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How did we approach A/B testing for the LA 610 mod? We made every effort to follow the AES test standards for mic preamps.  In cases where that could not be followed, we disclose the procedures used.

la 610 a/b testing and methodology

Below are several test plots of the LA 610. Every effort has been made to follow AES standards for each test. These plots show SNR, THD, THD+N, ITU-R and SMPTE intermodulation distortion, and impulse “chirp” based frequency response performance of a stock and a CC&E modified LA 610 measured using the following parameters:

  • For SNR, THD, and THD+N tests the input signal is -40dBV. This level was chosen because of its similarity to mic input level.
  • Mic preamp set at maximum gain (+10) and level pot set to 10 (as the AES test standard for testing mic preamps requires).
  • Input load is 2k.
  • For THD+N tests, output trim is set to yield a +4dBU (1.23Vrms) signal, and terminated into 600 ohms (+4dBm).
  • For IMD testing output trim is set to 0dBV and terminated into 600 ohms (0dBm).
  • Source impedance is 50 ohms.
  • Bandwidth is 5Hz-24kHz except for SNR which is 5Hz-20kHz.
  • All measurements are unweighted.

The idea behind tests of this nature is to put the device under difficult conditions in order to get an idea of its performance. It’s like a “how fast can you run that 5K uphill marathon” battery of tests. They show both how well the gain stage does its job as well as the way the output stage performs when driving a load. For a little more in-depth analysis of what we’ve done here, please check out the blog posts written about this procedure.

la 610 THD+N tests
chart1
The first plot shows the THD+N performance of the stock LA 610.

You can see the 1kHz fundamental whose amplitude is 1.23V, and you can see that the distortion behavior extends out from the 2nd, 3rd, 4th, 5th, and on to the 24th harmonic. THD+N makes up just over 35% of the signal, and SNR (Signal to Noise Ratio) is about 27 dB. For all intents and purposes, the device is behaving like an overdrive pedal in this test.

la 610 THD+N tests
THD+N performance of modified LA 610
The second plot shows the performance of the CC&E modified LA 610 under the same test

Again, you can see the 1kHz fundamental whose amplitude is 1.23V, and you can see its distortion behavior. THD+N makes up just over 0.24% of the signal, and SNR (Signal to Noise Ratio) is about 56 dB.  As an aside–this isn’t the exact specified THD of the original LA2A, but it’s pretty close.  (.35% @ +10dBm for the original vs .176% THD @ +4dBm here).

The second test performed is actually a group of tests

The first is something known as ITU-R, and the second is SMPTE IMD. Both tests are forms of IMD (intermodulation distortion) tests, and their purpose is to simulate how a device will behave when complex signals (like music) are present. The ITU-R test uses two equal amplitude tones, one at 19kHz and one at 20kHz. The tones influence (modulate) each other to produce distortion products.

LA 610 stock ITU-R
stock LA 610 ITU-R distortion test
To obtain the distortion figure for this test, we sum the amplitude of the three distortion harmonics relative to the amplitude of the test tones.

From the marker at M2 we can see that the 1kHz product is -17.2dB relative to the test tones, and the 18kHz and 21kHz products are -6.2 and -6.7dB.  I use the average of the two 3rd order mixing products (18kHz and 21kHz), which is -6.85dB.  If we add this level to the 1kHz harmonic, we can see that the LA 610 has a little more than 43% ITU-R IMD at +0dBm.

CC&E modified LA 610 ITU-R test
LA 610 mod ITU-R test
Here is the same ITU-R test performed on the CC&E modified LA 610.

As in the test with the stock unit, we measured the 1kHz harmonic and the 2nd order mixing products at 18kHz and 21kHz.  Together these give us an ITU-R figure of 3.2% at 0dBm

The second component of this test is SMPTE IMD.
LA 610 stock SMPTE test
LA 610 stock SMPTE IMD test

If we look at the chart below, we see the modulation tones that are occurring outside of the 7kHz fundamental once the 60Hz signal is activated.  The SMPTE spec is interested in ALL of the energy outside the 7 KHz tone.  Technically we should add them all together.  In this case, a measurement of ALL the IMD energy becomes somewhat overwhelming.  Thankfully we can get fairly accurate idea of the IMD behavior if we just consider the first two pairs.  Looking at markers M1-M4, we can see the amplitude of the distortion harmonics adds up  to -2.9dB relative to the 7kHz fundamental.  So based on these tests the LA 610′s SMPTE IMD figure is just over 71% at +0dBm.

Here is the same IMD test performed on the CC&E modified LA610.
modified LA 610 SMPTE IMD test
LA 610 stock SMPTE IMD test

In this case, the sidebands are -46.4dB and -47.3dB below the fundamental, yielding a SMPTE IMD figure of approximately 0.44% at +0dBm.

The third test is an impulse based frequency response test.

This test is performed using a short impulse “chirp” with a frequency spectrum that spans the audible band. Measuring the impulse at LA610′s output with a Fast Fourier Transform (FFT) shows us the device’s frequency response.  This test was performed at all  five gain settings, -10, -5, 0, +5, and +10, as a means of measuring response at each setting using a -40dB impulse.  ”Level” was set to 10 for the test.  As we pointed out in our blog post this caused saturation in the stock LA610, resulting in skewed frequency response readings, so the “Level” knob was reduced to 8.  This did not prove to be the case for the modified LA 610 though, so we performed the test with the “Level” at 10, as originally intended.

Below are the impulse responses of each of the five preamp gain settings for the stock LA 610.
stock LA 610 FR -10
Stock LA 610 Frequency Response

Here is the stock LA 610 frequency response with a -10 gain setting.  Remember: our -40dBV input pulse causes the preamp to overload on maximum gain and creates an unusable test reading.  In this test, the "Level" pot is set to "8".

Below are the impulse responses of each of the five preamp gain settings for the stock LA 610
stock LA 610 FR -5
Stock LA 610 Frequency Response

Here is the stock LA 610 frequency response with a -5 gain setting. Remember: our -40dBV input pulse causes the preamp to overload on maximum gain and creates an unusable test reading.  In this test, the "Level" pot is set to "8".

Below are the impulse responses of each of the five preamp gain settings for the stock LA 610
stock LA 610 FR 0
Stock LA 610 Frequency Response

Here is the stock LA 610 frequency response with a 0 gain setting. Remember: our -40dBV input pulse causes the preamp to overload on maximum gain and creates an unusable test reading.  In this test, the "Level" pot is set to "8".

Below are the impulse responses of each of the five preamp gain settings for the stock LA 610
stock LA 610 FR +5
Stock LA 610 Frequency Response

Here is the stock LA 610 frequency response with a +5 gain setting. Remember: our -40dBV input pulse causes the preamp to overload on maximum gain and creates an unusable test reading.  In this test, the "Level" pot is set to "8".

Below are the impulse responses of each of the five preamp gain settings for the stock LA 610
stock LA 610 FR +10
Stock LA 610 Frequency Response

Here is the stock LA 610 frequency response with a +10 gain setting. Remember: our -40dBV input pulse causes the preamp to overload on maximum gain and creates an unusable test reading.  In this test, the "Level" pot is set to "8".

Below are the impulse responses of each of the five preamp gain settings for the CC&E mod LA 610
mod LA 610 FR -10
CC&E mod LA 610 Frequency Response

Here is the CC&E mod LA 610 frequency response with a -10 gain setting. In this case, the circuit has enough headroom to accommodate the -40dBV input pulse without overload.  Level pot is set to "10".

Below are the impulse responses of each of the five preamp gain settings for the CC&E mod LA 610
mod LA 610 FR -5
CC&E mod LA 610 Frequency Response

Here is the CC&E mod LA 610 frequency response with a -5 gain setting. In this case, the circuit has enough headroom to accommodate the -40dBV input pulse without overload.  Level pot is set to "10".

Below are the impulse responses of each of the five preamp gain settings for the CC&E mod LA 610
mod LA 610 FR 0
CC&E mod LA 610 Frequency Response

Here is the CC&E mod LA 610 frequency response with a 0 gain setting. In this case, the circuit has enough headroom to accommodate the -40dBV input pulse without overload.  Level pot is set to "10".

Below are the impulse responses of each of the five preamp gain settings for the CC&E mod LA 610
LA 610 mod FR +5
CC&E mod LA 610 Frequency Response

Here is the CC&E mod LA 610 frequency response with a +5 gain setting. In this case, the circuit has enough headroom to accommodate the -40dBV input pulse without overload.  Level pot is set to "10".

Below are the impulse responses of each of the five preamp gain settings for the CC&E mod LA 610
mod LA 610 FR +10
CC&E mod LA 610 Frequency Response

Here is the CC&E mod LA 610 frequency response with a +5 gain setting. In this case, the circuit has enough headroom to accommodate the -40dBV input pulse without overload.  Level pot is set to "10".

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