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AustinJoeC

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  1. So I guess I had in mind a 2 ohm resistor, something that would keep total resistance above about 4 ohms. Sounds like the OLA tweeters in parallel are less of an issue because of their higher DCR, so this 2 ohm value comes from the minimum in impedance of the woofers, 5 ohms @ about 100 Hz, where with two woofers in parallel, 2.5 ohms. So with the additional 2 ohm resistor, the minimum would then be 4.5 ohms at the ~100 Hz minimum. So does this mean that the output at ~100 Hz is roughly halved because of the almost doubled resistance? Whereas at frequencies with higher impedance, the output reducing effect of the additional 2 ohms is reduced because it’s a smaller relative jump in impedance? If yes, then I can see where arbitrarily adding resistance might not be a very good idea... Guess I’ll be putting a high current amp on the birthday list!
  2. Thanks All, yes this was exactly my thought: run some additional resistance in series with the parallel pairs to keep the total resistance above a certain minimum level, thereby opening the options to use amps that are don’t have the power supply to run at low impedance. (Correctly stated?) I totally understand why you would add resistance when changing out the crossover caps so as to maintain the design ESR. But why would adding a series resistance to the entire system be a bad thing (other than throwing away some potential amplier gain?) Is it because, since the impedance varies with frequency, the addition of a fixed resistance will affect the different frequencies to a varying extent and unbalance the sound reproduction? Or something else? It may be a little academic, but it helps me think about how these systems actually work.
  3. Pete, if one were to simply add some series resistance to keep the impedance above, say, 3 or even 4 ohms, does that open up the universe of acceptable power amps that could drive the Double OLAs? (Recognizing that you’d need a more powerful (?) amp.). If this makes sense, any suggestions of a strategy to implement? Joe
  4. Martin, thanks for the comments as well as history! I’m going to move forward under the assumption that a crushed fried egg tweeter diaphragm is just not worth the effort. Some things don’t change in 40 years...back in my younger days there was the same “debate” about adequate amplifier power. Seems pretty clear to me though: an underpowered amp imposes an artificial ceiling that the strong transients just scatter off of, creating a mess. Still, to use the vernacular, “you gotta dance with them what brung yah”. So the first version of my new vintage stereo will be with my underpowered, but otherwise very nice Nakamichi receiver at moderate levels. To all: recommendations on a suitable pre-amp/amp combo for double OLAs? Pete B has recommended the Adcom 5x5 series for their low impedance stability. I’m open to both current or vintage. I’m not made of money, so I’m not averse to buying something that needs repairing (there’s a great repair shop within walking distance...) That’ll be the second generation. Joe
  5. Thanks for this Guido57! These forums are a goldmine of good information but I frequently find myself having to dig again to find those nuggets.
  6. Pete, thanks for this link. I’ve seen this previously and it does give me concern. How much better is the situation, impedance-wise if I’m using the OLA fried eggs and using either the Rev 2 or Rev 3 crossover? Should I simply not hook them up in parallel? Are the A/B outputs on a typical receiver a parallel output? (In which case, I’m thinking, it makes no difference to hook one pair to the A output and one pair to the B output, because they are both drawing from the same source.)
  7. Hello All, Whoa!! At first, after you post, you see the “views” pile up and wonder if anyone will throw you a bone and respond. Then, it’s off to the races! Thank you Kent for the welcome and the advice! I’m pretty sure I’ve seen your rebuild elsewhere on the site and was very impressed! The modern inductors seem so much better from a construction accuracy perspective. I could imagine that one could adjust the length of the coil easily enough to match those to 0.1% (though I most definitely do NOT plan to do that.) I see that you’ve spaced the inductors beyond the real estate of the original crossover assembly. Does that help reduce coupling between the inductors? When I looked at the bracing photo (nice, clean work, BTW) I was struck by the thought that the greatest insult to the integrity/rigidity of the box is to cut a great, honking hole in the baffle to accommodate the woofer. Has anyone looked at putting a reinforcing ring around the woofer cutout? I would think that a 3/4” thick x 1.5” wide MDF ring, where the inside radius is flush with the woofer cutout, and solidly glued, would really stiffen up the baffle and minimize any twisting of the woofer. Though no idea what it would do to the sound.... I’m dangerous like that. Ra.ra, thanks for the welcome and, having thrown out the phrase “simple compression struts” in casual conversation, I suspect you might have a useful opinion on the above reinforcement question/idea! I’ve already had to change directions multiple times on this project as I learn what’s most likely doable, so one of the first things I did was buy a cheap lot of four Advent tweeters (two OLA, one green 2.5” magnet Smaller Advent (are you still looking for that one Pete?)) and the aforementioned damaged, flush mount NLA. Looks like the mounting holes are a perfect match between the OLA and NLA fried eggs, so at least that’s not going to be an issue. Ajfink, thanks for the comments and the encouragement! I believe I’ve also seen the recommendation for 2 lb of fiberglass per cu.ft of cabinet, but I’m going to have to find that thread again. In the interest of maintaining design integrity, I definitely plan to put the 3 position high frequency switch in. However, based on what i’ve read about the dangerously low impedance in the Extended position on two pairs running in parallel, I will be scrupulously avoiding that setting...and I will try to avoid window rattling volume levels. Why the rev 2 and not the rev 3 crossovers? I’ve always assumed that the rev 3 was probably where they settled because it was the best optimum. Frank, beautiful setup! (Though I assume you don’t have any three year olds wandering around ( I KNOW I can climb to the top of that!)) While I agree in principle with your comments about enough power, all I can say is that when we added a second pair of Advents (in 1975) and drove both in parallel with my Dynaco 40 wpc amp, the sound improvement was awesome! Qualitatively better than the singles. So I plan to start with the Nakamichi, 45 wpc (which I think is a qualitatively better amp than the hand assembled (by 16 YO me and my Dad) Dynaco), and see what I get! Still, if you ever get tired of your Phase Linears.... Guido57, I’ve been reading extensively both this site as well as AK, so thanks for the confirmation that these are the best sources. What I’ve seen is that it’s the positioning of the tweeters (top speaker placed upside down on bottom speaker) that gives the best results. Still, as a high school kid, we placed both speakers upright and were thoroughly impressed. I’m going to have to again find the reference for the variable BSC using the rotary switch. That was a beautiful rendition of a really nice design. Thanks again, all! I will post updates on my progress, though I may not be able to start in earnest until we begin and finish constructing a garage apartment to rebuild and lodge them. I haven’t pick up a soldering iron in over 40 years...whew boy.
  8. Hello All, First post, I’ll do my best to be coherent (so go easy...). I really could use some guidance and reality checking. I’m trained as an engineer, so not a complete idiot, but electrical engineering was always confusing and acoustics, somewhat baffling (hah.) I’m trying to reconstruct the Double Advent system of my high school days, back when my older brother would come home from college and we’d pair up our respective pairs of The Advent Loudspeaker. Not being made of money, I concluded that rebuilding four speakers was going to be cheaper (well...we’ll see) than buying four refurbished. Plus, it would be fun. At this point I plan to drive them with my old but soon to be professionally refurbished, Nakamichi SR-3A receiver. In high school I had a Dynaco PAT4 preamp and Stereo 80 power amp. Based on my reading, I’ll probably install Pete B’s BSC circuit into the tape monitor, stuff the cabinets with fiberglass, perhaps add additional braces inside the cabinets. (Though I haven’t been able to find a thread that indicates exactly why this is helpful, can anybody help?) I managed to procure four New Advent Utility cabinets and separately, four steel frame, square magnet woofers. One set of woofers needs refoaming, I’ve purchased the surrounds for these from Rick Cobb (looneytoon2001). The second set has been refoamed...improperly, to the front of the cone face rather than the rear. QUESTION 1: Frankly, I’m inclined to let a sleeping dog lie, but: Does anyone have experience on the impact on sound quality when the refoam is to the cone front rather than the cone back (on the steel frame Advent woofers)? If I chose to re-replace the surrounds on the incorrect refoam, does anyone have tips on how to best remove the existing foam from the front face of the cone without damaging the cone? Being a chemical engineer by trade, I’m not necessarily shy about using solvents (properly), but that would not be my preference. I know that some of you could probably refoam these woofers blindfolded by now, so I could use your guidance. So, obviously, I’m on the track of rebuilding four The New Advent Loudspeakers (NLA)and not four The Advent Loudspeakers, (OLA) and this is where it has gotten tricky. There seems to be a large quantity of original, offset mount fried egg tweeters available, but very few flush mount fried eggs. Of the flush mount fried eggs that I’ve seen (including the one that I’ve purchased ?), a fair percentage seem to have decent sized (~3-4mm) holes poked into the diaphragm by the protective mesh or other trauma. I’m skeptical that such damage can really be repaired. I’m finding (and maybe to be expected) that there are also a large number of OLA crossover boards available and not too many NLA crossover boards. Realistically, I plan to completely rebuild the crossovers, even down to the Masonite boards if needed, so that’s really not a showstopper, but finding enough good flush mount tweeters seems to be a problem. I'm starting to leans towards using the OLA fried eggs instead of the NLA flush mount fried eggs, knowing that I’d need to use the OLA crossover design also. It seems to me that the only real difference, given my understanding that the steel frame woofers can serve as a drop in replacement for the Masonite woofers, is that the cabinet volume is slightly larger on the NLA due to the baffle being placed a little more forward since the flush mount tweeters don’t need the grill standoffs. QUESTION 2: Is reverting back to the OLA tweeters and the OLA crossovers in the NLA cabinet a stupid idea? If stupid, why? If not stupid, then I have a choice of either mounting the OLA tweeters directly to the baffle, where the tweeter will now stand proud of the cabinet face, requiring standoffs if I want to use the grills and looking a little wonky since the grills will now also stand proud of the cabinet face, or I could perhaps countersink the tweeter assembly by routing 1/4” to 3/8” of depth out of the front face of the baffle where the tweeter mounting flange is located. QUESTION 3: Does countersinking the tweeter assembly make sense? If I reinforce the tweeter mounting area by solidly gluing a square of MDF to the interior side of the baffle, right behind the tweeter assembly, is that likely to degrade acoustic performance? QUESTION 4: My Nakamichi receiver, is rated at 45 W/ch into 8 ohms (minimum), 16 ohms minimum with A+B outputs driven. I believe that it’s STASIS design may make it somewhat more vulnerable to low impedance loads, but that’s so far out of my wheelhouse that I’ve no idea what to think. I’ve seen measurements for this receiver down to 2 ohms. However, I’d prefer not to burn it up and I know the Double Advents, driven in parallel can go to pretty low impedance. I believe the OLA tweeters and crossovers are better in this sense than the NLAs. What things can I do to minimize the strain on the amplifier? Are there other things that I should watch out for? Sorry for the length of this post. Thank you for your consideration! AustinJoeC
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