QA472 Mic Preamp Analysis
The QA472 is an upcoming mic-preamp product to replace the QA471. The QA471 was based on an SSM2019, while the QA472 is based on the INA849. The INA849 offers much tighter gain limits, improved THD, and similar noise performance as the SSM2019. The mic pre offers, 0, 10 and 20 dB of gain, which are selectable using a front-panel button that cycles through the gains.
The QA472 also has an ultra-low-noise JFET amp with 30 dB of fixed gain.
The purpose of this post is to show how to make the measurements to characterize a mic-preamp. As the product being evaluated here is preliminary, certain specs may change prior to shipping.
QA472 Front Panel
The front panel of the QA472 is shown below.
PRE1: Mic Pre
From left to right, there are indicators for Power (the QA472 is powered by USB) and also +48V phantom. PRE1 has both BNC and XLR inputs. A push button to the left of the BNC will enable system phantom generation (+48V). To the right of the BNC inputs is an input select button, and LED indicators for BNC or XLR inputs.
To apply Phantom Power, the +48V button must be pushed and held for 1 second. To turn phantom off, a momentary push is needed.
Phantom is never applied to the BNC inputs–only to the XLR inputs. When you switch from XLR to BNC inputs with phantom power active, the phantom will be turned off and a short “bleed down” delay will be imposed before switching to the BNC inputs to ensure the full phantom voltage isn’t applied to whatever you have connected to the BNC inputs.
The XLR female inputs do not have 1/4 TRS, but a push-lock tab is provided. To the right of the XLR input is the gain select button, and LEDs to indicate the selected gain. And finally, a single-ended BNC output.
PRE2: JFET Pre
The JFET pre-amp is based on the TI JFE150 “ultra low noise” audio N-Channel JFET. This part has the ability to deliver noise performance about 5-10 dB better than the OPA1612 opamp with low (<1K) source impedances. And 20-30 dB better noise performance with higher-Z sources.
The QA472 front-panel settings are remembered through power cycles. So, if the QA472 is part of a measurement setup, a power cycle will restores the measurement settings you had in place. The QA472 is not USB controllable.
The measurements below were performed on REVC boards, with mods made that will be reflected in a REVD board, and REVD is expected to go to market.
Measuring PRE2: JFET 30 dB Preamp
JFET Amp Gain
A first measurement we’ll make of the JFET pre is the gain, because we’ll need to know this number for the “referenced to input” (RTI) noise measurements we make next. With the QA403 L+ output connected to the PRE2 Input, and the PRE2 output connect to the QA403 IN+ (and with IN - shorted), we can make a gain measurement as shown. Note that with 30 dB of expected gain, we’ll set the generator to -40 dBV.
Above we can see the amp delivering 29.89 dB of gain. This is a “no cal” amp design based on the TI app note for the part. TI’s overall circuit is a hybrid, using a JFET front end, and a precision opamp to close the loop. More units need to be measured to understand the spread of the JFET amp’s gain. But initial looks show them to be fairly tight and in good agreement with spice.
JFET Amp Noise
With the gain known, we can short the input of the amp. Because this amp is high-gain and also very low noise, it’s important to short the amp input with a true 0 ohm shorting block instead of the often used 50 or 75 ohm shorting block. Below we can see the thermal noise in a 20 kHz bandwidth of various resistor values. For example, a 120 ohm resistor normally has a thermal noise (in a 20 kHz bandwidth) of -134.03 dB. And with 30 dB of gain, we’d expect that to result in an amp output noise of -104.3 dBV. As we’ll see below, with this gain and noise, even a 120 ohm resistor will degrade the performance of this amp.
With the input shorted, we can see the A-weighted spectrum:
If we replace the 0 ohm shorting block with a 75 ohm shorting block, there’s about 1.5 dB of degradation:
The QA471 had a 30 dB amp based on a OPA1612 low-noise audio amp. The RTI noise of the OPA1612 is -129.42 dB. So, the JFET delivers about 6 dB better noise floor over the OPA1612. And as the source impedances increase beyond 1k, the JFET performance will get better and better due to to the current noise in the OPA1612.
JFET Distortion
Below is a sweep of input level versus THD. We can see at -20 dBV input (10 dBV output) that clipping is starting to occur.
Looking at the spectrum with -40 dBV input we see the 2H is about -105 dB below the fundamental.
Note the THD+N is -84 dB, which is considerably worse than the THD. We can see the N-D (noise minus distortion) is -94 dB. But now, let’s short the input again with a 0 ohm shorting block. Note the N-D figure improves about 10 dB, which means the THD+N is limited by the noise in the output of the QA40x analyzer, not the preamp. From this, we can roughly estimate the THD and THD+N are both about -103 dB at -40 dB input.
JFET Frequency Response
The plot below shows the frequency response of the JFET amp. The amp is bandwidth exceeds the bandwidth of the QA40x in 192Ksps sampling rate.
In the next week or so, the mic preamp measurements of the QA472 will be posted. The last board had some stability issues related to excessive capacitance due to switching topology on the INA849 gain setting resistor that will hopefully be resolved in the next board pass.
