[tbrooke]

This may have been covered before but I was reading an article about bi-amping at: http://sound.westhost.com/articles.htm .

It sounds like it is a great thing but in applying it to the VMPS speakers with the passive crossover it seems to me that you are not getting the benefit of biamping since you are basically running two amplifiers in parallel. It seem you would want to put the crossver in front of the amplifiers. Am I missing something?

Tom

[Housteau]

I know exactly where your question is coming from.  If you search my user name you will find very similar questions being asked not that long ago.  The forum members and Brian himself were very helpful with my questions.  I have always bi-amped using active crossovers myself.  It was when I was thinking about a new VMPS system that had me searching the passive route to bi-amplification.

There is a lot of detail required to answer your question completely and more importantly, accurately.  I am not really the one to do this, but I will try with a few highlights that I now understand and maybe someone else can help fill in and/or correct where I have gotten it wrong.

You already understand the benefits using the active approach from that article.  What that article does not mention is that some of them are more amplifier dependant.  While it is true that loads will be reduced to the amplifiers because they will not be amplifying the entire frequency range, it is also true that some amplifiers sound best when they are more under a load.  So, with some of them a reduced load is not doing them any favors.  However, is more common that the reduced load will help with most amplifiers rather than hurt.  The power rating and performance curves of the amplifiers in question also makes a difference in the benefits achieved.

With passive systems both amps of a bi-amped system do amplify the entire frequency range.  This is true.  However, they are not loaded on their output to the entire range due to the passive crossover they connect to.  As such, these amplifiers are not working under a full load (similar to an active system) even though their input sees one.  So, many of the same benefits are still present using the passive approach.

There may be issues in using the passive approach with some tube amps.  It has been reported in this forum and elsewhere that some tube amps will have problems not having the same load on their output compared to their input.  I understand that will depend on the circuit topology and tubes being used.  Many use tube amps for the higher frequencies in passive circuits without experiencing any problems at all.  I have just started doing this myself with some older VTL Deluxe 100 mono EL34 tube amps, and so far everything seems just fine.

[John Casler]

As a generality, I think it is that "passive" are usually designed for "speaker level" signal, and "active" are driven by "line level" input.

Not sure if I have ever seen a passive that was able to process line level, but must admit i haven't paid too much attention.

[RGordonpf]

Hi John,

Wouldn't the simplest passive line level crossover be a capacitor soldered across each of the input pairs of the tube amp?  With a properly chosen cap value you could roll of the bass frequencies being feed to the tube amp which drives the midrange and tweeter.  A full-range signal could be fed to the solid state amp driving the woofers since solid state amps usually have enough output that rolling off the top is not needed for best performance of the amp.

The only draw back to the above set up is that even a very expensive cap (Auricap, TRT, etc.) is audible on a high resolution system.

If your tube and solid state amps have different outputs levels which are too great for the L-pads to correct, you can always add passive stepped attenuators to the inputs of the higher output amp.  That will also be audible, but hopefully, the stepped attenuators will be added to the solid state amp rather than the tube amp.

[Housteau]

Wouldn't the simplest passive line level crossover be a capacitor soldered across each of the input pairs of the tube amp?  With a properly chosen cap value you could roll of the bass frequencies being feed to the tube amp which drives the midrange and tweeter.  A full-range signal could be fed to the solid state amp driving the woofers since solid state amps usually have enough output that rolling off the top is not needed for best performance of the amp.

The only draw back to the above set up is that even a very expensive cap (Auricap, TRT, etc.) is audible on a high resolution system.

If your tube and solid state amps have different outputs levels which are too great for the L-pads to correct, you can always add passive stepped attenuators to the inputs of the higher output amp.  That will also be audible, but hopefully, the stepped attenuators will be added to the solid state amp rather than the tube amp.

You nailed it.  It is a compromise sometimes.  For example:  In my case I could add the caps to the input of my tube amps, but if I did that I could not run my ‘bridging circuit’ out of them.  Since the bridging circuit needs to see the low bass frequencies.

I believe the imbalance of input sensitivities between my two amps are too great for the L-pads to adjust for.  The tube amps almost aways are the ones to need lowering in gain.  I agree that adding an attenuator to the input of the tube amps may degrade their sound, hence the need I have for the adjustable bridging cables that sets that balance I need instead.  So, in my case my tube amps need to see the full input signal. 

[tbrooke]

Housteau

I glanced at that thread a while back and I may check it again . As I think about VMPS is between a Bi Amp and Bi wire system in that the mid-High and ;lower aren't connected when using two amps and each amplifier only sees the load from the driver it is running even though it is being fed a full signal.  I am not sure of the effect of this. As you say it may depend on the amplifier. I seem to me as mentioned in an earlier post that taking this a step further and putting a high pass  filter and a low pass filter before the amps would give you alomost all the benefits of bi amping. The bass amp may not be critical but the mid high amp would only see the higher frequencies and  this should make it's life easier. I may try to check around and see what the perspective is from an amp point of view


Tom