[ProAudio] Microphones question
dan at dandugan.com
Wed Jun 23 09:33:45 PDT 2021
I think we are seeing this in field recorders such as the Zoom F6 and Sound Devices MixPre series II. The manufacturers aren’t talking about the topology.
> On Jun 23, 2021, at 9:21 AM, Chris Caudle via ProAudio <proaudio at bach.pgm.com> wrote:
> On Mon, June 14, 2021 2:49 pm, Dan Mills via ProAudio wrote:
>> The interesting takeaway from that calculation is that actually given a
>> reasonable ADC (-97dBu self noise or so), and a reasonable mic (-120dBu
>> self noise or so) we only need about 30dB of front end gain going into a
>> modern ADC to make the mic self noise completely dominant
> That is exactly the point made by Paul Frindle in one of my favorite
> preprints, pp4126 from the fall 1995 AES convention. Paul presented on
> the design considerations for the conversion system designed for the Sony
> Oxford large format digital console, and section 4.2 makes exactly the
> point you brought up:
> "As the input stage for the ADC converter, the microphone amplifier is
> required only to make up the shortfall in the dynamic range of the
> converter and provide impedance matching to produce the appropriate input
> referred noise specification in the digital domain.
> Beyond this, extra gain to increase the levels of signals is best
> performed in the digital domain. Therefore if a converter had a total
> dynamic range in excess of 128dB and input matched to 150R referred to
> 0dBu, no microphone amplifier would be required at all.
> "Similarly, for a converter with an input noise of 112dBu (with 0dBfs
> referred to 0dBu), the input noise specification could theoretically be
> achieved with gain of around 16dB providing no noise was added (i.e 128 -
> 112dB). However an amplifier with gain of 16dB and output noise below
> -112dBu is a difficult specification to achieve due to the relatively high
> impedances required within the amplifier circuit and the practical limit
> of power dissipation for the circuit."
> That entire section 4 of the preprint is good and covers lots of the
> practical considerations. I have not ever had access to one of those
> conversion systems, but from the description it seems that it is made for
> microphones or instruments only, it does not appear that it was designed
> to handle typical professional line levels (i.e. +20 dBu to +24 dBu range)
> without a pad. The input for full scale was +12 dBu, but I am not sure if
> the system used a 12 dB pad for line level, had a separate front end for
> line level, or just did not accommodate traditional line level signals.
>> All of a sudden those preamps with 60dB of gain range on the pot start to
>> seem rather pointless, just do a fixed +30dB and hit the ADC
> You probably still want some adjustment range to optimize the gain
> structure, but you won't need as much gain.
> That is all assuming you only care about getting into the digital domain
> as quickly as possible. If you have interoperability requirements with
> other equipment you might still need to be able to drive traditional line
> levels, but yes it does seem kind of pointless to take lots of pains to
> get large amounts of gain, to then throw it away when you have to
> attenuate the signal by a large amount to get into a converter running
> from +5V.
>> On the subject of Ein, does nobody discuss Iin, particularly for a bipolar
>> input stage it is often a more significant noise source then Johnson once
>> the source gets above a few k.
> I think that would primarily apply to instrument inputs where you are
> amplifying the output of a coil with a lot of windings, or a piezoelectric
> pickup. A microphone should only have output impedance in the hundreds of
> ohms, and a line level output would be in tens of ohms, so input current
> noise should not be dominant for those cases. From what I have seen the
> typical solution for instrument inputs is to put a JFET buffer in front of
> your mic amp circuit.
> Chris Caudle
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