[sdiy] Decent 192kHz USB audio-interface

Mike Bryant mbryant at futurehorizons.com
Fri Apr 30 11:40:48 CEST 2021

> "Then drops like a stone" seems to imply a brick wall filter. Aren't those expensive in analog implementations?

Not really - the combination of capacitors on the input transistor bases, then on the feedback of every op-amp, then the final RC feeding the ADC are all set to around 80-100kHz in most designs so as to make sure there's no response at the bottom of the long-wave radio band.

Just realised that the hyper-expensive AK4558 has a very extended frequency response in the slow roll-off mode so add that to the list, not that it's available at the moment due to the AKM fab fire.

There's also quite a few 384kHz devices available now, though some seem to start a slow roll-off at around 48kHz.  Still plenty of response at 80kHz but definitely not flat.

-----Original Message-----
From: Brian Willoughby [mailto:brianw at audiobanshee.com] 
Sent: 30 April 2021 01:58
To: Mike Bryant
Cc: synth-diy mailing list
Subject: Re: [sdiy] Decent 192kHz USB audio-interface

Thanks, Mike.

"Then drops like a stone" seems to imply a brick wall filter. Aren't those expensive in analog implementations?

On that note, if the AFE has a brick wall at 80 kHz, then it's not really relying on the ADC for the frequency response - at least the ADC isn't going to be able to be set to 384 kHz sample rate and change the frequency response much in that regard.

... not that we're talking about 384 kHz sampling - that would be crazy talk.


p.s. I'll mention that the Metric Halo Labs interfaces have DC response on the line level input path to the ADC. They don't maintain DC response on the mic preamp settings because of phantom power and other aspects of the microphone preamp stage. The Owners' Manual specifies a response of 1.8 Hz to 64.7 kHz +0/-1.0dB @ fs = 192 kHz. If you want tighter frequency response, it's 5.7 Hz to 42.1 kHz +0/-0.1dB at the same sample rate. 0.0005% THD @ -12 dBFS, 0.0015% THD @ -0 dBFS. I'm not sure which chips they're using, but it might be the AK4395.

On Apr 29, 2021, at 17:15, Mike Bryant wrote:
> Every mic preamp ADC I know of, including several I've designed myself which I won't plug here, primarily rely on the ADC itself to set the frequency response.  The AFE usually has a 3dB point of around 80kHz or higher so as not to affect the overall phase response, then drops like a stone to avoid any RF getting to the ADC.
> Also AKM and Cirrus ADCs seem to stick to a 40kHz bandwidth at 192kHz sampling so it may only be ESS or custom equipped units that can do 80kHz.
> On Thu, Apr 29, 2021 at 6:19 PM Mike Bryant wrote:
>> Most high frequency sampling ADCs aren't good to 80kHz.  They have a much more relaxed roll-off to about 40kHz to avoid phase effects on the audio, then dive.  Probably best to look at the ADCs datasheets from AKM, Cirrus and Analog Devices and find an ADC that does what you want, then look for units that use that device.
>> -----Original Message-----
>> From: Richie Burnett
>> Sent: 29 April 2021 11:44
>> Can anyone recommend a decent external USB audio interface that's capable of sampling at 192kHz in record and playback?
>> This is a bit off topic, but it's still audio related! ;-)  Some of my colleagues do underwater acoustic communications work and have been using a "Creative Labs Sound Blaster X-Fi HD 24-bit 96KHz USB Audio Interface" with great success to record and playback ultrasonic signals up to 44kHz.  Now they are wanting to do similar up to around 80kHz so I'm looking for an external audio interface capable of sampling at 192kHz with an analogue bandwidth out to around 80kHz.
>> The only application I'm aware of that requires this sort of analogue bandwidth is the delivery of composite FM stereo multiplex from studio to transmitter site over IP.  So, I know it is possible, but wanted to avoid buying lots of audio interfaces and testing them to see which are good to 80kHz!
>> Any tips/pointers/experience good or bad would be much appreciated.
>> Off-list is fine if high-bandwidth audio is not considered to be of interest to others.
>> Many thanks in advance,
>> -Richie,

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