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Lurch

System had a Brain Fart on Sunday

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Super Wammer
Posted (edited)

So Sunday was my 11th bake off as a host, and the 3rd one I've held this year and I have never had a problem, technical or otherwise. However unlike all my previous BOs, this one started off being quite stressful for me as my system had an unexpected brain fart.

My system was set up as follows:- All sources into a Mastersound PHL5 Pre, which has 2 pairs of outputs.

All sources & amps connected by late 90s/early 00,  0.5m or 1m Audioplan Musicable

1 Pair of outputs were connected to my IA  MB845 monos, the other pair were connected to a Rotel RB70 power amp.

The 845s were feeding the 989s via Kimber cable  and the Rotel, Goodmans Maxim2s via QED 79 strand.

The whole rig being run from a 2 month old Power Inspired AG1500S, all had been good via this prior to the BO and continues to provide a stirling service so definitely not at fault.

The problem I encountered was that when the IA MB845s where driving my Quad 989s, for some unfathomable reason the left channel was giving no output. The volume level on the right channel was going up and down like a port  whores knickers when the fleet is in.  I swapped over to the Rotel driving first the Goodmans with no problems and then substituted them with the Quads without a problem. I also repeated the above speaker swap after changing the ICs for some spares I had of the same brand, again both the Rotel and 845s happily drove the Goodmans but the 845s wouldn't play nicely with the Quads. I even tried running the 845s direct from the mains as opposed to the PI but this had no effect on the Quad/845 combo

After an hour of dicking around, I finally said Fook it, I and my guests will just have to put up the inferior SQ provided by the Rotel feeding the Quads.

Following a review of what I had tried and discussion of the issue with Dave (Bigfool1956) after lunch, he thought that maybe the 845s didnt like the Rotel  in circuit.  I couldn't see this as being the cause as the Rotel had been in the system for a couple of weeks without a problem, also prior to the Rotel I had a Nobsound Tune on Pre-out 2, doing power-amp duties without any problems.

Anyway I removed the ICs from between the Pre & Rotel as this was about the only thing I hadn't tried and bugger me!!!! all was good again and we could enjoy the sublime delights of 845s and 989s.

WTF was going on?

Note: ONLY 1 power amp setup was powered at any one time, with the other being switched off. Also the PI was only at 40% capacity with ALL components in circuit, and provides a constant 240V.


Does anybody have any Ideas as to why I had this problem?

Edited by Lurch

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Moderator

A guess, as there aren't any schematics I can find for the Icons or the Mastersound, never mind the RB-750, which Rotel don't even seem to have the specs for any longer.

The chances are that the outputs of the Mastersound are simply paralleled. The output of the PHL5 is therefore connected to the 845s, but whatever is in the input circuit of the RB-750 is going to be in parallel whether or not it's switched on.

The Icons use a 6SL7 input stage, so it's pretty likely that there is at most just a grid stopper resistor (if that). The Mastersound quotes 450 ohms output impedance and looking at the pics online, it looks like a strightforward capacitor coupled output, which is as expected.

What you've therefore got is a low impedance preamp with a capacitor in the output line driving a relatively very high impedance power amp (with the 845 connected). The Rotel is an older solid state power amp. At a guess, I'd expect a resistor/capacitor network on its input.

An educated guess is that by putting the Rotel in parallel, the RC network is acting as an unintended filter or possibly pushing it into oscillation.

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Super Wammer
Posted (edited)

Not questioning you're reasoning Rabs, as my electrial skills only extend as far as changing a fuse and plugging things in. But if the above is the case why were the Icons happy to drive the Maxim2s correctly but played up with the 989s, with one channel dead and the other oscillating? 

Would the plaited Kimber 8TC on the Quads Vs the QED 79 on the Maxim2s have exaserbated things, or possibly the different load type presented by the speakers? 

Edited by Lurch

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I hadn't spotted that the whole lot worked OK with different speakers.

Re-reading it, you mean that it was stable with both amps connected, but only the 845s powered up, driving the Maxims, but not the Quads?

If so, I haven't got a scooby, because that makes no sense at all. The output load on the 845s shouldn't matter a toss to the input.

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Super Wammer
10 hours ago, rabski said:

you mean that it was stable with both amps connected, but only the 845s powered up, driving the Maxims, but not the Quads?

Yep, 845s, qed 79 & Maxim2s all happy together. 

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