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Author Topic: Acoustic G120-112  (Read 541 times)

gbono

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Acoustic G120-112
« on: December 02, 2020, 08:35:54 PM »
Reassembling this amplifier and looking at the schematic I'm questioning the lack of large aluminum electrolytic in the output (quasi-complementary?)? I've seen pictures of amps and some have a 1000uF/50V in series with the output terminals. Also this amp has an extra large hole in the back panel for an isolating fiber washer(s) for the output jack but NOT for the extension speaker jack?? Doesn't this defeat the isolation?
« Last Edit: December 02, 2020, 09:09:00 PM by gbono »

gbono

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Re: Acoustic G120-112
« Reply #1 on: December 02, 2020, 09:13:23 PM »
Okay just noticed that L1 and R25 and the rest of the Zobel(?) network and there must be a misprint on the schematic since there should be a fairly large capacitor in the output. Still not sure about the isolated output jacks? Why does the ext. jack have small hole in chassis and no fiber washer???
« Last Edit: December 02, 2020, 09:15:03 PM by gbono »

Enzo

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Re: Acoustic G120-112
« Reply #2 on: December 02, 2020, 09:27:22 PM »
What?   Just what large cap were you expecting?  This is not a single rail amp like older Acoustics.   Look at your schematic.  RUns on plus 42 to minus 42 volts.  Not plus 90 and ground.  The output sets at zero.  No need for a fat cap.  The fact it is quasi doesn't matter.

Your schematic shows no output jacks.   But if I were to guess, I'd say the extension jack puts the extra speaker in series with the main, rather than parallel.  SO the sleeve can be grounded, and the tip cutout goes to the sleeve of the main jack.   Thus the main jack needs to be isolated.

Why would they do that?  If they made it parallel jacks, then any moron could plug a couple 4 ohm cabs in there and load the poor amp to 2 ohms and blow it up.  Now a couple 4 ohm cabs makes an 8 ohm load.

We do find schematic errors now and then, but I think it is wise to resist the urge to immediately label something we don't understand as a mistake.

gbono

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Re: Acoustic G120-112
« Reply #3 on: December 02, 2020, 10:04:31 PM »
Opps missed the -48V rail thanks for the help as usual.

gbono

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Re: Acoustic G120-112
« Reply #4 on: December 03, 2020, 02:39:48 AM »
Amp has a 120 Hz buzz - power amp seems to be clean so it looks like the preamp section is the issue. Also not sure this is the correct schematic for the preamp.

gbono

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Re: Acoustic G120-112
« Reply #5 on: December 04, 2020, 04:13:17 PM »
Need some help understanding the grounding strategy with this design.
Input/output jack and volume jack for footswitch are all isolated from ground with fiber washers. Power amp input and preamp out are NOT isolated.

Power supply consists of center tapped secondary with a bridge rectifier and two 4700uF off of center tap which is physically connected to chassis (ground).

Attached preamp schematic (not complete?) doesn't show how the rails are filtered or decoupled.

The amp passes a signal just fine but there is a very loud 120 Hz buss present. So power supply ripple (chassis ground) is mixing into audio path but where?
« Last Edit: December 04, 2020, 04:25:28 PM by gbono »

gbono

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Re: Acoustic G120-112
« Reply #6 on: December 05, 2020, 11:46:23 PM »
I noticed with a signal generator (with chassis ground connection) plugged into the input the noise drops 20dB but is still noticeable at idle. I plugged the sig gen into the amplifier input and I get a good clean output with no noise at all. The problem is in the preamp. Not sure if this is another issue but the amp was in pieces when I received it and I had to wire up the preamp board connections from some images someone sent me of their amp. If anyone has any experience with this model I sure could another pair of eyes or brain cells.

phatt

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Re: Acoustic G120-112
« Reply #7 on: December 06, 2020, 06:49:49 AM »
It's likely you have a ground loop or non grounded part of the circuit. You may need to research where the ground paths run. Generally you only want one Chassis to circuit Common point.
Normally schematics use an arrow head of horizontal lines for circuit Com and angled lines for Chassis Com,, but not always.

If you can trace the Circuit Com all the way from Power Com node through to preamp input com you may find there is more than one Com connection which could be the issue. i.e. a com point is wired back to chassis by mistake creating a ground loop.

Ground is always a problem because Schematics assume com and Chassis and are not usually shown. which makes it a tricky thing to get your head around.
Phil.