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Issue with LaScalas


mingham

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9 hours ago, mingham said:

I rechecked them and they are solid.  No problem with the Fortes. 

 the amplifier work properly on the 2 channels with the Forte  ,if yes ,  the amplifier is not the problem -

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11 minutes ago, JohnA said:

Will a McIntosh protection system do that? 

 

I'd have to look at the service manual for that amplifier, many of the McIntosh amplifiers don't use the same type of protection circuitry like other amplifiers, they use a limiter type protection that is located at the driver transistors. Too much current squishes the signal down at the driver stage to lower the output signal and hence lower the current. If it's the same type of system in this amplifier it could very well have a ramp up of signal over a short period of time vs relays clicking in and out, and typically an open relay protection circuit often needs a power cycle to clear but not always.

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OK.  First of all, I appreciate you guys helping me figure this out.  I hope I'm not being a PITA. 

 

I switched in the lugs adaptors on the side that was having issues.  Seemed like everything was working fine.  Then I  switched in the lugs on the other side.  When I turned the amp on, it seemed like it was playing lower and I heard static.  I turned it up a bit and the McIntosh Power Guard circuitry kicked in (if there is clipping, the tubes turn amber to warn you to turn it down.  They go back to green if you turn it down.  The manual also says the Powr Guard will kick in if there is an impedance mismatch).  This normally happens at 70-80% with the Fortes with corresponding super high volume.   The Power Guard is kicking in around 20-30% and the volume seems low for where it is.  This is now BOTH channels instead of just the one I was having trouble with previously.     

 

Triple checked polarity at the binding posts on both sides and the amp.  Triple checked polarity off the speaker cables to banana plugs.  All good.

 

Switched in some other speakers.  Everything works great.

 

I'm going to take the banana plugs off at the end of the speaker cables and just go bare wire into those lug adaptors.   Can't think of anything else.  Anyone got any ideas?

 

 

 

    

 

 

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Jeez.  Nevermind.  I have to learn to not panic and think the worst.  I think the banana plug exterior casings were touching.  I don't know why that is a problem as they are supposed to be insulated, but when I separate them, the problem goes away.  

 

 

I appreciate all the input (and patience). 

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15 minutes ago, mingham said:

Jeez.  Nevermind.  I have to learn to not panic and think the worst.  I think the banana plug exterior casings were touching.  I don't know why that is a problem as they are supposed to be insulated, but when I separate them, the problem goes away.  

 

 

I appreciate all the input (and patience). 

 

It's been a while but I always thought the McIntosh power guard circuitry has nothing to do with monitoring the output of the amplifier, it monitors the input of amplifier for too large signals, if too large an input signal is present an optocoupler reduces input signal so that it's not clipping. How this effects the output is when you amplify clipped signals or if the negative feedback cannot supply enough error correction to reduce distortion it will reduce the input signal to reduce distortion. Clipped signals contain a ton of odd harmonic content and can blow sensitive speakers or best just sound bad.

 

It's basically an input limiter/compressor.

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Sounds like an amplifier problem to me ,  you say it also shuts down on your Forte’s ,but at a different volume level . Check with a different amp so that you can rule  out the amp as the problem  . I’ve never had an amp shut down like you describe , shouldn’t be that sensitive. Good luck 🤓

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10 minutes ago, Tom05 said:

Sounds like an amplifier problem to me ,  you say it also shuts down on your Forte’s ,but at a different volume level . Check with a different amp , I’ve never had an amp shut down like you describe , shouldn’t be that sensitive. Good luck 🤓

 

It's not shutting down, these amps are more similar to pro audio vs hifi amplifiers in that they have an input signal limiter/compressor to ensure there are no clipped signals causing excessive distortion. As he described the sound lowered and over a period of time came back up slowly to full volume. It's not the same as output protection for over current and shorts, over temp etc..

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31 minutes ago, captainbeefheart said:

 

It's not shutting down, these amps are more similar to pro audio vs hifi amplifiers in that they have an input signal limiter/compressor to ensure there are no clipped signals causing excessive distortion. As he described the sound lowered and over a period of time came back up slowly to full volume. It's not the same as output protection for over current and shorts, over temp etc..

I understand , but is that circuitry working correctly? A simple swap would verify.I’ve had lots of amps go bad over the years , I’ve also got an avr that occasionally needs rebooting. Also at low volume there should be no clipped signal and therefore no excessive distortion , further pointing to an electronic  failure 🤓

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2 hours ago, mingham said:

 I think the banana plug exterior casings were touching.  I don't know why that is a problem as they are supposed to be insulated,

quality  banana plugs are made from 100% copper -----and 0 conductivity .............hold a magnet over these banana plugs , my guess is they  are magnetic , and as the 2 cases touch ..............voila .

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31 minutes ago, Tom05 said:

I understand , but is that circuitry working correctly? A simple swap would verify.I’ve had lots of amp go bad over the years , I’ve also got an avr that occasionally needs rebooting. Also at low volume there should be no clipped signal and therefore no excessive distortion , further pointing to an electronic  failure 🤓

 

The limiter is before the volume control in the amp, his setup is very simple, he drives his DAC directly into the MA252.

 

McIntosh's proprietary Power Guard circuitry, a special feature of this amplifier, monitors the summing point at the input differential amplifier. The amplifier input and feedback signals are fed, respectively, into the noninverting and inverting inputs of the Power Guard amplifier/comparator. Any voltage difference here means that the overall negative feedback is inadequate to cancel distortion; this might occur from voltage clipping, current limiting, or slew-rate limiting. The sensed voltage is rectified by a bridge rectifier, filtered by a capacitor, and fed to an LED. The front-panel LED indicators, one for each channel, illuminate if distortion is present, causing reduced resistance in a light-dependent resistor. This photoresistor shunts just enough input signal to ground to eliminate the overdrive.

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14 minutes ago, RandyH said:

quality  banana plugs are made from 100% copper -----and 0 conductivity .............hold a magnet over these banana plugs , my guess is they  are magnetic , and as the 2 cases touch ..............voila .

Yea , for now just use some wire .🤓

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12 minutes ago, captainbeefheart said:

 

The limiter is before the volume control in the amp, his setup is very simple, he drives his DAC directly into the MA252.

 

McIntosh's proprietary Power Guard circuitry, a special feature of this amplifier, monitors the summing point at the input differential amplifier. The amplifier input and feedback signals are fed, respectively, into the noninverting and inverting inputs of the Power Guard amplifier/comparator. Any voltage difference here means that the overall negative feedback is inadequate to cancel distortion; this might occur from voltage clipping, current limiting, or slew-rate limiting. The sensed voltage is rectified by a bridge rectifier, filtered by a capacitor, and fed to an LED. The front-panel LED indicators, one for each channel, illuminate if distortion is present, causing reduced resistance in a light-dependent resistor. This photoresistor shunts just enough input signal to ground to eliminate the overdrive.

That’s interesting ,sounds like your very knowledgeable on this unit, aside from the  chance that the input cables are bad , what components in the speakers would you be suspicious of ?🤓

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3 minutes ago, Tom05 said:

That’s interesting ,sounds like your very knowledgeable on this unit, aside from the  chance that the input cables are bad , what components in the speakers would you be suspicious of ?🤓

 

I would be fairly confident saying that the way he had the speaker cables connected caused an anomaly, since the input comparator senses the output via feedback it saw a difference it didn't like and engaged the input signal shunt circuit. I think the amp is functioning correctly but anything is possible. It sounds like when he drives his Forte's hard the power guard kicks in to reduce distortion just at a different level because it's a different load.

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On 3/17/2022 at 7:57 PM, RandyH said:

quality  banana plugs are made from 100% copper -----and 0 conductivity .............hold a magnet over these banana plugs , my guess is they  are magnetic , and as the 2 cases touch ..............voila .

 

I think you mean 0 resistance... you want conductivity.

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