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PGA2310 preamp turn on mute
PGA2310 preamp turn on mute
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Old 9th November 2017, 03:36 AM   #11
anatech is offline anatech  Canada
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PGA2310 preamp turn on mute
Hi BJosephs,
Quote:
when in mute mode, the outputs of the PGA chips are essentially grounded through a 10k resistor.
I would hardly call a 10K load, "grounded". Most signal sources will happily drive a 10K load forever and ever without much signal drop. This load is completely negligible from a signal point of view.

Now, if it were a resistance less than 10 ohms, that you could call grounded, not a great ground, but certainly signal loading and greatly reduced amplitude.

-Chris
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Old 9th November 2017, 10:15 AM   #12
belyakove is offline belyakove  Netherlands
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PGA2310 datasheet says this about how muting works in it "Muting is accomplished by switching the input multiplexer to analog ground (AGNDR or AGNDL) with zero crossing enabled." So this means that it shorts inputs but not outputs when MUTE pin is active. I've verified that thump comes exactly from it and from nothing else in the circuit. By the way the circuit is (RF filter -> DG409 input switch -> OPA2134 buffer -> PGA2310).

I've checked schematics for Arcam CD player and indeed found muting circuit based on transistors. However as I can see muting transistor is driven by some kind of mute driver circuit that is built on transistors too and has it's own dual power supply. Why is this circuit needed? Maybe this is a stupid question but I'm novice to all of this.

Also, can you please briefly explain why diode in series with base might help?
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Old 9th November 2017, 12:24 PM   #13
belyakove is offline belyakove  Netherlands
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I was investigating more the headunit solution and found one more surprising me thing. Mute transistors are connected with emitter to signal and with collector to ground. How does it work? Where does the current that goes into base flow? Into signal line?
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Old 9th November 2017, 12:49 PM   #14
anatech is offline anatech  Canada
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PGA2310 preamp turn on mute
Hi belyakove,
Well, to start, just because a circuit is designed a certain way doesn't mean it performs well. That's lesson #1.

Muting the output is always a good idea during start up and power down. Especially if the product is supposed to be a high quality one. That way you can be sure that even an induced noise will not make it out of your gear.

The diode in series with a muting transistor prevents reverse breakdown. Look at the situation again. Specialised muting transistors are designed with higher than normal reverse breakdown of the E-B junction. Normal transistors will generally not work for that function because they only have a 5 to 7 volt reverse breakdown, and they may leak before that. By putting a silicon signal diode in series with the base, you are preventing it from breaking down. This setup may cause other problems, I don't know. However it might work and allow you to use a normal, high gain BJT for muting. I would simply use a transistor designed for that purpose if I felt the need to use a transistor here. Another part that might work well would be a J-Fet. Again, I have seen them used, but never used on myself.

To my way of thinking, you have gone to the trouble of extracting the best performance from a circuit that you can. Why on earth would you then compromise it in the muting circuit?? Using a relay of some kind would tend to eliminate all influences of a muting circuit during normal circuit operation. BJT semiconductor junctions have a way of breaking down or becoming leaky under reverse bias conditions.

-Chris
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Old 9th November 2017, 12:50 PM   #15
anatech is offline anatech  Canada
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PGA2310 preamp turn on mute
Hi belyakove,
Any current into the base to anywhere will turn the C-E on. You would need to use a very high gain transistor because you are injecting current into the audio path. Not a solution I would use.

-Chris
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Old 9th November 2017, 05:04 PM   #16
BJosephs is offline BJosephs  United States
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Quote:
Originally Posted by anatech View Post
Hi BJosephs,

I would hardly call a 10K load, "grounded". Most signal sources will happily drive a 10K load forever and ever without much signal drop. This load is completely negligible from a signal point of view.

Now, if it were a resistance less than 10 ohms, that you could call grounded, not a great ground, but certainly signal loading and greatly reduced amplitude.

-Chris
We have a misunderstanding, but it's my fault because I was looking at the wrong datasheet (PGA2311) anyway. From said datasheet:

"MUTE disconnects the internal buffer amplifiers from the output pins and terminates AOUTL and AOUTR with 10-kΩ resistors to ground."

So, the internal amplifiers are not driving a 10k load, obviously this would accomplish nothing with regards to muting. The output pins are disconnected from the internal amplifiers and tied to ground through a 10k resistor, so no signal at all is present. My bad because I though all the PGAs accomplished muting the same way.

Last edited by BJosephs; 9th November 2017 at 05:13 PM.
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Old 9th November 2017, 06:47 PM   #17
belyakove is offline belyakove  Netherlands
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Quote:
Originally Posted by anatech View Post
Hi belyakove,
Well, to start, just because a circuit is designed a certain way doesn't mean it performs well. That's lesson #1.

Muting the output is always a good idea during start up and power down. Especially if the product is supposed to be a high quality one. That way you can be sure that even an induced noise will not make it out of your gear.

The diode in series with a muting transistor prevents reverse breakdown. Look at the situation again. Specialised muting transistors are designed with higher than normal reverse breakdown of the E-B junction. Normal transistors will generally not work for that function because they only have a 5 to 7 volt reverse breakdown, and they may leak before that. By putting a silicon signal diode in series with the base, you are preventing it from breaking down. This setup may cause other problems, I don't know. However it might work and allow you to use a normal, high gain BJT for muting. I would simply use a transistor designed for that purpose if I felt the need to use a transistor here. Another part that might work well would be a J-Fet. Again, I have seen them used, but never used on myself.

To my way of thinking, you have gone to the trouble of extracting the best performance from a circuit that you can. Why on earth would you then compromise it in the muting circuit?? Using a relay of some kind would tend to eliminate all influences of a muting circuit during normal circuit operation. BJT semiconductor junctions have a way of breaking down or becoming leaky under reverse bias conditions.

-Chris
Ok, you've convinced me, I'll use reed relays for this purpose. However I still have questions. I've seen in other circuits that resistors are used before and after connection to mute relay. Also I've seen that 10uF electrolytic capacitor is used between opamp output and muting point for dc decoupling. So the questions is what values of these resistors should be? And also if this electrolytic can be omitted? As I understand having electrolytics in signal chain is not very good thing.
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Old 9th November 2017, 06:48 PM   #18
anatech is offline anatech  Canada
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PGA2310 preamp turn on mute
Hi BJosephs,
Well, that makes more sense. The 10K resistor is there so that any charge on an output capacitor doesn't change. That would make a pop noise.

-Chris
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Old 9th November 2017, 06:55 PM   #19
belyakove is offline belyakove  Netherlands
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Quote:
Originally Posted by BJosephs View Post
We have a misunderstanding, but it's my fault because I was looking at the wrong datasheet (PGA2311) anyway. From said datasheet:

"MUTE disconnects the internal buffer amplifiers from the output pins and terminates AOUTL and AOUTR with 10-kΩ resistors to ground."

So, the internal amplifiers are not driving a 10k load, obviously this would accomplish nothing with regards to muting. The output pins are disconnected from the internal amplifiers and tied to ground through a 10k resistor, so no signal at all is present. My bad because I though all the PGAs accomplished muting the same way.
Checked 2311 datasheet and it indeed has different MUTE and Power on/off behaviour than 2310. Seems from description that 2311 doesn't have this problem. Pity that 2310 is not the same.

Quote:
Originally Posted by anatech View Post
Hi BJosephs,
Well, that makes more sense. The 10K resistor is there so that any charge on an output capacitor doesn't change. That would make a pop noise.

-Chris
Maybe my question is stupid but if output would be left just unconnected why would that affect output capacitor charge?
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Old 9th November 2017, 07:01 PM   #20
russc is offline russc  England
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Check easy stuff like: do you still get thump with pre-amp inputs shorted to ground?
Do you have any DC offset at output? Have you a buffer on the output and this is causing thump?
According to data sheet, gain is at zero on power up until volume setting is sent by processor.
So activating mute on power up does nothing.
My PGA2320 pre-amp signals power amps on from standby (5v trigger from Arduino pin) and no thumps.
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