No Gain, No Pain: Sonosax R4+ and AD8+ Gain Structure

Traditionally, when setting up input gain the engineer sets it so that a ‘healthy’ voltage is coming into the system. A certain level of headroom is maintained in order to allow for unexpected peaks.  24bit analogue to digital converters have allowed us more flexibility in leaving headroom before system noise could become a problem.

Here’s a more traditional input diagram from the previous generation SX-R4 recorder:

I’ve been using the latest generation of Sonosax preamplifiers for a while now. I’ve known that they can handle a huge dynamic range since using them but I’ve only just found this in the AD8+ manual:

The thing which is initially quite alarming is that the only gain before the analogue to digital converters (ADC) is a switchable +20dB.  Most professional equipment will have low cut filters, gain and analogue limiters before the ADC stage. If something overloads the ADC, it becomes digital garbage.

In this case, the paired ADCs can take a considerable amount of level- +21dBu (which is 24.6V) and +1dBu (2.46V) with the +20dB option enabled.  To put this into context, most amplifiers are designed to take +18dBu max at input.   So the only “real” gain setting is the +20dB option.  In the unlikely event that an even higher voltage is coming in, there are analogue pots on the line inputs of the SX-R4+ (the small 5 and 6 knobs on the front).

This means more information is coming into the ‘digital world’ earlier in the chain and there’s no limiter- everything is captured as it comes in. The gain on any of the knobs, rather than controlling an amplifier is effectively a multiply function.  Sometimes you’ll need to add gain just to hear a signal, however it now doesn’t really matter where it is added as long as the signal isn’t turned back into an analogue one,  it will have the same result in post production as it will from the machine directly.  Even if it’s sent over AES3, it’s the same 1s and 0s. Whether your post production facility want to work this way is another matter.

So, it should be possible to use an AD8+ as an analogue front end and get the same results without putting any gain on the inputs and adding ‘digital gain’ in another mixer or computer

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