[Housteau]

With the PBS option on the V60 combined with the separate subwoofer amps and the one for the top end, that becomes a tri-amped system.  If a preamp such as my Audio Research does not have a set of double output jacks, I would generally run a single pair of interconnects from it into Y-cables to supply a passively biamped system.  For a tri-amped system that will require another split of that signal line, with three amps in parallel. 

Is this a taxing load for a preamp (my preamp) to drive three separate amplifiers?  My manual states that the maximum load for the output connectors should not be less than 20K ohms for maximum sonic performance. 

[warnerwh]

You could certainly have a problem with input impedance with 3 amps. It's no joke either as I had that issue with an LS 16 I owned. It did have 3 sets of outputs but keeping the load high enough isn't that easy.

Output voltage of most any and probably all ARC pre's should be plenty. 

That is one issue I had with ARC that I won't have again as I won't use another one of their preamps. The sound quality however is excellent. Customer support and build quality are also excellent. I plan on continuing tri amping though.

A SS preamp that is not very common but has a smoothness that's amazing for an SS preamp is the Primare Pre 30. It is better than the LS 16 and Krell KRC 3 that it replaced. It's one component that's staying.

Another person here also believes this to be the case. He auditioned the Primare gear through new Martin Logan's and last I remember is seriously considering buying the Primare gear after he moves. You can occasionally find them on Audiogon for about 1000. The design is by Mike Bladelius who was with Nelson Pass. Primare gear is considered top tier in Europe and if you Google it you'll find rave reviews.

This preamp is very over engineered and build quality is as good as the Krell I had or better. List price is only 2k. I think Absolute Sound did a review of some of their gear a few months ago and gave it glowing praise.

I never discuss my preamp because hardly anybody has heard of Primare. It just happened that a guy who has a used high end shop here had one and I thought I'd give it a try. With the Vmps speakers it has been a blessing. Like I told the person in reference above, once I started using this preamp the "there" level reached a spooky realism the Krell or ARC couldn't match. A definite step above.

Please don't take what I said about ARC pre's as a negative comment. I truly loved the LS 16 until it needed to be hooked up to 3 amps. Plate amps generally appear to have unusually low impedances compared to standard stereo amps. I am using one of the HPSA plate amps on one of my subs now and it has the 20k input impedance.

Finding another pre with the ARC sound isn't easy. Just a touch of tubes and just right at that. Losing bass quality though is something I won't sacrifice. Now I use a Van Alstine Fetvalve hybrid to run my planar/ribbon section. An Earthquake Cinenova Grande 3 for the woofers and the extra channel to run the other sub. In my room it would be very difficult to get better sound. It's spectacular quality wows even experienced audiophiles!

Hope this helps! Best Wishes.

[Housteau]

That is what I needed to know, but not what I wanted to read.  These plate amps have an input impedance of only 12K.  I am not sure what it is for my VTLs, but my guess is somewhere between 50K and 100K.  I may be doing this wrong since impedance is not equal to pure resistance, but in either case (50K or 100K) add two amps at 12K each and you end up somewhere around 5.5K to 6.0K as a total.  Even with driving just a single plate amp and my VTL the total impedance is only around 11K.

So, how do I work around this?  Does the low impedance issue to the preamp from the plate amps only pose a potential problem to the high frequencies for those amps?  If that is the case and since these are bass amps, is there still an issue?  Will there be a high frequency loss issue to the VTLs as well?  What would the proper interconnect connection to this system look like to handle this issue?  Would you start with a Y cable at the preamp splitting it into two initial legs, with one going directly to the upper range amp?  Then split the other with another Y-cable at its end to connect into the two plate amps?

Why has this not been noticed before, or discussed as a potential problem?  Is there a way to increase the load to the preamp in some way by adding resistors?  Would you do this across the inputs of the plate amps?  What value should they be?

[Housteau]

I checked with some other tube preamp manufactures, such as Mapletree another Audio Circle member, and found out that the specs on my Audio Research are quite normal in respect to desired load impedance.  So, if this is an issue, it is not limited to just a few preamps.

[JoshK]

Running that kind of load (w/ three amps) is a tough number for *any* tube preamp.  Driving one SS amp with a tube preamp is already hard enough.  It isn't only the issue of impedance matches (which aren't usually congruent for tube pres and SS amps) but also an issue of (Miller) capacitance on the input stage of SS amp and the tube preamp not providing enough current to drive it, resulting in HF roll off.  When you match up 3 amps it must drive you are amassing quite a feat for the tube preamp.  I think a CF output on the tube preamp is your only hope of being able to do it. 

IIRC, the Modwright preamp uses a tube with a mosfet follower, so that may work, since the mosfet lowers the output impedance and provides current gain to driving the combined capacitances.

[Brian Cheney]

You may or may not have a problem with HF rolloff using your tube pre driving three amps.  The only way to find out is to try it.

Fortunately it is easy to compensate at the speaker.  There is 5dB of headroom in the tweeter relative to the midrange array, and its response tilts up due to the dehorning.  Even a 10dB droop at 20 kHz could be corrected with the level controls, or removing one of the waveguides as we did at CES, or both.

Another alternative is to switch preamps.  Here I can recommend the Ambrosia, the only SS preamp I have been happy with in the past 20 years.  Sonically it has surpassed the best competition, and it would have no trouble driving 3 amps at once.  It's only $5900, you could sell your ARC and use the proceeds towards the Ambrosia without suffering financial hardship.  I'll be happy to loan you mine for a tryout.

But, I suspect none of this will be necessary, as the flexibility built into the speaker should come to the rescue.

[Housteau]

Thank you for your response and the kind offer.  I would really like to use what I have now if I can.  However, I have done a little more research and depending upon the preamp design, there can be significant bass roll off at or around 20Hz as well with a load impedance below 20K.  Anyone biamping using a PBS will have a load impedance below 20K.  The thing is, I know I am not the only person with a tube amp that is planning to, or has already biamped / triamped a PBS system.  I am surprised (glad) that no one has reported to have noticed an issue.

[Brian Cheney]

The PBS permits up to 12 dB of boost at 20Hz, or any frequency up to 80Hz.  That should compensate for any LF rolloff.

We've had the PBS for over two years now, and many owners with tube pre's have reported good results while biamping or triamping.

[Housteau]

Can these plate amps be modded, or something added in line with them so the preamps can see more of a load, at least enough where no loss will occur?

I spoke to Jim Romeyn on the phone earlier today, because he wrote about the triamping issue quite a while ago and how he overcame it.  I am hoping he will chime in here and fill in more detail, but I will give it a shot.  He told me that he was using what Jack Elliano of Electra-Print Transformers calls a "bridging circuit".  This is basically a simple voltage divider circuit designed to drop the output of the mid / treble amplifier down to a line level signal.   It is then connected from the speaker outputs of that amp to the line in of the bass plate amp.  The plate amps gain control can then adjust to the proper level, as normal.  I had never heard of doing something like this before, but he swears by it and is currently using this type of circuit in his system.  This configuration allows the preamp to see only one amplifier in a biamp, or triamp set-up.  I know that you have commented positively on the sound of his system, so do you think this is a viable and simple option? 

[Brian Cheney]

Yes, that will work well, particularly at low frequencies where resistor noise is much less audible.

One way or another your problem will be resolved.

[Housteau]

Outstanding .

When he told me about it, my first thought was that the signal might have softened up a bit in the deep bass since it had already gone through a tube power amp and been amplified before being brought back to line level, and tube amps are not known for their bass.

[James Romeyn]

Outstanding .

When he told me about it, my first thought was that the signal might have softened up a bit in the deep bass since it had already gone through a tube power amp and been amplified before being brought back to line level, and tube amps are not known for their bass.

Yes, earlier I also considered your point above about using a tube power amp IFO another SS bass amp driving a sub.  The reason it's irrelevant (at least IMO) is that the descriptions of the sound quality of a tube amp in the bass range are...related to it's performance driving a woofer.  Woofers have more back emf than any other portion of the speaker.  Bass amps have more trouble (generally) driving woofers because the tubes don't make current.  Woofers are AC motors w/ coils, diaphragms, surrounds & spiders.  The usual method to attain the current is to convert the tube's output voltage into current via the output transformer, or in the case of an OTL by using a high number of tubes relative to the power rating.

But in Jack's circuit all the tube sees is...virtually nothing.  Well, nothing except for 9k series followed by 1k parallel resistors.  Vs. driving an AC motor. 

A long way of saying the rules about a tube amp's low frequency performance are IMO irrelevant in this case.  As long as some trace of the signal exists at 20 Hz, it seems like it should work.  Actually, I've had Brian's big woofers over here using the method described & it moved pants legs, just like they are supposed to.     

But I always want to learn.  If someone has tried it & had different results I'm interested. 

BTW, regarding stuffing a 20k series resistor (or similar) IFO an amp to simply raise the load impedance.  All this does is move the problem from one side to the other.  Yes, the preamp will be happier.  But the sub amp or any amp has a particular input resistance for a reason.  The preamp will have more current, but it is burned in the resistor before it gets into the amp.  Otherwise everything would just have a 100k input Z & we'd all be wondering now what really killed Anna, something inquiring minds want to know.....Oh if audio life were simpler, & we had no money & weren't wanting to triamp our speakers.......

My 2c.....

I like the $10 Vishay resistors from Michael Percy, maybe others sell them too.  Percy is absolutely quick & reliable.  Send money, get stuff.  Step down & very close are the 0.50c Dale.  Atma-Sphere's high-end option is the Caddock; they must be good.

Angela Instrument's website has some great details on comparing resistor performance.  I remember thinking when I read it that I heard the same qualities he did.  It's a great site.  I'll find it if requested.  It's a music instrument site, tons of DIY stuff, amps, etc...

 

[PLMONROE]

I spoke to Jim Romeyn on the phone earlier today. He told me that he was using what Jack Elliano of Electra-Print Transformers calls a "bridging circuit".  This is basically a simple voltage divider circuit designed to drop the output of the mid / treble amplifier down to a line level signal.   It is then connected from the speaker outputs of that amp to the line in of the bass plate amp. 

Ok, so that's the answer. Specifically how do those of who are electronically challenged idiots build one????

[Housteau]

A voltage divider is a very simple circuit.  It just consists of two resistors and they need to be in line between the Mid /Treble amps output and the plate amps input.



You would place these in parallel with the speaker cables running out of the high level amp to the mid / treble section of the speakers.  Or, you can tap directly off the speaker terminals themselves.  It depends on where your plate amp is, built into the subwoofer, or a stand alone component next to you upper range amp.

You would do one of these circuits for each channel to biamp, and just add in another in parallel to triamp.

[JoshK]

Now that I think about it, there is another solution.  You could use a 4:1 step down input tx on the plate amp.  This would make the reflected load to your preamp 4 x the nominal plate amp load (so for eg 4 x 12K = 48 K).  This trades the extra voltage gain for additional current too helping drive the input capacitance.  Input tx's aren't cheap though, so the voltage divider trick might work better.  Jack Eliano would also be the dude to go to for the input tx.  Since HF isn't needed for a woofer plate amp.  The available bandwidth could be tailored to optimize the bass range quite easily making the tx quite transparent. 

[Brian Cheney]

In my experience the simplest solution to a problem is often the best one.

Since the speaker allows you to compensate for many problems elsewhere in the chain, use its resources first if you have problems biamping or triamping.  So far, this has worked for everyone, and I mean everyone.

After that, the wise folks on this board and at the factory can help if need be.

[James Romeyn]

Brian, stop right now making this sound easy!  People are trying (tyring?) to solve an earth-shaking problem here (pun intended)!!!   

So the bass "rolls off" when the preamp output current is too low for the amp load.  Obvisouly, a level control & plenty of gain, followed by a 1024WRMS amp powering a VSS = no dearth of bass energy in a normal domestic environment.   

Do dynamics suffer w/ the bass rolloff?  If so, only in the bass range or throughuot?  The two symptoms seem to go together, such as ...passive preamp...lightweigt bass...soft dynamics... 

[Brian Cheney]

Or, people could do what you did and buy the Ambrosia.

Jim Bongiorno says one week delivery.

[Housteau]

the speaker allows you to compensate for many problems elsewhere in the chain

I think this is a great thing that will help many to integrate systems into their own environment.  Personally though, I prefer to use all of those adjustments to compensate for room interactions and acoustic issues, rather than correcting for an equipment mismatch.

[Brian Cheney]

I think it is possible for the speaker to do both--correct for the room and the associated equipment.

Jim Bongiorno reports (in response to my question) that the output impedance of Ambrosia is 50 Ohms and that it can drive 20 Ampzillas in parallel simultaneously.

Is there a word for a speaker that can use 20 amplifiers at once?