Mark K's MT RS225/RS28A

[Gir]

I've been planning on building my first DIY set of speakers and I thought that Mark K's MT RS225/RS28A looked like a good bet. Instead of buying the PE box I plan on building it myself, but I want it to be pretty much the same dimensions. I have two questions:

1) Will building my own box out of 3/4" MDF be any different than the PE box he used?

2) Could someone help me design a cheaper crossover (I am a bit low budget at the moment) and also recommend what different types of components I buy. I know there's a lot of different types of resistors, capacitors, and inductors that have different properties. I'm quite new to speaker building so an explanation on your selections would be great.

Thanks in advance

[WillyD]

1) Will building my own box out of 3/4" MDF be any different than the PE box he used?

No, as long as you brace it. The only difference is the 1" baffle (but you could just double up) on the PE box (and the fact that you have to build/finish them).

[Doug Lockwood]

Welcome to the forum.

Mark K's MT RS225/RS28A looked like a good bet.

This design looks excellent. Mark K does post here and may answer questions on the design.

2) Could someone help me design a cheaper crossover

IMHO, this is not really possible.
The problem is that the RS225 has a severe breakup that required a complex crossover to tame. A less complex crossover would make these sound harsh.
If I might suggest a similar design, The Modula MT uses the 7" Dayton Reference series woofer and either the Dayton Reference tweeter or the Seas 27TCFD. Because of the somewhat smaller driver, this allowed the crossover to be cost reduced. This version can be built for about $300.

Just my opinion.

Doug

[Gir]

IMHO, this is not really possible.
The problem is that the RS225 has a severe breakup that required a complex crossover to tame. A less complex crossover would make these sound harsh.
If I might suggest a similar design, The Modula MT uses the 7" Dayton Reference series woofer and either the Dayton Reference tweeter or the Seas 27TCFD. Because of the somewhat smaller driver, this allowed the crossover to be cost reduced. This version can be built for about $300.

Ah, that makes sense. Could you recommend what types of resistors, capacitors and inductors to buy? For instance what is the performance difference between an air core inductor and an iron core (or something like that)?

[EDIT]
BTW, if you're reading this Mark, what are the dimensions of the driver locations?

[Doug Lockwood]

Could you recommend what types of resistors, capacitors and inductors to buy? For instance what is the performance difference between an air core inductor and an iron core (or something like that)?

For myself, I buy Dayton Non-inductive resistorsExample , Dayton Metalized Poly Caps, Example and which ever air core inductor is least expensive for the required DCR.

All these choices are my personal compromises. Iron core inductors are smaller, cheaper, and have lower DCR. Because of the magnetic properties of the iron, they also have higher distortion. This can be acceptable in budget designs. I usually listen to the designer for advice on inductors. Zaph has specified iron core parts on several of his designs, and if I built one, that's exactly what I'd use.
Jon & Evil Twin specify air core inductors on "their" designs.

Capacitors have similar trade-offs. Non-polar electrolytic caps are less expensive, but generally have higher dielectric absorption (read additional distortion) and a shorter expected lifespan.
Inexpensive metalized caps seem to be a better value to me in most cases. Again, let the designer be your guide.

Hope this helps;

Doug

Edited for clarity

[oxcartdriver]

dimensions are here about the third/fourth post

https://www.htguide.com/forum/showthread.php?t=23335

[Gir]

dimensions are here about the third/fourth post

https://www.htguide.com/forum/showthread.php?t=23335

Ah, thanks, I couldn't seem to find it

[Mark K]

I knew I had posted the dimensions.

I'll get back to you on the thickness of the flanges. IIRC the RS28 is 4mm and the RS225 is 6mm but don't quote me on that just yet. Isn't it on the PE website...

I would not recommend a cheaper xover. If you pull out the traps you get a less smooth FR. Not worth it in my book.

[Gir]

No, they don't have the thickness posted on PE. I've searched around a lot but it's a tough thing to search for.

[JonMarsh]

I knew I had posted the dimensions.

I'll get back to you on the thickness of the flanges. IIRC the RS28 is 4mm and the RS225 is 6mm but don't quote me on that just yet. Isn't it on the PE website...

I would not recommend a cheaper xover. If you pull out the traps you get a less smooth FR. Not worth it in my book.

I'm a 150% with Mark on this one, obviously. It's only the secret sauce in this crossover design that makes it possible to do a two way with the RS225. Follow his recipe closely.

[Gir]

Is it acceptable to use a 5.1 Ohm resistor where a 5 Ohm resister is called for? What sort of margin of error is acceptable?

[ThomasW]

Is it acceptable to use a 5.1 Ohm resistor where a 5 Ohm resister is called for? What sort of margin of error is acceptable?

Of course.

[Gir]

I've recently been able to get some more work (and money) done on my speakers, and I'm about to build the crossover. For the wiring, I'm using (I think) 10ga plain 'ol house electrical wire. I'm lead to believe that this should work fine. My question is solder. I have some Silver-Bearing Rosin-Core solder from radioshack that I picked up a while back. Is this suitable for the job? Thanks in advance!

-Tyler

[JonMarsh]

The RS solder is OK, used to use it decades ago.

An easier solder for a new person to this work to use is Cardas Quad Eutectic, but it's a bit dear. Almost impossible to get a cold solder joint with, though. Google and you should be able to find small qty sales online. But the RS is OK, especially if you already have it.

[speedle]

Is your soldering tool big/hot enough for 10 ga wire? Cold joints are a bitch, no pun intended.

[oxcartdriver]

I used the Cardas Quad Eutectic per Jon's recommendation awesome stuff.

I've probably only used 6-8 kinds of solder, but the Cardas Quad Eutectic was extremely easy to use for cross over work.

I solder'd 10ga wire for the woofer connections with a 45W iron, but a bit more power would have been better (50-65W).

[Gir]

Yeah, I have a 30W iron, but it seemed to be just enough to get the job done. There was only one or two connections I was worried about a cold joint, but I tested it with a multimeter and there was a solid connection. I'm pretty happy with the handy work, except the solder joints are bit messy but definitely connect well. Just curious, do you have to worry about damaging any of the components with excessive heat? That was the one thing I was worried about since I'm used to soldering small electronics here and there.

Once my roommate gets back I'll steal his camera and upload some pics : )

[Gir]

Here are the pics of the low-pass filter. I'll have the other one done either tomorrow or wed. depending on how much work I have.





[EDIT]
Could someone please verify that I wired it right? I drew all the connections on the top of the board. Here are the schematics, thanks!

[fjhuerta]

Did you use 18GA inductors? I think 15GAs would have been closer to the inductors Mark used... I'm not sure how audible your change may be, but I'd guess you'll have a little less bass.

[Gir]

I used the 18's because of two reasons:

1) They're cheaper : )

2) They had the correct resistance and I wasn't sure if that would be more crucial than going for 15 guage, which brought me back to being cheaper : )

Shouldn't it only be a problem if I'm putting a lot of power through 'em? These won't be getting more than 110 watts until/if I get a better amp.

[Brian Bunge]

As long as the DC resistance of the coil matches what is spec'd in the crossover design then you are fine.

[kgveteran]

I'm building four of these for my surround channels.I have two done and they are fantastic.Music through them is great.Even though I have a shallow depth (8") they are an on wall delight.

As soon as PE gets in more 28a's I'll have the other two done.

I'm not sure what ga. inductor I used , but i tryed to get as close to the DRC as possible. Did I miss something about staying close to the DRC regardless of ga. ?

[Brian Bunge]

Nope, you didn't miss anything. The DCR of the coils is part of the overall crossover design. As long as you are close you're fine. I've built a few speakers where I had to put a resistor in series with the coil in order to get the appropriate DCR (each time DCR was > 1 ohm). IIRC, this was a case where even with an 18-20 gauge coil the DCR was so much higher than the coil's DCR that I had to compensate for the difference.

[Gir]

If you put an inductor in parallel with a resistor would that change any of the inductance and only the resistance?

[joecarrow]

That wouldn't work to give you a lower DCR since the inductor+resistor block would no longer work as expected- the inductor is being bypassed. If you think about it, anything above the inductor's cutoff frequency will pass right through the resistor, attenuated the same amount regardless of frequency.

[Gir]

That wouldn't work to give you a lower DCR since the inductor+resistor block would no longer work as expected- the inductor is being bypassed. If you think about it, anything above the inductor's cutoff frequency will pass right through the resistor, attenuated the same amount regardless of frequency.

So what you're staying is that it wouldn't work correctly then?

I'm building four of these for my surround channels.I have two done and they are fantastic.Music through them is great.Even though I have a shallow depth (8") they are an on wall delight.

Glad to hear you like them! I've been very excited to build these. It's a shame I can't finish it until the summer : (

[joecarrow]

That's correct, Gir. If you need an inductor with lower DCR, the the only way to get that is to use fatter wire, or less wire. It's expensive to use thick copper wire, so when people are on a tight budget and need a large inductor with low resistance, they usually use an iron or steel core instead of air core.

If you search around on this forum, there should be some good threads on the relative benefits of air core, and the drawbacks of iron and steel core. There are also some new toroidal core inductors available at Parts Express, for those who need extraordinarily large inductors with manageable DCR.

Good luck with your RS225/RS28a build- I don't think you'll be disappointed!

[Gir]

Thanks for the info Joe!

[Gir]

Quick question. When using the inductor calculator do you measure the diameter from the inside or the outside?

[EDIT]
nevermind, when I looked at the calculated results page it shows that the entered diameter is the center diameter.

[kgveteran]

Nope, you didn't miss anything. The DCR of the coils is part of the overall crossover design. As long as you are close you're fine. I've built a few speakers where I had to put a resistor in series with the coil in order to get the appropriate DCR (each time DCR was > 1 ohm). IIRC, this was a case where even with an 18-20 gauge coil the DCR was so much higher than the coil's DCR that I had to compensate for the difference.

Very interesting. So get as close as I can with the DRC and if need be, add a little resistance to compensate with a little in line resistor.Hmm

[Gir]

I just finished up the high-pass filter last night. Here's a few pics, let me know if you see anything wrong with the wiring.

[dlneubec]

FWIW, Darren over at PE used to say that you should not be able to look through the center hole of your inductor and see another inductor.

Yours look to be pretty far away from each other, so it may not be significant. However, based on his rule of thumb, both inductors one the end are not placed ideally (you can look through their holes and see the one in the corner between them). That would suggest that the ones on the ends would be better situated if they were turned 90º on their current vertical axis.

Of course, you would also have to take into account how the LP and HP filters will be mounted, relative to one another, in your box.

[joecarrow]

Dan's right, the ideal layout would have been to swap the rotations of the upper left and lower right. However, the upper right and lower left look to be nearly two diameters apart and it's not like they're coaxial or anything- I don't predict any major ill effects.

[Biff]

I always thought you shouldn't be able to see through the holes of TWO inductors, which would place their fields parallel, as long as they are 90 degrees rotation from each other on any axis, wouldn't that make all copasetic?

[Dennis H]

http://www.troelsgravesen.dk/coils.htm

[Gir]

Thanks for the pic Dennis, I was searching and searching for that pic all during classes today : )

Once I get some more hot glue I'll flip the inductors around, thanks for pointing that out.

-Tyler

[kgveteran]

I think I might need to recheck my layout

[Gir]

Got yours flipped around as well?

[kgveteran]

Looks fine.Second guess,second guess...it's a way of life .

[Gir]

Yeah, I'll get mine flipped around next weekend when I go home and can snag some more hot glue. I think I'll also finally be able to finish my cabinets. It's tough being at college

[kgveteran]

These are great surrounds.Don't hesitate.They put out tons of sound with very little effort.This is the thin version(only 8" deep).

Way to go Mark !

[Jed]

That room looks like audio nirvana. Very nice setup! I'm jealous.

[Gir]

Quick update: I managed to scrape together enough money to finish these puppies! I wasn't planning on having enough until the summer, but I just got a bunch of rebates back and I managed to make it through this semester with spending only around $120 (that's including books!!). I'll be getting a 4x8 sheet of cheery wood veneer as well for only $30 (good connections), so I'm looking forward to teaching myself how to veneer. I should hopefully have everything minus the veneer by Wed. Also, I just finished building one pair of cables with cat5e (thanks ssabripo!).

In other DIY news, a beer pong table painting as an old school Nintendo controller will be finished shortly! Hurray beer!

[mmoeller]

Hooray Beer!!!!!!

[Gir]

I just finished routing all the holes, and the speakers fit pretty well (the holes are just a wee bit too big). Here's some pics:










So, as I was dry fitting the speakers, I realized I over looked one teeny-weeny little problem... how the hell do I mount them? I've heard of people using t-nuts and such, but would it work if I took small pieces of plywood and put them on the back of the front baffle to hold the screws? I'm trying to save a trip to the hardware store : )