low output impedance
- Valve amps with no negative feedback around the transformer have a large amount of current control
- Rod Elliot of ESP (http://sound.whsites.net/valves/valve-trans.html#s4) does a worked example that shows that these differences can result in about
- Distortion harmonics
- This is the first one that’s actually related to technological differences between valves and solid state.
- A distorted signal has a higher RMS than a clean waveform. Valve circuits are often more distorted than solid state, even on the ‘clean channel’. However FETs and diodes can be used to distort solid state amplifiers.
- The distortion adds extra frequency content in the most sensitive part of the human ear.