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Modifying the ML Spires for line input drive to the bass amp
Modifying the ML Spires for line input drive to the bass amp
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Old 12th July 2018, 12:06 PM   #1
wynpalmer2 is offline wynpalmer2  United States
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Join Date: Oct 2016
Location: Greensboro NC
Default Modifying the ML Spires for line input drive to the bass amp

I have a pair of ML Spires that I want to modify to allow for a line input to the bass power amp so that I can integrate the Spires with a subwoofer and use a miniDSP setup to correct for a room resonance at 49Hz.

Does anyone have experience in doing anything like this? Is it even possible, and if so, how do I go about doing it safely?
I have an EE degree, and I have requested schematics from ML, but comments from anyone with relevant experience would be greatly appreciated.
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Old 12th July 2018, 01:47 PM   #2
jacek_k_wawa is offline jacek_k_wawa  Poland
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I am trying to get the same for my Summits. I have been looking to get schematic of x-over board but no luck.

From other hand made attempt to do reverse engineering of x-over PCB to understand how input stage is design and I have to admit that it is pretty difficult as - at least in Summits - PCB is multilayer.
From what I was able to figure out audio from bass binding posts goes to resistor network and then to the input of TL062 OpAmp.
So I guess this gives a chance to bypass/modify this input attenuator to tap low level signal to opamp.

But for sure - if there would be schematic available it would be much more easier and safer to do this mod...
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Old 12th July 2018, 03:05 PM   #3
wynpalmer2 is offline wynpalmer2  United States
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Join Date: Oct 2016
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It does seem likely that there is simply a resistive attenuator from the inputs to the non-inverting input of a buffering opamp.
However, if at all possible I'd like to be certain as the way the crossover for the bass speaker/panel is implemented is not specified anywhere as far as I'm aware and I can imagine ways in which the bass input is produced using the difference between the attenuated output of the high pass panel crossover output and the attenuated high level input, so just any old input signal level to the bass path would not work.
However, this seems unlikely for a non-DSP/passive design and if the two crossovers are independent then most likely a simple modification of the input attenuator can be made to work.
In any case, if I don't hear from ML or from any forum members concerning the actual design I'll follow your lead and try to trace at least the input circuitry out and report the results back.
I also have a pair of Montis, but I haven't summoned up the courage to even contemplate modifying those.
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Old 12th July 2018, 04:27 PM   #4
jacek_k_wawa is offline jacek_k_wawa  Poland
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From what I was able to trace looking at PCB of my Summits there are completely separate circuits - bass part goes to active xover on PCB and ELS panel bindings connects only to passive xover and step-up trafo.
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Old 12th July 2018, 05:20 PM   #5
wynpalmer2 is offline wynpalmer2  United States
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Join Date: Oct 2016
Location: Greensboro NC
Thanks. It appears that the Spires and Summits are similar.
ML responded to my inquiry and provided me with a schematic of sorts- one that doesn't include, naturally, their proprietary circuitry.
One of my Spires does not have a functioning bass output, which prompted my interest as I knew that I would have to go into the speaker anyway.
Per their instructions I swapped out the back panels and debugged the circuit and what has failed seems to be the opamp that you described, which is actually pretty convenient.
The input stage is a 15.8x in, 1x out (-24dB) differential to single ended converter/attenuator and the crossovers are indeed completely separate. The output from the opamp goes through what seems to be a zero ohm jumper (TP4) to the LP Xover board. The input stage can be converted to unity gain by changing two resistors- R1 and R4 from 158k to 10k- or it can be configured to match the power amp gain etc.
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