Jean Hiraga Altec A5 Crossover modification

[AllenB]

I suppose I'd work up some specs and look for power transformers or similar connected as autotransformers.

Or here's a thought, can you connect the highs to an 8 ohm tap and the woofers to 4 ohms?

[ticknpop]

I built LaScalas bottoms with EV woofers in an earlier lifetime with Altec 511 /802s and JBL 2405 tweeters, though the Klipsch mid horn went down to 400 hz. Really didn’t like Klipsh’s use of cheap drivers in general. That was many moons ago.................. now I use electrostatics at home and 1970’s JBL drivers and horns in black plywood 4 ways at work

[tizman]

Allen: Two autotransformers delivered were $186. Insanity. Would any power transformer that doubled or halved line voltage work? For example a 120V to 60V or a 120V to 240V? I also have a few pairs of different toroidal transformers that are 120V to 35V. What would using power transformers instead of autotransformers do to the sound quality? I have read about people using toroidal power transformers as output transformers for tube amps but have not tried it myself.

"Or here's a thought, can you connect the highs to an 8 ohm tap and the woofers to 4 ohms?" I do have a couple of amps that have dual secondary output transformers with 4 and 8 Ohm taps, but I did not know that both could be connected to different drivers in a crossover.

Also, if I use a L-Pad that presents the same impedance to the amp of 6 Ohms, of appropriate wattage, to take 6 DB of the LF output, why is that not a good idea?

[AllenB]

Originally Posted by tizman

Would any power transformer that doubled or halved line voltage work?

You could use these, that's a good suggestion. You could also use a split winding as an autotransformer. To that end..

When repurposing a power transformer I prefer not to mix dissimilar windings (thick and thin) in series unless I'm stuck for a value. Standard international split primaries should allow you to pursue 6dB +/- 1dB, or maybe a CT secondary fits the bill. Leave the unused winding open.

This is a lot like speccing an OPT. The self resonance of PTs is often above 1kHz, plenty for sub duty but measure if in doubt.

If you look at the impedance of the FH1 it is around 4 ohms at 20Hz, rising to triple that at 60Hz. You'll want to have higher than 4 ohms of reactance across half, or 16 ohms across the full winding at 20Hz.. or more than 250mH. The primaries on any power transformer smaller than 1kVA should have that at least, try to find one that has a resistance smaller than a few ohms.

What would using power transformers instead of autotransformers do to the sound quality?

You know the answer to this if you have experimented with output transformers. You manage the inductance, the bandwidth, the self resonance. Except that this is for sub duty, I wouldn't worry too much about the finer points of grain oriented steel or winding schemes etc.

I do have a couple of amps that have dual secondary output transformers with 4 and 8 Ohm taps, but I did not know that both could be connected to different drivers in a crossover.

Sure. From the secondary winding, the impedance looking back into the amp is small. In a sense the 4 ohm tap acts like a self contained autotransformer. The only drawback is that this represents -3dB. If you used 4/16 you'd get 6dB.

Also, if I use a L-Pad that presents the same impedance to the amp of 6 Ohms, of appropriate wattage, to take 6 DB of the LF output, why is that not a good idea?

This is the simplest solution. You might need to make minor response adjustments but it will work. Most of us have probably done it.

Some would declare it a waste of power. I notice that the FH1 is 103dB sensitive. What do you use on it, 10W? The L-pad would now use that as well.. less actually as you're aiming to be running the woofer lower than before.

[Pano]

97db is about right for the Altec A7/A5. That's the sensitivity of bass section - the horn is attenuated to match. In your unusual case the bass is louder than the horn, so you'll need some creative solutions. You could use two different power amps, of course.

The best is to get some practice with REW. Post you results here and we can look at them. Absolute SPL numbers aren't important at the beginning, just relative responses between bass and horn. It may end up that 104dB is just the peak of the bass and it's response needs to be shaped to a lower level, just like the 511 horn.

Measurements will tell.

[freddi]

in piddling with a few Klipsch - style setups, I don't think the FH1 would need attenuation. If the amplifier involved is ~constant voltage, then a transformer could be used on the mid - but still don't think needed.

my little RCA-Fan 8 ohm driver horn measures a bit hot compared to ancient 16 ohm SAHF driver. There should be some bandpass gain too besides what the ear likes.


(fwiw, I have more fun with a Karlson 12 than FH1}

Inroom


Outdoors

[tizman]

Originally Posted by Pano

97db is about right for the Altec A7/A5. That's the sensitivity of bass section - the horn is attenuated to match. In your unusual case the bass is louder than the horn, so you'll need some creative solutions. You could use two different power amps, of course.

The best is to get some practice with REW. Post you results here and we can look at them. Absolute SPL numbers aren't important at the beginning, just relative responses between bass and horn. It may end up that 104dB is just the peak of the bass and it's response needs to be shaped to a lower level, just like the 511 horn.

Measurements will tell.

Pano: I will do some measurements with REW. I am new to REW, having started using it a week or so ago. Should I do a measurement of the entire speaker, or do the drivers separately? Should I move the microphone so it is the same distance from each driver and directly in front of the driver being measured if I do them separately, or keep it in the same position?

[tizman]

Originally Posted by freddi

in piddling with a few Klipsch - style setups, I don't think the FH1 would need attenuation. If the amplifier involved is ~constant voltage, then a transformer could be used on the mid - but still don't think needed.

Freddi: In your graphs, the LF and HF appear to be closer in amplitude than in mine. I will post a whole speaker measurement shortly. The amp I am currently using is a single ended EL84 triode connected amp. I like to keep available wattage on the low side for testing, as I have damaged drivers before by connecting the wrong lead to a more powerful amp.

[tizman]

Originally Posted by AllenB

This is the simplest solution. You might need to make minor response adjustments but it will work. Most of us have probably done it.

Some would declare it a waste of power. I notice that the FH1 is 103dB sensitive. What do you use on it, 10W? The L-pad would now use that as well.. less actually as you're aiming to be running the woofer lower than before.

AllenB: Normally I use low wattage SET amps of less than 10 Watts. I will do some measurements and post them with the speaker as is.

[tizman]

These are four measurements made with the microphone placed about a meter away from the front of the speakers, and placed slightly above the top of the bass cabinet so that it is pointing at the lower third of the 511B. I have included three images. One is the combination of both speakers (combo), one is for just the HF on, and one is with just the LF on. I also included a measurement of one of my La Scala speakers for reference.