Filter, Limiter and Insert in TM301 Mic Pre

TM301 Mic Pre with Monitor Mixer
Limiter - High-pass and low-pass filters - Insert - Record Output

On of the major problems of any live recording is that you can't start all over again. Any problems that occurs during the recording can spoil the entire production or cause time wasting repair work. So you can't control the levels while the show is running, overload is the most common problem. For this reason the TM301 contains a very fast to brickwall limiter that protects the inputs of the A/D converters from overload.

The TM301 Limiter
The Limiter of the TM301 is optimized for very fast reaction to levels above threshold in combination with inaudible regulation of short peaks. The system uses a feedf-orward regulation with a high precision VCA, brand THAT in combination with a peak rectifier. This principle makes it possible to reduce the delay between the time when the momentary value of the input signal exceeds the threshold below 5 micro seconds. Even with 96 kHz systems, the limiter will react within the sampling time. The fast reaction allows to use normal levels near the clipping point of the digital system without the risk of distortion with unexpected peaks. The attack time is fixed to the fastest possible value. The setting of the relase time is crucial for the low frequency distortion under regulation. If it is set to very fast values, regulation takes place withing the period time of the audio signal. On the other hand long values cause audible pumping effects. Even though 0.3 seconds can be considered as a good compromize, the release control of the TM301 limiter gives the choice to find the best compromize for the particular application. The threshold is controlled by another pot in a range from +20 dB (= + 26 dBu) down to -10 dB (-4 dB). A multi-color LED indicates the operation of the limiter. It changes its color from green to yellow as soon as the level exceeds threshold and changes via orange to red at a gain reduction of 6 dB. The LIM switch inserts the entire section into signal chain. Using an appropriate release time setting, the limiter can fade down peaks of up to 6 dB without any audible side effects. It also will not alter the frequency response under regulation.

High-pass and low-pass filters
The TM301 includes sweeped high-pass and low pass filter with appropriate control ranges to reduce subsonic noise and high frequency disturbance. The high-pass filter is maximum flat type filter with 24 dB/oct steepness. The sweep ranges stars below 20 Hz and reaches up to 600 Hz. The low-pass filter with a steepness of 12 dB/oct can be swept from above 20 kHz down to 1.5 kHz. The filter section is inserted directly at the output of the mic pre by the FILT switch.

In addition to the filter section, very soft subsonic and RF filtering is integrated with the mic pre itself to protect the system from overload caused by very low frequencies below 10 Hz and RF interferencies above the long wave band. These filters have a very flat phase response and will not effect the audio band.

Insert
Even though simple 'loading' of tracks is done with most live recordings, it might be necessary to install some processing gear in a couple of channels. De-essers for the vocals might be necessary, some EQ's may improve the quality of the recording and avoid time wasting repair work in the final mix. With the TM301, this can be done on the fly. The record chain contains a fully buffered, balanced insert point at line level that is available on two 1/4" TRS jacks on the connector panel.

The insert is located post filter and pre limiter. The output is always availale on the connector that is half normalized with the insert input. The INS switch actives the insert input in the module. Like the entire audio signal chain, the insert can handle levels of more than + 30 dBu. The TO MON switch above the INS switch rerouts the entire insert section to the monitor chain.

TM300 Live Recording Home

TM301 Mic Pre & Mon Mixer

TM301 Limiter, Filter, Insert

TM301 Monitor Mixer Chain

TM311 & TM310 Master

TM 300 Frame & Power

ToolMix Compatibility

TM301 Block Diagram

TM300 Frame Rear View

TM301 hi-res View

high resolution picture

Record PFL and Meter
The PFL REC switch feeds the signal at the record output directly into the PFL bus of the mixer. You can control the output signal that feeds the recorder inputs at any time. The Meter is also assign to the record output but it can be swtiched to the monitor chain as well. If the meter displays the level pre or post the limiter is determined by an internal jumper

Record Output
The record output is balanced, has a source impedance of less than 50 ohms and drive levels of more than + 30 dBu into a load of 1200 ohms. It is also stable against capacitive loads of 10 nF without reduced power bandwidth and additional phase shift. If you are interested in these qualities, click on the icons below to open diagrams of the output properties.

max. Level vs
load resistance

max. Level vs
capacitive load

Output source
resistance

output CMRR