AR-3a’s – padding the mid driver in order to get more from tweeter

[onplane]

Well after thinking about this for several months, I finally put a 2 ohm resistor in series with the mid drivers. Just to recap, I soldered the mid driver connection right to the top of the pot leaving the 16 ohms in parallel with the driver. The tweeter pot is totally by-passed. Purpose in doing this was to get more SPL from the tweeter and I got a little, but not much. The 3a tweeters were still way, way behind the titanium domes in my TSW’s.

So, the issue becomes is there a way to get more out of those old tweeters.

I speculated that by padding the mid driver, to put it behind the tweeter (and also behind the woofer, which is bad thing), then when I increase the volume on my mid/tweeter amp (I bi-amp) to bring the woofer and mid back in balance the tweeter will see more current.

Both Carl and Roy indicated that this might work. Roy also speculated that some of that additional current will result in heat as opposed to SPL and potentially cause the tweeters to fail sooner. Roy’s position is quite reasonable and only time will tell if this happens.

Anyhow, I made the change and it did increase the tweeter output a bit. Are they now comparable the titanium domes in the TSW?? NOPE, the titanium domes still put out more sound, but the gap has closed significantly and I actually prefer the 3a tweeters now. They are not as bright and metallic sounding as the TSW’s. In short, cymbals sound as I remember them to sound.

I did learn some interesting things that I though I’d share. Before making the change I used my dual trace scope to measure the voltage across the mid on one channel and the tweeter on the other. I then manually scanned the audio range to see what’s going on.

Now, as we’ve discussed on many posts, totally by-passing the tweeter pots causes the net resistance to increase and this should cause a drop in the xover frequency (i.e. more current and voltage at the lower frequencies). Well, prior to the addition of the 2 ohm resistor, the frequency at which the voltages across the tweeter and mid driver were exactly the same, was 6,500 Hz! I expected this to be around 4500 Hz, so found this interesting. As I went up in frequency beyond 6,500Hz the mid driver’s voltage declined while the tweeter voltage increased.

(I neglected to mention that I also tested new caps at the same time on the tweeter and could see no difference whatsoever! Nothing, nada! In short, the voltage across the tweeter was identical with the new or the old caps across the entire frequency range.)

So, I figure once we pad the mid with the 2 ohm resistor, we should see this “equal voltage” frequency drop a little, perhaps 500 Hz. Well, another surprise, the new equal voltage frequency became 5000 Hz!

Is this good or bad?? Well, I honestly don’t know. Clearly, we are now asking more from the tweeters. That is, not only are they getting more currently, but also over a larger frequency range. Now, if as many speculate the tweeters are no longer converting electrical energy into SPL efficiently, this might be a bad thing.

All things consider, was it worth doing? Probably not. The change is still pretty small. You can achieve a much bigger change by simply using the treble tone control. Then there is the risk, that Roy mentions, of the accelerating the tweeter’s demise.

My take on that is … “if they die, they die!” I’ll worry about it then, but in the mean time I like the realistic cymbal sound.

Regards,

Jerry

[RoyC]

Hi Jerry!

Just saw your post down here:-)! Interesting experiment, especially since AR's specs say that the AR-3a tweeter/mid crossover is 5000hz. As you know, I believe the old tweeters are rolling off more steeply in their old age, as their suspensions lose compliance.

Anyway, ever since Ken's Kantor's tweeter tests, I am more convinced than ever that the old tweeters are, at BEST, variable in response these days. It seems we are now pretty much in agreement. There is no reason not to try to coax more output from them as the need arises.

Roy

[onplane]

Hi Jerry!

Just saw your post down here:-)! Interesting experiment, especially since AR's specs say that the AR-3a tweeter/mid crossover is 5000hz. As you know, I believe the old tweeters are rolling off more steeply in their old age, as their suspensions lose compliance.

Anyway, ever since Ken's Kantor's tweeter tests, I am more convinced than ever that the old tweeters are, at BEST, variable in response these days. It seems we are now pretty much in agreement. There is no reason not to try to coax more output from them as the need arises.

Roy

Hi, Roy!!

Well, Roy, that is exactly what I am doing .... trying to coax more out of the old tweeters. Frankly, I would have never tried if you and Carl hadn't mentioned that it might work.

So far, at least, it appears to be working. Time will tell if they completely fail soon. My sense is what's more likely is they'll require more and more power to produce sound. At some point, we'll just have to give up and get new tweeters.

As for the "equal voltage" frequency, I was also very surprised that with the pot by-passed it wasn't a lot lower. Nice thing about the AR's is we can measure these voltages without having to open the boxes. It's really easy to see what your xovers are doing from the front terminals. All you need is a dual trace scope and an audio oscillator..

Regards,

Jerry

[Guest alternety]

Jerry,

How have the tweeters held up. I realize it is a relatively short time since you changed them. I am interested in doing the same or similar thing to my AR3a's.

[onplane]

Jerry,

How have the tweeters held up. I realize it is a relatively short time since you changed them. I am interested in doing the same or similar thing to my AR3a's.

Good question!!

Tweeters are working just fine, but ... but I swear that after several months of getting more voltage they have "loosened up" a bit. That is, they seem to be putting out more sound today than they did after I completed the mod.

My only way to compare is against my TSW's. Now, it could be that the TSW tweeters are putting out less sound, but I seriously doubt that. All I know is the gap is very, very small today.

Further, confusing things is, I put the 3a's back on my stands (7 inches) and that puts the tweeters closer to ear level. So maybe all I'm hearing is the effect of the change in position.

Please remember, the TSW's are a taller speaker and the tweeter is just naturally position much higher.

Hope this helps ...

Regards,

Jerry

[Guest alternety]

Thanks. I think I will do your mod. I will probably not replace crossover caps.

I have a system that uses Audyssey. The process tests all the speakers and adjusts the listening position for proper results. It considers frequency response of speakers and room conditions. It should attempt to deal with low response from the tweeter. I have not done the setup yet because I can't find my camera tripod to hold the test mic. I am now looking at a floor lamp and some tape. The mike has to be properly positioned and be quite steady during the tests.

[onplane]

Thanks. I think I will do your mod.

OK, below are the directions for bi-amping with an HT amp:

First off, your HT amp must have an "all stereo" mode. What this does is send a complete Left and Right channel to ALL amps. Next, vast majority of HT amps provide the ability to adjust the "relative" power put out by each amp in 1 db increments. You'll need this feature for the bi-amp to work.

To start, you should set the tweeter pot full ON and the mid-pot at half rotation (rotate it “end to end” and find the mid-point).

So, now to jerry-rig your speakers:

1. Connect the BLACK speaker output on the FRONT amp to terminal #1

2. Connect the RED speaker output on the FRONT amp to terminal #2

3. Turn the amp on and verify that you have a decent full sound from the speakers. If OK, then remove the lead on #2 and apply black tape over this lead (this is just temporary).

4. Connect the RED speaker output on the SURROUND amp to terminal #T

5. Turn the amp on and once again verify that you have a decent full sound from the speakers. If OK, then, you are all set to bi-amp. Turn the amp OFF.

6. Remove the lead on #T and the strap on #T then replace the lead.

7. Now remove the strap on #2, the black tape from the lead and re-connect the lead to #2

8. Turn the volume control to zero and then power ON the amp.

9. Slowly increase the power on the amp and you should hear all of the drivers working.

10. The sound, however, will be very bass heavy and not at all pleasing. So, to bring the mids and tweeters back in balance try +3db on the surround amp. If this is too much, back off to +2db. If insufficient, try +4db.

Notice that with the amp "relative power controls" and the mid-pot, you have complete control over speaker voicing. This should make the tweeters … wake up!

Now, last thing to do is tell us what you hear!!

Regards,

Jerry