Lowther DX4 in MLTL driven by Paramount 300B

[Thoburn]

Hello Bottleheads,

I recently built a set of Bob Brine's LT-2000 speakers. They are a Mass Loaded Transmission Line design. I am using Lowther DX4 drivers from Lowther America. I am driving them with a highly modified (better caps and resisitors) Golden Tube Audio SE-40 Special Edition. I have a Basie pre-amp from diyHiFiSupply that I built several years ago (no longer available). My source is a Linn Genki.

First off, Bob Brines will tell you that you can't get much bass out of a DX4 in a MLTL. He recommends a DX2 or DX3. Jon Ver Halen of Lowther America (very helpful and knowledgeable) loaned me a DX4 (stock) and a PM6A (15 ohm, silver voice coil) to try. The PM6A was nice, but the DX4 blow it away. This speak design incorporates a Baffle Step Correction and a Zobel ciruit. The BSC in my setup is a 3.3 mH copper foil inductor in parallel with a 15 Ohm Dueland resistor on the + side. The Zobel is a 6.8 uF cap in series with a 10 Ohm Mills resistor across + and -. The combination brings the DX4 down from 99 Db to 95 Db. You can read all about MLTL and BSC circuits at Martin King's web site http://www.quarter-wave.com/

In my current system the new speakers sound great. True, the bass is a bit weak. But, the is enough for me given the improvement in the mids and highs and imaging over my previous speakers (Vandersteen 2Ce Signatures). In fact, the overall improvement was such that I have sold the Vandersteens. (out of space. to be continued as a reply)

[Thoburn]

(continued from original post)

Now that I have a high efficency speakers I want to try a 300B SET amp.  Dan at Bottlehead was kind enough to loan me a set of 300B Paramounts to try. Well, the mids and highs and the imaging and just about everthing was improved. Not by a hugh amounts grant you, but enough to justify the expense. There is one little problem. Although there is about the same level of bass. The quality of the bass is kind of vague. For lack of a better word I'd call it mushy. Bob Brines believes the problem is the low Q factor of the DX4 and the low Damping Factor of a 300B SET amp. Jon Ver Halen of Lowther America recommends that I remove the correction circuit and have the correction built into the pre-amp. So my question to the group is, what do you think?

Respectfully,
Thoburn

[Thoburn]

One thing I forgot to mention. My listening room is also our living room, so there are very high WAF issues to overcome, which is why I didn't build a Hedland or some other flavor of horn. If you re interested you can see my speaker at

http://www.quarter-wave.com/Gallery/Gallery.html

Mine are labeled Toby Patton

[Paul Joppa]

... The quality of the bass is kind of vague. For lack of a better word I'd call it mushy. Bob Brines believes the problem is the low Q factor or the DX4 and the low Damping Factor of a 300B SET amp.

The low damping factor of a non-feedback amp will indeed cause some frequency response changes, especially in a system that relies on carefully managed resonances such as a MLTL. A driver with more damping than the one for which the cabinet is designed should solve that problem.

Another possibility is that the BSC wastes too much midrange and high frequency energy, so the amp is running out of steam. Check to see if the bass improves if you turn it down to a quieter listening level. If that is the problem, a BSC between the preamp and the amp will help some. An example is shown on the SEXy Speaker page of the Bottlehead web site.

[Thoburn]

Thanks for the reply Paul. Bob Brines has suggested that with low damping factor amps you don't need as much BSC. So, I have ordered a 1.5 mH inductor and an 8 ohm L-pad. Unfortunately, I had to send one of the DX4 drivers back for repairs. It seems the voice coil may be rubbing agains the magnet. When I get the driver back I will try your suggestion as wells as the low BSC and maybe even no BSC at all just to remove it from the equation. I sure hope a solution can be found that does not include sub woofers.

[John Swenson]

I also have Lowthers in a MLTL design based on Martin King's work (but using concentric cardboard concrete forms as the "cabinet") using my own concoction of a single ended amp using 813 pentodes. The correction circuit sits before the amp, its much nicer to do it that way. I made a testing version of the circuit with pots and spent about 6 months trying different settings until I got things just the way I wanted them, then built the circuit with the final values into the input of the amp. The adjustable circuit gives you great flexibility in tweaking things to match the speaker amp interface. I used the BSC circuit that PJ mentioned, it works very well.

My experience has been that Lowthers really LIKE high impedance amps (but you do need to tweak the correction circuit). If the Paramounts come with a 16 ohm tap I would highly recommend trying that as well as the 8 ohm tap and see which you prefer.

John S.

[Thoburn]

Hi John,

Thanks for the feed back. Which Lowther model are you using? Also, when placing the BSC between the preamp and the amp do you start with the same values as Martin King and Bob Brines suggest for placement inside the cabinet? How about the Zobel? Before driver or before amp?

FWIW, Jon at Lowther America is sending me the plans for his Medallion III. I enjoy building speakers and it will be fun to see which cabinet I prefer.

Toby

[John Swenson]

The correction circuit is completely different than what MK uses. His is designed to run between amp and speaker, this one is designed to be run between preamp and amp, it uses just resistors and capacitors. By using variable resistors you can significantly change the behavior of the circuit and home in on the right combination. I do not use a zobel, with the high impedance amp I found it was no t needed.

I'll try and dig up the values I used for the circuit.

I'm using EX3s.

John S.

[dady]

Thanks for the reply Paul. Bob Brines has suggested that with low damping factor amps you don't need as much BSC. So, I have ordered a 1.5 mH inductor and an 8 ohm L-pad. Unfortunately, I had to send one of the DX4 drivers back for repairs. It seems the voice coil may be rubbing agains the magnet. When I get the driver back I will try your suggestion as wells as the low BSC and maybe even no BSC at all just to remove it from the equation. I sure hope a solution can be found that does not include sub woofers.

I am new in the forum, one year ago, I had the same problem with the DX4, I open the magnet (following web instructions) I put a card with a scotch tape and remove the dirt inside the gap. After that I close the motor in horizontal position with a 50Hz signal moving the system until the noise disappear. The choice could be send them to England and spend more than 350 euros each.