VOTT A7-500 EQ Circuit confusion (30923 vs N501-8B)

[altec-man]

Seeing that this setup is a common standard mid 70's A7-500-8, I thought that this issue would have been sorted out years ago. I don't want to go active, I just want some advice from someone who has carried out the mod for the horn compensation (midrange shout).

[GM]

GM
So if I'm using N501-8A, 802-8D, 416-8B & 511B components in a 825 box what would be the best solution??

'Best' is a floating target based on a number of variables with the two major ones being the app and the individual's hearing preferences. The 'textbook' answer is to make the speaker system nominally flat in-room throughout the desired listening area from at least its Fb to 20 kHz. This will entail considerable EQ of various types if an active solution isn't used similar to what Old Guy recommends, to which I'll add multi-amping. Typically, system efficiency will only be around 93-96 dB/W/m depending on the room's contribution down low.

For just passive horn EQ, ideally you'd either drive it with a ~matching impedance amp with variable DF as it was originally designed for or measure both its and the woofer cab's response and use an XO design program to modify the XO and design the L-R-C band-pass notch filter required to flatten/extend the horn's response, then fine tune the components by ear for any other amp, etc. combination.

Otherwise, for the casual DIYer, there's two options with the cheap one being to shelve the horn using the XO's pot as required to make it sound tonally balanced through the XO BW, i.e. only up to ~2 kHz for a nominally 500 Hz/2nd order XO and experiment with various value (by-pass) caps wired in parallel around it to allow some portion of the HF BW through ~unimpeded, starting with 1.0 uF for 8 ohms and work up-down from there. Obviously, this will require removing the XO and its cover and making air-tight the cab's mounting cutout during this fine tuning process.

A good argument for permanently mounting them out in the open on top or at least creating a sealed cavity for it and some type of plug-in system as 'voicing' can be a lengthy process if more than one person will be listening regularly, especially if any women are part of the 'mix', and why I prefer to use their generally superior hearing in our acute hearing BW as the reference.

A bit more involved procedure that IME yields better results overall is to disconnect the woofer from the XO, set the shelving pot to zero and wire a cheap trim pot of at least 25 ohms such as the RS 271-265 http://www.radioshack.com/product/in...ductId=2062299 in series with the XO and horn to set relative flatness through the XO-mids-lower HF BW, then experiment with cheap by-pass caps wired in parallel around the pot to let some portion of the HF BW through. Once the horn sounds smooth enough with no perceived harshness and with good HF extension, then reconnect the woofer and use the XO's pot to fine tune any excess horn gain, though IIRC the A5, 7 won't need any in a typical HIFI/HT app unless well away from any boundaries in a large enough room to allow such placements. In this case, the pot's added resistance may require a fine tuning of the trim pot which in turn may require a different by-pass cap.

Of course once the trim pots, by-pass cap values are dialed in, they ideally need to be replaced with quality components, i.e. fixed resistors and the best caps your budget can justify, with 1% mil spec and PIO being the goal IME since it's our acute hearing BW being heavily EQ'd.

GM

[GM]

Seeing that this setup is a common standard mid 70's A7-500-8.........

By the mid '70s, the A7 wasn't much of a 'player' anymore (at least around here) plus by then bands were using mixers to EQ them, so all they were really concerned about was how loud they'd play without blowing up and why first, loading caps became standard, then increasing the XO point until finally most of its horn gain was EQ'd out, ditto most of its 'shout'. Unfortunately this left a hole in the critical mids response you could drive a truck through, so of course 'filling' it was just a flick of a slider away, causing even more failures and the swap to JBL, EV or (pick one) components that didn't sound all that good, but didn't as readily 'let its smoke out' in the 'heat of the moment'.

Since the remaining few folks who tolerated these big 'dinosaurs' in their homes were wedded to their equally 'Stone Age' tube systems, none of these vanishingly low output impedance driven issues were a problem. I remember one famous reviewer remarking around this time that about all one could do without the right electronics was to turn the horn around backwards and fire it at the wall after B0$3 to diffuse its mid-range 'shout' and excessive HF beaming. I actually tried it without any filter and once I'd found the 'close enough' time delay offset I had to admit that in some ways it sounded better overall, but couldn't deal with the 'look' plus I still preferred my mouth forward filtered version.

GM

[Old Guy]

Seeing that this setup is a common standard mid 70's A7-500-8, I thought that this issue would have been sorted out years ago.

GM said it well...

"Best" is a floating target. Your "best" might not be the same as mine. Hearing, therefore audio, is very subjective and absolutes just can not be applied. Plus, the crossover responds to the amp as much as the speaker...so unless you use the exact same amp and get my ears transplanted on your head, you won't get the same results. Then, you might have different tastes in music....

Please note the ear transplant isn't recommended. Unless you wish to be nicknamed "Dumbo".

I have BTW played with that and similar circuits, my solution was switchable values. Try one cap, play with the EQ, then try the other and repeat. It's all about giving yourself options...you are the final authority, the only one who needs to be pleased....unless you are married...

YMMV= your mileage may vary.

[altec-man]

Thanks for all the information, I think. What part does the 6uF cap play in the 30923. I hate to appear ignorant but could someone please explain the capacitor's purpose and why the N501-8B capacitor is smaller in value.
Once again thanks for your replies.
PS
What I was trying to say about the best solution was what gives the most linear frequency response, I suppose the same way they try and sell a loudspeaker by showing how flat the response curve is by displaying the frequency response graph.

[GM]

I see our attachments are MIA.....

You're welcome, I think!

It's a by-pass cap.

Don't have a schematic, so don't have a clue other than caps are required as some part/all of a network's high pass section, though considering its relatively low value I assume it's a by-pass cap also.

Anyway, as I previously noted, getting a 'flat' response with any degree of accuracy over a wide BW will require the ability to accurately measure everything, so unless you're willing/able to spend the time/$$ to achieve it, then you'll probably have to pay someone to come into your home and do all that's required since it's the in-room response that matters, not some half space theoretical ideal.

GM

[Art J.]

"I see our attachments are MIA..... "

Here are the direct links until this new site becomes dependable.

http://webpages.charter.net/artj/Altec3090430923.jpg

http://webpages.charter.net/artj/AltecN-501-8a.jpg


"could someone please explain the capacitor's purpose"

The capacitor value determines the frequency in which the attenuation begins.
Same as a first order crossover.
The resistor values will determine the amount of attenuation.
Its OK to change any of the values to suit the application.

[altec-man]

Art J
So what I said earlier about the 6uF cap being about 3300 Hz and the 3uF about 6500 Hz is correct if this applies to a first order network. If that's the case why was the N501-8B crossed over so high.

[Art J.]

Art J
So what I said earlier about the 6uF cap being about 3300 Hz and the 3uF about 6500 Hz is correct if this applies to a first order network. If that's the case why was the N501-8B crossed over so high.

Yes, you were correct. I am not familiar with the 8B but they must have been starting the attenuation at a higher frequency. If they were using a 16 ohm diaphragm, 3uf would give the same results as 6uf for 8 but I would think the resistor values would also be doubled.


.

[altec-man]

Art J
I have attached the schematic of the N501-8B that I had attached earlier but now has vanished. I assume that it is a 8 ohm circuit e.g. 8B, the values for the xover's main circuit seems the same as the N501-8A.