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Correcting Headphone Jack to cut Speaker out - Yamaha G50

Started by SemiConductive, May 03, 2022, 04:43:03 PM

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SemiConductive

I recently picked up a Yamaha G50 Series III. Nice amp, glorious cleans. Solidly built.

BUT, it turns out the headphone jack doesn't cut off the speaker output. Odd, and useless for late night practice while the household sleeps.

Circuit diagram, click on it to zoom once you're there: https://i.imgur.com/E30dpvS.png

I'm thinking that I could just use a jack that switches the off the signal feed to the power amp when a plug is inserted into the headphone jack. On the schematic, there's a "T" junction where the signal splits and heads down to the power amp section and also up to C164, and IC109 to split into left/right legs for the headphone jack.

HOWEVER, I'm a newb. I don't know if it's safe to just cut that feed of the power amp signal, so I could use confirmation on that plan. Also, do I need to have a pulldown resistor and/or something else to prevent a serious "pop" on insertion?

Lastly, do they commonly sell 1/4" jacks that also have an independent switching function built in in addition to the more common the signal switching? I know I've seen that feature in some pulled Fender board mount jacks, but I haven't see other generic jacks for sale that have the feature. What's the industry name for that sort of jack?  Also, looking at the chassis, the jacks are definitely the isolated type with plastic housings. So it needs to meet that bar too.

Enzo

I can't read yours, but I have the G100-3, which looks the same other than power.

May I suggest, just unplug the speakers?  The schematic and parts layout shows the speakers plugging into the rear panel jack.

g1

Enzo's approach would be the simplest. 
But if you really want to automate it, there is already a relay engaging the speakers, you could just toggle that.
The type of switching jack you mean is probably with 'isolated' switch.
The switch could replace the connection between D112/C183 junction and R1998/R198 junction.
I believe that would leave the protection function via pin2 of IC111 enabled.
Full service manual attached.

SemiConductive

#3
Quote from: Enzo on May 03, 2022, 06:25:25 PMI can't read yours, but I have the G100-3, which looks the same other than power.

May I suggest, just unplug the speakers?  The schematic and parts layout shows the speakers plugging into the rear panel jack.

That's relatively easy. Is is safe to run without speakers connected? I thought that was a no-no.

SemiConductive

#4
Quote from: g1 on May 04, 2022, 12:55:00 PMEnzo's approach would be the simplest. 
But if you really want to automate it, there is already a relay engaging the speakers, you could just toggle that.
The type of switching jack you mean is probably with 'isolated' switch.
The switch could replace the connection between D112/C183 junction and R1998/R198 junction.
I believe that would leave the protection function via pin2 of IC111 enabled.
Full service manual attached.

I see that relay... so I guess my only question would be the same one as above: is it safe to run with the speakers disconnected from the output transistors in the power amp? I'm guessing so since you both suggested it, but make me feel good about the lack of danger. :-)

What's the function of IC111 i.e. what fault is it protecting against?

Thanks.

phatt

A whole lot of muckin about that may well cause complications.
As Enzo & G1 have said just unplug the speaker. A lot less work and no chance of stuff-ups.

There is no issue with the speaker unplugged on SS amps ,, Valve Amps Always need a load on the output, while SS amps Do Not.

Hard to read the blur but IC111 looks like a protection circuit,, don't mess with it.

Phil.

Enzo

That^^^^

SOlid state amps do not need a load.  TUBE amps always do.

SemiConductive

Slightly OT: For you guys who can't read the schematic I posted... did you click on the graphic once you got there to zoom it? Inquiring minds want to know (so I can plan better next time if needed).


SemiConductive

#8
I may go with the "unplug it" option... I just prefer a passive solution that I won't forget to do... then strike a hard rock power chord at 11:30 at night when everyone is sleeping.


Enzo

I think the lesson there is don't just flip an amp on and crank a power chord without at least checking the amp.  AANy amp, ever.

I did, it grew.  I had options of huge or tiny.  I like a pdf so I can grab the part I want and make it the size I want.   I'm not complaining, I am just picky.

g1

Quote from: SemiConductive on May 04, 2022, 03:58:20 PMWhat's the function of IC111 i.e. what fault is it protecting against?
It senses DC at the output and kicks out the relay to protect the speaker.  It's also supposed to be able to prevent over-current but I'm not sure if that function is being used.  (see datasheet attached)

Quote from: SemiConductive on May 04, 2022, 09:26:37 PMSlightly OT: For you guys who can't read the schematic I posted... did you click on the graphic once you got there to zoom it? Inquiring minds want to know (so I can plan better next time if needed).
My computer is old and my eyes are too. :)  The picture of the schematic only offered me one level of zoom and it still was too small for me.  Probably the zoom function was not working right with my old browser.

SemiConductive

#11
Quote from: g1 on May 05, 2022, 03:00:11 PM
Quote from: SemiConductive on May 04, 2022, 03:58:20 PMWhat's the function of IC111 i.e. what fault is it protecting against?
It senses DC at the output and kicks out the relay to protect the speaker.  It's also supposed to be able to prevent over-current but I'm not sure if that function is being used.  (see datasheet attached)

I see, thanks. Makes sense to keep it in place then. I could obviously still have an issue even with the speakers disconnected. Better to have it .

--
Taking my own thread off topic for a moment.. Just for curiosity to expand my electrical/electronic knowledge while we're here: what sort of current are we looking at for this speaker that's getting 50 watts at 8 ohms? 

If I use this formula, is it correct or am I missing other factors of audio and A/C signals?:

ohms = watts / ampsĀ²
8 ohms = 50 / ampsĀ²
2.5 = amps

And does that mean this is valid?:
volts = amps * ohms
volts = 2.5 * 8
volts = 20

SemiConductive

Back on topic, I have another question on this amp. (This might be answered by a user manual, but there are none that I can find. Service, yes, user, no). 

Anyway... there's an LED on the Parametric EQ section (Level, Frequency, Q are the control pots). There's an LED on the front panel in this section. In the circuit that LED103 / LED P on the right side.

It appears to be controlled by IC 103 1/4 and/or a feed labelled "P" from TR109's emitter, above, in the Parametric EQ section.

In practice, it doesn't ever seem to be off, in any user control setting. Would that be a surprise? Does the circuit suggest it would be off in some particular state? Seems like a lot of circuitry to use if they only wanted to send it a "light me" voltage full time.

Enzo

You have three LEDs - plus power indicator -  A, B, and P?  I'd wager channel A, channel B, and Peak.


I'd check the IC for function

g1

Is there a jack for PEQ footswitch?  I think you may be able to turn it off and on so you can use it like a lead boost etc.