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Advent experiment - mod - much better sound IMO


Pete B
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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.

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Guest russwollman

Pete, I don't quite understand what you have built/added/done, but I am very curious. I have lots of Advents and like them, though I'm always on the lookout for workable, practical improvement. Can you explain it in simpler terms?

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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.

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Guest Ken Perkins

I posted this yestersay on the Parts Express forum. It's a performance upgrade for the original Advent Loudspeaker. It does not address the baffle step but it does improve detail resolution and imaging and gives them that "disappearing" quality mentioned earlier that they lack stock. I've compared my modification before and after and it will not affect or alter the original voicing.

Also, I highly recommend the foam surround replacements from www.rssound.com as they have the correct 17mm wide roll like the woofers originally had. Most of the refoams I've seen use a 14mm wide roll and this will restrict xmax and the dynamic range of the woofer.

http://www.pesupport.com/cgi-bin/config.pl?read=189156

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Guest russwollman

Yes, Pete, I understand the acoustical explanation but I'm still not sure how to create the device. I'm not skilled at reading schematics. So could you kindly bring it down a few more notches so I can create this thing and try it? I am definitely interested.

I have an older NAD preamp with an external processor loop so I'm sure it's an easy connection.

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Guest russwollman

My Advents are a newer, series A4, and I have replaced the capacitors and tweeters—poly caps and the newer tweeter from Layne Audio. So it could be this idea won't work as well.

I just don't know much about this stuff!

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Guest Ken Perkins

First an apology as it seems like I'm hijacking this thread with my post...my intention was that the BSC mod might work well with my upgrade.

Also, I'd be interested if there's a way to adjust the amount of BSC this circuit provides? I've always found for my taste that a full 6 dB is too much and prefer more along the lines of 3 to 4 dB of boost.

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Guest Ken Perkins

Oh, and one other thing, regarding the baffle step compensation. I've managed to get some of it mechanically by placing my Advents on "stands" made of plywood boxes filled with cat liter. They're made to match the width and depth of the enclosure and raise them 10" off the floor. This 14"x10" surface gives me 140 in^2 of "added" baffle area and it helps ALOT. I've tried them on conventional 12" high pedestal stands and the lack of reinforcement makes them sound thin. They are 1.5' from the rear wall, toed in slightly and about 8' apart. I live in an A-frame house so the wall behind them slopes inward toward the center of the room and I sit about 9' away from them in the middle of the room. This non-cubicle environment is fantastic for low coloration bass performance.

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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.

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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.

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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.

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Guest russwollman

Thanks for your details, Pete. It's clearer. I really want to try it, and I know it isn't difficult, but if you are so inclined, can you detail it just a little more for the uninitiated?

It looks like the device requires twice everything you've listed since there are 2 channels. Correct? Here's your list, with my questions below.

OK to build it, get a dual RCA jack:

---one pair for input, one pair for out?

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.

---these two are wired in parallel?

---one pair for each channel?

twist 4.7 K with .047uF twist hangs in air.

one end of twist to hot out.

other end of twist to gnd.

---I see the value of the capacitor has become smaller.

If you can make a drawing of this, suitable for someone like me who knows not the terminology or much of anything else, that would be very kind. I can email you my fax number if you'd be willing to do that. Whether my iMac will open attachments you'd send is doubtful.

If you can help this way, I will build it and you will know the results. This is exactly the sort of thing I'd like to do. I have the time now, and as they say, an idle mind is the devil's playground, so maybe you can do your part and keep me out of hell's jaws?

Thanks very much, Russ Wollman

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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.

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Guest Ken Perkins

Pete,

You asked me earlier about my amp. I have a Musical Design D-75, dual mono with 75 w/ch driven by a B&K Pro 10mc preamp. The two are total synergy and the Advents seem to like the amp.

As for measuring the red tweeters, I've only measured the Fs of three of them...my two from 1976 and one I picked up on Ebay that's so old looking, you'd think it was a prototype (there's no date on it). They all measure 700 hz +/- 50 or so hz with an amplitude of about 12 ohms and a low of about 5 ohms. It makes you wonder how thay got the crossover at 1000 hz (so close, when supposedly you can't do that) but I suspect that the shunt inductor in the highpass is damping the tweeter's Fs, similiar to what an LCR trap would. In a normal second order electrical highpass the shunt inductor comes after the series cap and before the padding resistor. In the Advent it's located AFTER the padding resistor and is placed directly across the tweeter. So I have to assume damping is part of the reason Kloss put it there.

As for SPL and such, I have none of these measurements. I will say that XMAX seems rather low since when the donut part is pushed gently, there's little or no movement. The donut/dome assembly seems pretty stiff, in other words. I think it's this combination of mechanical and electrical damping is what makes the tweeter play as low as it does and it's probably being pushed to it's limit, as witnessed by the number of people who have had to replace blown ones over the years.

A note about the surround replacements. I've noticed some of the "Advent foam replacement kits" being sold and used are the same generic 14mm wide foam used for most any 10" woofer. I read about the rssound foams as being "true authentic" so I bought a pair. At $5.75 /each you can't beat the price. When I got them, I saw that they measured 17mm wide, just like the original foam and decided for myself, they were closer to the original and are what I was going to start recommending. The foam kit I had used before was from Parts Express and I've seen a picture of a refoam done by Simply Speakers, and they both use the generic foam. They can be identified by the fact that you can see a small, even ridge, just inside the perimeter of the masonite ring. That ridge should not be there as the foam should be exactly the width between the cone's flat outer lip and the rings inside edge.

The sonic difference between the two is subtle. With the wider foam, the bass is not necessarily any deeper but it has a stonger output with more sense of power than the 14mm wide foam. Highly recommended. I'd advise anyone who is having someone else do their refoam inquire whether the proper sized foam will be used. If not, keep looking for someone else or DIY.

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Guest russwollman

Thanks, Pete. My amplifier is a NAD 2600 with 150w/ch, and it drives 2 Advents/ch wired in parallel.

I don't mean to be thick. Perhaps it's just the words, but when you say "twist", what exactly does that mean? Twist both ends together? And "hangs in air"?

I can send you a stamped envelope for a drawing. Let me know.

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.

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Guest SpruceMoose

just a quick question: anybody have the T-S parameters for the 1st and 2nd generation large advent woofers? i would love to see them modeled in different size boxes on the pc.

sm.

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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.

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Guest Ken Perkins

Here are mine, averaged from two woofers, dated May 1976.

FS - 20hz

Qts - .44

Qes - .48

Qms - 5.57

Re - 5.5 ohms

Le - 1.5 mH

Vas - @250 liters

BL- 7.39

mmd - @30 grams

Vas is hardest to measure acurately, IMO, because it can be affected by temperature and humidity and you have to have the added mass exact for the Delta Mass method. I generally measure it 5 times in a row and average it then approximate it when averaging the two. The thing about acoustic suspension is that minor errors will cancel themselves out.

My Advents, when measured with the woofers sealed in the box and their inductors in series with the woofers (other crosssover parts and the tweeters disconnected) measure an FB of 43 hz with a QTC of .9 which I think is pretty decent and probably close to original. I used a Woofer Tester that used to be available from Parts Express to get all of these measurements.

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Guest russwollman

That's looking good, Pete. Thanks for persevering with my questions. I think I can take it from there.

All my Advents have newer black-domed tweeters and new poly caps (in the same values as the originals).

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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.

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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.

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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.

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