This project takes the popular Heathkit IG-18 (and 5218) audio generator to a whole new level of performance, replacing the main board with a design based-on the HP-339A. The write-up is in the form of what I call construction notes, which is brief.
I did not notice much hum and the 20-1000Hz distortion measurement was at the floor of the analyzer. It's possible that the folks who dealt with the transformer were actually chasing a ground loop problem.
The new board layout has the power supply section comfortably spaced from the audio circuits. I was also very careful about ground connections, as is reflected in the layout diagrams and schematic.
So I didn't find a reason to change the transformer situation.
Let me know how it goes!
Posted by
Larry
June 11, 2011 - 11:02 pm
Thanks for link Steve. Regarding the power tran, I note you did not relocate,shield or change to toroid as some others have indicated. Did you see no improvement in doing so? I have a HP339A so I know what this circuit is capable of doing. Good work.
Posted by
Steve L.
June 11, 2011 - 10:15 pm
Hi Larry, The HA3-2625-5 is available from Newark electronics here: Newark link
The listing is a little confusing but it IS in the DIP package and they have 400 in stock and they will sell quantity-1 for $8.55. Note that after stock is depleted, they will only be available in multiples of 500.
I did not try any other opamps, since I felt confident that this one would work well there (per the original HP design). However, I imagine that there are some high-performance opamps which could match it. This one has 100MHz gain-bandwidth, 35V/us slew rate and works from ±15V power supplies.
I will be happy to help if you have any other questions.
Steve
Posted by
Larry
June 11, 2011 - 09:52 pm
Have you a source for the HA3-2625-5 IC? If not have you tried another low distortion opamp?
Posted by
Steve L.
February 13, 2010 - 08:11 pm
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Finished IG-18SL
Bottom view of the oscillator board wiring
Oscillator board
Oscillator board installed. New range switch with cap board mounted.
