FR125SR/LM3875 - Best configuration

[zebra100]

Hi all,

I'm currently running an LM3875 Gainclone, unregulated 2x 18-0-18, 120VA with a passive attenuator. I'm using it to drive Tannoy Mercury M1 speakers, sourced by a Toshiba SD100E DVD, (2.2v output).

I like my music to be accurate, and I'm mainly interested in midband detail and sound stage, rather than all out bass grunt. So to that end, I want to experiment with some full range speakers to eliminate crossover distortions & colouration.

I've seen the CSS FR125SR box design on planet10's site which utilises a 7 litre enclosure, and I have a couple of questions;

Firstly, would 3/4" birch ply (A-A grade) be ok to use? Secondly, what are the difference between ported and sealed in this application (aside from the bass extension)?

Regards,

Chris.

[chrisb]

Re plywood, Dave & I have been known to get on a soapbox on the subject of Baltic Birch (or equivalent) - but here goes: "A-A grade" may only refer to the quality / matching of face veneers- the more important feature that HDP (high density plywood ) can bring to the table is its far higher stiffness per thickness/mass.

There are some very pretty face veneered multi-core plywoods that use much softer veneer species and or MDF/ particle or stranded materials for internal crossbands. With appropriate joinery/construction techniques these are more than adequate to pass millwork trade specifications & inspections for structural integrity, but may not necessarily sound much better than regular particle board, and certainly inferior to HDP.

Re the CSS FR125 - to make it short, these tend to deliver less troublesome performance in sealed or highly resisitve vented boxes - the original mini-Onken being great example for a wide-band enclosure for these drivers. If you're not committed to the FR/WR125, the latest generation of 3-4" Mark Audio drivers are also worth a serious look.

[zebra100]

Thanks Chris,

The mini-Onken does look a fantastic design, however well beyond my limited woodworking skills. I'm going to use the CSS FR125SR in a 7 litre enclosure as it's probably about my level. At 7 litres, would they benefit from being ported, or do they loose an element of control?

So HDP is the order of the day to ensure density and stiffness. Are there any issues with using glue and screw construction, or are screws a big no-no?

Regards,

Chris.

[planet10]

The miniOnken for the FR125 was the 1st of the family and is special because of that. But with more experience with the FR125 we found that the bass could get a bit out of control so i am recommending a 4.5-7 litre aperiodic over anything else. The easiest way to achieve this is to build the 7 litre slot port enclosure and then insert some open cel foam into the slot. There are lots of sources for this, but the source of my favorite is from Apple parts service boxes. The bigger ones have a circumferential "ring" of fairly high density foam that is just about perfect and easy to trim with a utility knife/box cutter. A query to the local Apple service shop might get you an empty box with the foam.

I have a ton of it... if you get desparate i can always mail some. I like to use it for packing and suggest people hang on to it for future aperiodic projects, so some is already in the UK.

The thing you are looking for in the plywood is a large number of plies (and no voids). The BB multi-ply we use has 13 plies in 18mm, 11 in 15mm, 9 in 12mm. We would use 15mm for the boxes in question.

dave

[chrisb]

Originally Posted by zebra100

Thanks Chris,

The mini-Onken does look a fantastic design, however well beyond my limited woodworking skills. I'm going to use the CSS FR125SR in a 7 litre enclosure as it's probably about my level. At 7 litres, would they benefit from being ported, or do they loose an element of control?

that was our experience with "standard" ported enclosures for these

we did have more success with this one:

http://homepage.mac.com/tlinespeaker...odic-plans.pdf







The close to aperiodic loading was achieved through the multiple small holes on the rear panel and judicious use of dense foam.

So HDP is the order of the day to ensure density and stiffness. Are there any issues with using glue and screw construction, or are screws a big no-no?

Regards,

Chris.

I wouldn't say no to using screws (have to carefully parse that sentence), but provided your glue surfaces are cleanly cut and using a decent wood glue (yellow PVA or polyurethane) a good overnight clamping should be all you'd need.

of course if you're lazy or impatient like me, a few brads from an air powered nailer can speed things up - they need no pre-drilling and take a lot less filling

[chrisb]

two, two that's two replies for the price of one

[zebra100]

Thanks chaps,

Now the multi hole speakers look good (although I'll probably use the 7 litre design). How are they for bass & control?

Chris.

[Scottmoose]

You mean the aperiodic? Like a sealed box, but an easier impedance load (which most gainclones will thank you for).

[planet10]

Bass is good, and pretty extended... you trade them off with how much and how dense the foam in the slot is (you can also use polyfluff, wool, or fiberglass insulation but they aren't quite as elegant)

dave

[zebra100]

Thanks,

I'll have a play with both designs and see how they turn out.

Regards,

Chris.