What's the current thinking on muting transistors? What's still available? (2SC2878, etc)

Jump to Latest
#23
grunf said:
2sc3326 from Mouser
Click to expand...
I have successfully used these in multiple circuits. If there is a particularly large switch on or off thump, you can add a second one after another series resistor.

If you put the collector to ground instead of the emitter it is less effective, but the muted residual is less distorted.

Here are more high Vebo options from Mouser: https://www.mouser.com/c/semiconduc...-bjt|~Emitter- Base Voltage VEBO&sort=pricing

Back to back series 2N7002 are also effective, and can achieve a lower, but less linear Rout. Dual SMT 2N7002 are small and low cost.
 
#25
Sorry for resurrecting this thread. But I also need to replace a 2SC2878 transistor. It is located on the output of an audio IC as a mute circuit. At the base there is already a 15K resistor and a diode in series

Will the 2N3904 work well there? I probably don't need to add another diode right?

I also didn't know about these "muting" transistors. As I see in the datasheet of the 2SC2878 it has a very high Vebo at 25V. The 2N3904 only has 6V

Also, is Hfe important here? 2SC2878A is in the 200 - 700 range and 2SC2878B is in the 350 - 1200 range

There are a lot of A versions on eBay and not as expensive as the B versions. My schematic says 2SC2878B and the one I took off is also a B. I can't find any other difference in the datasheet

What do you say about replacing it with a jfet? Like a 2SK117 or 2SK170? These have very high Vgds around 50V
 
#28
electrocutioner said:
ok I understand that jfets are very sensitive to ESD.
Click to expand...

You have to be pretty determined to zap one and its only an issue while handling them. Once in circuit the ESD risk vanishes.

hFE should not be an issue for the normal transistors but the 2SC's are unusual in that they are symmetrical at low operating voltage meaning collector and emitter can be reversed and it will still work as a transistor.

There is nothing lost trying whatever you have. Nothing bad will happen.
 
#29
JETS aren’t especially sensitive to ESD. Mosfets with small input capacitances are. The problem with JFETs is the high ON resistance compared to a bipolar. In shunt configuration, they let more signal leak through. Two cascaded stages, however, can be used when isolation requirements can’t be met with a single shunt device.

Whether another diode is needed or not depends how the circuit is configured. If there is a resistor hanging off the base directly to a negative rail, the additional diode is required with “regular” transistors. If it’s just in series with the series (base current limiting) resistor, then no. Diode are often used as a wire-or, where multiple signals may be used to turn the audio off.
 
#30
A JFET to ground will not mute well if the series resistor is a sensible value, as their on resistance is is not necessarily very low. Back to back MOSFETs in a series then shunt arrangement can work, but they are not entirely out of circuit when unmuted.

The 2SC2878 is still available in SMT SOT23 as the 2SC3326, if that helps. ROHM also have some high Vcebo muting transistors still available, but again SMT.

(just seen already mentioned above. Mouser is one of the few catalogues that have Vebo as a searchable item.)
 
#32
Pssst! There are devices that can short outputs to GND and have no detrimental side-effects as they are not in the signal path. They are called micro relays.

One of the worst (but cheapest) ideas in audio was to use transistors for muting. Todays trend with muting-ICs is not better either as they fail rather quickly.
 
Last edited:
  • Like
Reactions: AudioPackrat
#33
A regular transistor like the 3904 cannot replace a dedicated muting transistor: it has a considerable offset voltage and is completely asymmetrical.
You would need a much lower base drive resistor, and even so it will still leak some signal, distorted if the drive isn't strong enough.
The series diode is required to avoid parasitic conduction in the off state.
A jFET is generally not going to achieve the same level of muting. You need a low Rds device, like a J110, but the drive is completely different: it has to be negative for the off condition, and the direction of the diode has to reversed (with a pullup resistor between S and G).
jFETS are significantly more sensitive to damage than BJT's, but that's mostly the case for small geometries, not switching FETs like the J110
 
#34
If you use regular transistors and the series diode, a negative bias is REQUIRED in the off state, just like a JFET. If not, they distort quite a bit. Took me a while to figure out what the hell was going on there. 100k to the -15 rail and it would stay OFF all the way even with 7 or 10 volt peak signal. Slightly larger geometry devices like MPSA06 tend to work better, offering ON resistance an order of magnitude lower than the J11x series.
 
#35
jagwap said:
The 2SC2878 is still available in SMT SOT23 as the 2SC3326, if that helps.
Click to expand...
Aaaand this is needed as well.:wave2:

SOT23_adapter.png
 
You must log in or register to reply here.
Top Bottom