Tube Preamplifiers – My thoughts…

I prefer to design tube preamplifiers and solid-state amplifiers. The reason for this is simple. Tubes amplify voltage. Transistors amplify current.

So, what is the purpose of a preamp?

In modern terms, it is a simpler function than it used to be. Today’s sources have adequate gain to drive most amplifiers to full volume. Audiophiles eschew tone controls due to modern purist trends. Balance controls can be desirable, and they also add another component to the signal path.

Of course, the preamp serves as source select, volume control and gain. Although these days, not much gain is needed. So, aside from gain, why not just use a passive attenuator and high-quality selector switch? The answer to this, is impedance!

In my opinion, one of the biggest things that a preamp does, in addition to gain, is to provide the ideal impedance relationship with the amplifier. The rule of thumb for ideal preamp to amp impedance relationship is 1:100. If you have an amp with a 50K input impedance, you want to be sure that your preamp has an output impedance of less than 500 ohm. When matching tube preamps to SS amps, this is worth considering, because some tube preamps can have fairly high output impedances, often > 1K. If you are matching that tube preamp with a SS amp with a 10K input impedance, it may not be ideal. I find that bass suffers first in these cases.

So, a preamp must:

Provide adequate gain.
Provide volume control.
Provide source select.
Provide a low enough output impedance to match the desired amplifier(s).
Provide adequate drive to overcome cable losses and impedances.
And most importantly, do this WITHOUT noise, added distortion or sacrificing frequency response.

This sounds simple enough, but as in most things, details matter.

The most important things that I consider when designing a ModWright preamp are:

Frequency response.
Noise floor.
Output impedance.
Distortion.
Musicality and tonality.

I use tubes because I like their musicality and tone. Low impedance, low distortion, low noise floor and wide bandwidth, can all be achieved, but details matter!

In my next blog entry, I will explain the design topology of our LS 300 Reference Balanced Tube preamp https://www.modwright.com/products/ls300-balanced-tube-preamplifier/ that we will be using at Capital Audio Fest in Rockville, MD https://www.capitalaudiofest.com/event-details/capital-audiofest-2025 in a few days.