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Analog Line Level Preamplifiers , Passive Pre-amps, Crossovers, etc.

PGA2310 preamp turn on mute
PGA2310 preamp turn on mute
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Old 8th November 2017, 10:20 AM   #1
belyakove is offline belyakove  Netherlands
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Default PGA2310 preamp turn on mute

Hi. I'm trying to design PGA2310 based preamp and I guess last one problem left to solve is turn on/off thump. It's pretty heavy and if it will thump into 120w power amp it will be very loud or even can kill speakers I guess.

As far as I know the easiest way to do this is to use relays that will short output of PGA2310 to ground. But one of my requirements is no relays to make preamp completely silently operating.

I saw in some car audio head units that output mute is done using simple bipolar transistor. I've attached schematics of one of these solutions. I tried to do the same with 4n35 optocoupler lying around but with no luck, the thump is still pretty loud even though it's a bit softer than it was before. Honestly I don't understand why this head unit solution works at all because as I understand open bipolar transistor conducts current only in one direction thus eliminating only half of the signal and another half will still produce noise.

I'm also thinking of trying to use something like TLP227G or maybe analog switch capable of 5V single supply operation. (I'm using Arduino to control PGA2310).

What would you recommend to use for this case?
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Old 8th November 2017, 10:43 AM   #2
OlegSh is offline OlegSh
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PGA2310 preamp turn on mute
Did you try to hold the mute pin of the PGA2310 low while powering up?
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Old 8th November 2017, 11:02 AM   #3
belyakove is offline belyakove  Netherlands
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Quote:
Originally Posted by OlegSh View Post
Did you try to hold the mute pin of the PGA2310 low while powering up?
Yes, I tried that. Didn't help unfortunately.
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Old 8th November 2017, 11:13 AM   #4
OlegSh is offline OlegSh
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PGA2310 preamp turn on mute
How does your power up sequence look like? You should first power up the arduino so it can set its pins to proper state before the PGA2310 gets the power. Otherwise if arduino pins are still in high-Z state and the PGA2310 gets the power then the mute is off and you have a turn on thump. Same on power down. Sequence the turn off such that the PGA2310 shuts down first and only then the arduino.
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Old 8th November 2017, 11:15 AM   #5
OlegSh is offline OlegSh
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PGA2310 preamp turn on mute
If all this doesn't help and the mute function of the PGA2310 is ineffective during power up then it will be difficult to avoid relays.
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Old 8th November 2017, 11:24 AM   #6
belyakove is offline belyakove  Netherlands
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Quote:
Originally Posted by OlegSh View Post
How does your power up sequence look like? You should first power up the arduino so it can set its pins to proper state before the PGA2310 gets the power. Otherwise if arduino pins are still in high-Z state and the PGA2310 gets the power then the mute is off and you have a turn on thump. Same on power down. Sequence the turn off such that the PGA2310 shuts down first and only then the arduino.
This is exactly the case. Arduino is powered first from always on +5v source, then after signal from remote it turns on +/-15V power supply that powers up PGA2310. I tried even putting MUTE pin of PGA2310 directly on ground making it always on but still there's thump during power on and even worse during power off. (Since during power off rails voltage lowers slowly because of PSU capacitors it results in prolonged noise in speakers).

I understand that some kind of mute circuit has to be built around but want to find some alternative to conventional mechanical relays. Something like PhotoMOS relays or transistors or ...
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Old 8th November 2017, 11:27 AM   #7
OlegSh is offline OlegSh
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PGA2310 preamp turn on mute
Unfortunately I can not help you with electronic switches due to lack of experience. I use relays in most cases. Maybe someone with real experience in using electronic switches for line level audio applications can chime in.
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Old 8th November 2017, 10:36 PM   #8
MAAC0 is offline MAAC0  Portugal
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On many Digital gear like CD players they use mute transistors on the output. Grab a schematic and study the mute transistor connection.
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Old 9th November 2017, 02:37 AM   #9
anatech is offline anatech  Canada
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PGA2310 preamp turn on mute
Mute transistors have very high E-B reverse breakdown voltages. A normal transistor will be from 5 ~ 7 VDC. I don't know if you looked carefully, but the transistors in the schematic are "digital transistors" that also have a resistor in the base circuit. You must use the exact part number shown as these transistors are specifically designed as muting transistors. The 2SC2878 was an early, well known muting transistor. A signal diode in series with the base might solve the reverse breakdown problem you have.

Personally, I'll take a distortion-free relay to ground mute over a transistor mute any day of the week, and twice on Sundays! You can get reed relays that are pretty quiet, and you do have to watch that the reed relay is designed for audio use (distortion problems). Although, that shouldn't be an issue shunting the signal to ground.

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Old 9th November 2017, 03:00 AM   #10
BJosephs is offline BJosephs  United States
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IIRC, when in mute mode, the outputs of the PGA chips are essentially grounded through a 10k resistor. Additionally the device should default to muted at startup regardless of the mute pin state (admittedly Iím not as sure about that). Are you sure the thumps arenít down stream of the PGA? Does the system really only thump with PGA in line?
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