Dynaco A25 XL Crossover assistance

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Mark Korda

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Re: Dynaco A25 XL Crossover assistance
« Reply #20 on: 5 May 2013, 05:09 am »
Hi Dchristie, I have the answer for you,but first,I owned a pair of Dyna A-40XL's and they were the best! At the time I was living like a loser in my mothers basement. I had 2 Dyna Mark 3 amps. The A-40XL's produced a church organ note as well as a subwoofer. I don't know if it was the cement floors?. Well Dean,I was out tonight with some thing bothering me,I think I had the solution to the unknown coil. I found the answer in David Weems speaker building book. There is a chart that shows the cross over network design of values of inductance and capacitance with speakers with impedance from 4 to 32 ohms. I'm to out of it to make any sense now,but would be glad to send a copy to you. It looks like the coil is .16mh at a cross over at 1200 hz with 8 ohm speakers. Is that what Wayner said?. Dean ,the book is Designing,Building,and Testing Your Own Speaker System wih projects by David Weems. Keep in touch, lets solve the mystery....Mark Korda

Wayner

Re: Dynaco A25 XL Crossover assistance
« Reply #21 on: 5 May 2013, 11:58 am »
Mark,

I have that book, too. The chart on page 80 is for first order cross-overs. If you go to page 84, you will find that Butterworth and Linkwitz-Riley slopes need to have the values from page 80 modified by the formulas for the particular slope type. So the chart is good, but not the right numbers for a 2nd order (12db slope).

Wayner

Mark Korda

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Re: Dynaco A25 XL Crossover assistance
« Reply #22 on: 5 May 2013, 09:50 pm »
Hi Wayne, I was wondering how you determine that the A-25XL has a 2nd order 12db slope. I'm not very acknowledged with cross overs. Isn't the second order cross over used for 3 way designs?On page 79 of that book it says when  a high pass filter and a low pass filter are wired together,the combination makes a full first order crossover. Isn't that what the Dyna-A-25XL cross is? I'm not dis agreeing with you,I'm trying to learn this stuff. I do get that the resistors change the cross overs frequencys  as you already stated.....remember, I went to summer school for various math studies,you got to go slow with me.....thanks Wayner....Mark

Dchristie

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Re: Dynaco A25 XL Crossover assistance
« Reply #23 on: 5 May 2013, 11:07 pm »
Mark - if you look at textbook crossover designs, a first order crossover has a single capacitor in series  with the tweeter ( high pass) or just an inductor  in series with  the woofer ( low pass), respectively,  to create a 6db slope. To create a 2nd order ( 12 db slope) high pass, you would then also add a coil ( inductor) as a shunt to the tweeter and then  also a capacitor as a shunt to the woofer for the low pass. The 1st order only has one component for each driver ( either a capacitor for the tweeter or a inductor for the woofer). The 2nd order crossover has 2 components for each driver -  a capacitor and inductor - you just flip flop the positions of the components to create the high pass slope on the tweeter or the low pass slope on the woofer.

If you look at the crossover components of the A-25 XL, you have a 8.o uF capacitor in series with the tweeter and also a coil ( inductor) as a shunt to the tweeter (2 total components ). This would constitute a 2nd order crossover. The resistors are in the circuit just to adjust the level of the tweeter.

dB Cooper

Re: Dynaco A25 XL Crossover assistance
« Reply #24 on: 6 May 2013, 11:07 pm »
deleted due to lack of math savvy

Mark Korda

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Re: Dynaco A25 XL Crossover assistance
« Reply #25 on: 7 May 2013, 03:40 am »
Hi Dchristie, thanks for that explanation. You made it simple to understand and very well written.If you couldn't get your hands on the meter Wayne suggested, I think if you bought a couple coils that you thought were close from Parts Express and found one that matched the size and weight of your damaged coil, their pretty good about returns, you would be up and running. Thanks for filling me in on this and you too Wayner.....Mark Korda

Wayner

Re: Dynaco A25 XL Crossover assistance
« Reply #26 on: 7 May 2013, 11:50 am »
The coil wire of the Dynaco is really a small gauge. I don't think anyone makes coils now days with such small diameter wire. One way the coil could be measured is by undoing the wire and measuring the length, then measure the wire diameter (to determine the gauge). This would then tell what the induction would be, as I have several books here that have charts for this. But then the coil would be destroyed. No good.

Wayner

neobop

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Re: Dynaco A25 XL Crossover assistance
« Reply #27 on: 9 May 2013, 11:44 am »
You might find this calculator useful in ballparking crossover component values.
http://www.diyaudioandvideo.com/Calculator/XOver/

There are 3 different 12dB filters listed, with different component values.  They have different Q values.

It might be easier to buy an inexpensive multimeter with inductance/capacitance measuring capability.  Parts Express used to have one for around $15, I think they still sell it.  You can order a coil made with 20 Ga wire.  I think that is the smallest ready-made Ga.  It might be better to stick with, or close to orig gauge.  The coil will have higher DCR than a heavier GA and increased resistance might be part of the design.
neo

Mark Korda

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Re: Dynaco A25 XL Crossover assistance
« Reply #28 on: 10 May 2013, 12:26 am »
Hi Dchristie, this might not mean anything but I saw a picture of the coil for the Dyna-A25XL in Audiocarma or something like that and it was fine wire like Wayner said but it was of the color purple not copper colored. It was as purple as the Minnesota Vikings football helmets. Maybe a clue, maybe not....Mark.

neobop

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Re: Dynaco A25 XL Crossover assistance
« Reply #29 on: 12 May 2013, 01:00 pm »
I just reread this thread and Wayner is right about inductor value for 8 ohm tweeter with 8uF cap.  2.04mH for 1250Hz. 
The problem is the resistors will change the impedance of the tweeter.  Also, it's unusual for a 12dB x-over to be in normal polarity.  At the x-over frequency there will be a cancellation, a null in the response.  This is true for both 12dB Butterworth, Linkwitz and Bessel.   With a x-over point like this, I wouldn't guess that a cancellation would be part of the design. 

Years ago, 12dB Butterworth was the network of choice for manufacturers.  In phase, the drivers sum to 180 degrees out of phase.  Reverse polarity sums to + 3dB.  This would be what I would guess, the orig configuration.  Maybe the speakers were repaired previously and hooked up wrong?  Anyway, differences should be obvious once they're repaired and tried both ways.  Speaking of which, Dchristie, you said that the coil leads were broken off and unrepairable.  If you still have the broken leads it would be much easier to repair the coil.  The coil wire is coated with lacquer which first has to be sanded off.  Then you solder it back together.  A small difference in length will virtually make no difference.  Just rewind the coil back to original length.  If this can be accomplished, it is by far the easiest solution.   

Butterworth makes sense because inductor values are smaller for a given x-over frequency.  2.04mH is for an 8 ohm driver - Linkwitz, but 8 ohms might be incorrect.  If you calculate 12dB Butterworth, 1250Hz, 11.25 ohms = 8uF, 2.03mH.  The same.
http://www.parts-express.com/pe/showdetl.cfm?partnumber=255-054
neo

Dchristie

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Re: Dynaco A25 XL Crossover assistance
« Reply #30 on: 13 May 2013, 03:08 pm »
Thanks Neo.

My speakers do not appear to have been repaired but it would be interesting to see If Wayner's A25 XL speakers are wired in phase (like mine) or reverse phase for the 2 drivers as you suggest in your post.

Wayner, you could just pull your tweeters and see if the white cable is connected to the + terminal of the tweeter. If so, I assume they are wired like mine ( in phase) unless the white lead in your speakers is also connected to the negative terminal of the crossover which would indicate that mine are wired wrong from the crossover ( highly unlikely).

Wayner

Re: Dynaco A25 XL Crossover assistance
« Reply #31 on: 13 May 2013, 03:31 pm »
They are the same.

Wayner

bregez

Re: Dynaco A25 XL Crossover assistance
« Reply #32 on: 13 May 2013, 03:47 pm »
I have a "new" pair of these also and the white wire goes to the red on the tweeter.

BNR_1

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Re: Dynaco A25 XL Crossover assistance
« Reply #33 on: 3 Oct 2013, 02:37 am »
Hello Everyone:

I must have read this post so many times to get a better understanding of what folks are describing to determine the value of the air core inductor that is installed in the A25XL crossover.  In one of the postings, Dchrisite references an old spec sheets for the speakers.  I was browsing the net and by luck I came across these old spec sheets.  I attach the reference here to see if anyone can use the information to get us any closer to the value of the inductor.  In these sheets they make reference to crossover frequencies, fs, 6dB/octave so I was hoping with this information we (actually you because I have no clue) are more equipped to figure the value out.

Main site
http://www.seas.no/

On the far left click "SEAS VINTAGE DRIVERS"
http://www.seas.no/index.php?option=com_content&view=article&id=29&Itemid=63

Select "VERY OLD"
http://www.seas.no/index.php?option=com_content&view=article&id=33&Itemid=57

Then the scanned documents
Woofer - "H085 25F-EW"
http://www.seas.no/images/stories/vintage/pdfdataheet/h085_25f-ew.pdf

Tweeter - "From the 70's #2"
http://www.seas.no/images/stories/vintage/pdfdataheet/from_the_70s_2.pdf

Thank you