The Griffin Audio Preamplifier

You can learn a great deal from your students. Many times you learn where your ignorance lies when you cannot give a complete answer to one of their questions. This is a problem with yourself and if you are worth your salt, you fix the problem. Sometimes you learn about other problems. You find problems with student understanding, manufacturer’s equipment, or institutional processes. It was during one of these problem finding moments that I found the need for what would become the Griffin Audio Preamplifier.

The Problem

For those of you who have used the REI© TALAN™, you have probably found that it is much like an old handyman. He’s good to have around, and he does a lot of different things, however he’s not exactly a master at any of them. For instance, the audio amplifier on the TALAN™ does what should. It amplifies audio. When compared to a piece of equipment whose sole purpose it is to perform the same function, for instance REI’s© CMA-100™, the TALAN’s™ amplifier can only give about half the performance. On the other hand, the TALAN™ provides one the ability to push up to 90 VDC down a pair of wires. This ability is unheard of with any other commercial amplifier. So at least in this case, the TALAN™ is the master. So what do you do when you want the amplification of one amplifier with the line drive capability of another? You do what a few industrious students have tried. You clip the two of them together and attempt to grow an extra set of arms in order to run two pieces of equipment at once. It’s an effective but sad sight that usually ends in frustration.

The Requirements

Any solution devised needed to solve several issues:

  1. The solution needed to provide an equivalent amount of gain as other single purpose amplifiers on the the market.
  2. There needed to be a way of delivering a substantial amount of live drive voltage.
  3. The footprint of the solution could not not overly burden the operator.

The thought that immediately came to mind was to build a preamplifier of sorts that would attach to the TALAN™. I knew the footprint could be kept small and out of the way but I didn’t know how to deliver line drive voltage with this system. The big hurdle was going to be answering one question. How do you design an preamplifier that both amplifies audio and allows line drive voltage to pass through it at the same time?

The Solutions

The preamplifier part of the solution was solved using the SSM2019 chip development by Analog Devices. This chip is a preamplifier that is used in radios and digital audio systems. It uses a small number of external components and has fantastic gain characteristics over the audio band. The line drive part of the solution was much less straight forward. After many failures I was discussing the problem with a colleague and with his help was able to devise a solution based off POTS telephony. In this solution transformers are used to allow AC audio to pass into and out of the preamplifier while preventing line voltage from damaging the input of the preamplifier. The same pathway that routes line voltage away from the preamplifier also allows the user to push DC line bias down the line without harming the audio circuitry. It’s very much how a central office operates in a POTS setup.

The Results

The Griffin gives a lot of gain for a  preamplifier. For the majority of the audio range (900 Hz to 20 kHz) it maintains a voltage gain of over 50 dB. The chart below shows excellent frequency response all the way to the top end of the audio range.

chart_2

On the DC voltage side of things, the Griffin can attach to lines that have a maximum of 100 VDC present. Also, the Griffin can pass line drive through itself up to 100 VDC.

Unintentional Benefits

Several other benefits have become apparent during testing. The first is that the combination of using the Griffin with the TALAN™ helps reduce noise that is on the line. The SSM2019 chip in the Griffin was designed to help reduce common mode noise and this becomes very apparent when connected to the TALAN™. Secondly, using a current probe with the Griffin is not a waste of time. The Griffin provides so much gain that this normally weak sensor becomes a fantastic tool. Finally, the preamps maximum current draw is only 15 mA of current.  With a normal 9 Volt alkaline battery, the Griffin can be run continuously for over 40 hours!

Questions

If you have any questions about the Griffin Audio Preamplifier, please do not hesitate to leave a comment or email me. Thanks again! Any product or brand names contained in this article are used only for identification purposes and are trademarks or registered trademarks of  their respective holders.

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