Unscientific comparison of Rodgers M10 with and without JBL 075 Bullet Tweeter

[Eric Mack]

I want to share what I am learning with anyone who might be curious about the bullet tweeter and what they add to the sound.

I have a Rodgers M10 Speaker (eight 8" round speakers and one JBL 075 Bullet Tweeter)

In several conversations on this forum, toodles has strongly encouraged the removal of the JBL 075 tweeter from the M10 or RW3 because it can sound harsh in a small room.

I plan to follow his advice, but I was curious to see what difference it would make, so I did an unscientific test using an M10 connected to the BRASS channel of my Rodgers 340 Analog Organ.

I placed my iPhone in record mode near the M10 speaker which was laying on the couch with the bullet tweeter face up.

I then played a few notes of the the HORN with and without the JBL 075 Speaker Installed. Listen to Horn You can hear the difference.

Next, I repeated the test with the TRUMPET, again with and without the JBL Speaker Installed. Listen to Trumpet

The bullet tweeter clearly adds a brilliance to the sound, but as toodles pointed out it is probably too bright for my small room.

Finally, just for fun, I ran a third test in which I held down a note first with the JBL 075 and then with a tweeter I removed from an Allen HC12. No changes in circuitry, just a swap. You can barely hear the difference with and without the tweeter from the Allen HC12 installed but it is there. When I look at the waveform in Audacity it adds a very small amount of brightness to the sound. Again, this was a very unscientific experiment and sub-optimal conditions, but there you have it.

Listen to all three



For those of you who know what an analog electronic trumpet and horn should sound like, I would be curious to hear your comment on the first two recordings.

UPDATE: The forum won't allow me to upload .WAV files so I have attached a .ZIP with the three files
UPDATE2: Apparently the forum will only allow 97 kilobyte uploads, so no audio. Sorry, I tried.
UPDATE3: OK I temporarily set up a dropbox to let you hear the audio. I will delete in 48 hours.

[jbird604]

Eric, this is certainly interesting, and a little surprising to me. Thanks for posting. The Trumpet sound actually seems little changed (to my ears) with or without the tweeter. That's probably because the trumpet sound is harmonically concentrated in the center of the audio spectrum, not really having much output in the upper frequency ranges, thus not much affected by the presence or absence of a tweeter. And the 8" drivers you have are pretty well able to handle the majority of the audio range anyway.

The test with the Horn sound though is puzzling. Rather than a clear difference in tone, I only detect a quite large difference in VOLUME. I can't account for that, as the Horn sound would also seem to be more concentrated in the middle of the audio frequency range, and you'd think the tweeter would make very little difference on it either.

The final test with the JBL vs no tweeter vs the Allen dome -- more in line with what I'd expect to hear. The bullet tweeter is obviously extremely efficient, so the sound of the note you chose to hold down (fairly high note, I assume) seems to be quite a bit louder with the tweeter attached. The tone with the tweeter removed is then much softer. The Allen dome tweeter adds SOMETHING to the sound, but not nearly as much as the JBL because dome tweeters are normally far less efficient than horn tweeters.

You will find, as you study this field, that "tone" is very subjective and largely a matter of taste and opinion. When I was younger, I tended to really love the super brilliant tones, and when I go back and hear an organ that I voiced 20 or 30 years ago, I'm often surprised at how bright I made them, how much emphasis there is on the mixtures, for example. My later work tends to be more mellow.

So once upon a time I probably would've favored the loud, bright tone of the speaker with the JBL attached. Nowadays, I wouldn't necessarily prefer the very dark tone of the speaker with no tweeter at all, but I would probably find the sound of the Allen dome the most pleasant of all, to my ears.

The great thing about electronic organ voicing is that there are usually some adjustments available. The JBL tweeter probably would have had a level pot in the circuit so the voicer/installer/user could adjust the brilliance over a wide range to suit his preference. The Allen HC-12 and -15 cabinets used to have a slider resistor on the tweeter, but in later production that was replaced with a fixed resistor, and the voicer was expected to make all changes by adjusting the tone pots in the cage (or by adjusting the tone in DOVE for the later models without any pots).

Nice work. Thanks again for sharing all this with us.

[Eric Mack]

Thanks for the feedback., John. I'm happy to share. It's the least I can do for a group of people that have been so helpful to me.

I was a little surprised by the results in terms of the two tweeters but that is why I was careful to indicate that this was an unscientific test. I'm sure that the crossover or adjustment pot could have been changed to affect things. I simply wanted to see the difference. I'm eager to repeat this experiment on my other channels. I think (if you ask me today) I might keep just one JBL on the brass channel and eliminate the others. We'll see. I am collecting parts to build the T3's so I'll share those results when I have something.

[jbird604]

Another observation about tweeters that I've made over the decades -- the larger and more reverberant the room, the more treble you need from your speakers. Take a small, relatively inert space, such as many of the smallish Baptist churches where I installed organs in the 80's and 90's, with padded pews, carpet on the floor, popcorn ceilings -- such a room cannot tolerate much treble in the organ tone. High frequencies will seem harsh, piercing, ugly, distressing to the ears. So an organ installed in such a space might sound best with the tweeters turned down a lot, or with the treble pots on the TG boards turned way down.

But in a big open lively room, such as one church I service which is made of cast concrete with stone floor, a high smooth wooden cathedral ceiling, bare pews, large expanses of glass -- a church like that needs LOTS of treble in the organ tone, and high frequencies don't seem harsh or overbearing in such an acoustic setting. The fact that the sound can reflect and bounce and roll around in there, mix and blend and "bloom" (as voicers say) means that everything gets smoothed and prettied up. Also, reverberant rooms enhance the perception of bass frequencies, making the ear more receptive to sparkling highs. So in a room like that the speaker systems will need efficient high-output tweeters such as horns. I've even actually added extra horn tweeters to the existing speaker cabinets in a room such as this in order to get enough treble.

In a lively room, the "chiff" and air sound and crispness of organ tone that would seem nasty in a dead room is now lovely and sweet.

I say all that just to point out that your experience with the tweeters might be vastly different if you were doing this in a big lively room rather than in a living room at home. Still, the experiment is valid because it reveals exactly what these tweeters do without any enhancement from a room.

[toodles]

Does your "M10" with 8 inch speakers and a JBL 075 bullet tweeter have a Rodgers serial number tag on it? I ask, because as far as I know, Rodgers never built a cabinet that fits that description. All the M10's I've ever seen, and Rodgers's literature shows them the same, have six each 6x9 drivers with 4 piezo electric tweeters. I am wondering if that cabinet was someone's "home brew" project.

I would not expect a lot of trompette sound to come through the bullet, as the formant (band of frequency emphasis) for the trompette voice is in the range below 1kHz. The JBL 075 has a frequency response from about 3kHz to 15kHz.

As to replacing the JBL 075 with an Allen tweeter, the issue is comparative efficiency of the different speakers. The 6x9 speakers are generally more fairly efficient than the woofer, midrange, and dome tweeter used in the Allen speaker cabinets. And the M10 uses 3 parallel branches of 2 speakers in series--this increases their volume by 9.5 dB so the dome tweeter is going to be much softer than the 6x9's. If you use just two of the 6x9's in series, the balance between the 6x9's and the Allen tweeter will be much closer.

The JBL 075 is much more efficient than the 6x9's--its sensitivity is 110 dB per watt (super loud). For reference, the threshold of pain is 120 dB on average, and it would only take 10W of power to get the 075 to 120dB volume level.

One of the issues with the 6x9 speakers is that the specifications for them are not available.

[Eric Mack]

It's possible that these are not Rogers official speakers. There is no serial number but they did use the orange 8 inch speakers. My cabinets did not have for tweeters but one JBL 075 bullet Twitter. I had one that I'll describe it as an M 10 one that I will describe as an M6 and one hybrid cabinet that I will describe as an M13 but it also had a mid range added to it. So it could very well have been a custom build of some kind.
I found that I actually did like the brightness of the bullet tweeter on the brass at times and I'm thinking now that I will just add it to be really controlled with the stop tab perhaps at the back of the room like an echo trumpet when I want brightness, Perhaps with using second touch.