Help With Electrical Calculation

WntrMute2

Super Member
I am trying to work on altering the crossover point of the HF driver in this Altec network. I am trying to calculate the resistance of this HF circuit maybe with the pot at either extreme of its range. Any assistance for a complete novice would be helpful.
TIA
Dave
N-800F Schematic copy.jpg
 
Ideally you'd need to know the inductance of the tweeter, though you can just do it for 16 ohms to get at least some answer. What you do need is the frequency, as the results will depend on that. Easiest way to do this is just enter the parts in the free LTSpice program.
 
I down loaded the program as well, thanks C.

I can understand the desire to figure it out but from recent experience to 'tweek the tweeter', that cap can be changed from 4uf to 2uf if say you want a tweet to sound brighter or sound or go a little lower on the uf if paper, soft cone or titanium dome? example: I just rebuilt my custom speakers with new titanium tweets same realistic mid soft con and replaced the 12" woofer with a spare jbl pair. My orig. realistic xovers don't have pot dial in but I could add a pad or two easy. But with the new titaniums from dayton and an assortment of caps from 8-2uf poly's and I found the sweet spot at least with no measurements on acoustic suspension. Also depends on acoustic suspension or ported speakers shell game shuffle just like impedance matching your amp to speakers generally.
 
One extreme is it 20 ohms in parallel with 16+25 ohms = 1/((1/20)+(1/(16+25))) = 13.44 ohms
Other extreme is 20+25 ohms in parallel with 16 ohms = 1/((1/(20+25))+(1/16)) = 11.8 ohms

Of course this assumes the tweeter's impedance is actually 16 ohms at the frequencies concerned. Easy to measure this with a signal generator, VTVM, and known resistor.

we really need spice for this ?
 
Keep in mind that in the middle position, the resistance will be higher than at either extreme,

i.e. R= 1/((1/(20+12.5))+(1/(12.5+16))) = 15.18 ohms

If you're really interested, make an excel spreadsheet for, say, 0.5 ohm changes in the position of the control, then put this into the crossover formula and you can graph how the crossover point changes over the range of the control. The crossover point for the tweeter will be highest at the two extremes, and lowest in the middle.

The variation isn't that great though, Altec knew what they were doing, those resistor values are carefully chosen to keep things more or less in balance.
 
Thanks maxhifi. Those of us without EE education need all the help we can get!
That really helps clarify things.
 
Thanks maxhifi. Those of us without EE education need all the help we can get!
That really helps clarify things.

Maybe this will help a bit more:

https://www.electronics-tutorials.ws/resistor/res_5.html

For the purpose of this calculation, consider the tweeter as a 16 ohm resistor, until you actually measure its impedance at the applicable frequency.

Electrical stuff is actually not so bad, it's very predictable and objective, numbers are just numbers.
 
"I am trying to work on altering the crossover point of the HF driver in this Altec network."

I think of resistors in this tweeter circuit as attenuating the volume, not altering the crossover point.

It's a 2nd order high pass filter with the capacitor managing the cutoff frequency, and the coil shunt making the cutoff slope steeper. Changing the crossover point would require changing the Cap and Coil values. Yah?

Check out this simple calculator page. It's not gospel but it is simple.
https://www.v-cap.com/speaker-crossover-calculator.php

Using the calculator I see around 1250 Hz as the Tweeter crossover point. In the 2nd Order Calculator, I chose Linkwitz Riley, and input 1250 Hz, along with your 16 ohm Tweeter. I assumed a 8 ohm woofer, and you'll see it delivering close to your 4 uF and 4mH values in the Tweeter filter, and reasonably close to the Woofer values.

Look at the L-Pad attenuation calc for changing the volume. That would help you determine the right Pot for adjustable attenuation. Any chance the value for R2 is 2.5 ohms not 25? (also, going full OCD, I would label the R going to Positive as R1 and the shunt as R2)

“You needed a new pancreas. The one we bought for you frees you from a dangerous dependency.”
Armitage
 
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For the tweeter crossover frequency point I would agree with 1250Kz for a LR slope. I also agree with the 8ohm woofer value.
With regard to the L-pad, assuming a 16ohm tweeter, 20ohm parallel resistance and 0-25ohms series resistance - Bearing mind as the pot is moved it will add 25 - 0 ohms to the parallel resistance as the series resistance changes from 0 - 25ohms - I calculated the L-pad to vary tweeter attenuation from 0 - 11.62dB.

Notably, an L-pad should create attenuation without changing the tweeter impedance as seen by the crossover. This one is miles off. As the level control moves from zero to maximum attenuation the impedance as seen by the crossover will vary from 11.8 to 34ohms. What this will do is to affect the actual crossover point because the driver impedance forms part of the crossover calculation.

Hope this helps a bit.
 
For the tweeter crossover frequency point I would agree with 1250Kz for a LR slope. I also agree with the 8ohm woofer value.
With regard to the L-pad, assuming a 16ohm tweeter, 20ohm parallel resistance and 0-25ohms series resistance - Bearing mind as the pot is moved it will add 25 - 0 ohms to the parallel resistance as the series resistance changes from 0 - 25ohms - I calculated the L-pad to vary tweeter attenuation from 0 - 11.62dB.

Notably, an L-pad should create attenuation without changing the tweeter impedance as seen by the crossover. This one is miles off. As the level control moves from zero to maximum attenuation the impedance as seen by the crossover will vary from 11.8 to 34ohms. What this will do is to affect the actual crossover point because the driver impedance forms part of the crossover calculation.

Hope this helps a bit.
Yes it does help tremendously! The last few posts helped my understanding by orders of magnitude. Much appreciated.

Now the question arrises - did Altec intend the impedance to change? Or was just this what they did for lack of L-Pads or to save money?
 
Apologies, I didn't truly read the question. I just jumped on to the fact that changing the loading will change the response from the C and L. I'd think what you'd really want to know is the effect of the adjustment on the tweeter level for a given frequency.
Here's the response for 0 and 25 ohm pot settings from 500 to 20,000 Hz. Response goes up as you'd expect for a tweeter, phase is the downward sloping curves.

AK_cross.jpg
 
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1 - The simple way
Provided that the crossover frequency of about 1250Hz is O.K you can probably leave the crossover alone. What is needed is to measure the relative sound pressure level of each driver in the cabinet with a sweep generator, mic and analysis software. This will tell you how much attenuation is required for the tweeter. You can then easily calculate the Lpad resistor values required. Try this - http://www.mh-audio.nl/att.asp There are also other loudspeaker calculators on the same site which are very useful.

2 - The hard way
Unfortunately - although this might not be initially helpful - in this case I would start by measuring the drivers for their impedance curve, measure their frequency response using active crossovers and analysis software (as I would not have the Thiel Small parameters for the drivers), and then start again with the crossover.

You asked the question of whether Altec purposely varied the effective tweeter impedance on purpose. In truth they probably didn't worry about too much about it as long as the speaker sounded more or less O.K. This sounds like I'm accusing Altec of not knowing, or not bothering about what they were doing. I'm not. This kind of thing happened a lot, with engineers designing speakers and accountants/salesmen overruling them on cost and/or marketing criteria. It is worth noting that this situation is still rife, especially in the budget sector - i.e. electrolytic caps in crossovers and oversimple crossover networks being used. It is usually price related in a cutthroat market where everthing has to be cheap.

Also, over the years, a lot has happened regarding real understanding of loudspeaker design and tuning.
 
I don't know if things are different now, but way back when I was fooling with speaker design, no matter how much I measured parts and calculated the design, the final tuning of things like tweeter level was always by ear.
 
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