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Muting Transistor Attenuator Circuits and the 2SC2878

Among semiconductors, the muting transistor and its audio application is perhaps the most obscure and poorly understood of all. There is extremely little information on this subject available on the internet and what is available is sketchy. This discussion is an attempt to tie the pieces together and to add some insight –I have been working on this off and on for some time and for me it has been a learning experience. While I do not claim expertise in this subject, I question if such experts actually exist.

Schematics

Applications for muting circuits

Applications for muting circuits abound. Included are synthesizers, audio amplifiers, pre-amplifiers, audio mixers and audio line & level equipment (both consumer and professional). In amplifiers and pre-amplifiers, the muting circuit is intended to prevent the popping of loudspeakers when power is applied. In mixers and audio line & level equipment, the muting circuit may also be employed to select /deselect various input channels –such must be done cleanly or quietly.

Types of muting circuits

Obviously, the function of muting circuits is to mute (kill) the audio signal. This can be done in a number of ways including switches, relay contacts, junction FETs, analog switches and bipolar muting transistors. Switch and relay contacts can be noisy and intermittent due to contact bounce. Furthermore, properly applied contacts must be dry circuit rated due to both the low current and potential across the contacts –this forces the use of bifurcated dry circuit contacts that are special and expensive. While a junction FET is sometimes applied as a variable attenuator, its use as a muting device is a misapplication due to the lack of symmetry because it cannot support more than about 600mV due to its intrinsic body diode. Analog switches can do the job if applied correctly, but lack the audio range required in professional line and level applications, may be subject to crosstalk between sections and are also very sensitive to ESD. On the other hand, the bipolar muting transistor does everything well and inexpensively.

What is unique about the bipolar muting transistor?

The standard NPN transistors is symmetrical in that it consists of 3 layers, 2 layers of “N” material (collector & emitter) separated by a layer of “P” material (base). However, that is as far as the symmetry goes as the layer thicknesses and doping varies greatly. The standard transistor still functions with the collector and emitter terminals reversed, but the hFE (current gain) in this mode is reduced by perhaps two orders of magnitude and the reversed voltage rating (Vebo) generally limited to about 5V.

In comparison, the muting transistor is a good deal more symmetrical –it has high reverse hFE and some types offer a 15 to 40V base to emitter reverse voltage rating (Vbeo). In effect, it is rated for AC operation and this suits it well for audio voltage signals that are AC in nature. Whichever terminal (emitter or collector) is the most negative becomes the effective emitter terminal. If sufficient base drive is applied, it will saturate either polarity, and this is exactly what is required to implement the mute function.

Two categories of muting transistors

The two categories are graded by the base to emitter reverse voltage rating (Vbeo).

The 1st category Vbeo is limited to 5V and is suited for consumer electronics that has an average program signal level of -7.8dbu (0.315Vrms or 0.445Vpeak). A 5V rated device provides 21db headroom to handle program material transients.

The 2nd category Vbeo starts at 15V and is suited for professional studio line and level devices that have an average program signal level of +4dbu (1.228Vrms or 1.736Vpeak). A 15V rated device provides 18.7db headroom to handle program material transients.

Preferred devices for the experimenter

Toshiba 2SC2878, TO-92, Vebo = 15V, reverse hFE = 150 typ, obsolete but many remain in the surplus pipeline –this is the only thru-hole device available –get some from eBay while available, $0.85 each. Every experimenter should have some of these in his bag of tricks. Datasheet link

2SC2878 on ebay

Rohm 2SD2407K, SO-23, Vebo = 25V, DigiKey 2SD2704KT146CT-ND, $0.52 each, Datasheet link

Interesting datasheet circuit

2SD2704K R-on measurement circuit

Other misc part numbers in excel spreadsheet format

This is a fairly comprehensive list that you will not find elsewhere.

Typical domestic audio muting application

Sony Playstation output schematic

The muting transistors are Q403 & Q404. BCW60 devices are used, but this must be an empirical design because the BCW60 is a standard transistor type and not specified for muting applications. Note how they are driven via VT2 & VT1. Compliments: http://www.dogbreath.de/PS1/output/output.html

Muting transistor failures –frequent repair item

Muting transistors are susceptible to ESD and have a high failure rate. This is not because they are different than other semiconductors, but where they are used in the circuit –at the output connector. I believe that the main reason for failure is the widespread use of the ancient, sub-standard RCA phono jack connector –the long center pin establishes initial contact directly to the muting transistor before the ground sleeve makes contact forcing any charge directly into the transistor. This discharge destroys or degrades the transistor. ESD protection may be provided or added, but I believe that it is still a race as to which device fails first: the ESD protection device or the transistor. To avoid ESD damage, the best preventative measure is to touch the sleeve of the connector to the chassis before plugging in the connector –and remember that cables have two ends.

Note that only the relay method is not susceptible to ESD.

RCA Phono Plug Photo

RCA Phono Plug

Driving the muting transistor –IMPORTANT DETAIL!

When OFF, the muting transistor base drive must be high impedance over the entire signal voltage swing –in other words, the base may not simply be grounded because it would act like a diode when the signal swings negative. Base drive is best provided via a PNP transistor connected to the positive 15 to 24V rail –a series resistor limits the base drive to approx. 1mA. Multiple muting transistors may be driven by the same PNP transistor because all base inputs tend to act as diodes when OFF and all ‘anodes’ may be effectively tied together thus preventing reverse base current flow or interaction.

Attenuation

A muting transistor has very low ON resistance –in the order of 1Ω. With a 1K audio signal source resistance, it provides 60db attenuation –this is much better than the best analog switches in the ON mode. In audio mixer applications additional attenuation (perhaps 80db) is recommended to completely silence unused inputs. The additional attenuation may be provided simply by cascading two attenuators.

Measured attenuation = 51db (R5 = 22K, Ib = 650uA). This attenuation could likely be significantly improved by reducing the value of R5.

Note that this is not effective as a linear (adjustable) attenuator.

Other uses for the muting transistor

Other uses include low voltage synchronous transistor experiments, and any application that needs a high Vbe reverse voltage rating.

Oscillographs

89 Oscillographs

Photos

For the future

  • Open collector outputs and AC loads
  • Enhanced analog switch muting circuit

6 Comments

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  • Meter

    Great article, but I have a question. Would you please elaborate on the statement, “While a junction FET is sometimes applied as a variable attenuator, its use as a muting device is a misapplication due to the lack of symmetry because it cannot support more than about 600mV due to its intrinsic body diode.”

    It’s my understanding that MOSFETs have an intrinsic body diode and that JFETs do not. Also, many JFETs can have their drain and source’s swapped. Is there something I’m missing?

    • Jim Keith

      @Meter, you are correct on this detail.

      This is what I should have said:

      “While a junction FET is sometimes applied as a variable attenuator, its use as a muting device is a misapplication due to its relatively high on resistance. For example, the popular J107 JFET has a typical Rdson of 8Ω, while the bipolar 2SC2878 muting transistor has an on resistance of only 1Ω.”

  • Francois

    Hi Jim,

    I appreciate your article, and your explanations!

    I am currently repairing a 5.1 Pioneer Amplifier, with this problem. (I assume) Two of these muting/ attenuating transistors have gone South, and I have just ordered some of these 2sc2878 transistors. I ordered 40, as they were just a fraction of the delivery cost! (U$0.05 each!)

    When these fail it appears to always be in Mute/attn mode, hence it mutes the output.

    Kind Regards

  • Pentaconto

    Dear Jim Keith:

    I’m Electroschematics.com habitual follower, but I do not know enough english, I’m spanish. For a better understanding of this article, I just do not understand some expressions:

    What is “headroom”? In Google I found that: “Headroom (audio signal processing): the difference between the nominal signal value and the maximum undistorted value”

    What is “program material”?… (Signal for audio edition and/or distribution?)

    Can you make a short comment on these expressions?

    Thank you.

    • Jim Keith

      Program level is the normal audio signal level of voice or musical program material.

      The ability to handle and/or reproduce normal signal levels is insufficient for high quality because PEAK audio levels greatly exceed the normal level. e.g. shout, symbol crash or crescendo when it gets very LOUD! To avoid distortion under these conditions, the system must be able to handle much higher signal levels or it sounds very bad. This additional signal level is referred to “headroom”. This is what drives the quest for very high audio amplifier power levels –makes it appear that a 200W amplifier is overkill when normal program level is only 10W or so.

      To keep radio transmitters at a high modulation index, audio compression amplifiers are used to reduce the dynamic range (low to high level) of program material. As a result, AM radio can never truly reproduce orchestral music. See my post on the audio compression amplifier:
      http://www.electroschematics.com/9400/audio-compressor-agc/

      This is akin to headroom in a small car where one does not want to feel claustrophobic or to keep from bumping his head. Lots of headroom is much more comfortable.

      This is a favorite website that touches on this stuff:
      http://www.sengpielaudio.com/calculator-db-volt.htm

  • Jim Keith

    Afterthought trivia using DC measurements:

    Emitter grounded
    DC input = 0V, Muted DC output = +3.4mV
    DC input = -3.1V, Muted DC output = -15.6mV
    DC input = +3.1V, Muted DC output = +16.5mV

    Collector grounded (collector & emitter swapped)
    DC input = 0V, Muted DC output = 0.1mV
    DC input = -3.1V, Muted DC output = -14mV
    DC input = +3.1V, Muted DC output = +20.7mV

    What this indicates is that with a base current of 662uA, the attenuation is essentially 45db, and that it is more symmetrical with the true emitter terminal grounded.

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