While I wait for my new system to arrive, I am using part of that time to prepare for it the best I can. For the passed 17+ years my goals have been centered around reaching a specific synergy in components and connections based on the Infinity RS1b speaker system. Those speakers were a 4 piece system actively biamped with a relatively high crossover frequency of 150 Hz going to the flanking bass towers. Most of everything I have learned over the years I now need to leave behind as I move into something completely different with a full VMPS RM V60 system.
So, what cables I have been using may not have the same results between my amplifiers and new speakers. I do have several different older sets of both speaker and interconnecting cables. Now is the time to lay them all out and prepare and/or change the ends for test connections within this new system. That much is fairly easy to to. The largest change I will need to work through is going from a previously active biamped system over to a passively biamped, or triamped one. And that is what I would like to explain in this thread.
There have been previous mentions of possible issues and problems in passive systems, but at the time they did not concern me, and so I did not really pay attention to what was being said. However, as I recently started to investigate this passive approach for this new system, I found myself remembering bits and pieces of threads discussing things that will now directly impact what I wish to do. These are all things I had not needed to consider before with an active crossovered system.
I use an older Audio Research preamp. It needs to see a load above 20K for full performance. A load below that can cause a loss at both frequency extremes, as well as a loss in dynamics. The AR preamp always saw a high impedance load in my previous system, but that was active and now I am switching to passive. There will be two to three amps in parallel needing to be driven by that preamp. That would be a difficult load for any tubed preamp. But the real kicker is that these bass plate amps have a load impedance of only 12K. So, with even with just my VTL mono amps in parallel with a single plate mono amp, that load to my preamp would be way below that 20K needed. Anyone planning to passively biamp using one of these plate amps with a tubed preamp may have this same issue of driving a very low impedance.
If I do this will my system suffer from these previously mentioned possible problems? If it does, I could use controls wisely built into the speaker system to help compensate for them. But personally, I want to save the speaker adjustments for room interactions if needed, and not to correct for electronic mismatches. Besides, a loss in dynamics is hard to compensate for, if even possible. My choice was clear. I want to eliminate this issue before it gets to the speakers. After all, why buy a world class speaker system and then place it behind an eight ball right away. That just doesn't make any sense to me.
My solution was found in some older posts written by Jim Romeyn (RibbonSpeakers.net). These are ones that I only just briefly glanced through, because like I had mentioned, they didn't relate to me.
http://www.audiocircle.com/index.php?topic=31209.0He was using what I thought was a very unique method of passively biamping a tube and SS amp in his system and was able to match the levels between them. I was considering triamping by using plate amps for both the VLA bass towers and on the V60 itself. I was considering the plate amp for the V60, because it has a gain control to allow level matching, something very necessary when mixing tubes on top with SS on the bottom. But, here Jim was using one of his own bass amps without such a control in a similar circumstance. His tube amp was also an integrated without a preamp out circuit. I now found all of this simply amazing and extremely relevant. I gave him a call and he took the time to explain the mechanics of this 'bridging circuit', first described and recommended to him by Jack Elliano of Electra-Print Transformers.
All it is is a simple voltage divider circuit where you tap off the output from the high frequency amp, either in parallel with the speaker cables leaving the amp, or at the speakers input terminals themselves, and break it back down to line level. This line level signal is then the input to the bass amp. The preamp is only seeing the the single load of the tube amp even though it is driving two, or more amplifiers. Here it is in a simple drawing:
A simple 9K resistor in series with the signal followed by 1K going to ground is all it takes to use one of these plate amps without adding a stressing load to my Audio Research preamp. Now, to use my own bass amp for the V60 a similar circuit is needed, but since my bass amp does not have a gain adjustment, one needs to be in this 'bridging circuit'. Jim's solution to this is also quite simple. Those previously fixed resistor values now get replaced with an Alps pot. His personal choice was to use a 100K linear taper instead of a 10K audio one, then convert it to an audio taper by inserting a 10K resistor between the wiper and ground. So, I decided to do this as well.
I decided to make them as mono cables and hardwired as all my amplifiers will be close to each other. This is what each one, one fixed and the other variable, have ended up looking like.
I have been able to test both circuits and they appear to work just fine.
