What Is A Preamp Buffer?
To answer the question “What is a preamp buffer?” let’s start with a preamp that has no buffer, i.e. a passive preamp.
At a minimum, a passive preamp provides volume control (attenuation). Optionally, a passive preamp can also provide input switching between multiple connected sources. Passive preamps control volume by using either a potentiometer (i.e. “pot”), a stepped attenuator (series of switched resistors) or, in the case of Tortuga Audio, light dependent resistors (i.e. LDRs). All these devices use adjustable resistance. Another approach is to use switched pairs of transformers which operate electromagnetically.
Passive preamps can provide exceptional detail, clarity, and uncolored performance because no power supply touches the audio signal. Tortuga Audio’s line of LDR passive preamps are examples of the best passive preamps available today. Still, passive preamps do have their challenges.
When you increase the input impedance of a passive preamp to reduce the load on your source and improve performance, the output impedance will increase along with the input. Moreover, the output impedance will vary with volume level (see yellow line in chart below). Keeping the output impedance both low and constant can often improve overall performance by making it easier to deliver the audio signal to the amplifier. This is where the preamp buffer comes in.
Adding a preamp buffer to the output of a passive preamp will keep the preamp’s output impedance low and constant. Adding a preamp buffer to a passive preamp is the essential definition of an active preamp. The term “active” is used because all buffers have a power supply that “acts” directly upon the audio signal.
Preamp buffers can be summarized as follows;
- All preamp buffers isolates an audio source from its load
- Some preamp buffers can provide voltage gain to the input
- All preamp buffers deliver current gain to the output.
- Some preamp buffers impart their own sonic signature on to the audio signal
Let’s unpack this in more detail.
Preamp Buffer As Isolation Device
The job of an audio source (a CD player, DAC, phono preamp, tuner, etc.) is to deliver high quality audio content. Audio content is represented by the voltage signal from the source’s line stage output. Every line stage output has a finite amount of power delivery capacity (power is defined as Current x Voltage). If the downstream load (current demand) is too high for your line stage, the music may sound flat with flabby bass and poor dynamics.
Adding a preamp buffer between your source and your amp (the load) isolates the source from the amp. Preamp buffers have high input impedance so your source doesn’t have to work very hard (minimal current demand) to deliver it’s audio signal (i.e. the voltage).
This begs the question, why not just use a source with a more robust line output stage and skip the buffer? Why not indeed. Many sources today do have very robust line stage outputs. In which case a good passive preamp is all that’s needed. However, some do not. All audio components are compromises between performance and cost. An preamp buffer is a specialized line stage device that performs better than the output stage of most audio source components.
Preamp Buffer As Input Voltage Gain Device
Preamp buffers can optionally boost the voltage level of the audio signal coming from your source. Boosting the audio signal voltage is referred to as adding “gain”.
All things being equal, adding gain (i.e. increasing signal voltage) gives your system more volume headroom. Since gain (volume increase) is the opposite of attenuation (volume decrease) why add gain to a system only to attenuate it?
Gain is useful if the voltage level of your source’s line stage output is somewhat on the low side plus perhaps you have this marvelous sounding low wattage tube amp connected to great sounding but not the most efficient speakers. Sure would be nice to have a bit more gain to get the volume up. Many buffers provide a range of adjustable voltage gain. Those that don’t are referred to as unity gain or voltage follower buffers.
Preamp Buffer As Output Current Gain Device
As we’ve already discussed an preamp buffer’s key purpose is to provide robust delivery of the audio signal power. Delivering the audio signal with authority to your amplifier means having enough current drive to get the job done.
Even though the current demand of an amplifier input stage is quite low (due to the relative high impedance of most amps), it’s not zero. The output stage of an preamp buffer is designed specifically to deliver greater and more dynamically responsive current that the music source can by itself. Done right, the result is better sounding audio.
Preamp Buffer As Sonic Coloring Device
The topic of sonic coloring can be controversial among audiophiles. If the objective of high performance audio is the honest recreation of the recorded music why would you want your hardware adding its own flavor to the sound? The reality is all audio hardware imparts some it’s own characteristics on the audio signal – for better or for ill.
Coloring refers the addition of certain tonal qualities or harmonics (distortion!) to the audio signal such that the audio signal is enhanced in a manner that is (hopefully!) pleasing to the listener. Do you want to color your audio? That is mostly a philosophical question. Another word for coloring is euphonics which is defined as “agreeable sound”.
Audio buffers come in three broad types: solid state, tube, and some hybrid mix of both solid state plus tube. Solid state types can use discrete components, integrated components (op amps), or some combination of both.
Without a doubt the leading example of sonic coloring is the thermionic vacuum tube or “valve” as some refer to it. Many audio enthusiasts embrace tube based gear as the pinnacle of high performance audio. The reality is that all tubes impart varying amounts of second order harmonics and possibly some compression to the audio signal. Words like bloom, warmth, euphonics, tubiness etc. are used to describe this coloration. In some tube based gear this coloration is subtle and in others it can be very pronounced. Ironically, when we speak lovingly of that “tube sound” we are actually extolling the virtues of certain type of distortion!
Not all preamp buffers use tube buffers. Buffers can also be solid state or some hybrid mix of both solid state and tube. Arguably, solid state type preamp buffers add the least amount of sonic coloration. Tube enthusiasts argue that solid state is too dry and clinical compared to tubes.
There are no right or wrong answers here, only personal preferences.
Do Preamp Buffers Improve Performance?
They can, but not all do.
Case in point. If buffered attenuators (a.k.a. active preamps) were all inherently superior to passive preamps you could reasonably conclude that buffers always enhance the sound. Tortuga Audio has a significant base of customers who have owned a wide array of active preamps yet strongly prefer Tortuga Audio’s LDR based passive preamps (sans buffers) over any of their previous buffered preamps.
As with so many things in audio, the devil is indeed in the details.
The buffer stage of every active preamp introduces another power supply into the audio signal chain. Every time you touch and manipulate an audio signal with another power supply you invite the possibility of degrading that audio signal.
It’s not an exaggeration to say that the very definition of audio is a DC power supply output with an AC music signal superimposed on top of it. That is why power supply quality is so critical to high performance audio. Literally, audio is the power supply. You can do everything else right but if the power supply quality isn’t top notch then it doesn’t matter. Of course even with a perfect power supply the rest of the design has be done right too or it can negate even the best power supply.
High quality power supply and audio design costs more. It just does. The materials cost more, the components cost more, the assembly labor costs more, the sunk development costs are higher, you get the picture.
There are countless brands and models of active preamps on the market both new and used. There are far fewer stand alone preamp buffers. Most are fair to middling, some are quite good, and a very few are spectacular.
We are quite proud of our line of LDR based passive preamps. Our customers rave about the incredible open, transparent, articulate, and musical sound quality from our passive preamps.
Can we do even better? We think we can.
To that end we plan to release our new TPB.V1 Tube Preamp Buffer by mid February 2017. The TPB.V1 will complement the already wonderful sound quality of our passive attenuators. We’re going to put a little kick in our passive’s not to mention some joyous euphonics!