Pete B

Your observations on the Dayton make sense since it probably has low Xmax but then so does the Advent. I was thinking that the Dayton might need some mods and since the price is so good it looks interesting. Then I looked at the review and noticed that the response has gotten worse on the latest version when I thought I read at another moder's page that it had gotten better. I think it might need more damping in the pole vent, but who knows probably not worth the time and there's not much that can be done about Xmax. I agree about the SEAS, I've not used that particular model but have used several others and they are very good, the TFFC did happen to be my next pick. It has the same .5mm one way Xmax as the North. I also noticed that the Advent tweeter suspension was stiff but as I mentioned it's required to get the correct Fs with a highish moving mass. As long as the compliance and Bl are constant then the driver is linear, but with an equal hung coil it will most likely soft compress with more excursion. Some drivers do very well exceeding the theoretical Xmax and it's hard to predict without Bl curves or distortion tests. I agree that it might be interesting to put some damping in the chamber that you mention. Did you notice if the pole piece is extended in the Advent tweeter? Anyone have specs on the Layne black dome replacement tweeters, or have a dead one to disect?

Thanks Ken, very, very much for all this information. When I looked at that tweeter years ago I thought it must be fragile with that small voice coil and I didn't know at the time that it was crossed over so low. Recently, as I've been stating here I thought that it's probably long throw or at least fairly robust and thanks for confirming that. The thing that jumps out is that the gap height and coil height are equal which means that the theoretical Xmax is zero. There will be some fairly linear displacement as a result of the fringe field which makes it an underhung coil in a sense and that the BL reduction is proportional to the fraction of the coil that's out of the gap. It is therefore slow and progressive. If we assume no fringe field as an approximation and allow a 50% reduction in BL then this results in an Xmax50 of 1.5 mm and from here we can calculate a VD for this imperfect case which can be compared to other tweeters. Many Morel tweeters also have a linear Xmax of zero and I've never used them for this reason, most others North, Scan, Vifa, Seas most have .2 to .5 mm theoretically linear Xmax. Let's take the North for example with an 1.8 mm underhung coil and a 2.8 mm gap height which results in a linear Xmax of .5 mm and an Xmax50 of 1.4 mm. This is close but the dome area and therefore VD are also lower than the original Advent. Still all these numbers are theoretical and the NorthD25 test data looks very good so it still may be an excellent replacement, at least it has a theoretically linear region. I've also recently noticed that the Parts Express (DAYTON DC28F-8) 28 mm dome tweeter is going for only $12.50: http://www.partsexpress.com/pe/pshowdetl.c...r=275-070&DID=7 It's been noted on the web that this is a sort of hybrid copy of the Morel tweeters, a Morel replacement dome even fits it. It probably has a zero theoretical Xmax but then so does the original Advent tweeter so perhaps this would work well also. It would be interesting to test these bargain tweeters that seem to have potential as replacements.

Hi Ken, I'd guess that those inductors were overheated at some point and that lead to the deterioration of the insulation. I've never seen such small inductors in a speaker. Hope the inductance value is correct, yours agrees with the schematic on this site so it's probably fine. The air core inductor plus resistor is fine to retain the original transfer function, it's good that air core coils do not saturate. I noticed that you measured the woofer in box, I was wondering if you could measure Qms in box to determine how much it is lowered by the damping material if it's not too much trouble. I expect it to be a fairly significant amount. Anxious to hear what you think of the BSC!

Hi Ken, Just wondering what you used for dust caps?

The crossover design looks to be exactly the same configuration as the one shown at this site, however yes this system (1972 time frame) has a 16 uF capacitor where the 8 uF "increase" cap is shown. I'm positive that the system is stock. The tweeter shunt inductor is definitely 2.2 ohms, I measured both systems, and this system has never been stressed too badly, never had any tweeter failures. A low reading could be due to burnt and shorted insulation, these inductors are not going to handle much power at all. It's physically .5" in diameter painted red on top and the wire is VERY light gauge. The lower DCR, assuming it is correct, will help LF rejection and power handling. I doubt it will change the voicing very much since this is mainly below the tweeter passband. I prefer the newer 8 uF for the "increase" cap. Actually, I'd put 4 uF across the 3 ohm directly to compensate for the 15 kHz rolloff, then an additional 4 uF when in the increase position - a minor point. The other "decrease" inductor measures .6 ohms DCR, same diameter and is painted white. I've not put the system on the bench so I've not yet measured inductances and transfer functions. The woofer inductor DCR is the same as you measured. The woofer inductor is nearly falling off the back and the other system has a loud buzz coming from the back when just the right mid bass note plays. These just need to be remounted probably with screws and hot glue. I wasn't aware that you matched the coil resistances, so the only thing I'd check is that the old inductor does not have physical damage. So are you going to try the BSC Ken? It should work fine with the newer XO since their very similar especially in the "normal" position and none of the systems seem to have BSC compensation.

Hi Ken, That's a good amount of power so that's good. Thanks for the tweeter info. I've seen the R come before the L before and it usually helps raise the input impedance in the tweeter range, it also makes the rolloff heavily damped. I've actually simulated a very similar design and this configuration usually provides a very over damped response, at least -6 dB at the crossover point, perhaps more. This helps but even -6 dB is not that much. I'll try to simulate the actual Advent configuration sometime soon. Looking at one of the orange tweeters the dome is very far forward relative to the magnet top plate, there's probably a long aluminum former that serves as a heat sink, very curious to find out the VC winding height, I'd bet that it's much greater than most tweeters providing much more Xmax. The diameter is about 1 3/8" inches using one half of the roll as is suggested for woofers so it's also got more surface area than most tweeters. Interesting design. Anybody have a fried orange tweeter to disect and measure the voice coil height? The orange tweeters I have are dated Nov 1, 1972 and they also look like prototypes. The suspension may be stiff, but that large roll should give it long throw, stiff with a low Fs means that the moving mass is fairly high. Thanks for all the foam replacement info, the old one here looks like it was done correctly. I assume you do yours with the roll inverted as it was in the original? I'll be getting the ones you suggest. I was looking at your crossover upgrade and I'd guess that the DC resistance of the coils is lower. The woofer coil will tighten up the bass a bit which is fine especially with a tube amp. I'd expect that the Erse inductors have decent distortion characteristics have you seen any measurements? I just measured the old tweeter shunt coil at about 2.2 ohms and the new lower resistance coil will alter the Q of the network. It's good that it's air core, and some series R could be added so that the total resistance matches the old inductor if one wants to copy the old tonal balance with the advantage of lower distortion. It's a minor point.

Two moderately priced drivers that come to mind to replace the woofer are the Madisound Eclipse 10", I've used the 8" version and it's one of the best for low bass, don't know if the midrange is well behaved on the 10" so it's a risk. The Peerless 10" CSX is another good candidate expecially with the shorting ring, but have not tried it either. Would probably want to simulate first and perhaps add a touch of series R to adjust Qtc. Actually the BSC provides more bass so a slightly lower Qtc would probably work out just fine. Just to mention, if I remember correctly the Advent woofer has a .75" long VC and a .25" top plate which results in a one way Xmax of .25" or 6 mm.

Hi Ken, Thanks for all the driver data. The theory that the XO frequency should be higher than Fs for a tweeter applies sometimes but it really doesn't always. A 4th order LR rolloff can be made by having Fs = the XO freq with a Q of .707, and a 2nd order electrical network providing the other section. I do think the XO is close to an inphase type as it seems to have decent vertical off axis performance. A low XO freq is more a matter of tweeter excursion and heating, if the XO freq is high it's not much of an issue, but this is fairly low and I think the VC is only .75" in diameter which does not offer much thermal capacity, unless it's long. I wonder how long the VC is, like to take appart a blown one. Distortion is the other issue with such a low XO unless it has a very long Xmax.

Hi all, I just wanted to mention that in case I didn't make it clear - this is a jaw dropping improvement, not at all subtle, like a completely new and much better speaker! I'm still shaking my head thinking this was too easy and shouldn't sound this good. Hope others will try it. On most music without much bass below 40 Hz and moderate levels the bass is very close to the reference. The midrange is also very similar but very slightly more laid back - but in a good way. I've forgotten and had to check, on several occasions, to see which speaker was playing during casual listening. Now I'm thinking that the iron core inductors in the tweeter circuit are very small, TINY, and probably cause some distortion during peaks. I'll try to look into it at some point, no guarantees.

Hi Russ, Twist one lead from each part C2 and R2 as shown below, the two twisted leads should not connect to anything else so it hangs in the air. Let me see if I can get my scanner going today, then I'll mail it if not. Here's the baffle step and HF compensation network: (found the check box so that the board doesn't strip spaces) IN O--------- R1 4.7 K --------------O OUT |--- C1 .0047uF ---| | | C2 .047uF | | < twist | R2 4.7K | GND O---------------------------------O GND I happened to find an old frequency response curve for the "Small Advent" of the same time frame which was supposed to sound like the Large but be less efficient. It used the same tweeter. It clearly shows baffle loss, with the response falling about 10 dB (7 dB if a narrow band peak is ignored) from 1.2 kHz down to 100 Hz. It also falls from this peak at 1.2 k down to 5 kHz. This supports what I heard. It is about -3 dB at 15 kHz and about - 10 dB at 20 kHz relative to 10 kHz. This suggests that the compensation is about right.

Hi Russ, My scanner recently quit on me otherwise I'd have it for you now. Your right you need one for each side. I think if you get started it will become obvious. The caps are smaller because the resistance is higher. I'll see if I can get a new scanner bulb or scanner soon. By the way, how many watts are you running? Anybody know the specs for the original orangey-red tweeters? Fs, sensitivity, Xmax etc. They must be long throw to go so low.

I'd like to hear impressions concerning the compensation circuit if people decide to try it. I also wanted to mention that the NorthD25 tweeter looks initially as if it would make an excellent upgrade/replacement for the original tweeter. They're economical at $36/pair, they come in matched pairs, they have high Xmax (good for low XOs) and George at North Creek has high standards often using premium drivers: http://www.northcreekmusic.com/Specials.htm I'll have to look into the Advent more to see if any XO mods are required for the North D25. Here are some tweeter distortion tests including the North D25. It keeps up sometimes doing better than the best of them. http://www.euphase.com/Design/DriverCompar...rComparison.asp Keep in mind that this distortion test is provided by a seller of the Usher drivers.

Let's scale the impedance for your tape monitor loop: To use 4.7K resistors then the .1 becomes: (2.2/4.7) * .1 = .047 uF, the .01 would be .0047 uF or 4700 pF. I'd suggest this version for the tape monitor loop. Let's call it the PB_BSC (baffle step comp) OK to build it, get a dual RCA jack: wire gnd in to gnd out. wire 4.7 K hot in to hot (RCA center) out. wire .0047 uF hot in to hot out. twist 4.7 K with .047uF twist hangs in air. one end of twist to hot out. other end of twist to gnd. tape out to PB_BSC in. PB_BSC out to tape in.

Hey Ken no problem about the thread, the source for surrounds is useful. I'd try it with the 6 dB, it sounds real good. How much power do you have? To calculate the step just treat it as a voltage divider to determine the max gain. 6 dB is an AV of .5 hence the equal value resistors to get 1/2 the voltage. For 3 dB AV = .707 and then compute resistors for this voltage division. The circuit can also be impedance scaled say for less loading on the preamp. To use 4.7K resistors then the .1 becomes: (2.2/4.7) * .1 = .047 uF, the .01 would be .0047 uF or 4700 pF. I'd suggest this version for the tape monitor loop. One might want to go to 22K if say a tube power amp is used with a .5 or 1M input impedance. The caps would then be .01 and .001 uF.

Hi Russ, It sounds like your not familiar with baffle loss. If you put speakers in wall as is sometimes done for home theater the driver sees a large baffle and there is no baffle loss. When a bookshelf size speaker is pulled out away from the wall as most do for serious listening the drivers no longer "see" a large baffle at low frequencies and there is a loss of amplitude, usually no loss at about 1 kHz, about - 3dB at about 400 or 500 Hz and - 6 dB at about 100 Hz and down until room gain kicks back in around 50 Hz and lower. Old designs do not usually compensate for this, whereas newer designs do. The circuit I show is placed between the preamp and poweramp input, or in the tape monitor loop, and provides boost from about 1 kHz leveling off at a maximum of at about 100 Hz, matching this loss. If we imagine leaving out the .01 uF cap for the moment, at low frequencies the .1 uF is an open circuit so the signal is passed with almost no attenuation, whereas at high frequencies the .1 uF is a short and the two 2.2 K resistors divide the signal in half which is -6 dB. There is a smooth transition in between. The .01 uF shorts the first resistor at very high frequencies, I think I made it +3 dB at 15 kHz to compensate for these old tweeters that didn't have great extension. Does this help? I'd be curious to hear what you think of the difference, if you try it.

These are very old, early 1970s, first generation Large Advents by the way.

I posted this to another list, for what it's worth, be interested to hear from others if they try it: There was talk sometime back about large Advent speakers and I'm always surprised when people comment about how good they are, I suppose they might be good for their day or good for the low price point. I noticed fairly serious midrange coloration from the first listen. Still, they had the best bass and a real tweeter when compared to the competition of the day, Dyna A-25, EPI 100, AR2. The Dyna A-25 and EPI100 both had much better midrange smoothness. I happen to have a pair of large Advents here where one was reconned about 10 years ago and the other needs to be reconned. These are otherwise original, never had any tweeter problems. I've always thought the Advent (about $250/pr in the 1970s) would be a good candidate for mods since it seems as if the basic components are good. I hooked the good one up in my main system with an A/B switchbox to compare it to my commercial floor standing 3 way reference (about $2000/pr). The reference was 3ft out from the wall and I first placed the Advent on top. The Advent was much louder in the tweeter range, very bright sounding, there's a 3 position tweeter level control and in the "increase" position it sounds like a shrill transistor radio with bass, not even a good transistor radio, shouty sounding to an extreme. The other positions sounded better but still something drastically wrong in the midrange. My kids gave a big thumbs down for the Advent and thought the A/B switch was very cool. I guessed that the coloration was midrange peaking and there was also a boxy quality to the sound. Definitely like sound from a speaker rather than music in the room and I thought they'd need a lot of work. The A/B switch also provides line level switching so I inserted a volume control to match levels. Matching was hard due to the frequency response differences. Next I put them on the floor which helped bring up the mid bass to some extent, still not right, not even convincing on vocals. I happen to have a passive line level circuit that provides baffle loss compensation (6 dB) with also some high end boost, mostly above 10 kHz. Boost this high tends to add air to the sound rather than brightness and as I recall these older tweeters start to roll off above 15 kHz. I didn't think it would be this simple but now they were very close at least at moderate levels and with the tweeter switch now in the "extended" position. The Advent runs out of displacement capability with any demanding bass material and higher listening levels. Now the reference required some level reduction due to the 6 dB of loss in the baffle compensation circuit. The Advent was now heavy in the mid bass, and elevating them 10.75" was a bit too much, sounding thin, 4.75" was about right overall. The two sound very similar with this setup, the reference is a touch cleaner, smoother, and more transparent but they're very similar at moderate levels, as I said the Advent runs out of steam in the bass, and the reference also has more extended very deep bass but these are design differences. The difference is much more subtle having to listen for it rather than being in your face. I believe that the baffle step compensation is a bit too much, 4-5 dB is probably about right, still much better even at 6 dB. I thought the speakers would require much more work, they're not perfect but the difference is dramatic. Advents have never had that disappear, music in the room quality, they do with this shaping network. I'll probably do some driver measurements, don't expect to do full measurements of the Advents, just don't have the time. Might work up crossover mods so the line level circuit is not needed. These are bookshelf speakers that one might think should not need baffle step compensation when used in bookshelf applications but I've heard similar colorations even on the floor against a wall. Actually, on an ear level shelf they'd see only one close boundary, the wall behind. Placed on the floor out in the room should provide a similar one close boundary response and therefore the compensation is probably about right even for elevated bookshelf applications, adustments to this circuit are easy: Here's the baffle step and HF compensation network: --------- R 2.2 K ---------------------------- |---- C.01uF -----|.....| ...........................C.1uF ............................| ..........................R 2.2K ............................| ------------------------------------------------ ignore these: ..... they're needed because spaces are removed on this board Pre should have < 100 ohm output Z, power amp > 22K input Z.