Use of chopping transistors in power amp circuit...?

[wayward bluesman]

Here's a schematic for a Soundtech C600. It's essentially a 2-Channel power amplifier which feeds signal to a third woofer amp. The woofer amp uses a fairly common topology except for those FETs right at the input. According to the data sheet, they're "chopping transistors" although I'm not sure this is how they're being used. Can anyone tell me their purpose here?

Also, their static D-S resistance is nominally 40 ohms, 35 in reality. When in parallel as they are in the Soundtech circuit, they form an input resistance of about 17 ohms just past R436! Can anyone shine some light on why this is normal?

Thanks in advance!

C600 Woofer amp.pdf

[Jazz P Bass]

Those FET's are J112 jfet's.
"N-Channel Switch. This device is designed for low level analog switching"
They are like a normally closed (on) switch.
Hense the low ohm's reading to ground.
Now give the gate pin a voltage & walla, they are turned off.
I think what you are looking at is the 'mute' circuit.
Not being aware of what is on pin WCH, I would guess it is a voltage that ramps up at power on.

[Enzo]

Looking at the connector, I'd wager the LCH, RCH, and WCH are probably signals to the power amp inputs:
Left Channel
Right Channel
Woofer Channel.

Clearly WCH in this page is the input to the amp. There are two power amp circuits, the signal feeds both, though the upper one has an inverter stage at its input, so I assume the two channels act in bridge to drive the subwoof speaker?

If I had to guess further, I might think they use this same board for a stereo amp as well. Maybe a couple different jumper wires and the L and R inputs become active instead of the W input


Q412 at the input is a mute, or at least a clipping limiter. Note Q410 and Q422 over on the right side. They are driven by current sampling of the output stages, and if either channel has excess current flowing, they will allow Q412 to turn on, either killing or limiting the input signal.

Q411 and Q423 are also mutes, they are power up mutes, one for each amp channel. From the connector on the right side, see the -27vDC line over to their gates. Once -27v is present, it turns off the JFETs at their gates.

As Jazz mentioned, a JFET is ON until turned OFF by a voltage at the gate.

[Jazz P Bass]

Oops.
I missed that -27 volt line for the unmute voltage.
Thanks Enzo.

[wayward bluesman]

Thank you, gentlemen. It's the first time I'd seen the JFETs used in this way; it's always good to learn something new.

[Enzo]

There is a first time for everything, but really this is a very common application.

Used in shunt, JFETs make mutes. Used in series, they make signal switches.

[wayward bluesman]

Any idea why, if the designers wanted to "mute" the circuit, they used the FET to shunt the incoming signal to ground as opposed to using the FET in series and opening the circuit? It seems, from the point of view of the preamp, that an infinite input impedance would be preferable to a near-zero impedance, even if the signal is only line level or smaller.

[Enzo]

The signals don't care either way. But an amplifier with a grounded input is much more reliably silent than one with an open at the input.

[J M Fahey]

Agree and add: when turned on, amplifiers are "stupid" for, say, a second, until capacitors charge to operating voltages.
This almost always causes a pop or thump.
If you add a muting circuit which needs some voltage to work, it may also be useless during that first second ... just when it's needed most.
*But* using fets as shown here, they *start* in the muting state and only after they receive those 27V they unmute.
Cool.
Modern chipamps (LM3886/TDA7294) already have a mute circuit built in, which enables them after a couple seconds.

[wayward bluesman]

As always, great information for the bench notebook. Thanks again, guys.