Hi Bryan
It is maybe a wise idea to try to measure the amp's actual output resistance. If this is known a much more accurate calculation of the reflex tuning is possible. Keep in mind that with high driving impedance a driver with lower Qts can/must be used than with transistor amps in order to achieve a flat response. Do you still want to use your REL sub with it ?
Regards
Charles
It is maybe a wise idea to try to measure the amp's actual output resistance. If this is known a much more accurate calculation of the reflex tuning is possible. Keep in mind that with high driving impedance a driver with lower Qts can/must be used than with transistor amps in order to achieve a flat response. Do you still want to use your REL sub with it ?
Regards
Charles
A lot to consider in the several previous responses 🤓
Fabrica-I’ll have a look at the Celestion this morning, interesting crossover, reverse polarity on the cd and no resistors, I’m interested to see how that models but love the simplicity.
Thanks for all the help guys, there’s just too much to know for a novice, I’m trying to internalize this stuff as quickly as I can but it’s just going to take time and a lot of elbow grease.
I’m not looking for perfection as much as some feeling that whatever results I end up with are intentional and not just blind hope. I’ll do the best I can but at some point soon I’m going to have to just have a shot on goal before the defenders take the ball away.
And yes Charles, I’ll definitely be using the REL, I just love how smoothly it integrates and gives me that juicy extra bit in the bottom end.
So GM, I definitely think it’s safe to assume a low DF here, but as per your previous suggestion and that of phase_accurate, I’m intending to measure output impedance if I can gather my wits and get the process figured out. The Beyma 15cxa400 has a Vas of 196L so I’m still looking at a pretty big box at 136L with your formula, but I’ll work up some proportions and see if it can work in my space, might have to stretch the height a bit to a full meter, but I may have to use a driver with smaller Vas. Fortunately I’ve got a few .1-.01 caps around so can play with the treble response, I really love cymbal work and don’t want to lose anything in the 10-12k region. I’ve been spoiled by my RAALs, this will be a pretty different presentation.
Fabrica-I’ll have a look at the Celestion this morning, interesting crossover, reverse polarity on the cd and no resistors, I’m interested to see how that models but love the simplicity.
Thanks for all the help guys, there’s just too much to know for a novice, I’m trying to internalize this stuff as quickly as I can but it’s just going to take time and a lot of elbow grease.
I’m not looking for perfection as much as some feeling that whatever results I end up with are intentional and not just blind hope. I’ll do the best I can but at some point soon I’m going to have to just have a shot on goal before the defenders take the ball away.
And yes Charles, I’ll definitely be using the REL, I just love how smoothly it integrates and gives me that juicy extra bit in the bottom end.
So I have a simple request. Both GM and now Charles have expressed how important it is to measure the output impedance and know what the damping factor is for the amp I'll primarily use with these speakers.
I don't own a scope or analyzer. I've read all the old posts in this forum I could, including those linked in this thread, and watched videos, and read a bunch more on the web. And still, I have little idea how to perform this measurement. Older posts with links to programs that would use a PC as a scope are now dead. Most of the talk about it is amongst people already at a certain level of accomplishment, so they say a lot about what they do but not much about how. I'm talking the "for dummies" version. I'm not a dummy, just a novice. I really need info at the level of showing a high school class how to do this, and if someone would take the time to clearly explain the process to me I'd be super super grateful. Or an actually informative link that isn't full of math that's useless to me, not because I didn't get an A in algebra but because I have trouble putting everything in context.
I'd really like to learn this part of the speaker game. Do you have to have a scope to do this?
I don't own a scope or analyzer. I've read all the old posts in this forum I could, including those linked in this thread, and watched videos, and read a bunch more on the web. And still, I have little idea how to perform this measurement. Older posts with links to programs that would use a PC as a scope are now dead. Most of the talk about it is amongst people already at a certain level of accomplishment, so they say a lot about what they do but not much about how. I'm talking the "for dummies" version. I'm not a dummy, just a novice. I really need info at the level of showing a high school class how to do this, and if someone would take the time to clearly explain the process to me I'd be super super grateful. Or an actually informative link that isn't full of math that's useless to me, not because I didn't get an A in algebra but because I have trouble putting everything in context.
I'd really like to learn this part of the speaker game. Do you have to have a scope to do this?
You’ll need a scope on the output to make sure the signal wave form is smooth and not distorted, otherwise the test is kinda meaningless in application. You can find used scopes for pretty cheap. Knowing what you’re looking at on screen is the challenge.
Not sure how we got here though from selecting a 12” coax to worrying about how a SET amps damping factor will deal with your alignment as a primary concern……synergy is waaaay overblown IMO…….there’s more important areas of focus that can and will have profound results on your listening experience.
Not sure how we got here though from selecting a 12” coax to worrying about how a SET amps damping factor will deal with your alignment as a primary concern……synergy is waaaay overblown IMO…….there’s more important areas of focus that can and will have profound results on your listening experience.
Thank you mayhem. I think we got here because when I heard about the DF thing it put the fear o' the gods in me, because it was new information and it sounded like a make-or-break part of the process. I really didn't want to overthink this project, but I do definitely want to avoid really bad mistakes, so it seemed like a bit of science here might do that. Heck, I'm just happy at this point to have gotten decent with WinISD and REW, which I've used quite a bit actually GM, but what does REW have to do with DF alignment?
And it's a 15" coax, either the Beyma 15XA38ND or the 15CXA400ND. Both 98db sens., low Mms, and model extremely well at least in WinISD.
If I leave the output impedance/DF aside for now, the biggest question for me is still whether I should start the rolloff earlier with a smaller enclosure in order to conserve power (this would mean max flat pretty much, tuned above Fs), or whether to tune to Fs, if there's something important about that. I apologize but I've gotten a bit confused by responses coming from different points of view.
Which "areas of focus" are you referring to in that last sentence mayhem13?
Okay, here is a way to get a number for amp output resistance.
Acquire an analog VOM with a 20 vac scale or 50 vac scale. These cost about $30 on ebay. Not a DVM, except $180 fluke RMS DVM will do this (waste of money). Acquire an 8 ohm 10 watt resistor or higher wattage. (your 2 watt amp may not be the last time you want to do this.)
Acquire a sine wave track. Hammond organ tracks are easy to find if solo. I have 4 actual hammond organs. There are also test records, test CD's, or sine wave generators. For hammond use a mid frequency tone, the bass tones are distorted from sine wave to fatten up the sound. Pipe organ flute tones are also sine waves, not the principal pipes. Turn the amp up until the sound is not clipping, does not sound like a siren. (requires switching between resistor and a speaker to check the sound. ) Measure AC voltage out on resistor=V8ohm
Now disconnect the resistor. Measure the voltage out of the amp on speaker terminals at the same gain setting.
V open circuit is what the amp is putting out. Voc-V8ohm=IsumR(internal resistance of amp). Isum=V8ohm/8
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I assumed that since it measured T/S that it would display its impedance plot, ditto DATs, but if not, then maybe this freeware demo or Google for more options.
And i too apologize for possibly being responsible for some of that confusion…..some clarity. My secondary trade is a live and recording/mix engineer…..I’ve dabbled in mastering but it’s just too boring. But my point is that I look more at the environment and the device producing the sound as a whole……because it is just that…..interchange either and the performance is completely different…….so there’s some focus points here for those interactions. First and foremost is listening distance and that relationship to room boundaries…..let’s get started
Correct me if I’m wrong, but your space is pretty compact yes?…..and I read earlier that you plan on listening to 15” coaxial drivers somewhere in the 2 meter range?……..i dont wish to further confuse the particulars here.
Thank you so much Jo for this clear "for dummies" instructional! I've ordered a nice metal housed Vishay 8ohm resistor already, and I do have an old Ratshack VOM, I'll post a photo and you can tell me it it's OK, the Vac scale only covers 10v so maybe not applicable.
And thank you GM for your continued support, I'm learning a ton here and it will al be great to have behind me in future builds, which I will definitely be pursuing. I'm a woodworker/carpenter and so making cabs is fun for me and as easy as some of these electronic brain-breakers are for some of you guys. I'll get down to studying the links later today.
Yes, the front wall is 13', and the room is 12'deep. Ceiling height is 10' fortunately. I have several 2'x3' absorption panels at first side reflection, back wall across from the speakers, and also a 4'x4' ceiling cloud at the FR there. The room is nicely damped, with a big couch serving as a bass trap haha. It's definitely a challenging room but not terrible. I've measured it a few times and a buddy made me up a great convolution filter that I've got plugged into Roon, and it really evened things out. The one thing that surprised me when I measured was how the panels originally behind the speakers on the front wall seemed to be doing absolutely nothing, which kind of makes sense since they're only 2" of rockwool and don't touch the lower frequencies that get behind the speakers. Eventually the listening room will be in the bigger space next room over as soon as we can finish my lady's studio upstairs in the barn, it's a dream space for me, 14'x16' with a big full height bay behind where the system would sit, and the listening position 4' off the back wall, not up against it like now, which to me is the biggest compromise of all in my current room. No matter how I position my speakers it's always the same - if I lean forward even a foot, the whole presentation blossoms and wraps my head.
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Your RS VOM has a 50 vac scale, not a 10 vac scale. It probably has silicon diode bridge inside, which with 1.4 v drop takes up 1/3 of 2 watt amp output. 2 watt = (4^2)/8 .
If you use the RS 10 vdc scale and put a schottky or germanium diode bridge ahead of it, you may be able to get a measurement that means something. 2 schottky diodes is 0.6 v . Minus of bridge to meter - plus of bridge to meter plus. I have a simpson 266XLPM which I believe uses selenium diodes on the 2 vac scale. Selenium diodes are high resistance, but 0 forward voltage drop.
I listen to my Peavey SP2(2004) with 15" woofer only 3 m away. The tweeter horns point down 10 deg and the speakers are on stands over the Hammond organ, so my head on the couch is in the sweet spot. With the wide dispersion design of the SP2 I can walk all around the room or out to the kitchen 11 m away and the tonal balance doesn't change.
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Well this meter doesn't sound like the best tool fot the job, so I'll have a look on Ebay and find one. I love the older Simpsons, but they're a little pricey. It's a worthwhile investment, I'll be doing this again.
Nearly all the cheap meters have silicon diodes for the AC scale. Not too many have a 10 vac scale. Invest in 4 schottky diodes and a little board to put them on. You can't buy selenium diodes as an individual.
Analog VAC meters are useful for measuring watts out of amplifiers on an 8 or 4 ohm resistor. The best test to determine if the amp is sounding polite because you are hallucinating or the rail caps are restricting power to 1/8 or 1/20 of specified watts. I used a RS meter to condemn an Allen organ amp that the pros on organforum all say "Never EVER need new electrolytic caps". S100 amp was putting out 2 watts. All new ecaps, 100 watts.
Analog VAC meters are useful for measuring watts out of amplifiers on an 8 or 4 ohm resistor. The best test to determine if the amp is sounding polite because you are hallucinating or the rail caps are restricting power to 1/8 or 1/20 of specified watts. I used a RS meter to condemn an Allen organ amp that the pros on organforum all say "Never EVER need new electrolytic caps". S100 amp was putting out 2 watts. All new ecaps, 100 watts.
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Very good stuff to know, even if it's useful only now and then. I think I'll just find a decent older Simpson, I can find one probably around $50-60, better than ordering diodes and making the whole affair more complicated. At some point I could see myself getting a used scope too, and a month in trade school with a bunch of 20 year olds to learn how to use it 🙄
How about this little beauty? If not, if you could tell me what to look for I'll find it. Lots of Tripletts around too:
https://www.ebay.com/itm/235249465199?hash=item36c5f5f76f:g:lMIAAOSwDlNlHEnM&amdata=enc:AQAIAAAA0KUdI2IdWalsjjmsNvd4/HjDjyKivBrn5OfY96RZAclvYbIBOYcRcnMXfxq84tTx97s+pjv5k4a7shoevJ+36D1S8OgxNWkzxr1N4Ylnng9lXMYdO8cFj3/xoCxVPHOCHxpp8CjG89h77ldI2mAOaCnkwkzU/+urdfa3ILQY5yWiEtvaGdP+DPu396uGWwxzSTGSQ97LLzebbITNrCThjEXvzHhBFwF/EzR5RhpW2SpDYN3/gjbPpL9TkBiZSwWPk8MkHWPyEnZOP+obP4rKa/s=|tkp:Bk9SR6SM7fLlYg
But wait! I stumbled on a whole ton of very nice looking Leader and other scopes, many of them under $200, would this be a better way to go? Sure has more functionality.
https://www.ebay.com/itm/4044921509...C9Lp|ampid:PL_CLK|clp:4429486&epid=1500709524
Also lots of new ones but not sure if they're any good:
https://www.ebay.com/itm/3256769004...U6Z|ampid:PL_CLK|clp:4429486&epid=12057344707
https://www.ebay.com/itm/235249465199?hash=item36c5f5f76f:g:lMIAAOSwDlNlHEnM&amdata=enc:AQAIAAAA0KUdI2IdWalsjjmsNvd4/HjDjyKivBrn5OfY96RZAclvYbIBOYcRcnMXfxq84tTx97s+pjv5k4a7shoevJ+36D1S8OgxNWkzxr1N4Ylnng9lXMYdO8cFj3/xoCxVPHOCHxpp8CjG89h77ldI2mAOaCnkwkzU/+urdfa3ILQY5yWiEtvaGdP+DPu396uGWwxzSTGSQ97LLzebbITNrCThjEXvzHhBFwF/EzR5RhpW2SpDYN3/gjbPpL9TkBiZSwWPk8MkHWPyEnZOP+obP4rKa/s=|tkp:Bk9SR6SM7fLlYg
But wait! I stumbled on a whole ton of very nice looking Leader and other scopes, many of them under $200, would this be a better way to go? Sure has more functionality.
https://www.ebay.com/itm/4044921509...C9Lp|ampid:PL_CLK|clp:4429486&epid=1500709524
Also lots of new ones but not sure if they're any good:
https://www.ebay.com/itm/3256769004...U6Z|ampid:PL_CLK|clp:4429486&epid=12057344707
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The simpson 260 you linked to had 2.5 vac and 10 vac scales, also 50 vac. Useful for amplifier debug on music. Input to a power amplifier is bypically 0.7 to 1.4 vac. After first transistor 5 to 10 vac. Note the ebay listing is an auction. Expect the price to go way up 1 second before auction end.
I've bought 3 used scopes. After about 15 hours work I got one to function. Scopes are chock full of expired e-caps. Sweep quite after 30 minutes use on last one I bought. Inexpensive scopes are okay for a 2 watt amp, but for PA amps of 400 w above the 80 v limit of vertical scale is a problem.
My Simpson 266xlpm is 28 years old, has no electrolytic caps in it to expire from old age.
Debugging with a "PC scope" or soundcard can blow up your PC if your device under test has a problem like strong DC. For this test 2 w amp test that works, would be an acceptable risk.
I've bought 3 used scopes. After about 15 hours work I got one to function. Scopes are chock full of expired e-caps. Sweep quite after 30 minutes use on last one I bought. Inexpensive scopes are okay for a 2 watt amp, but for PA amps of 400 w above the 80 v limit of vertical scale is a problem.
My Simpson 266xlpm is 28 years old, has no electrolytic caps in it to expire from old age.
Debugging with a "PC scope" or soundcard can blow up your PC if your device under test has a problem like strong DC. For this test 2 w amp test that works, would be an acceptable risk.
So my final thoughts here as I don’t think I can be much help with the design trend. Take my experience within the realm of production and recording for what they are…classic operation that have defined listening trends for decades…..
The presentation and illusion created by STEREO has some major constraints if one wishes to get as close to what the artists and engineers intended…..and that’s a point source…..the smallest one possible. It’s the 3D space and power response around the source point that allows the engineer to place distinct sources using panning, Spacial effects and mid side production technique. In my own personal listening experience, a 15” woofer for on axis listening beyond 1khz violates the point source constraints enough to become intolerable to listen and quite fatiguing in a short time…..the point source illusion is gone…..just a big set of headphones blaring out. Sure….no or low HD, smooth response, and an absolute bore.
But give me a pair of shoebox 2 ways of boot box 3 ways on stands and 3 well placed subwoofers in a small space like yours?……..it’s a different sport altogether. Even though non concentric, a classic 180mm midwoofer and 1” dome is still a much smaller point in space that the 15” driver…..and the contrast in power response is stark.
Best of luck on your journey and glad to have been of help somewhat.……While still not ideal in a small space, I’d much rather see you take the 12” coax path.
The presentation and illusion created by STEREO has some major constraints if one wishes to get as close to what the artists and engineers intended…..and that’s a point source…..the smallest one possible. It’s the 3D space and power response around the source point that allows the engineer to place distinct sources using panning, Spacial effects and mid side production technique. In my own personal listening experience, a 15” woofer for on axis listening beyond 1khz violates the point source constraints enough to become intolerable to listen and quite fatiguing in a short time…..the point source illusion is gone…..just a big set of headphones blaring out. Sure….no or low HD, smooth response, and an absolute bore.
But give me a pair of shoebox 2 ways of boot box 3 ways on stands and 3 well placed subwoofers in a small space like yours?……..it’s a different sport altogether. Even though non concentric, a classic 180mm midwoofer and 1” dome is still a much smaller point in space that the 15” driver…..and the contrast in power response is stark.
Best of luck on your journey and glad to have been of help somewhat.……While still not ideal in a small space, I’d much rather see you take the 12” coax path.
Thank you mayhem, I appreciate your well-informed and sobering response, and am taking it seriously. I guess there is such a contingent of 15" old-schoolers out there that it's easy to overlook the shortcomings of such a large source. But isn't most of the detailed information above the xover point still emitting from a very small sourcepoint? I guess you're saying the integration just below that point is where the loss is enough to be a problem.
My main speakers are a pair I built a couple of years ago, 20L or so, a Jeff Bagby design. RAAL 70/20 ribbons and Satori 6.5"MW, and they present music beautifully in every way, with help from a REL.
I agree with you about the 12" coax, and it's with great trepidation that I move to 15" and only because I need the extra sensitivity. Your response is compelling enough to make me consider ditching this SET amp, which I've put a lot of work into, and either finding one with more power (even a 2A3 has 2x the wpc) or just enjoying my current amp with 25-35wpc and building the 12", which would be easier in a lot of ways, especially cab volume.
Well it's Sunday, happy Sunday to all, I'm going to have a nice walk with my lady who's just returned from a long and difficult trip overseas, and clear our heads and take in the lovely Vermont autumn atmosphere, get some perspective, hopefully.
My main speakers are a pair I built a couple of years ago, 20L or so, a Jeff Bagby design. RAAL 70/20 ribbons and Satori 6.5"MW, and they present music beautifully in every way, with help from a REL.
I agree with you about the 12" coax, and it's with great trepidation that I move to 15" and only because I need the extra sensitivity. Your response is compelling enough to make me consider ditching this SET amp, which I've put a lot of work into, and either finding one with more power (even a 2A3 has 2x the wpc) or just enjoying my current amp with 25-35wpc and building the 12", which would be easier in a lot of ways, especially cab volume.
Well it's Sunday, happy Sunday to all, I'm going to have a nice walk with my lady who's just returned from a long and difficult trip overseas, and clear our heads and take in the lovely Vermont autumn atmosphere, get some perspective, hopefully.