THD Improvement kit for IG-18 series audio generator test equipment (meter buffer board ONLY) - IGA-18-2

$ 24.95 $ 24.95 24.95 USD

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Let's improve your audio meter.

No need to spend hundreds to upgrade your IG-18!

Many voices, one kit. Lots of articles have suggested improvents to the Heathkit IG-18. But until now, no one offered a complete kit to make these modifications easy for the DIY'er.  We have incorporated the best of those published ideas and now offer a complete new kit to help you upgrade your Heathkit IG-18. 

Highly compatible. This upgrade can be performed on the IG-5218, SG-18, and the SG-5218.

Confirm your desired kit format. Our upgrade kit is available in two formats:

  1. A complete upgrade kit including all the parts that need to be changed in the original IG-18, and
  2. This kit, which is a smaller 'Meter Buffer Only' kit, for owners who have already made the basic changes to the Heathkit IG-18 circuit and now want to add only a meter buffer.  

This kit is for the supplemental Meter Buffer Board only (see below).

Whichever meter type you have ...this kit handles it. We constantly strive to improve our products. It happens a very few of the last IG-5218's use a meter manufactured in Japan. It has the same electrical specifications as the earlier meters, but a smaller physical spacing between the meter terminals.  To ensure compatibility, we've gone the extra mile, and designed and manufactured for you a new PC board that fits both styles of meters.  Either way you're covered.

This kit is for the Meter Buffer Board.


This kit assumes that you have already made the easy changes to your IG-18's power supply and oscillator circuits, but either stopped short of adding a meter buffer circuit or--like many others--you implemented the additional buffer circuitry on some kind of prototyping board.

This kit is intended to replace your meter buffer-circuit breadboard if you have one, or add that meter buffer circuit to your IG-18 if you don't. This kit does not include the other components necessary to modify the existing boards in the IG-18. Just the meter buffer board (PCB and its components).

Let's get technical. The meter buffer is a crucial part of improving the THD specification of the IG-18. The non-linear loading of the analog output can create additional harmonic components due to the non-linearity of the diodes used to drive the meter.  By adding an additional transistor to increase the impedance of the meter driver and to isolate the rectifier diodes for the meter circuit, you remove this effect from the analog signal path.

Let's check out your choices.

Are you ready to go pro?

Odoo - Sample 1 for three columns

Wrong choice: Cheesy homebrew breadboard.

Here's a typical meter buffer installed using a breadboard technique. (Or maybe your kid brother did this for you.)

Odoo - Sample 2 for three columns

Right choice: Official Heathkit PCB.

Here's the IG-18 Meter Mod board included in this kit. Looks better, doesn't it.

OK, how does it do?

Let's see some proof.

THD: Improved.

Here's the graph of an IG-18 with the modifications as plotted on an HP 8903B Audio Analyzer (THD).

RMS Output: Flat.

Here's the graph of an IG-18 with the modifications as plotted on an HP 8903B Audio Analyzer (Output Level).
Odoo - Sample 1 for three columns

Gosh that's flat.

Here's a view of a Modified IG-18 FFT spectrum on a 60dB scale at 1 kHz out to 20 kHz.

Odoo - Sample 2 for three columns

Try finding some THD. Go ahead. Try.

Here is a view of a Modified IG-18 FFT spectrum on a 72dB scale at 1 kHz. The approximate 68dB floor represents a good solid 0.04% THD spec. This is the limit of our spectrum analyzer.


We thought so.

Get on back up there to the top of this page and order yourself one, before we run out.