Fishman Loudbox Headphone Mute Not Working

[bobloblaws]

I have a Fishman Loudbox 100 here and the speaker is not being muted when inserting a TRS plug into the headphone jack. I am attaching the pdf with the schematic showing the circuitry for this function. Can anybody explain to me how it actually works and where I should measure voltages etc. to determine what is failing? For starters, I know that op-amp U4D is 1 of 4 voltage comparators contained in the LM339M IC. From what I read though, the VREF voltage is normally attached to the inverting input (e.g. Electronics Components: How to Use an Op Amp as a Voltage Comparator - dummies), but in this case the Loudbox diagram shows that VREF is attached to the non-inverting input. Anyway, I doubt I would attempt to replace any of the miniaturized surface mounted components on a PCB like this myself, but I'd like to know how this switching system works and be able to actually identify what is causing the problem. Thanks in advance!

- Bob

[Mick Bailey]

The first thing to do with a comparator is to establish what the input voltages should be.

Vref on pin 11 is given on the schematic as 5v.
Now look at pin 10. A voltage divider is formed by R140 and R148. One end is grounded through the headphone socket switching contact, and the other connected to 14v. The voltage at pin 10 is 10/110*14=1.27v

It's easier to think of a comparator by looking at the +/- signs on the inputs and applying the rule 'whichever one is more positive swings the output in that direction'. I visualize them as high/low logic states.

With no headphone plugged in then, pin 5 (+ input) is higher than pin 10 (- input) and the output swings towards the +ve supply rail. It's also helped up to +14v by R147. This turns on Q21 and Q22, allowing a signal to pass.

When you plug the headphones in, the ground side of the voltage divider is lost and R140 acts as a pullup resistor raising pin 10 to +14v. so now the - input has the highest positive voltage and the output swings negative. This in turn turns off Q21 and Q22.

So to check the operation, make sure pin 11 has 5v, pin 10 switches between 1.2v and 14v, and pin 13 toggles high/low.

[bobloblaws]

The first thing to do with a comparator is to establish what the input voltages should be.

Vref on pin 11 is given on the schematic as 5v.
Now look at pin 10. A voltage divider is formed by R140 and R148. One end is grounded through the headphone socket switching contact, and the other connected to 14v. The voltage at pin 10 is 10/110*14=1.27v

It's easier to think of a comparator by looking at the +/- signs on the inputs and applying the rule 'whichever one is more positive swings the output in that direction'. I visualize them as high/low logic states.

With no headphone plugged in then, pin 5 (+ input) is higher than pin 10 (- input) and the output swings towards the +ve supply rail. It's also helped up to +14v by R147. This turns on Q21 and Q22, allowing a signal to pass.

When you plug the headphones in, the ground side of the voltage divider is lost and R140 acts as a pullup resistor raising pin 10 to +14v. so now the - input has the highest positive voltage and the output swings negative. This in turn turns off Q21 and Q22.

So to check the operation, make sure pin 11 has 5v, pin 10 switches between 1.2v and 14v, and pin 13 toggles high/low.

Mick, thank you, exactly what I was hoping for. I have a couple of follow up questions.


1. I don't understand the part about the output at pin 13 being "helped up to +14v by R147".
I thought the output was +14V by virtue of the fact that the supply voltage (Vcc) at pin 3 on this chip is +14V.

2. Can you explain what is going on specifically with Q21 and Q22 (and the diodes)? I understand that with 0.4V on the gate of that transistor it will start to conduct so I assume that represents the "on" state that would effectively "unmute" the speakers. But I don't understand how +14V or -14V at pin 13 works with the transistor, diode, resistors and caps to turn the transistor on and off.

Incidentally, I took the measurements you suggested. The only anomally was that with the phones plugged in the voltage on pin 10 was 8.5V rather than the 14V we expected. I suppose that shouldn't affect the operation as 8.5 is still higher than 5.

[Mick Bailey]

1. Apologies for the confusion - the 10k resistors are pull-up resistors. If you notice the other comparators use them too. Whilst the operation of a dedicated comparator IC is similar to a regular opamp, the output operates more like a switch to ground - there's usually an open-collector transistor in the output. The 10k resistors ensure the output is pulled up cleanly to close to the positive rail and they form the supply path for the positive voltage to the output, rather than the comparator output itself doing this.

2. Q21 and Q22 are FETs. They conduct with no voltage applied to the gate and switch off when the gate is lower (more negative) than the drain and source. That is, when no voltage is applied to the gate it floats at the same level as the drain/source and conducts. R142,C94 and R146, C96 are there to slow down the switching transition. Without them the switching is abrupt and can cause a loud pop. The diode ensures the gate can be pulled down lower (more negative) than the drain/source to turn it off, but blocks the +14v from appearing on the gate and possibly causing it to conduct.

[bobloblaws]

1. Apologies for the confusion - the 10k resistors are pull-up resistors. If you notice the other comparators use them too. Whilst the operation of a dedicated comparator IC is similar to a regular opamp, the output operates more like a switch to ground - there's usually an open-collector transistor in the output. The 10k resistors ensure the output is pulled up cleanly to close to the positive rail and they form the supply path for the positive voltage to the output, rather than the comparator output itself doing this.

No apology required. Any confusion is due to me being a complete newbie. Your explanations are very clear and thorough and much appreciated.

2. Q21 and Q22 are FETs. They conduct with no voltage applied to the gate and switch off when the gate is lower (more negative) than the drain and source. That is, when no voltage is applied to the gate it floats at the same level as the drain/source and conducts. R142,C94 and R146, C96 are there to slow down the switching transition. Without them the switching is abrupt and can cause a loud pop. The diode ensures the gate can be pulled down lower (more negative) than the drain/source to turn it off, but blocks the +14v from appearing on the gate and possibly causing it to conduct.

I'm not clear on why +14V on the gate "causing it to conduct" would actually be a problem. My take away is that in the absence of a voltage it is conducting already and only ceases to conduct when it sees the -14V.

Are the pin numbers for the FET(s) correct? On the schematic it looks like pin 1 is the source, pin 2 is the gate and pin 3 is the drain. But on the datasheet it shows pin 1 as the drain, pin 2 as the source and pin 3 as the gate.

[Mick Bailey]

The gate-source is a P-N junction. In normal operating modes there's no voltage applied to the gate (on) or that junction is reverse-biased (off). The arrow in the symbol gives a clue to the junction, as with a BJT. With an N-channel Jfet a positive voltage will usually forward-bias that junction and allow current to flow from gate to source. This junction is not designed for forward-bias operation and is easily destroyed. The additional diode in the switch circuit ensures the gate can never go positive.

The schematic shows pin 1 drain, pin 2 gate, pin 3 source. Agreed, though - this doesn't tie up with the spec sheet for that device.

[lowell]

Got one in that Channel 2 doesn't work. The Ch1 jack is working and is not the reason. The ring works, just not the tip. Can one of you post the entire preamp or entire amp schematic? Meanwhile waiting on reply from Fishman.

[bobloblaws]

Got one in that Channel 2 doesn't work. The Ch1 jack is working and is not the reason. The ring works, just not the tip. Can one of you post the entire preamp or entire amp schematic? Meanwhile waiting on reply from Fishman.

Here you are...

Loudbox100Preamp2schematicRev4P4.zip