A Deep Dive into Passive Preamplifiers

Everything You Need to Know About The World's Most Transparent Preamplifiers

While many hi-fi components aim to shape or enhance the sound, a passive preamp’s job is the opposite: to serve as an invisible control hub, allowing signals to freely pass through without having any effect at all, other than switching and volume control. So, if you’ve ever wanted to hear your music with the kind of transparency only a passive preamp can deliver, buckle up—we’re diving into everything you need to know about them. Let’s start with some basics: 

What is a passive preamplifier?

Simply put, it’s a high-quality volume control, a few resistors and some switches in a box. No power supply, capacitors, transistors or any powered electronics. This allows you to control the volume, switch between sources and configure multiple outputs with absolutely no audible change to the sound. 

Passive vs. active preamps

Active preamps amplify the signal, boosting its voltage to drive your power amp. They’re great for low-output sources, but they can introduce noise, distortion or coloration due to the extra circuitry. On the other hand, passive preamps don’t amplify the signal—they just control volume and switch inputs/outputs. This results in a cleaner, more transparent sound.

Sounds great, so what’s the downside?

“Downside” might be too strong of a word, but passive preamps do have some limitations, all of which can be mitigated if the preamp is well designed and your system is compatible.

System compatibility

The most obvious limitation is that passive preamps have no gain, meaning they don’t amplify the signal. For proper volume, your source needs to have a high enough output voltage to drive your power amp adequately. As a rule of thumb, your source output voltage should be at least half your amplifier’s input sensitivity. This ensures that you will have sufficient voltage to drive your amp to near full volume. Digital sources typically have enough output that this is not a concern, but phono setups require more attention. Here, you’ll need to consider both the output voltage of the cartridge and the gain of the phono stage. Send us the details of your phono cartridge, phono stage, and amplifier, and we can help you check compatibility.

Output impedance and cables

The other limitation of passive preamps is that they can have high output impedance if not designed properly, which unfortunately is true for a lot of passives on the market. High output impedance is bad for a number of reasons. The first is that you need to ensure careful system matching to avoid losing maximum volume (see Impedance Matching below). The second, is that higher values introduce more noise, reducing the signal to noise ratio of your system. The third and perhaps most serious is that a high output impedance will combine with the capacitance of your output cables to form a low pass filter, rolling off some of the high frequencies. If you were to combine a 12.5 kOhm output impedance (a typical figure on some poorly designed passives), with 3 meters of average RCA cable, you get a -3 dB point of around 10 kHz! This is well below the upper limit of human hearing and will have a noticeable effect on the sound. 

How to avoid a high output impedance?

It’s quite simple really, use a 10 kOhm volume control of the right sort, such as a potentiometer or SMD stepped attenuator like in our Mini Passive Preamp. Avoid using higher values, and avoid shunt attenuators of any value. Our Mini preamp has a worst case output impedance of 2.5 kOhms, meaning those same 3m cables now have a -3 dB point of 80 kHz, well outside the limits of human hearing, ensuring the sound is not affected.

"Impedance matching"

A common misconception about passive preamps is that they require “impedance matching”. Hi-fi systems actually use what's known as impedance bridging, where the source’s output impedance is significantly lower than the load input impedance in order to avoid unintentionally reducing the volume level. Despite what many people believe, impedance does not have any effect on sound quality itself under normal circumstances, only volume. So when does impedance become a concern? Some passive preamps have an output impedance that’s high enough to cause some significant volume loss when connected to an amplifier. In contrast, the SMD attenuator used in our Mini preamp has an output impedance of 0 Ohms at full volume (a straight-through connection), meaning that impedance bridging is not a concern. You will not lose any volume, even when driving multiple amplifiers at the same time.

Different types of volume controls

Potentiometer? Shunt attenuator? What are all those? These are some different types of volume control. There are a few different types that all have different pros and cons. Let’s go over them now:

Potentiometers

The most basic type of volume control found in the vast majority of audio equipment. They use a sliding contact that rotates around a resistive element.
Pros: Cheap and cheerful. Do a good enough job for 99% of applications.
Cons: Are often of poor quality, and they tend to have very poor channel balance at lower volume levels.
Impedance characteristics: Ideal for passive preamps when using a 10 kOhm value. They have a fixed input impedance, and their output impedance will always be equal to or lower than their value divided by 4. E.g. a 10 kOhm attenuator has a fixed input impedance of 10 kOhms and a maximum output impedance of 2.5 kOhms.

Stepped attenuators

There are three main types of stepped attenuators: series, ladder and shunt. See this image to help you visualise the differences. Perhaps we will take a deep dive on attenuators in another blog post, but for now we'll just cover the basics. 

Series stepped attenuators

This is the type of attenuator we use in our Mini Passive Preamp. They work almost identically to potentiometers, except instead of a resistive track, the contacts sweep across a circuit board with lots of tiny individual resistors on it.
Pros: Higher quality and more precise than potentiometers.
Cons: More expensive.
Impedance characteristics: Identical to potentiometers.

Ladder stepped attenuators

These have similar performance to series attenuators, except they use a complex switch to select between pairs of resisters. 
Pros: Fewer components in the signal path at any one time.
Cons: They can produce switching noise (pops through your speakers). Requires double the amount of resistors to make an attenuator compared to a series configuration, making them more complex and expensive.
Impedance characteristics: Identical to potentiometers, if designed properly.

Shunt stepped attenuators

These are essentially an attempt at reducing the expense of ladder attenuators by replacing one half of the ladder with a fixed value resistor. While this sounds good in principle, making the input resistor a fixed value causes far more problems than it solves. They have a substantially higher output impedance than other configurations, causing all of the aforementioned problems such as high frequency loss. The fixed resistor is always in the signal path, meaning there is no "straight-through connection" at full volume, so impedance bridging becomes a real concern. In addition to this, they introduce a whole new problem: the input impedance now changes with the volume, and at lower volume levels it can get low enough to cause impedance bridging issues with the source as well! To put it bluntly, shunt attenuators are a solution looking for a problem and are not well suited to passive preamps.
Pros: Only two resistors in the signal path (though they are suboptimal value, which negates any benefits).
Cons: Requires extremely careful system matching and short, low capacitance cables to combat the high output impedance. No "straight-through connection" at full volume. Impedance bridging issues may arise with both the source and amp.
Impedance characteristics: High output impedance with a potentially low input impedance. The opposite of what we want in a passive preamp.

Exotic volume controls

Relays

Relay volume controls are essentially a ladder attenuator, except instead of using a big rotary switch, they use an array of relays to switch between resistors. If well designed, this can produce fantastic results. The downsides are that they’re difficult to design well, they’re quite expensive to make with high quality components, and multiple relays will click on and off every time you adjust the volume, which can only be described as sounding like a toddler rummaging through a box of Lego.

Transformers

Transformer volume controls, or TVCs, are an interesting one. On paper they are worse than potentiometers in many ways, offering higher distortion and complicated impedance characteristics that vary with the volume level. High quality transformers are also not cheap, and poor quality transformers can have terrible performance. They are the volume equivalent of using a quantum computer to perform basic arithmatic. They look a bit like one too. All of that said, there's something fun about such an absurd degree of overengineering, so if you have the budget and are willing to do some work to ensure your system is compatible, they can be a fun alternative to resistive volume controls.

Conclusion

Passive preamps offer a uniquely transparent listening experience, but only if they’re well designed and matched to the rest of your system. At Tisbury Audio, we’ve spent years refining our Mini Passive Preamp to avoid the typical pitfalls and deliver precise, transparent performance. Got questions about compatibility? Just drop us a line — we’re happy to help.