SDA-1A.1TL modification

[altecman]

Finally done it and started to modify my SDA-1A's into pseudo 4.1TL's whilst waiting for delivery of the new silk tweeters and deciding which tweeter would be better. After looking at the 4.1TL & SRS 3.1TL schematics I must say that the RD0198-1 looks the best bet. I suppose if they used the SL3000 (RD0198-1 replacement) for the SRS3.1TL I gather that it's sensitivity must be higher than the SL2000 (RD0194-1 replacement). Either way, once I finially receive and fit them (xover mod yet to be incorporated) I'll let you all know.

PS - I found a good use for the SL2000 tweeter, remove the magnet and use the plastic frame as a template for making the blanks that cover the old tweeter cutouts.

I am not too worried about the fact that the tweeter sit proud on the baffle as I'll probably place felt around it to reduce edge refractions caused by the tweeter body. If not has anyone got a suggestion what color I should paint these blanks.
Will send more photos as I proceed with this modification.

PSS - I forgot to mention that by placing the tweeter in the middle has made a big difference to the sound, the treble seems more focus now and no more SDA artifacts.

[avguytx]

If not has anyone got a suggestion what color I should paint these blanks.

Um....basic black maybe with black screws would blend it in nicely.

[zingo]

That is quiet the project and I'm excited to see how it turns out. I'll be interested on how you think the two extra drivers sound verses the standard 4.1TL.

[daifanshi]

PSS - I forgot to mention that by placing the tweeter in the middle has made a big difference to the sound, the treble seems more focus now and no more SDA artifacts.

Hi Altecman,

It's great that you're not afraid to work on your speakers. This is good stuff!

The original location of the tweeters near the upper corners of the cabinet would certainly produce measurable diffraction ripples in the treble response. How detectable this is by human ears? That's hard to say. But the fact that you can hear a difference means something real has changed by moving the tweeter to the middle of the baffle.

The cabinet looks to have very sharp edges. At some point in my past, I rounded off the edges of a front baffle with a inside radius of around 1-inch. This done for the sake of measurement and no other reason. That radius seem push the diffraction effects outside the detectable region of a typical tweeter's output. Not something you'd want to do with your original cabinets though...

The top edge of the cabinet is still very close, but the other edges are much farther which should help a lot. Since it's hard to move the tweeter away from the top edge, Your idea of putting absorbing material near the tweeter is a good one. As you mentioned the faceplate edge is the first discontinuity, the next major ones being the cabinet edges.

Are you still looking at a tweeter substitute? GV#27 has identified a very good modern tweeter from Seas (H1212 I think) that would be easy for you to sub in since your already have the additional adapter plate.

Cheers.

[heiney9]

Great your willing to experiment with the older SDA design. To Daifanshi remember the outer tweet on this generation SDA was for dimensional sound only not regular "highs" so they would not compete with each other.

It is good to move a single tweet to the center but there are still some issues as I fear it's too low in the in the radiating pattern to be optimal, but it's probably better than the alternative.

Good luck

[ben62670]

Nice work. The single tweeter mod is much better than the side by side of the original design. Shoot me a pm when you are ready to do the XO mod.
Ben

[daifanshi]

Great your willing to experiment with the older SDA design. To Daifanshi remember the outer tweet on this generation SDA was for dimensional sound only not regular "highs" so they would not compete with each other.
Good luck

Hi Heiny9,

FYI, the effect you are describing with the second competing tweeter is called "combing" or "comb filtering". It's basically frequency dependent constructive and destructive interference due to the relative phasing between the two sources. Like you mentioned the original design would not have as severe a problem with comb-filtering because the local stereo signal is attenuated going that dimensional tweeter. But it is not completely eliminated as you can see from the crossover design.

Diffraction is caused by discontinuities in the baffle like raised faceplates or abrupt cabinet edges. The original poster's concern was with diffraction effects.

They both can cause response problems, but only diffraction induced problems can be helped with absorbers such as felt or wool sheeting.

Cheers.

[Face]

They both can cause response problems, but only diffraction induced problems can be helped with absorbers such as felt or wool sheeting.

Or velvet. :)

[altecman]

It is good to move a single tweet to the center but there are still some issues as I fear it's too low in the in the radiating pattern to be optimal, but it's probably better than the alternative.

Good luck

Heiney9,
Can you please explain. Are you talking about the crossover wavelength distance for the tweeter and midrange at the xover point (2000Hz). Remember also that the tweeter's attentuation resistors (the series 3.5 ohm resistor and possibly the tweeter's parallelled 2.7 ohm resistor) will be adjusted for the 3 dB gain due to there only being one.

[heiney9]

Heiney9,
Can you please explain. Are you talking about the crossover wavelength distance for the tweeter and midrange at the xover point (2000Hz). Remember also that the tweeter's attentuation resistors (the series 3.5 ohm resistor and possibly the tweeter's parallelled 2.7 ohm resistor) will be adjusted for the 3 dB gain due to there only being one.

I'm simply basing it on my 1C's and the way the tweeters are arranged in the baffle(height wise). Plus the 1C's use a progressive point source.. Since you are compensating for a single tweeter perhaps things will be fine. I was more or less throwing it out there. None of us will know the effects for sure until your done and can listen to them since your mod has never been done before (as far as I know)

I was more concerned with the radiating pattern of the tweeter in it's current spot in relation to the radiating pattern of the non-dimensional midwoofer(s). Experimenting will surely tell.

Good luck and keep us updated.

H9

[ben62670]

The Morrel MDT20 would be great for this project. Use the SL3000 XO. I have tested the 3000, and MDT20, and they are an ideal swap.
Ben

[ben62670]

Hi Heiny9,

FYI, the effect you are describing with the second competing tweeter is called "combing" or "comb filtering". It's basically frequency dependent constructive and destructive interference due to the relative phasing between the two sources. Like you mentioned the original design would not have as severe a problem with comb-filtering because the local stereo signal is attenuated going that dimensional tweeter. But it is not completely eliminated as you can see from the crossover design.

Diffraction is caused by discontinuities in the baffle like raised faceplates or abrupt cabinet edges. The original poster's concern was with diffraction effects.

They both can cause response problems, but only diffraction induced problems can be helped with absorbers such as felt or wool sheeting.

Cheers.

SDA's with side by side tweeters use the outer tweeter for dimensional duties. They did this on the first generation only for a reason;) Both tweeters ran the same frequencies.
Ben

[altecman]

More mods carried out
I removed the parallel 2.7 ohm tweeter resistor and replaced this with a wire link. The 3.5 ohm series resistor was removed and replaced by the previously removed 2.7 ohm resistor, all the bypass caps were removed as F1 has stated previously in other posts. All I can say is brilliant - who needs 2 tweeters in any format unless it's of a very large vertical array design. I still can't believe that in the 22 years since this model was last made that no one else has done this modification -what's happened to all the DIYer's (only joking).
So basically the tweeter circuit is as per the SDA 2B which uses the SL2000 (can't wait for the RD0194's-6 more weeks to Oz). Therefore there is no need to use the RD0198 as the SL2000 and it's replacement (RD0194) will provides enough energy in the top end.
I hope now, that people who have this model will have no excuse but to carry out this modification.

[ben62670]

The single tweet is definitely a nice mod.
Ben

[zingo]

Nice work matching the x-over for the SL2000 per Polk specs and not just plugging in different tweeters or components. I'm sure your ears and reasonable audiophiles with thank you.

I like the idea of your newly created SDAs and I would be very interested in hearing them and how they compare to other models.

[altecman]

I like the idea of your newly created SDAs and I would be very interested in hearing them and how they compare to other models.

I too would like to compare my modded 1A's to the other SDA's but alas I live in the land of Oz and therefore this request would be difficult if not impossible. The reasoning to compare would be to see the difference between the 1B & C's(tweeter progressive point source). I myself have not been particularly fond of point sourcing in standard array speakers, unless there are of the large line array types, I can understand this application as applied to the larger 1.2's but applied to the much smaller non array types like the b & c's seems futile. Therefore I will try and find some 1C's in Oz to make this comparison and hopefully change my conception.
What should these speakers be called, I think 1A.1TL might not be appropriate.

[altecman]

Or velvet. :)

I am still deciding which material would be best to stop the diffraction between the tweeter and the top part of the non rebated midranges. I know this might sound strange but has anyone used scotch brite scouring pads (4mm thick) as a diffraction medium. The reason why I was thinking of going this way is because the scouring pads are coarse this might help prevent the sound from reflecting back. I've looked at felt but this is still a hard surface to some degree, it will stop some reflection but not all. Any ideas!!!
By the way I'm still enjoying the sound that these modified 1A's are making, has anyone carried out this modification yet?

[Face]

Felt, wool, or velvet would work fine. As for a scotch pad, it would probably work also, but would bother me knowing what it is. :D

[altecman]

Done some more work, painted the wooden blanks black and made a small angled tweeter mounting plate. The reason why I made the angled face plate was that after some listening I was finding that the treble was placed too far outside the sound stage area and by angling the tweeter slightly inwards brought the treble more into the sound stage. After reading the original patent I realize that the tweeter wasn't really mention only the SDA aspect.
Only 3 weeks before I get the RDO-194's, can't wait. Overall I am very pleased with was I have done so far, the next step will be a xover upgrade and some internal dampering mods.