For the LSC or LSS gurus

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tmac
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For the LSC or LSS gurus

Post by tmac » Thu Aug 05, 2010 6:29 am

Hi!

I'm an Express 5:25 owner. As you may know, its power selection circuit is not channel assignable like on the LSC and LSS.

I do know my way around a tube amps to some degree, and I want to check if a modification can be done on my Express. Perhaps it is possible to imitate the LS amps?

I know the Mesa amps use sophisticated mute circuits to eliminate switching pops, at least in the preamp. How is this done in the poweramp of the LS amps when each channel has been set to a different power mode.

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Re: For the LSC or LSS gurus

Post by thom » Thu Aug 05, 2010 1:27 pm

On my LSS there is a very noticeable pop when switching channels that are set on different power modes.

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Re: For the LSC or LSS gurus

Post by tmac » Sat Aug 07, 2010 3:47 am

Thanks! Perhaps MB never uses any poweramp muting for such switching on their amps.

On my E5:25, there a very noticable noise pop when doing 5W -> 25W. The noise is weaker in the opposite direction. Its not loud compared to rock band gigging volume/band practice.

Obviously, it is independent of any knob settings since it takes place in the poweramp.

It would be a really nice feature if I could mute the pop, so I'll have to look into how that can be done. I'll probably put this off for a while, since I need to do some work on a different amp.

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Re: For the LSC or LSS gurus

Post by ricardo loma vista » Sun Aug 08, 2010 1:55 am

hmmm..absolutely no pop on my LSS when switching regardless of power settings
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Re: For the LSC or LSS gurus

Post by plan-x » Sun Aug 08, 2010 2:18 am

tmac wrote:Thanks! Perhaps MB never uses any poweramp muting for such switching on their amps.

On my E5:25, there a very noticable noise pop when doing 5W -> 25W. The noise is weaker in the opposite direction. Its not loud compared to rock band gigging volume/band practice.

Obviously, it is independent of any knob settings since it takes place in the poweramp.

It would be a really nice feature if I could mute the pop, so I'll have to look into how that can be done. I'll probably put this off for a while, since I need to do some work on a different amp.
I had a Peavey Valve King that would drop out momentarily upon ch switching. Maybe it had some sort of ch switch muting. If that was the case, I dumped that amp because of that.

I saved an ABY pedal switch design I found on the web because it incorperated a resistor on the jacks to absorb the pop. Take a look. Maybe these things will help you piece the puzzle together.

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Re: For the LSC or LSS gurus

Post by tmac » Tue Aug 10, 2010 4:52 am

ricardo loma vista wrote:hmmm..absolutely no pop on my LSS when switching regardless of power settings
Interesting... Perhaps the amount of pop depends on the power tubes.
plan-x wrote:I had a Peavey Valve King that would drop out momentarily upon ch switching. Maybe it had some sort of ch switch muting. If that was the case, I dumped that amp because of that.

I saved an ABY pedal switch design I found on the web because it incorperated a resistor on the jacks to absorb the pop. Take a look. Maybe these things will help you piece the puzzle together.
Yeah who knows, it might have been muting for too long a time. Thanks for the pic, it seems to use 2.2Mohm resistors across all jacks. Those resistors do help against leaking DCV from other units which heave leaking input and/or ouput coupling capacitors. DVC on the switch would cause a pop.

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Re: For the LSC or LSS gurus

Post by djw » Fri Aug 13, 2010 1:46 pm

I'm not sure if this info actually applies, but I'll throw it out there. I have a 50/100 LSC, and I had my tech friend look at the possibility of adding the 10 watt option to my amp. After some back and forth with Mesa and his own analysis, he found that adding a switch to make BOTH channels go to 10 watts was a snap -- but making them independently assignable to 10 watts amounted to a board swap. I'm lost as to exactly what the deal was, but basically the circuit design, specifically in the output stage, didn't allow a way in. So we bailed on the idea (though I think I might do this anyway, once my warranty expires).

So, I'm going to guess that making each channel's power output independently assignable would be a pretty significant alteration to your Express.
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Re: For the LSC or LSS gurus

Post by tmac » Fri Aug 13, 2010 3:06 pm

djw wrote:So, I'm going to guess that making each channel's power output independently assignable would be a pretty significant alteration to your Express.
Well, I would need two have two switches identical to the current 5W/25W switch. Their function would be to select between the two modes independently for each channel.

Secondly, I would need a relay (or several) to select which of these that are part of the circuit. This relay would be powered by the same current that energizes the preamp relays when changing channels.

I'm assuming that there is enough current on tap to power the extra relay(s).

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