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Active bi-amping of ET LFT-8b

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Posted on June 28, 2016 at 16:24:08
BDP24
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Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 I am doing research on the subject of actively bi-amping my Eminent Technology LFT-8b loudspeakers. Bruce Thigpen recommends bi-amping by adding a couple of parts to the existing passive x/o, and connecting the two power amps to the separate woofer and magnetic-planar M/T panel binding posts on the speaker. Though I am going to be auditioning other woofers in place of the stock ET 8" cone (both a pair of Tympani T-IV woofer panels, and a pair of GR Research OB/Dipole subs), I am also interested in hearing the stock woofers with an active x/o, doing it differently than Bruce's suggestion---by employing an active line-level x/o before the power amps, not using the internal speaker-level x/o at all. That appears to be no problem for the panel, as the x/o leg for it is simply a single capacitor that creates a 1st order HP filter. But the leg for the woofer contains other parts, that do more than just create the complimentary LP filter. I am no electronic engineer, so can a more technically educated member here enlighten me on how to proceed? On how to include the woofer-compensation parts into the active line-level x/o? Thanks---Eric.

 

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RE: Active bi-amping of ET LFT-8b, posted on June 28, 2016 at 18:38:18
Davey
 It's more complicated than it might seem.
I believe I would leave the entire passive portion of the mid/tweeter intact. There is a large value capacitor in series with the mid/range that creates a very low electrical cutoff, but I think you could let that remain.
Implementing the woofer portion of an active crossover is where it gets tricky because the series inductor (it's the only component in the woofer leg) creates an electrical roll-off that is very non-generic. :)

I think I would think about this a bit before you proceed further. I would try the shunt capacitor on the woofer driver as I outlined a while back before I progressed on to a possible biamp setup.

Cheers,

Dave.

 

RE: Active bi-amping of ET LFT-8b, posted on June 28, 2016 at 23:40:23
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 Thanks Dave. I did see your post about adding the shunt capacitor to the woofer, and as you am mystified that Bruce didn't do it himself! I just soldered it across the woofer's plus and minus terminals? I had already assumed I might end up leaving well enough alone with the stock x/o on the panels, as it is so simple there is little to be gained by going active, infact in all likelihood just the opposite. I'll just add the extra capacitor on the amp's input jacks to create the 30Hz high-pass filter Bruce recommends, so the power amp doesn't see the very low frequencies.

I'm assuming no one has substituted a line-level (active OR passive) low-pass x/o in place of the stock speaker-level one in the LFT-8 for the exact reason you state: to duplicate the electrical characteristics of the x/o at line level would be difficult. I will almost certainly not end up using the ET woofer anyway, as the GR Research OB/Dipole Sub is a perfect replacement and superior woofer system anyway. I am also going to give a shot at using the two bass panels of my Tympani T-IV's in place of the ET woofer, crossing-over at the same 180Hz using the Dahlquist DQ-LP1 I already have.

Thanks for your advice Dave, I believe I'll heed it!---Eric.

 

RE: Active bi-amping of ET LFT-8b, posted on June 29, 2016 at 07:28:16
Davey
 Also on these, I find the tweeter too hot even in the "low" setting. I inserted an external 12 ohm resistor attached between the "high" screw terminal and the driver and that yields a nice top end.

Dave.

 

RE: Active bi-amping of ET LFT-8b, posted on June 29, 2016 at 08:42:30
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 I was thinking of an ET LFT8 mids Frankenpan with either a TIV/IVa bass (for myself) or H/W frame dipole bass (in general) with a double stacked ET panels and the maggie ribbon. I think the mids of the ET are superior to single sided maggies and I like their open frame over even the Neo8 with its holes. There is no cavity resonance problem with the ET and though it is controllable in the neo8 in arrays, it is better not to have the problem in the first place.
The ET panels are too short to match with the TIV bass and maggie ribbon so I thought of stacking the ET mids, which made the project notably more expensive than a Neo8 line array.
The other issue was the Neo8's higher sensitivity.

 

RE: Active bi-amping of ET LFT-8b, posted on June 29, 2016 at 18:15:52
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 Dave, it sounds like you might have preferred the earlier "a" version of the LFT-8, with the original tweeter. Some were complaining that even with a maximum output connection the LFT-8a had too soft a high end. With my high-frequency hearing loss (that's what playing drums for fifty years will get ya, along with tinnitus) I barely need the tweeter at all!

Satie, the LFT-8 midrange driver is around 42" tall. I don't recall how long the T-IV tweeter is---I'll see how the two line up when I get them set up next to each other. But your NEO 8 conversion appears to have advantages over the magnetic-planar driver, from what you and Guitar Slim say. Higher efficiency is nice, but I'm interested in the dynamic and low-volume resolution capabilities of the NEO line. By the way, I have my GR Research OB/Dipoles in W-frames at the moment, but have a pair of H-frames I need to build out soon to move them into. The H are much more robust, being CNC cut from 1.5" MDF! Comparing them to the T-IV bass panels is going to be very interesting. Whichever wins will be relieved of reproducing the bottom octave by a pair of Rythmik F15's.

Thanks to both of you, I really appreciate your advice!

 

RE: Active bi-amping of ET LFT-8b, posted on June 29, 2016 at 20:13:56
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 I didn't remember the LFT8 mids being that long, IIRC the TIV mids were 48 or 50". and I thought they were too short. I was intending to go for 8 drivers per side but stopped at 6 since that already did all I wanted from it and didn't require a new frame.

There are other issues, The sensitivity is >91 db for the Neo8 array - supposed to calculate out to 95-6db but I have doubts about that figure. vs 83 db for the LFT8 mids, the other thing was max spl which came out to 1% THD @110db 1m 500hz and up on the neo8 array again with the caveat that it is from memory from BG white papers they pulled off the web years ago. The ET mids reach excursion limits in the low 100s db - someone's measurement I don't remember where from..

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 01:44:39
Roger Gustavsson
Audiophile

Posts: 2055
Location: Huskvarna
Joined: February 12, 2010
 Years ago, I was also looking at the ET LFT drivers as a possible partner for my old Tympani basses. The LFT drivers need a lot of power and I decided not to use them. Not sure if there are any other issues but they are not line source drivers and stacking them would make a very tall speaker.

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 05:09:46
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 I've been making my way back in time through the old threads here, and now know that I am not the first to have brought up the topic of bi-amping the LFT-8's. In doing so, I have become somewhat confused, so I'll be asking questions for a few days. Thanks for your patience!

First up: Since the leg of the x/o going to the magnetic-planar panel is just a single cap, is there any reason I couldn't (or shouldn't) bypass the stock x/o for the panel entirely, hooking up the panel leads directly to the power amp driving the panels, with a HPF installed on the amp's input jacks? I hope I am not revealing my ignorance by asking ;-)! Wouldn't doing so be of benefit if for no other reason than preventing the amp from seeing the low frequencies in the signal? I could install a pair of binding posts onto a piece of MDF on top of the LFT's bass enclosure, connecting the panel leads and my speaker cables to them. Would doing this be likely to reward me with any audible improvement? I realize this tactic won't work with the woofer, for reasons already covered in this thread.

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 06:12:20
Davey
 The crossover for the planar portion is more than just a single cap. Take a look at the schematic on Page 22 of the manual.

Dave.

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 06:52:46
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 Right you are Dave. I don't recall where I got that idea. Well, that settles that!

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 07:09:32
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 All right then, second question: Bruce specifies a 30Hz high-pass filter be added to the panels when bi-amping. Why so low? Why isn't it better to make the frequency higher, thereby keeping more of the bass out of the M/T power amp? I'm assuming there is a good reason, my ignorance preventing me from knowing the answer.

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 07:24:28
Davey
 In this case the acoustic crossover is quite a bit higher than the electrical HP filter. He's using the natural roll-off of the midrange driver to form most of the crossover. The low-frequency filter is more about protection than crossover function. It's an "interesting" design that is not nearly as simple as it looks.

In your case, it seems your intent is to match a completely different woofer setup to the system. So, I think it's up to you to implement it however you see fit. However, I think your woofer system should have viable response for the entire bottom decade (20Hz-200Hz) of the audio band. In fact, a bit higher than that would be preferable.

Dave.

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 07:54:37
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 Ah, okay. Obviously then to raise the 30Hz figure would be to decrease the panels output between 30Hz and 180Hz, and disrupt the balance between panels and woofer---not a good idea.

You're right Dave, just for fun I'll be trying the panels without the ET woofer, using in it's place 1- the bass panels of a Tympani T-IV, which are used in that speaker up to around 400Hz, and 2- a GR Research OB/Dipole Sub, which can be crossed-over to as high as 300Hz. I'll try them both at the stock LFT x/o frequency of 180Hz, 1st order.

An idea occurred to me, which I am expecting to receive an education on ;-). What would you expect to happen if I were to use a symmetrical 4th order x/o at the same 180Hz, instead of the stock 1st order, rolling off both the ET M/T panels and either substitute woofer much faster than stock? The phase relationship between panel and woofer will remain the same, correct?

Who knows, I may end up just staying with the stock woofer. No harm in trying the others, ay? Thanks---Eric.

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 08:30:10
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 You can run an LR4 2 way at line level rather easily as it would protect the mid/tweeter and provide a sharp cutoff for the bass and you just need to keep track of your polarities and time align your panels. I am not a fan of LR4 high passes on sonic effects of having so many caps in the filter and the input 2Xgain and 2Xbuffer circuitry you need to execute it in a commercial XO or a Marchand custom piece. My usual choice is 3rd order LP with 1st order HP (passive) and working with the natural rolloff of the driver (2nd order hp) to make a 3rd order net HP. That leaves you with the same issue you have with biamping the LFT8 with the recommended 2 poles at 30hz and 180 hz.

In any case, biamping with the amp on top being relieved of bass duties will make a huge difference in its performance and overall system performance.

 

RE: Active bi-amping of ET LFT-8b, posted on June 30, 2016 at 18:49:31
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 Got it, thanks Satie.

 

RE: Active bi-amping of ET LFT-8b, posted on July 1, 2016 at 06:40:22
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 Having done some more reading here, I saw some things I found thought provoking, particularly in regard to using Tympani T-IV bass panels as woofers (thank you Satie!). Crossing-over at 180Hz to Eminent Technology LFT-8b panels, I'm going to put the left and right channel T-IV bass panels right up against and perpendicular to either side wall, to prevent cancellation and create reinforcement. Free bass! Whether or not it also creates boom remains to be seen.

The LFT-8's will sit right beside the T-IV panels, but facing the listening position straight-on, so there is the possibility the imaging will suffer due to the distance difference between the LFT-8 and T-IV panels. I just want to hear what T-IV bass can sound like so positioned. With the x/o being at a relatively low 180Hz, do you think imaging WILL be affected?

The T-IV panels will be 5' from the front wall (front meaning the one the listener is looking at), so bracing the panels to it with a length of wood (as Satie does his) is out. I have been thinking I would build some new 2' deep feet and rear supports (a la Mye; a set of them would be great, but they would run me about what I paid for the T-IVs!)), and I still think so. But, as I was installing a new spring-loaded shower curtain rod in my Sister's bathroom today, the proverbial light bulb above my head lit up. Can you guess? It's worth a shot, so I'll get four short length spring-loaded rods and install them between my room's 8' ceiling and the top of each T-IV bass panel. Funny! It won't brace the middle of each MDF panel frame, but it will keep the panels from swaying. It's only temporary, unless it works better than I expect.

 

RE: Active bi-amping of ET LFT-8b, posted on July 1, 2016 at 11:39:27
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 I can show you how to do the bracing for the top of the TIV bass to your walls or rafters if either of them are accessible or are strong enough (i.e. not regular drywall). There are a couple of tips on how to do the wall loaded bass panels - position, toe in etc.and matching the XO to it so as to avoid damaging imaging.

I am going to guess that the bass will overload your room with full wall loading and positioning in the near middle to catch the transverse room mode.

 

RE: Active bi-amping of ET LFT-8b, posted on July 2, 2016 at 03:38:39
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 The room's walls and ceiling are typical modern West Coast textured drywall---no wood on the walls, no exposed rafters. I'll have to make the legs with bracing out of 3/4" x 2" x 2' pieces of oak. The legs will bolt onto the panels where the factory feet do, the bracing coming up from the rear end of the legs to the top of the panels.

I was thinking of the 5 ft. distance to get the 10 millisecond spacing between front and rear waves. That will put the tweeters of both the T-IV's and ET's 8 ft. from the lp, and 8 ft. apart.

I'm looking at the Marchand XM44 or First Watt B4 active crossovers to let me play with filters without soldering resistors and/or capacitors in and out. After finding the best settings, I can then solder in parts for simple single-pole filters if that's what I want and need. I'm assuming I'll do that atleast for the M/T panels, thereby avoiding an extra set of cables and the components of the B4. The B4 does look really nice though, all discrete no opamps. High input impedance and low output impedance as well.

 

RE: Active bi-amping of ET LFT-8b, posted on July 2, 2016 at 09:36:43
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 The B4 is probably the better choice since you alter the filters by using plug in jumpers. The Marchand required desoldering and resoldering in the components on the frequency boards. that is time consuming and an annoyance. .

Wall loading not only reduces dipole cancellation on one side but if placed about 45% into the room will capture the transverse mode of the room which has the lowest frequency of any of the major room modes. If there is one mode you do want to excite then that is the one. Since that is usually in the 30 hz range. The narrow wall mode is one you want to minimize as it is likely to fall in the 80-100 hz range and also produce nulls at that freq in various spots in the room. To do that you place the bass panels flat face forwards with no toe in.
When using the wall loaded setup and you have the panels face forwards then the bass panels will be offset from equidistant and will be out of time alignment so would have to be low passed steeply so that they don't interfere with the mid panels and smear the imaging. I am still trying to make the most out of this config so that you can get closer to the perfect imaging of the equidistant arc setup. .

 

RE: Active bi-amping of ET LFT-8b, posted on July 2, 2016 at 16:28:27
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 So with the T-IV bass panels placed equidistant with the M/T panels, and only say 3' away from the front wall, the problems created with the 5'/straight ahead positioning can be avoided, ay? I'll try them there ofcourse, as soon as I can get going here in the new room. I'm chomping at the bit! Thanks again Satie, your advice is of great help.

I looked again at the First Watt B4, and realized it was the original version (from '12) that had an input impedance of 100k. The '14 revised current version has only 10k---no input buffer. My pre-amp (EAR 868L) has a pretty healthy output at the studio-standard 600 ohms, which I guess will be okay looking at 10k, though 100k would be much better. I didn't know the XM44 required soldering in parts---don't the changeable cards do all the adjusting of frequency and slope?

 

RE: Active bi-amping of ET LFT-8b, posted on July 3, 2016 at 00:14:24
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 Yes the cards do all the changes, but you still need to solder the parts for each freq and slope into the card. And if you want to reuse the card for a different freq or slope you need to desolder the old parts out and put the new parts in. If you don't recycle them then you end up with a big pile of freq cards. I tried using micro pots (wirewound) but they were difficult to turn precisely and have no calibration marks.

If you end up with the tweeters just 3' off the front wall then you will need to do some absorption or quite a bit of diffusion over the wall.

Lots of things to try out.

 

RE: Active bi-amping of ET LFT-8b, posted on July 3, 2016 at 03:18:40
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 I'm pretty sure I'll end up with the panels 5' off the front wall (I have JUST enough space to pull that off), all equidistant and aimed at the listening position. The M/T panel will be separated from the two bass panels of course. I'll switch out the T-IV M/T for the ET's and Quads, trying all three with the bass panels. Having the First Watt B4 will make it much easier. Is the B4 reputed to have a little better sound than the Bryston 10B? Looks like used 10B's go for about the same as a new B4.

 

RE: Active bi-amping of ET LFT-8b, posted on July 3, 2016 at 03:44:44
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 I would go with the B4 for flexibility and Pass' voicing. I have not come across a heads on comparison, but it seems the 10B has better bass but the B4 is better in everything else. Both are regarded as transparent.Both use discreet parts rather than op amps. Marchand manages to do quite well with the op amps but they are still noticeable. Taking out an idle gain and buffer on the marchand was a relief,

Do try the equidistant arrangement on the corner to corner plane, I am curious and can't really try it in my room any time soon.

 

RE: Active bi-amping of ET LFT-8b, posted on July 5, 2016 at 13:29:31
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 I happened to go onto Audiogon yesterday not long after a First Watt B4 x/o was listed. It's the revised 2014 version, and less than a year old. I made an offer, it was accepted, I sent payment, and it'll be on it's way to me today. What luck!

 

RE: Active bi-amping of ET LFT-8b, posted on July 5, 2016 at 21:00:18
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 There is serendipity for you. If your preamp sags on driving it (loses the freq extremes) then you can increase the input impedance by replacing the input resistor to a 20k, most tube pres do perfectly well with that load.

 

RE: Active bi-amping of ET LFT-8b, posted on July 7, 2016 at 16:11:14
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 Serendipity---exactly Satie! I took another look at my pre-amp's (EAR 868) specs. I thought I remembered it having an output impedance of 600 ohms, but the spec sheet actually reads "5 volts output into a 600 ohm load". Tim de Paravicini, the pre-amp's designer, does a fair amount of work in the professional recording field, designing products for the studio environment, where 600 ohms is the de facto standard. Nelson Pass told me yesterday that the EAR will have no trouble driving the B4.

 

RE: Active bi-amping of ET LFT-8b, posted on July 8, 2016 at 07:04:28
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 Pass and TDP know each other's equipment so I would expect you would have no problem if Pass does not expect one.
BTW I used TDP's TC4 MC trannie for a while and it does deserve its high reputation, definitely bested the internal MC trannie in the 834p, The phono stage benefited from a cap diode and resistor upgrade in a big way, made it sound more like the higher end TDP designs.

 

RE: Active bi-amping of ET LFT-8b, posted on July 11, 2016 at 14:11:48
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 I know TDP is not a believer in boutique parts, using garden variety one's in the EAR products. The 868 sounds darn fine as is, so I'm not gonna tinker with it just yet!

 

RE: Active bi-amping of ET LFT-8b, posted on July 11, 2016 at 19:20:39
Satie
Audiophile

Posts: 5426
Joined: July 6, 2002
 Well, he tends to make the best of a price target with robust designs that can handle some component drift. That is why he is TDP. Does not mean that the end user can't mod the unit with boutique parts and obtain a significant improvement at a reasonable cost that would have multiplied the price had it been done at the OEM.

 

RE: Active bi-amping of ET LFT-8b, posted on July 13, 2016 at 01:53:05
BDP24
Audiophile

Posts: 1070
Location: Vancouver, Washington
Joined: September 12, 2013
 I'm going to take a look inside the 868 after I get set up and running in my soon-to-be new room. I remember being surprised by how few parts it actually has.

 

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