[Dchristie]

Hi folks, I am new to the AudioCircle and need some assistance identifying the inductor coil value in a pair of A25 XL speakers. I recently purchased a pair of A25 XLs and unfortunately discovered that the very fine coil leads for the tweeter inductor had been broken from the base of the coil and may not be repairable. My next option would be to replace the coil, but upon searching the net, I have not been able to find any source that provides the actual value ( mh) of the inductor. The 8.0 uf cap and 3 resistors are all readily identifiable; only the coil is unknown at this point.

I seem to recall seeing somewhere on this Site a member ( maybe Wayner)  mentioning that they thought the coil value was 0.75 mh but I would sure like to be sure before I replaced the damaged one.

If anyone can help me, I would really appreciate it !

Thanks,
 
Dean

[hdspeakerman]

See if you can find someone nearby with a meter that can measure the inductor value.  Google search for crossover if you have not tried that.
Howard

[martyo]

I seem to recall seeing somewhere on this Site a member ( maybe Wayner)  mentioning that they thought the coil value was 0.75 mh but I would sure like to be sure before I replaced the damaged one.

You are correct. Wayner did refurbish a pair. They are in his normal rotation of speakers in his lab. He should notice this thread, or you could "pm" him.

[Dchristie]

Thanks for your responses. Hopefully Wayner or maybe someone else can help me. I have intensively searched the internet ( Google) to no avail and even tried to contact SEAS since the drivers were theirs and I hoped they might be able to provide some direction. Don't really have any possibility of finding someone local with a bridge to check the inductance here in Spokane, WA either.

The H086 tweeter used in the A25 XL has a resonance point of 1000hz so I really wonder how they ( Dynaco) realized the published acoustical crossover point of 1200hz for this speaker. There must be a lot of overlap with the 25 F-EW mid/bass due to it's natural roll-off as it provides a lot of signal up to 2000hz according to what I have read. I wonder what the electrical crossover point ( not final acoustical crossover) for the H086 actually is in the XL - I assume it is much higher than 1200hz as it would not have to add much below 2000hz due to the 25 F-EW carrying the majority of the signal from 2000 hz and below. Seems to me it would also be prone to power failure and also distortion if it were used much below 1800 hz or so. SEAS only recommended it as useful from 1500 - 3000 hz with a recommended power handling capacityof 5 watts. Probably why Dynaco switched to a 2nd order crossover in the XL model for this 1 inch tweeter compared to the 1st order used for the 1.5 inch dome tweeter in the classic A-25.

Makes me also wonder if another more modern tweeter could not be substituted for the H086 with a simple 1st order crossover as is done with the classic A-25. I also wonder how much different ( or alike) the 10 inch bass drivers are in each of the speakers ( original vs XL). If they are more alike than similar, maybe the current production Morel or SEAS tweeters mentioned elsewhere as suitable substitute  for the A-25would work in the XL as long as the levels were matched to the bass driver.

But, I would prefer keeping the speakers as originally designed at the present so hopefully I can repair or replace the inductor.

[Mark Korda]

Dchristie, The A-25XL is crossed over at 1500 hz. The Dynaco A-35 and A-50 were crossed at 1200 hz. I am working on a pair of Dyna-A-25Mark2's. They are the A-25XL later, 1978 with the SEAS-86 tweeter mounted right above the woofer, not to the right like the A-25XL. Wayner told me,he guessed,that the coil was .75  like you said. My speakers use a 8 ohm L-pad instead of 3 resistors like yours. Also my coil looks different from the one I saw in the A-25XL. Some where on the computer is the schematic for your cross over. I will do my best to find it. I'm going to try to send you a picture of my crossover.

I know I can find it...Mark Korda

[Mark Korda]

Dchristie, I was wrong,it is 1200 hz for the crossover. Mine with the L-pad are 1500 hz....sorry....Mark. I tried every thing I could on the computer and could not find it. If you ever do measure the mh of the coil, I would really like to know too....thanks,  Mark Korda.

[dB Cooper]

Wow, a two component crossover. Who would do that today. The Dyna low-parts-count approach extended to their speakers too, obviously.  The original A25 simply had a capacitor to roll the low end of the tweeter and used the inductance of the woofer voice coil to roll its high end. They got all all fancy with the XL 

[bregez]

Just as a visual, I am assuming your speakers have the original Dynaco A25 XL coil as shown in the image.

[Mark Korda]

Dchristie, this might be harder than cracking the German (Enigma Machine),I'm with you all the way brother! The value (mh) of the coil in my crossover is cemented on the board with silicone offering no data to be noted. I just got out a book that said with conventional shop equipment there is no way to measure inductance. There is a short or not. The book says all service data gives the DC resistance of coils, but Dchristie since we don't have that,I saw the bare coil of the A-25XL, and my coil has no info ,wouldn't we have to weigh the wire on a pot,whoops,a postal scale noted the mil. of the wire? ....Mark Korda

[Mark Korda]

Hey Dchristie, I just re read your question about the Dyna tweeter. The ultimate tweeter that could match the Seas woofer was the Dynaudio A-28. This tweeter could go down to 1000 hz. I made a pair of speakers with them. Dynaudio does not sell their raw drivers anymore to guys like me. There was a great article in Speaker Builder magizine about the mod. I would be glad to share my copy with you...Mark Korda

[Wayner]

This topic again. Well, if I could find a LC meter that went down to say .5 mh, I'd buy one (if anyone knows of one) so this riddle is an interesting one.

I used to think that the coil was .75mh, but have decided that there isn't enough facts to make the call. Of course, the internet has nothing but the truth with in it. I am interested where the 1200hz cross-over point came from, cause I haven't seen it for fact, only opinion.

Anyway, we do have two clues and they are known facts. One being the cap in series at 8mfd, and there is a coil in parallel with the tweeter, so we can also safely assume that it is a 12db per octave slope.

If the cross-over were truly around 1200 (actual 1250), then the coil would be 2mh, and this would be a Linkwitz-Riley slope. I also noticed that someone said that the tweeter has a resonance frequency of 1000hz. So at 1250, one octave down would be 625hz and the signal would be down 12 db. This means that at about 1000hz, it would be down only about 6db @ the resonance frequency. Yikes.

Now, one other possibility is the good old Butterworth slope. With an 8mfd cap and a 1mh coil, that would put the cross-over at about 1750. One octave down would be 875 (close to the stated resonance frequency of 1K) with a 12db output reduction.

Or, the third possibility is that someone was "creative" and used an unknown value for the coil and we are all guessing.

So there is no final solution, other then best guess, or have someone actually measure a coil or two and finally end the guessing game.

Hint: There are speakers with much lower cross-over points, such as the Altec Lansing Santiago or Valencia that could have either a 500 or 800 hz cross-over point. That is fricken' low, man.

Wayner

[Dchristie]

Thanks everyone for your responses.

Wayner, the 1200 hz crossover came from High Fidelity Test Reports 1976 where they compared some of the design differences between the XL model A-25 and the Standard model. The original A-25 had a crossover of 1500 hz according to the test report. ( I have the test report).

The resonance frequency came from an old  detailed fact sheet from SEAS ( which I also have) which provides a lot of info on the tweeter, but interestingly, does not provide the nominal impedance info for the H086 which would have an impact on the crossover slope if indeed we are talking about a 2nd order slope. This same sheet also lists the usable frequency range for the tweeter as 1500 - 30,000 hz and the usuable range for the 25 F-EW as 35-1500 hz .

I also have the same questions about a crossover point actually being at 1200 hz as you do - if it were true, makes me wonder how all the A-25 XLs did not end up back in the shop for repairs if this figure is real (1000 hz resonance).

Now if the tweeter were truly an 8 ohm driver, textbook cap and coil values would be 7.0 uF and 0.90 mH respectively for a 2000 hz high pass crossover which seems more reasonable for a design like this and are close to the 8.0 uF cap and your estimated 0.75mH values.

If the bass driver actually has fairly substantial output up to 2000 hz or so, a 2nd order crossover on the tweeter around the same frequency might make sense.

So, the question remains unanswered at this point. Too bad  there are no info sources for schematics for some of the Dynaco equipment like there are for Acoustic Research but I am sure there is a schematic for the XL crossover somewhere in some file.

Dean

[Wayner]

OK Dean, good info. If in fact the cross-over is at 1200hz, then I would go for the 2mh coil. My H086 tweeters all say 8 ohms, with a resistance value of about 6 ohms. I would guess that the cross-over designer simply used the 8 ohm value to calculate the needed components. I do have a mint pair of H086 drivers that I got off of screw-bay, but they are 4 ohms, and that is why they sit in a box.

Now to the A25. I thought it had a simple 5mfd cap in series with the 8 ohm tweeter. At that point, it calculates out to 4K as the cross-over point, or do I have the cap value wrong, or is there a coil in the circuit (don't own an A25)?

So to sum up the A25XL cross-over design (at least at this point):

1) The woofer has no components and rolls off naturally (we think about 2Khz)
2) The tweeter crosses over at 1200hz, using an 8mfd, 50volt cap in series, and a 2 mh coil in parallel with the tweeter (making a Linkwitz-Riley 12db per octave slope).
3) has a 3 position, progressive resistive level switching technique. (I believe the values of the resistors are 2.2, 3.9 and 15 ohm). I think they are 10 watt, but correct if I am wrong.

I also wonder if the tweeter's polarity is reversed or not (usually done with a 12db slope).

Wayner

[Dchristie]

Wayner, you are correct in that the original A-25 ( not XL) only has a simple cap ( 5.0 uF) in series with the tweeter and 5 resistors. No inductor. looking at the tweeter crossover in my XL speaker, it appears to be in phase ( not reversed) with the woofer. I just put a battery to both drivers and they both tested in phase ( cone/dome move out with + terminals connected to + end of battery), so since each is attached the same way to the + and - terminals on the crossover board, they must also be in phase as well.

Speaker builder did a modification on the A-25 years ago and from the article, the actual crossover of the A-25 is only at the specified 1500 hz point ( as published in all of the Dynaco literature) when the series/shunt resistor combination was 11/10 ohms respectively. As you adjust the level control from this point the actual crossover for the tweeter shifts to a final value of 2500 hz where the series/shunt values end up as 2.2/18.8 ohms respectively. So the actual crossover for the A-25 speaker is variable based on the resistor combinations which makes sense.

So, if we look at the XL crossover, you have the same type of impedence shift as you add in resistance  via the level control and so I assume the crossover also varies according to the resistance added into the circuit. What I can see from looking at the crossover in my speaker is that the coil is always shunted across the tweeter + and -  terminals. Then, as you adjust the level switch from maximum output to minimum the series /shunt resistance ( in ohms) after the 8.0 uF cap adjusts ( via the different resistor combinations) as follows: 0/21 ohms, 2.2/18.9 ohms, and finally 6.1/15 ohms.  I would think that this would also affect the actual crossover point for the speaker as well.

So, maybe with the variable resistance in combination with the tweeter impedance and coil resistance, we end up with a crossover point at 1200 hz for at least one of the switch positions. We still do not know what the final resistance value would be taking everything above into account so I am not sure we can derive the final value of the coil for a 2nd order crossover without an actual measurement since the tweeter impedence is based on the whole package of the components.  Be much simpler without all of the resistors and the coil itself in the circuit

So, now my disclaimer for everything I have written above, I am not a speaker builder nor an electrical engineer so I am limited to my limited knowledge of these things but sure do wish we had a schematic. Hopefully someone more knowledgeable about such things can enlighten me. 

[Wayner]

I just did some math with regards to total resistance of the circuit with the 3 step resistor control. Total resistances are 5.8, 6.5 and 7.3 ohms.

When we leave the cap value at 8 mfd with the 3 different resistance values for a Linkwitz Riley, the 2 other unknowns can be solved.

So:

5.8 ohms = 8 mfd cap-----1mh coil-----cross at 1700hz
6.5 ohms = 8 mfd cap-----1.37 mh coil------cross at 1500
7.3 ohms = 8 mfd cap-----1.72 mh coil------cross at 1350

If we think it's a Butterworth

5.8 ohms = 8 mfd cap-----.5 mh coil------crosses at 1825
6.5 ohms = 8 mfd cap-----.64 mh coil------crosses at 1625
7.3 ohms = 8 mfd cap-----.86 mh coil------crosses at 1450

Here are some problems with some of the earlier statements. A 1200hz cross-over point is not possible with an 8 mfd cap. It would have to be over 10mfd. Since the 3 settings produce 3 different cross-over points (with the same coil value), the review of the speaker could not have published just one cross-over point, unless they chose the middle one (as an average).

If I were a betting man, I would pick the 2nd Linkwitz-Riley values.

Reverse engineering is a bit_h.

Wayner

[Dchristie]

I believe you are correct in that Dynaco only adressed one crossover value for their literature as they did the same for the A-25, A-35,  and A-50. All show just a single crossover point of 1500 hz, 1200 hz, and 1000 hz for each model  respectively in the literature that I have.  High Fidelty Test  Reports proably just used the manufacturers literatue for their information.

But you do have to wonder how they ( Dynaco) came up with the value of 1200 hz with the 8 uF cap in the circuit. The other thing to take into account is the values you derived in your calculations all assume a true 8 ohm driver load. Wonder what the actual impedance of the H086 is at the respective crossover frequencies as the hz increase or decrease according to the switches.

Also, that small of a coil is going to have a somewhat high DCR due to the resistance of the fine wire although coils are usually not relevant as long as their DCR is 1/20th of the driver voice coil DCR ( according to Dickason).

Dynaco appeared to be pretty thorough with their engineering ( think I read somewhere that David Hafler was involved with the development   A-25) so I have to believe their published specs for the XL have some merit to them.

Where is that ex Dynaco engineer when you need him !

Thanks.

Dean

[Dchristie]

Wayner,

Also, would you please share with me your solution to dislodged/broken terminals on the 25 F - EW 10 inch bass driver. Mine are not totally off the frame yet but they are very loose and won't be attached for long.

Thanks !

Dean

[Wayner]

PM me.

Also, I think you have to use the tweeter's "nominal" impedance (8 ohms) when figuring out the cross-over as the impedance curve of the tweeter changes with the frequency.

Wayner

[DavidS]

apologies for a little off topic.  I always see these speakers for sale in good condition for $200 or less.  How do they sound - are they worth picking up.  Seems even if they don't work out with original drivers and crossover you could update with new A26 woofer and seas tweeter but not sure if this is going to be something I am happy with or just a fun experiment (and something that will sit unused in closet).

[Dchristie]

Hi David -once I have the crossovers completed, and the speakers back together, I will be glad to shoot you an opinion. I am recapping and also changing out the resistors in the crossover all with Mundorf components. Will also probably upgrade the wire to Supra 13 gauge and deoxit the level controls.

Actually, you should be able to get a better opinion from Wayner as he has rebuilt his A-25 XLs and also replaced the original cabinets with much higher quality cabinets. He has lived with his A-25XLs a lot longer than I have.

As to  how they sound, there is a huge following for Dynaco speakers - especially the original Dynaco speakers which I feel also includes the A-25 XL and A-40 XL models. If you read enough sites ( like AudioCircle) you will find some folks that absolutely love the sound of the A-25 XL and also some that prefer the original non-XL A-25 speakers which some have christened the "classic" A-25. I actually like them both and I do not feel you can go wrong with either model but it all depends on what your priorities are. To my ears they both have a very easy sound with fairly natural mids ( no bass below 45 hz or so) and smooth highs. I prefer the XL models as they have a little more high frequency extension but as I said , either is a very easy listen.

They are in pretty good demand right now so if you pick up a pair for a descent price, you can most likely resell them if you do not like them. Just be sure to listen to any used speakers carefully as a lot of them have been abused in their past lives.