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Q: what is correct way to use CD compensation with active crossovers?

74.219.107.178
Posted on March 18, 2015 at 06:29:56
deathtube 667
 Finally got 808-8A installed on 811B horns.

The 808-8A was fitted with aftermarket titanium drivers by the seller, a prosound company in California.

Didn't get a chance to measure, but my first listening impressions are that the 808A needs some CD compensation to boost the high end.

I am biamping with an older XM9 active at 1K cycles on the horn.

Most of what I have read seems to be for passive networks or says use an active crossover with built in CD comp networks.


 

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RE: Q: what is correct way to use CD compensation with active crossovers?, posted on March 18, 2015 at 06:40:49
Bill Fitzmaurice
Industry Professional

Posts: 5370
Location: New England
Joined: October 20, 2002
 Use active EQ. Even the flattest responding speaker made isn't flat when you put it in a room, so you should have it anyway.

 

RE: Q: what is correct way to use CD compensation with active crossovers?, posted on March 18, 2015 at 10:04:30
Inmate51
 By "CD", do you mean "constant directivity"?

If so, be aware that the Altec 811B wasn't designed as a constant directivity horn. It WILL beam, regardless of the driver.

:)

 

RE: found problem with 808-8A frequency response ..., posted on March 18, 2015 at 16:15:17
deathtube 667
 I got around to measuring the 808-8A with ARTA and it was not good.

Tried adjusting the XM9 controls and various compensation caps at the + of the driver, all no good.

-3dB at about 2500 cycles. -12dB at 14500 cycles.

Next I measured the Dayton D250P-8 and got decent flat bandwidth.

-3dB at 14500 cycles adjusting XM9 controls and using 3.2uF compensation cap at the + of the driver.

As soon as I hooked up the 808-8A's, I knew something was not quite right.

Got the Dayton drivers on the 811B for a listen after measurements and they are killing the Altec drivers which should not be happening.

Sent an email to GPA to see if they can save the 808-8A, but they are useless to me in their present condition.

Oh well, live and learn.

Edit : one other thing I noticed was that when I put a metal object to the mounting screw of the Dayton driver, there is a very strong magnetic pull.

When I do the same on the Altec driver, the magnetic pull is very weak.


 

RE: found problem with 808-8A frequency response ..., posted on March 19, 2015 at 05:30:48
Bill Fitzmaurice
Industry Professional

Posts: 5370
Location: New England
Joined: October 20, 2002
 >When I do the same on the Altec driver, the magnetic pull is very weak.

That might indicate that the magnet needs re-gaussing. A weak magnet will result in high Qes, causing a loss of high frequency sensitivity.

 

RE: found problem with 808-8A frequency response ..., posted on March 19, 2015 at 05:55:31
deathtube 667
 I asked the seller if they charged the alnico magnet and measured the frequency response of the driver, but they said they only replace the diaphragms and do some sort of basic signal testing.

Since I bought from a prosound company, the drivers were probably used in a PA or some high-SPL environment and got abused since the diaphragms were replaced.

The box they were shipped in looked OK, so I doubt they got mishandled.

Cosmetically, they look like new units.

Edit: spoke to Bill at GPA.

He said the problem might be dirt down inside the screen/throat of the driver and to look there first.

He said he has found all sorts of weird things like bugs or whatever and that might be clogging the outlet.

The magnetism would not be strong on the outside of the alnico unit, only at the gap, so testing at the outside of the case or at the mounting screws would not determine anything.

He also said the early rolloff due to demagnetization would affect all the bandwidth from the crossover point up, so my situation might be some other issue.

Also take note, GPA does not do any bandwidth testing after they remagnetize the drivers. I was quoted $30 for 2 units.

GPA has not tested aftermarket diaphragms, so he couldn't say if that was contributing to the problems or not.


My plan is to take the 808-8A apart and look for obstructions of other problems.

 

RE: found problem with 808-8A frequency response ..., posted on March 19, 2015 at 11:19:47
b.l.zeebub
Audiophile

Posts: 9361
Location: 52deg 28'N,1deg56'W
Joined: April 17, 2006
 Being AlNiCo drivers high SPL (read high temperature) doesn't bother them. Indeed less than any other magnetic material, AlNiCo is the only one which remains magnetic even if heated to red hot.

However AlNiCo is VERY susceptible to physical shock. They will weaken considerably if dropped or generally being thrown around in their enclosures. Unfortunately this is also common if used in a touring PA.

 

RE: found problem with 808-8A frequency response ..., posted on March 19, 2015 at 15:38:41
djk
Manufacturer

Posts: 6135
Joined: June 17, 2000
 If you take off the top plate it will need to be remagnatized.

 

RE: found problem with 808-8A frequency response ..., posted on March 20, 2015 at 03:50:24
deathtube 667
 I removed the back cover and diaphragm so I could shine a light thru the phase plug and see if the bug screen needed cleaning.

Bill at GPA suggested removing the bug screen, but I have read that it can collapse the magnetic field of the driver, so the bug screens were left alone.

I was not sure how to properly take the driver apart any further without damaging it, so that was as far as I went with the process.


There is a local prosound/MI/electronics repair depot close to home and I dropped off the drivers for their speaker tech to evaluate.

One of the sales guys said they might have re-mag equipment in the shop, perhaps later today I will know if they can help or not.

Edit : the local repair depot failed to produce results, so I sent the 808's off to GPA.

Bubble wrapped and doubled boxed them to survive UPS delivery goons, but IME they can trash anything.

 

RE: both 808-8A down 1.2K gauss ..., posted on March 27, 2015 at 12:25:36
deathtube 667
 GPA finished re-magnetizing the 808-8A drivers, both were down 1.2K gauss.

Not sure if that was off spec enough to roll off the top end response or not.

Bill at GPA said they sounded good to him.

Hope to get them back next week for additional measurements and subjective review.




 

RE: found problem with 808-8A frequency response ..., posted on April 5, 2015 at 22:00:33
John Sheerin
Audiophile

Posts: 524
Location: PA
Joined: July 28, 2003
 Cutting out the bug screen won't do anything to the magnetic field unless you pull apart the entire motor assembly to get the screen out.

In one of your previous posts, you noted your 808-8A's had a weak pull on a screw driver compared to a Dayton driver. The 808's have a center Alnico ring inside a pot structure. This is very well shielded - it doesn't have a lot of stray flux outside the structure, thus weak pull for a screwdriver. The Dayton driver has a ceramic ring magnet on the outside of the magnetic system. This has tons of stray flux and thus has exerts a high force on a screwdriver held near it. In other words, that observation is completely as expected. I guess a bit late now as you already had them remagged.

 

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