Eminent Technology LFT8B’s reversing woofer polarity

@ledoux1238:

1: You sure can. However, if the two amps don’t provide the same gain (fairly likely), you WILL need a means of adjusting the volume of the higher gain one. Some power amps include a volume control, but these days that’s rare. A simple outboard volume "pot" at the input of the power amp will work, but will of course necessitate an extra inter-connect.

By the way, if you use two identical amps, you can bi-amp either horizontally (one stereo amp for the panels, one for the woofers) or vertically (one stereo amp for each complete loudspeaker).

2: Exactly correct.

a: This subject has been discussed in detail many times before on Audiogon, so do a subject search; there are lots of products to choose from. I have the nice little x/o Nelson Pass offered in the First Watt B4, but it is no longer offered factory-assembled (kit only now), though it is more x/o than you need for this application (plus it retailed for $1500, perhaps more than you want to spend). There are LOTS of self-contained volume pots (even from ebay vendors), one from HSU (the maker of subs) for under $100. Cheap volume pots are notorious for channel imbalance, so get a good one! When he was alive, Roger Modjeski was offering a "Volume Pot In A Box" pre-amp (he was a proponent of passive pre-amping). Contact Music Reference to see if there are any left in stock.

However, the best way to do the filtering (possible because that needed is the simplest of all types) is a passive, 1st-order (6dB/octave). All that requires is one cap and one resistor per channel, which can easily be soldered onto the input jacks on the power amps, inside the amps. Why is that ’best"? No extra inter-connect required, no extra breaks in the signal path. Less is more! (that goes for you too, musicians ;-)

If you have a good tech in your area, having him do the work won’t cost you much. I had Tom Carione at Brooks Berdan Ltd. in SoCal install the exact filter you need in my Bedini 25/25. I had him replace the stock binding posts with Cardas at the same time.

b: already answered.

c: ditto.


I hope this doesn’t sound too daunting. Used as delivered from the factory, the LFT-8b already sounds great (have you read the review by Robert Greene in TAS, or the reviews in UK mags?) . I would listen to the speaker stock, see if you like it enough to take it to another level. Be sure to get the dedicated stand Sound Anchor makes for the LFT-8b. If you’re familiar with that stands they make for the Vandersteen speakers, you’ll know what they look like. Eminent Technology stocks and sells them.

@bdp24  I greatly appreciate your answers. As it turns out one of the amplifiers that I will be using, Mactone mh 300b,  has a built in volume control.  It put out 22 watts and I am using it right now to drive the LFT-8b's to satisfactory level. But I intend to use it to drive the M / T panels in the bi-amp set up. There will be a 100 w tube mono's returning from repairs. These will drive the woofer. I will start with no x/o and use the volume control of the Mactone to match levels for now. 

I also got in touch with Bruce T. and he recommended I look into x/o's from Marchand Electronic which I believe you have commented on in previous post. 

@ledoux1238: Yeah, the Marchand x/o’s are very well respected. They make filter "cards" for every conceivable need, the 1st-order you need being the most simple.

As for using the volume control of the Mactone to balance the panels and woofers, remember this: a volume control doesn’t provide gain, it only attenuates the signal sent it (with the volume control set fully clockwise, it is proving zero attenuation). For your plan to work, the Mactone will need to have greater gain than do the woofer amps. If the woofer amps have higher gain than does the Mactone, the volume control on the latter will not be able to bring that amp up to the level of the former. Even wide open, the gain of the Mactone will remain less that that of the woofer amps. On the other hand, if the Mactone has greater gain, that amp’s volume control will allow you to attenuate (lower) its output, thereby providing a means of balancing the panels and woofers. See what I mean?

@bdp24 Regarding the potential mismatch of the two amps, your point is crystal clear. I will know in two weeks time.

If there is a mismatch, the issue of which x/o remains. The initial thought was a passive x/o, 1st order. However, several comments, including yours, I believe, indicated that the particular passive x/o from Marchand was not as good as their active tube. You have pointed to the less than stellar parts quality in the passive. There is also a clear price difference between the two units.

I suppose going active will provide the most flexibility for future subwoofer upgrade, or, heaven forbid, a change from the ET’s to other transducers.

Regarding your earlier post on the TAS reviews of the ET’s, I think I’ve read all the available professional reviews and most of the forum posts on the LFT 8b’s before I contacted Bruce T. I have had the pair for over a year.  In terms of placement / interaction with my listening room, I believe they are optimized. I would like very much to push to optimize the speakers in other ways, hence the bi amp queries. And eventually, I ‘d like to upgrade the the internal x/o caps, and try OB subs. 

My list of active crossovers includes:

Marchand XM-44
Beveridge RM-3
Accuphase F-25

With each of these I have the flexibility for different slopes and frequencies. I can also attenuate (Beveridge, Accuphase, Marchand) or add gain (Marchand). I've used these frequency dividers with various panel speakers and I'll never go back to passive crossovers again. Unfortunately the LFT 8b's crossover is too high for my taste, I prefer to keep it under 100 Hz (and currently use 80 Hz). Not sure why Thigpen chose the higher crossover frequency, but I'm sure he had a reason.