Bi amp pros out there I could use some help! First time Bi Amping...


Just picked up a BAT vk 200 for the bass and using my Pass Aleph 5 for the mids and tweets. Ive never played around with bi amping so I apologize in advance for any lame questions My speakers are Dunlavy SC3's original 5.5 nominal load. The pass is 90 wpc at 4 Ohm and the BAT is 200 wpc at 4 ohm so Im guessing around 75 wpc off the Pass Amp and 150 plus with the Bat Amp. My pre amp is a Aleph P and Im running the Single ended through a XLR adaptor (cause the Bat is Balanced inputs only) and the pass Aleph 5 off the XLR outputs of the pre and inputs of the Pass amp. The PASS Pre Amp manual says there is a 6db differential between the RCA and XLR outputs  two and both can be driven at the same time. So the RCA is 9db and the XLR is 15 db. Gain is within 2db on each amp. So whats the best way to do this? Get a custom XLR "Y" connector and drive both off the XLR output of the pre? Or is there a way better way to get the magic? This is past my "WORLD" Map and experience so Id thought Id ask for the smart people for advice. 

Thank you in advance!

-ALLGOOD
7e8adcbb d3c6 4988 9f12 da3326830708haywood310
If you do not have gain controls on at least one of the amps you have a problem. The major benefit of biamping is to be able to bypass the crossovers in the speakers and instead use an electronic cross over between the preamp and the amplifiers such as the DBx 223xs. This allows  you complete control of levels and cross overs. I do not know of any unit that gives you just gain control although you could rig a box with just potentiometers in it. 
Specifically, the amp with the higher gain ( the amp that plays louder at the same preamp level ) would need to have a stereo input level control, so you can turn it down a bit. The Bay has plenty, just type in " passive level control ". Douk Audio comes to mind ( they make xlr and single ended ), and Schiit ( the company ) makes a passive called the Sys, which is single ended only. Hope this helps. Enjoy ! MrD.

In addition to what has already been posted; the Dunlavy's with their 1st order cross-overs have significant overlap, ideally you would be using two identical amplifiers.

I would not recommend bypassing the Dunlavy cross-overs, as they are custom tweaked for each pair of speakers to a lab reference.

A line-level cross-over can not necessarily be used in place the speaker-level one installed inside that speakers' enclosure. The speakers' cross-over often contains electronic parts that are used in "compensation networks" needed to optimize the behavior of the speakers' drivers. You need to get a schematic of the cross-over, to see what you are dealing with.

Some loudspeakers come from the factory set up to allow bi-amping---for instance the older .6 series Magnepans, but most speakers aren't. Are your Dunlavy's?

There's a couple of issues. The preamp has relatively high output impedance, making it a less than ideal candidate for using 2 amps at the same time.

The other is the possibility of having the amp gain mismatched. You'll need some way to adjust the levels appropriately.
Post removed 
I have bypassed quite a few cross overs although I can't say how many.
A good active crossover can pretty much compensate for anything. If it is a 3 way speaker you want to bypass only the woofer section leaving the midrange to tweeter cross alone. 
Even the good active x/o's (Bryston, Pass, First Watt, Marchand) provide only "textbook" filtering: 1st/2nd/3rd/4th-order (6/12/18/24dB per octave). None of them provide compensation networks, which lots of loudspeaker x/o's include, even some employing 1st-order filters (such as Thiel). Look at the x/o schematic for your speaker to see how complex it is.
Or is there a way better way to get the magic? 


Yes. Its called a better amp. Take whatever you spend on bi-amping. Two amps. Two power cords. Two speaker cables. Do you use a shelf? Cones? Twice as many of those. It can take a very long time and a lot of wasted dollars chasing the fantasy that you can do better splitting your funds into twice as much stuff. As cool as it may make you feel, which if that is the goal have at it, you will impress a lot of gear heads easily impressed with gear, but it just won't ever deliver "the magic" you can get with twice as much to spend on each component.
Correct crossover integration with passive or active components is a giant PITA. There's no free lunch.
At best, if you have separate low and high pass filter sections, you can use set them a couple of octaves away from the actual crossover points.


For instance, setting the high pass at 1 kHz, and low pass at 5 kHz when the actual crossover is 2.8 would probably be OK.

I agree with Millercarbon, use the KISS approach. You don’t have the ideal pre and your using different amps. IMHO, the Alpeh 5 is not the best match for the Dunlavy’s with what I believe to be a 3 Ohm minimum impedance.

I suggest you sell both amps, and get one more appropriate for your needs.

If you absolutely want to bi-amp; get a different pre with dual identical outputs that mate with identical matching amps that better suit your speakers.

@Haywood310 After looking at the descriptions, specs, and/or manuals for the equipment you’ve listed, if (a)you are satisfied with the sound you are presently getting with the configuration you have described, and (b)assuming the specs on the equipment are reasonably accurate, and (c)the lengths of the cables between the preamp and the amps are not unusually long (for example, longer than 15 feet or so), I’m going to offer a radical suggestion for you to consider: A good course of action may very well be to do nothing :-) Just enjoy the configuration you have described.

It appears to me that the present configuration is fine with respect to all of the major technical pitfalls that can be encountered with a passive biamp configuration:

1) Gain matching: The BAT amp has a specified gain of 25 db. By providing it with a single-ended input, while there is no change in its gain per se (i.e., the relation between its output voltage and its input voltage), overall system gain is 6 db less than if you were providing it with a balanced pair of signals. In effect that reduces its gain to 19 db. While the gain spec provided in the manual for the Pass amp appears to have a typo (it says that its gain is "20 dB balanced, 20 or 26 dB balanced") after reading the text I’m fairly certain that its balanced input provides a gain of 20 db. So you only have a 1 db gain difference between the two signal paths. Room effects most likely make a good deal more of a difference than that.

And if you were to split the XLR outputs of the preamp and drive both amps with a balanced pair of signals you would be **introducing** a significant gain imbalance, that would require insertion of a suitable attenuator between the preamp and the BAT amp.

2) Impedance compatibility: As is often the case, the signal provided by the preamp to its RCA output for each channel is the same signal that is provided to pin 2 of the XLR connector for the corresponding channel. So that signal will be loaded by the input impedances of both amps. But the numbers are such that that is not a problem in this case.

3) Upper treble rolloff that in some cases may be caused by the interaction of preamp output impedance and the combined capacitances of the cables to both amps: Again, not a problem in this case assuming the cables are not unusually long.

4) Inability to utilize most of the power capability of the higher powered amp without clipping the lower powered amp: In this case the roughly 2:1 disparity in amplifier power capability **might** result in not being able to turn the volume control high enough to utilize perhaps 2 db or so of the power capability of the BAT amp, without driving the Pass amp into clipping, but that’s probably not a major issue. If you were passively biamping say a 400 watt amp with a 50 watt amp I would not say that.

5) Degradation of the common mode noise rejection that can be provided by the amp receiving the balanced signals, due to the differing load impedances presented to the two signals in the balanced signal pair: The output and input impedance numbers in this case suggest that is unlikely to be an issue.

Others have suggested changing the equipment. That’s of course something that can be considered, now or in the future, but if you are happy with the present equipment in the present configuration it seems to me that doing nothing is also a reasonable course of action.

Regards,

-- Al
AL, my thoughts exactly ! lol 
Adding powered subs is a form of bi-amping, providing many real benefits with no penalties.
I had my Martin Logan Vantage speakers modified for biamping mainly to bypass the in unit power amplification for the bass units which while Ok ,could IMO be improved. I use a 90wpc @ 8ohms Rogue 90 tube amp to drive the electrostatic panels and an Acurus 250wpc @ 8 ohms for the bass units. Technically I have no idea what the modifier did re crossover bypassing but the result is much deeper and tighter bass. Well worth the expense which was not great. I also have a powered sub which I hardly ever use because it can tend to be overpowering in a small/medium sized room.
Greetings,
Hope I can help, I worked as a soundman and discovered the magic of bi-amp and tri-amping. It brought much better, cleaner and louder to my PA system even when I had a mixed bag of amps. The multiple gain controls and adjustable crossover points and a summed subwoofer output gives many ways to match up with almost any amp, speaker or room. If you have an RTA you can tweak it to perfection and get great sound from mediocre equipment.
Before buying my Peachtree nova 300,  I used a 5ch B&K Sonata (105rms) per channel with independent gains. It sound very full range and sparkly clean with my B&W 565s and a infinity sub.
Takes a little time to tweak set up, but that's half the fun.
Go on Ebay or your local musical equipment store and get a quality DBX, EV, or a drive rack for a few bucks more.
You might consider a DBX 223xl.  Available fairly cheap.  Balanced input and outputs, master gain control, and individual gain control over hi/lo.  I use two of them as three way xovers.  Very flexible, all pots are stepped.  You can change the frequency on the fly, without a noise penalty.
haywood310
Just picked up a BAT vk 200 for the bass and using my Pass Aleph 5 for the mids and tweets.

This should sound very good.

Because you have amps that are not identical, vertical bi-amping is out.
You’ll have to do horizontal bi-amping.
http://www.av2day.com/wp-content/uploads/2014/05/biamp1.jpg

And because there’s no way both amps will be equal in gain, you’ll have to put a passive volume control at the input of the loudest amp so you can lower it’s volume to be identical to the other amp.
https://www.schiit.com/products/sys
Then your main preamp volume control will be the master volume control for both amps that now have identical gains.


Cheers George
And because there’s no way both amps will be equal in gain, you’ll have to put a passive volume control at the input of the loudest amp so you can lower it’s volume to be identical to the other amp.

Hi George,

See item 1 in my post earlier in the thread.  As well as the statement earlier in my post that I'm assuming the specs are accurate.

Best regards,

-- Al


Hi, Im about to take delivery of a set of floorstander speakers that do have a separated passive crossover with a pair of speaker connecting posts for the mid/treble, and for bass as do many modern speakers. Fortunately I have four mono Musical Fidelity mosfet amps, a bit old yes, but all 50 watt.
The simple reasoning that supports biamping in this case is that the mono amp supplying the mid/treble speakers will have considerably less load drawn from it during heavy bass passages of music.
There is no doubt that an active crossover system would be best of all as that same mono amp would only have to amplify a given range of frequencies.
However as I firmly believe that this new speaker set has been designed by engineers etc with infinitely better experience and measuring equipment than I have, the best option that I have is to maximize their design by biamping them.

I will leave a report on the differences I find between single and biamp on  this discussion in a couple of weeks. I have many years listening experience, so it should be relevant. So if you would like to know, then click the `Follow this discussion` at the top!
To the naysayers: biamping is definitely advantageous with the separation of reactive energy from the woofers from the mid/high drivers. You will experience cleaner midrange and treble if you use amplifiers rated with enough power to stand on their own in non-biamp configuration. I recently upgraded my amps, quadrupling the available power to drive the same speakers, and the difference is very noticeable. 

Next stop: experimenting with subwoofer swarms. 


Thank you all for the detailed responses! It means a lot to a guy like me, or just me. Take care and Happy early Memorial Day weekend to any Vets.

-Allgood 

PS I didnt add that I have two Hsu Research ULS 15 sealed subs that I am running stereo via speaker level from the Dunlays but no high pass just for the foundation. Was thinking of adding a third to try and get that swarm effect.Thank all of you!
Looking at some rear panel photos of the Hsu ULS-15 it appears that it may have a two-prong IEC receptacle for its power cord. If so I don’t see any issues regarding the presence of the subs in the system.

However if they have a three-prong receptacle, and a three-prong power plug, I would suggest that you connect their speaker-level inputs to either the output terminals of the Pass amp or to the mid/hi terminals of the Dunlavys. Connecting the negative speaker-level input terminal of a powered sub that has an AC safety ground connection to the negative output terminal of a fully balanced amp (such as the BAT), which as a fully balanced amp has a full amplitude signal on that terminal, may cause problems and conceivably even damage depending on the impedance through which circuit ground and AC safety ground are interconnected within the particular amp and sub. And of course that would also apply if the connection is made to a speaker terminal to which the negative output terminal of the balanced amp is connected.

In the biamp configuration you have described the Pass amp won’t be supplying a great deal of current or power at bass frequencies, of course, but since it will be approximately gain matched with the BAT amp the subs will receive essentially the same full-range signal voltage and hence similar bass information regardless of which amp they are connected to.

Regards,

-- Al
Ive decided Bi amping is just too much for me... Not to mention my OCD is off the charts with all the variables that were discussed in this thread. So Im gonna stick with the Pass for the Dunlavys and try to Swarm (mutil sub) the bass. Ive spent at least 60 hours A/B ing the BAT and the Pass. I really really like the BAT VK 200 and it  sounds just like a Levinson 334 to me and I cant recommend this amp enough to anyone especially given the used market price. There was one review I read that said "It just sounds good enough" and I cant agree more. But there is something about that 2 stage Aleph that grabs my soul every time I listen and I just cant get rid of. I cant even turn off songs I dont like. Its a trophy wife well actually much better I had one of those. haha. Thanks again for all the input.   

@sleepwalker65, I’m all for bi-amping, but haywood310 needed to be alerted to the fact that to do it correctly is much more involved than merely putting an external x/o and a second amp onto a pair of loudspeakers. The internal speaker-level x/o must be discarded, the outboard line-level x/o doing all the filtering. And that will NOT work if any of the drivers used in the speaker required any compensation network filtering to sound as the designer intended them to.

As I said above, Magnepan used to make their speakers so as to be easily bi-amped, and endorsed doing so. The .6 and earlier Maggies had parallel cross-overs and two pair of speaker cable connectors (which made bi-amping simple), but switched to series x/o’s for the .7 models (bi-amping them requires cutting into the x/o, installing another set of binding posts, etc.). Some of the monitoring loudspeakers used in recording studios come with filtering and a separate power amp for each driver, but they sound very different from audiophile speakers.

On the other hand, adding a sub or four to just about all loudspeakers provides many of the benefits of bi-amping, plus more. They present their own challenges to optimize in a room and system, of course.

o do it correctly is much more involved than merely putting an external x/o and a second amp onto a pair of loudspeakers. The internal speaker-level x/o must be discarded, the outboard line-level x/o doing all the filtering.
The whole idea was simple, to vertical bi-amp using the speakers internal xover.
All the other talk now has just confused the s**t out of OP into now giving up, so sad.
He could have have a great sound with the KISS horizontal bi-amp setup with these two amps.
Cheers George
The thing is, to bi-amp using the internal speaker-level x/o is to not reap some of the major benefits of bi-amping: keeping the woofer frequencies out of the m/t amp, eliminating the effect of the woofer's emf on the m/t drivers (they remain electrically connected through the x/o), etc. May as well just bi-wire, in the opinion of some.
The thing is, to bi-amp using the internal speaker-level x/o is to not reap some of the major benefits of bi-amping:

I totally understand where your going, but it’s obvious or not in tune with what the OP is capable or wanting to do.
It’s his thread so lets keep it to his speed., he has a bruiser of an amp for the bass and a delicate amps for the mid and highs, and he simply needed to passively bi-amp them.

Cheers George
Ah, NOW I get it George. As usual I was thinking in perfectionist terms; gotta keep the op more in mind.
REALLY GUYS!!! Gonna "Nube me in third person" as my son calls it on Fortnight and Minecraft. LOL. Allgood and sorry to cause a ruckus. I really do appreciate all the feedback. Ive wasted money and hope to more well informed decisions in the future. Hence going to the Old pros...

*Allgood
My perception over the years is that many members here have achieved good results with horizontal passive biamp configurations. I recognize that in general a vertical passive biamp configuration, using identical amps, is likely to be preferable from a sonic standpoint. And that inserting an active crossover ahead of the amps can **potentially** provide added benefits, while at the same time creating opportunity for error that would compromise the results. And that removing the internal crossover of the speakers (if practical) can **potentially** provide further added benefits, while also creating greatly increased opportunity for error that would lead to poor results. And that in many and probably most cases a well-chosen single-amp approach can provide better results than all of the above, while at the same time minimizing opportunity for error.

In this case the OP **already has the amps,** and as I explained earlier the configuration he described in his original post would avoid the various technical pitfalls that can arise when passively biamping with different amps. Others have recommended a KISS approach. It seems to me that under the circumstances biamping as the OP described in his original post would in fact be a KISS approach, as George also indicated. Which as I say corresponds to an approach that has been successfully used by more than a few members here.

Regards,

-- Al

I have been an advocate of passive vertical biamping forever, because, speakers such as the Dunlavy line, easily show sonic differences between amplifiers, and to maintain coherence, it is the only way to go. Not to mention, the amplifiers maintain much better control in this configuration. I urge the OP, to seek out another exact Pass amp, and vertically biamp the Dunlavys, because it has been my experience, he has not heard what the Dunlavys are capable of. Understanding how difficult it would be to attain another Pass amp, it would be worth seeking out anyway, or, finding another pair of high quality stereo amps and vertically biamp. Of course, this could lead to another thread, as to finding a pair of amplifiers to replace the sonics of his Pass, but passive vertical biamping has been most impressive for me over the years. Just my final thoughts on the matter, understanding this thread has come to an end. Enjoy ! MrD.
haywood310:
 I'm gonna stick with the Pass for the Dunlavys and try to Swarm (mutil sub) the bass.


Smart move. Massively more bang for the buck this way. The reviews on the Hsu ULS-15 MkII sound like you add another one or two of these and you will have AWESOME bass probably beyond your wildest dreams. Four of those in your room should go flat to subsonic with hella headroom. 
Bdp24 you are completely missing the point. The OP wanted to use the bi-amp feature designed into his speakers. In other words, the way they are supposed to work. The jury is still out on power ratio as far as I’m concerned. Bottom line is that the optimal configuration for his speakers is bi-amp. 
I am little late for the game and may have missed some fine points in comments, but wanted to add 2 cents. I have 4 way crossover with identical amplifiers (except for subs) and one of the issue I had not figured out was speaker impedance matching. The woofer (in parallel) had 3.5 ohms and midrange (in series) had 8 ohms. All experts told me that would not cause problems but every time, I increased the volume, the sound stage would move forward. Once I matched all speaker impedance, sound stage is stable. 
In the spirit of KISS as others have, here is a simple way to have more control and get as complicated as your OCD allows, add a Mini DSP 2x8 Active Crossover. It makes bi-amping super simple and you can integrate your sub swarm by ear, or get sophisticated and use room correction. The computer interface (connect via USB only while tuning) is clean and intuitive with visual on screen crossovers (start with what the manufacturer recommends) and adjust levels to match your amps. 

I had horizontally bi-amped for years with matching amps and it sounded good but recently vertically bi-amped my set up and it has much more punch. So I recommend finding another like model Pass amp to do it and you’ll love it. 

I am happy to discuss how to set it up if interested. To illustrate how far you can go: I’ve gone off the deep end with four subs (two in stereo) integrating seamlessly with my vertically bi-amped 20.1 Maggies. There are 12 bi-quad filters in each speaker created by Multi Sub Optimizer from data captured with Room EQ Wizard. The  crossovers look crazy and the result is jaw dropping.  
Back in 2001 or 2002, I did passive vertical bi-amping with Musical Fidelity A3cr preamp and two power amps. These amps got great reviews, the only real shortcoming being lack of power (at 120 watts per channel, and they were not "stiff" watts). To address this in response to the earliest reviews that mentioned the lack of power but otherwise superb sound (especially for solid state), the manufacturer made a change in production and built a line level pass-through into the amps. You could thereby daisy chain the amps for easy vertical bi-amping.

I was using Sonus faber’s original Concertino at the time, which were lowish in sensitivity. They really came alive with the extra power, and I was surprised just how much clean power they could handle in this configuration. (In the early days, Sonus faber didn’t even make tower speakers. Bi-amping their refined bookshelves with monstrous amps was kind of a thing that yielded shockingly big sound from tiny enclosures. This practice reached its zenith, at the time, with the original, if no longer "bookshelf" reference Extrema stand-mounted design.)

I also used B&W 602 during this sojourn. These were higher in sensitivity than the Sonus faber, but the old B&Ws had a way of drinking up all the power your amps could muster, and still wanting more. They sounded great with less power, but were capable of much better with more power still. (Not sure whether this has changed with the latest D3 versions.) Unlike the Sonus faber, the 602s didn’t sound good at low volume levels, losing too much life and subtlety. However, with bi-amping came life at low-level listening through the 602.

Now I’m using Musical Fidelity A308cr components, which have been my reference for well over a decade. These were big brother to the A3 and A3.2 series, predecessor to the Tri-Vista kW components, and some of the best solid state Musical Fidelity made before production shifted outside the UK. These amps also have line-level RCA pass-through to allow easy passive vertical bi-amping. But who would need to bi-amp with 250 watt amps?

Well, I recently fulfilled a long-standing ambition to bi-amp with two of these 250-watt bruisers, using JMLAB/Focal Electra 915.1 tower speakers. The speakers are rated 91.5 db sensitivity, so don’t need a great deal of power, except they drop to 3 ohms in the bass, typical of JM designs. The A308 amp never wanted for power with the JM speakers. I wanted to try it nevertheless.

I expected the volume to sound twice as loud and wondered if I’d have to use the preamp at the lower end of it’s volume control. What I found instead was increased subtlety and finesse, with greater detail, larger soundstaging, and more density and complexity to the sound. It was not a knock-you-over-the-head kind of difference in loudness, drive, or bass control, as it had been with the smaller A3cr amps, and bookshelf speakers disguising deceptively demanding loads. It was more a difference of greater refinement and naturalness; an ability to see deeper into the soundstage. I presume the difference is because the amps are coasting in this configuration, and maybe mitgaing the EMF of the woofer in a vertical bi-amp. I use identical interconnects and speaker cables.

I couldn’t say whether they’re harder to find than the Pass Aleph, but the A308cr amps are no longer plentiful on the used market. I waited years until I found one in suitable condition and had spare funds for it.

My experiences with passive vertical bi-amping, therefore, have been entirely positive. However, I have done it with identical cables and identical amps, with speakers designed for bi-wire/bi-amp, and with amplifiers whose manufacturer made provision for easy vertical bi-amping (rather than needing a preamp with multiple outputs).

Speakers have generally gotten easier to drive in recent years, and many manufacturers have stopped offering the bi-wire option, which used to be de rigueur for "serious" designs in the ’80s and ’90s. B&W have long been known to benefit from bi-wire/bi-amp setups, as have the old Sonus faber designs. I find it disappointing that Focal does not offer this capability on its current lineup. In terms of semi- affordable statement speakers, my experience suggests a big speaker like the Sopra No 3 could really benefit from having its woofer driven separately, especially if you’re putting most of your budget into new speakers and can’t afford the tariff for a high quality 400- watt amp. It’s an expensive loudspeaker. Why can’t the manufacturer offer the courtesy of bi-wire flexibility? For many of us, it’s easier to bi-amp with a less expensive high-quality design whose sound we already know than trying to find a more powerful design that retains everything we enjoy about our current reference. This is one reason I’ve stuck with my older Electra speakers and A308 amp, even though I might otherwise aspire to a Sopra design or the Musical Fidelity kW 750 (which sounds decidedly different from the A3, A3.2 and A308 series).
@readargos 
That’s why I bought the Magnepan 20.1, my dealer told me I literally got the last pair assembled. The new Maggies have their crossovers built in, I always dreamed of having an active set up and bi-amping so the 20.7 was out. The difference in sound quality is significant taking advantage of this feature. 
@hifidream, the pre- .7 series Maggies are just about the easiest to bi-amp speakers available. The .7 have series crossovers, so it takes internal surgery to bi-amp them. The earlier models have parallel crossovers, and as you know a line-level x/o can be easily substituted for the stock speaker-level one. In the owners manual for my Tympani T-IVa, bi-amping is encouraged. I have a First Watt B4 x/o for just that purpose.
@bdp24 I love Nelson’s First Watt work. I’m sure the B4 sounds great. The Tympani speakers move a lot of air. . . I think the new 30.1s can take a bi-amp configuration as I read in the set up from Magnepan that wire needs to be run between the base and mid/high panels. They have even more surface area than the Tympani speakers. Perhaps I’ll get a chance to hear them sometime and see for myself. They look awesome. . . I find it interesting that the 30.1s moves back to the true ribbon tweeter as well which both the Tympani and 20.1s have.

I had a long listening session with my wife’s friend who said the system sounded better then a “live.” It was a lot of fun, I so enjoy exposing people to the hobby. I wish it were more mainstream. 
I have a First Watt B4 x/o
I want that, where do you live?
 https://i.ebayimg.com/images/i/132493410045-0-1/s-l1000.jpg

Cheers George
Hi George,

Check out Nelson’s site:
http://www.firstwatt.com/b4.html

His First Watt work is being distributed by Pass Labs dealers. You can pick one up for $1,500. Reno Hi-Fi will sell them as a special order for example. 

I bet bdp24 has a good security system like all of us! No chance of snagging one from him :)

Have fun! 
Steve


Too much for for me, that equals out to $2200aud, my limit for any active xover is $500aud used

Cheers George
I got my B4 used for $750, I think it was, and that was the last one I've seen come up for sale. I read Nelson discontinued it as a finished product, offering it only as a kit, but Steve (hifidream) may know better.
Bdp24 you’re right it was the case that he had stopped making them and you could get a kit but he made some tweaks to the design and it looks like he has a new model that is in production on a special order basis. . . I like to keep up with his designs in First Watt. He’s so innovative!

How much fun would it be to sit and listen to all the different designs he has come up with one after another and hear how they differ. Speaking of bi-amping, most of his work in First Watt are for efficient speaker designs and being able to vertically bi-amp them would allow them to be used with a lot more speakers. One can dream. . . 
@georgehifi - Something to consider: (http://www.allegrosound.com/Dahlquist_DQ-LP1_AllegroSound.html) (https://www.hifishark.com/model/dahlquist-dq-lp-1) Lots of ways to upgrade/tweak and still supported by it’s builders(well- some of the guys that worked there, anyway). (http://www.regnar.com/dahlquist-dq-lp1-parts---upgrades.html) Hard to find, ’cause they work great and are still sought after, by many. btw: If you’re interested, PM me and I can send a PDF copy of the manual(no, I don’t still have a DQ-LP1, but- do miss it, sometimes).
Thanks rodman99999 just another opamp based one though, I have tried a quite a few and in the end was disappointed, then spent the time and built my own discrete active 4th order xover @150hz just for the bass of my Monoliths, It’ just I’ve always wanted Nelson Pass’s B4 as it's discrete also.

Cheers George