Matching output impedance question

[catalin gramada]

Hello. Supposing I wanna use a 2 way speaker cab but get different impedance speakers for bass and hights : bass at 8 ohm , the highs at 16 ohm. The speakers have low / high pass 2nd order filters in respect with their impedance. My question is : may I use the 8 and 16 ohm output OT taps simultaneously for this aplication please ? In other words: the bass speaker with its filter to 8 ohm tap and the 16 ohm high speaker to 16 ohm tap in the same time?Thanks.

[g1]

No, you can't use multiple taps of the OT at the same time. Not so much that it will cause any problems, but because neither tap will be it's correct impedance anymore.
There was a thread about this long, long ago, and I think someone ran through the math. It was fairly complex I seem to recall.

[catalin gramada]

So is better to set the crossover filters in parallel to the same output tap ? Say 8 ohm tap output with low pass 8 ohm bass speaker and 16 ohm high pass speaker in paralel please ? Of course with the price of 3 db loose from high section...

or better to adjust the high section impedance to 4 ohm (there are two 8 ohm units), modify the high pass filter to 4 ohm in respect with new speaker Impedance, and use an l-pad tailoring the sensitivity of high output ?...using the same 8 ohm output tap...

[Enzo]

REmember the response curve of the speaker drivers are not flat lines. That 8 ohm woofer impedance is only at one frequency. And the woofer and tweeter are not in parallel, only their circuits are.

Use the woofer impedance as the cab impedance, your 16 ohm tweeter won't materially upset things, though it will affect the component values in the crossover.

[loudthud]

What you want to do is run the 8 Ohm woofer and it's low pass filter from the 8 Ohm tap, and run the 16 Ohm tweeter and it's high pass filter from the 16 Ohm tap. This will keep the output tubes seeing the OT's primary impedance across all frequencies. Note that each speaker needs a filter designed for it's impedance. You shouldn't use an 8 Ohm crossover on a 16 Ohm tweeter. Also note that since tweeters are typically not able to absorb full power in a two way speaker system, you can toast the tweeter with feedback or heavily distorted program material.

[catalin gramada]

Hey. Thanks. Yes , my question was if I can use the output taps simultaneously? My project looks like that: 4x8 ohm bass woofers for 8 ohm total impedance and 2x8 ohm mid woofers for 16 ohm impedance. The crossover frequency will be 250 cps around. The filters will be 2nd order in respect with a total impedance of each section: 8 &16 ohm.I just wondered If I can conect it to the relative OT taps in the same time...?

[Dave H]

I just wondered If I can conect it to the relative OT taps in the same time...?

As loudthud said above it's OK as long as you have the appropriate crossover networks. What you don't want to do is to connect an 8 ohm speaker to the 8 ohm tap and a 16 ohm speaker to the 16 ohm tap without crossover networks as the output tubes would then see half the required impedance..

[pdf64]

I’d be a bit wary of putting stuff between OT and speaker. Might the crossovers cause significant phase shifts around the crossover freq, which might act to increase anode dissipation? I suppose that happens anyway at bass resonance, without bad things happening

[Enzo]

I have to observe that crossovers have been put between OT and speakers for decades and decades. It seems to work.

[pdf64]

Of course, but I’m unsure about their use in our application, ie continuous high power signal, typically with some degree of overdrive, valve amps?

[Enzo]

Why would that matter? Certainly multiple thousands of watts PA systems do it with their speakers, all day long. Overdrive is in the tubes, not the speaker. You could overdrive your speaker, but the cone eventually shreds. ANy distortion your guitar amp makes can be amplified in my mega=PA system.

[pdf64]

I don’t see that addresses my point, ie an application that overdrives a valve amp into a load that contains a passive crossover.
The concern being that the 2nd order filters will cause significant phase shifts, which by changing the relationship between load current and load voltage, may affect anode dissipation.
eg Zollner shows that with overdrive and real speaker loading, anode dissipation in a Super Reverb can reach 100W.
So my thought was that the introduction of further significant phase shifts, especially within a frequency band where there’s lots of signal content, might exacerbate that.

[Enzo]

Perhaps, I don't claim expertise. Just pointing out that in all the years of tube amps, it hasn't appeared as a problem. I suspect your 100 watts in the example would be instantaneous readings on individual peaks.

[J M Fahey]

I would worry about the Bass speaker, which is the "main customer" by far.

So your cabinet has an 8 ohm woofer, a 16 ohm tweeter and already a crossover that works that way?

Then you have an 8 ohm cabinet, for all practical purposes.

Conect it to amp 8 ohm tap and sleep like a baby.

[catalin gramada]

I would worry about the Bass speaker, which is the "main customer" by far.

So your cabinet has an 8 ohm woofer, a 16 ohm tweeter and already a crossover that works that way?

Then you have an 8 ohm cabinet, for all practical purposes.

Conect it to amp 8 ohm tap and sleep like a baby.

No. There is nothing built yet. There are just 4 bass woofers and 2 mids woofers, all at 8 ohm impedance. Thinking to build one 4x10 bass cab and a 2x10 mid on the top, crossed at 250-400 cps around.