Email Address:
Password:

Lost your password?

This is the legacy website; please use the new website.

Versatile 4-Input Mixer

This low-cost 4-input mixer features low-noise input preamps, each of which can be configured to suit a wide range of signal sources: microphones, guitar pickups, tape decks, synthesisers or CD players. Other features include a built-in equaliser with bass, midrange and treble controls along with a monitoring amplifier which can drive stereo headphones.

Specifications

Input Sensitivity (for 2.0V RMS output, each main preamp configuration):
Dynamic mic, low impedance: ............................................................ 2.6mV RMS
Electric guitar: ......................................................................................28mV RMS
Tape deck: .......................................................................................... 145mV RMS
CD player: ..........................................................................................463mV RMS

Frequency response:
..................... -3dB at 23Hz and 40kHz, -1dB at 40Hz and 22kHz
(with tone controls flat; see Fig.4)

Maximum output:
.................................. 3.2V RMS (9V p-p) before clipping; see Fig.6

Output noise level (with respect to 2V RMS output, maximum gain & volume,
tone controls flat, inputs terminated with 1kW, unweighted 22Hz-22kHz bandwidth):

CD player input, .......................................... -92dB unweighted; -96dB A-weighted
Tape deck input ........................................... -92dB unweighted; -96dB A-weighted
Guitar input ................................................. -85dB unweighted; -89dB A-weighted
Low-Z mic input .......................................... -67dB unweighted; -70dB A-weighted

Total harmonic distortion (THD):
.................. Less than 0.01% up to 3.2V RMS output

Graphic equaliser:
Bass: .........................+13dB & -12.5dB at 100Hz, ±18dB at 40Hz, ±0.5dB at 1kHz
Mid Range:.................................. ±11dB at 1kHz, ±1dB at 100Hz, ±2.5dB at 10kHz
Treble:.................................... ±10.5dB at 12kHz, ±1dB at 1kHz, ±11.5dB at 15kHz

Headphone amplifier:
Output voltage before clipping: ........................... 590mV RMS into 2 x 33W loads
THD for 500mV RMS into 2 x 33W loads: ......................................................0.8%

Supply voltage
................................................................12V DC (nominal) – see text

Maximum current drain:
....................................................................................45mA.

Back in January 1992, we published the design for a low-cost four-input guitar mixer module for small bands and groups.

It turned out to be very popular and the kit people tell us that kits for it were still selling steadily until quite recently.

However, in its original form, it apparently wasn’t quite as flexible as many users wanted, particularly in terms of the ability to configure the input preamps for signal sources other than guitar pick-ups – eg, for dynamic mics, tape decks, CD players and synthesisers. It also didn’t include a built-in headphone amplifier for monitoring.

These shortcomings have been eliminated in this new design. It retains all the features of the original January 1992 unit but there’s now more flexibility in configuring the input preamps, together with a built-in headphone amplifier.

Block diagram

Fig.1 shows the block diagram of our new Versatile 4-Input Mixer. As shown, it still provides four inputs, each with its own preamp stage and gain control. However, unlike the earlier design, each of the four input preamps can now be configured by the user, to provide the appropriate gain and input impedance values to suit a wide range of signal sources – from the millivolt or two of a low-impedance dynamic mic to the 1-2V signals of a CD/MP3 player or keyboard synthesiser.

This makes the new unit much more versatile.

Following the input gain controls, there’s a standard mixer stage, to allow the signals to be combined in whatever proportions you wish. The resulting composite audio signal is then fed to a 3-channel "mini equaliser" stage, where three tone controls (bass, mid-range and treble) allow you to adjust the tonal balance.

This equaliser stage is basically an expanded version of a standard "Baxandall" feedback tone control, with three controls instead of two.

From there, the output of the equaliser stage is passed to the master volume control and finally to the output jack via an output buffer amplifier operating with a gain of 2.2.

This section is similar to the 1992 design but the headphone amplifier (shown above the output buffer) is a new addition. It simply allows the output audio signal to be monitored via a pair of standard stereo headphones.

Share this Article: 

Privacy Policy  |  Advertise  |  Contact Us

Copyright © 1996-2018 Silicon Chip Publications Pty Ltd All Rights Reserved