I mentioned in our first post that the LA 610 and the original “green” 610 aren’t really the same design.  There are similarities, yes. But the differences between them are very important in our opinion. So now that we have functional T4B modules inside our friend’s pair of LA 610′s, it’s time to start making the mic preamp into as much a copy of the original as we can.

That means changes are in order for the input impedances and bias points of each stage. We started by making changes to the input stage and EQ so that they were reflective of the original green 610.

Another difference is the output stage. The LA2A and the LA 610 use different output circuits. The LA2A uses an output circuit that delivers much more drive current than the LA 610 does.

Why does this matter? Because of the extra current, the output of the LA2A conducts signal with less distortion than the LA 610. So the logical choice is to convert the output of the LA 610 into something like the LA2A uses. This is a little complex because the circuits are so different. But fortunately the space exists to make these changes successfully.

Once we made these modifications to the LA 610, it was interesting to see what the test results looked like.

Below you can see the results of a THD+N, THD, and SNR test we performed using the same methods mentioned earlier: 1kHz -40dBV input, maximum preamp gain, and +4dBU output into a 600 ohm load (technically +4dBm). If you remember the earlier test, you’ll likely remember that THD+N was a little more than 35%. In this test, we can see that THD+N is a little more than 0.24%, and THD is 0.176% at +4dBm. Compare this with the LA2A, which is spec’ed around 0.35% THD at +10dBm. Not 100% identical but certainly close.

THD+N performance of modified LA 610