Transistorized Mixers . Com

Transitorized mixers were developed when the elecytronics industry was transitioning into "the silicone Age". Transistors were "smaller" and lighter" than their "bulky" Vacuum Tube Grandparents. This meant that circuits were smaller and more compact and they could put alot more active components into ever shrinking spaces. To make it more appealing, the transistor utilized lower voltages, and drew alot less power (except for those psky "Power Amps". The real revolution was the reduction in size and power requirements for the pre-amp and support circuitry, not to mention the radio receiver circuits. While tubes were "Voltage Amplifiers" - the Transistor was a "Current Amplifier".

The transistorized mixer can be as simple as a "single" transitor circuit. In this circuit the transistor is a "single stage amplifier" that compensates for mixer matrix signal loss due to the attenuation of the signals in the resistive combining circuitry. It can also be a multiple transistor circuit made up of individual "single transistor pre-amp stages that pre amplify the signals prior to mixing, and even incorporating a transistor line amp driver to amplify the output signal.

It should be noted that pre-amplification is more expensive than amplification after the fact. It is also prone to "more noise" and is undesirable in an economy mixer design. It is desirable when dealing with low mic levels or low audio signal levels that "need" amplification to keep the useable signal above the base noise level of a circuit. You can attenuate a signal to a level that makes it unuseable (or indistinguishable from circuit noise).

So you see - everything is a trade-off. A) Do you need alot of preamplification? B) Can you deal with or accept the additional noise? C) Can you design the circuit with minimal noise, allow for preamplification of input signals of extremely low levels, and provide a useable signal output with ufficient level to overcome ambient circuit noise?

Then there you have it!!! A useable and practical Mixer Design!!!

Here is a common and simple transistor mixer that combines only two audio sources. You will notice the "simple Resistor Network" and the simplicity of the circuit.

In fact, with such a simple circuit you can eliminate the transistor all together. Resistive losses of such a small circuit are minimal and the transistor does not have much loss to overcome.

The only reason to have a transistor "buffer amplifier" at all is to eliminate interaction with the following audio stage. The transistor amplifier "will" provide isolation and eliminate possible interaction when levels are adjusted. If you absolutely "must" have the audio adjustments for the two channels, then the transistor buffer amplifier is a possible must. But if you can get along with just a simple non-adjustable mixer matrix circuit - You Can Eliminate The Transistor altogether. Life is a trade-off, what do you wnat the outcome to be?.

Here is a common and simple transistor mixer that combines four audio sources. You will notice that the "simple Resistor Network" is expanded, and that the circuit simply "has more channels".

But it is not quite as "simple" as before. You have "more" channels to mix, and more resistive circuit losses to contend with. But resistive circuit losses are still not really high and the transistor still doesn't have allot of work to do. But, this circuit does need isolation buffering to keep the circuit stable when audio adjustments are being made.

Just for "grins" Here is a common and simple transistor mixer that incorporates a FET (Field Effect Transistor). You will notice that this circuit is essentially the same as the 2-Input single transistor mixer. And you are right!

So why use an FET? Because it has a high gain, and better isolation properties that that "plain ole transistor". It is included here as an example. Your choices are not just limited to NPN or PNP Transistors, You can use MOSFets, FETs, or UniJunction Transistors (but I should point out that unijunctions do not perform well in the audio frequency band and are prone to RFI (Radio Frequency Interference). MOSFets are also prone to RFI, but their extremely low power requirements off set their RFI concerns when designing a circuit to run on Batteries.