In many condenser microphone circuits, the impedance converter circuit often consists of two stages. Of course, first, there is the JFET stage or JFET Op-Amp, but this is often not capable of directly driving a microphone preamp with an input impedance of 1kΩ. An output impedance of max 200Ω is required for this to be feasible. Preferably, even lower. The drain side of the Schoeps impedance converter is usually on the order of 2kΩ, and of the KM84 in the order of 6 kΩ. Directly connecting such outputs would cause too much signal loss and could also increase distortion and noise in the mic preamp. So it’s obvious that we need an additional impedance converter stage when the output impedance of the first stage is too high. It may seem like an advantage if such an additional stage would not be required, but this is not necessarily the case as we’ll see. Let’s explore some common output stage configurations and learn their properties, how they can be improved if required, and how the extra stage allows the addition of a High-Pass Filter (HPF, aka Low-Cut filter).
Basic output stage configurations
There are three different output stage configurations will be covered here. These include:
- Transformer based.
- Transformerless (i.e. electronic with BJTs), single-ended, AC-coupled, impedance balanced.
- Transformerless, differential, DC-coupled.
Transformer based
In the tube area, and later when the first JFET solid-state circuits made their way into condenser microphones, transformers were the norm. In many modern microphones, they have been replaced by much cheaper and often superior transformerless circuits. Yet, they are still popular among many who believe that transformer circuits sound better or give them that special vintage sound. Before diving into some secrets and peculiarities of the output transformer stage, you’ll first have to be familiar with the basic function of the output transformer and transformer physics. If you’re not, then here’s an accessible article as a preread for the rest.
To illustrate that there’s more to transformers than just the basics from the mynewmicrophone.com article and that transformers and their sound are not easily explained, here’s an interesting thesis to read before I continue with this article.
Transformerless, single-ended
TBD
Transformerless, differential
TBD