A1 preamplifier, de-esser

The A1 is our first analog input/output only product. A state-of-the-art microphone/line pre-amplifier with a de-esser in the popular API 500 Series format. The side-chain of the de-esser uses a DSP – unique but advantageous for the precision of the side-chain and enabling the use of a very low noise gain element. Two modules can be side-chain linked for stereo operation.

It has a transformer balanced wide level range input with a pad for line level sensitivity. The input gain is set with a sixteen position switch. A low cut filter is useful for microphone work. Phantom power can be switched on or off.


The output is also transformer balanced and has an additional potentiometer to set the output level independently from the levels the de-esser is running at. A polarity switch allows to change the absolute polarity of the output signal.

The de-esser section uses a bandpass filter to split the audio band. The bandpass filter parameters are the center frequency and the Q. The Threshold parameter defines the sensitivity of the de-esser side-chain. A few attack and release times and ratios can be selected via DIP switches at the bottom of the unit. A LED indicates whether there is any gain-reduction happening and a “listen” switch allows to listen to the signal filtered by the band-pass. Useful to zoom in on e.g. specific sibilances. The de-esser can be bypassed with a switch. Several LEDs allow to monitor the input and output levels.

Main Features 

Input, pre-amplifier

The A1 microphone pre-amplifier stage uses a transformer-balanced, discrete, class A design with a current-feedback amplifier architecture for low distortion and wide bandwidth across the full gain range. The pre-amplifier sports very low noise for a wide range of source impedances, across the full gain range. The gain is variable in 3 dB steps between 15 and 60 dB. A 24 dB pads allows to feed line levels as well. A high pass filter can be switched in. The corner frequency can be selected to be 40 or 80 Hz.


The de-esser section uses an advanced mixed signal design: The direct signal path is fully analog while the side-chain works in the digital domain. This enables the use of a low distortion precision MDAC (Multiplying D/A Converter) as the variable gain element. The side-chain is built for very low latency, using a 500 kHz sampling frequency.

Front panel controls

The front panel controls for the de-esser section are knobs for frequency, bandwidth and threshold. Further parameters (ratio, attack and release time) can be set via DIP switches.



The output also uses a transformer balanced design with a high maximum output level of +27 dBu. An additional output gain potentiometer allows to set the output level in a wide range.

Reviews and user feedback 

 Professional Mastering Engineer

“I've mastered all the national anthems for the FIFA World Cup in Brazil. Your A1 unit helped with the de-essing. Great de-esser!!!”


"I have 2 of the Weiss A1 Preamp and I can tell you that from classical music to jazz stereo recording to pop vocal or guitar or whatever you want to record these Preamp are just AMAZING !!!

No noise - great sound and the De esser inside it is just worldclass.
I record pop vocal with a lot of air EQ in the monitoring and use the de esser to record with that setting. It works great. I have SSL - Neve - Aurora - Audient - Studer Preamp but
Weiss A1 is the BEST the very BEST I've heard after more than 30 years in the recording field."

Dom Torche at Studio Relief, Switzerland


"My engineer said your de-Esser in your analogue 500 series is the best de-esser he has EVER heard anywhere!"

Andy Bramhall, UK

"One word..... CLEAR! High headroom and great presence evenly across the frequency range! The desser is probably the best when it comes to hardware units and definitely the best as it relates to the 500 series format. Think GML mixed with a pinch of Avalon M5 (Open Top End)with a extremely small sprinkle of Neve (transformers). There's 2 Lundalh Transformers on here so it adds a nice touch of color to give some weight but its not over bearing. This is the most open and full sounding mic pre I've ever heard in the 500 series format."

Brandon W., USA


"Outstanding product. I bought this because I had heard the de-esser portion was second-to-none. That’s an understatement. Even though you can’t bypass the pre, it’s clean enough to pad down and use at line-level. Though the pre is fantastic and totally capable of standing on its own, I’m using my John Hardy and several 500-series modules before the Weiss. As a de-esser, it’s amazingly transparent. It’s better than any hardware or software unit I’ve found. Even really clamping the sibilance, it never gets that dull lispy quality caused by pretty much all other de-essers. Using more moderate settings, I find it to be just as good as manually attenuating individual sounds."

Emmett A., USA

Technical Data 

Dimensions (cm)


  • Compatible with API Series 500 power racks
  • Current consumption ±16 V supply: ≤ 150 mA
  • Current consumption +48 V supply: ≤ 15 mA



  • Depth: 15 cm
  • Width: 3,8 cm (1 slot)
  • Height: 13,3 cm





  • Input gain range, pad off: +15 dB to +60 dB, 3 dB steps
  • Pad attenuation: 24 dB
  • Maximum input level at minimum input gain, pad on: +29.7 dBu (for THD+N ≤ −80 dB)
  • Input impedance at 1 kHz, pad off: 8.4 kΩ
  • Input impedance at 1 kHz, pad on: 2.8 kΩ


  • Output gain range: −∞ dB to +12 dB, continuous
  • Maximum output level, 100 kΩ load: +27.0 dBu (for THD+N ≤ −80 dB)
  • Output impedance at 1 kHz: 290 Ω

The A1 is used as a microphone pre-amplifier for recording or for live purposes with the option to do de-essing on the fly. As a bus de-esser it is used during mix down or mastering in a stereo fashion, i.e. by linking the side-chains of two modules.






For Frequently Asked Questions and discussions about this and other products our new Forum will be available soon.


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