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Old 5th March 2011, 06:30 PM   #71
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I have an interesting question. Puzzling really.
I just acquired a pair of 1+1 type narrow "C" mod like charlesp210 pictured above but with the back mounted large cap. I have gotten into the habit of wiring up just one single 9" panel and though it is a bit "challenged" it sounds quite good and loud considering. I have used a stock non-Medallion set of interfaces from a set of Model 3 along with a high current Harman Kardon power amp model PA-2400. At 170wpc it works quite well and I never though the amp could do any damage. Well when I received the smaller case sized 1+1 Medallion units it was a nifty coincidence that you could bolt a couple "sticks" to the interface and they were just the right width to screw in a loose panel. Now I have had no instance with the non-Medallion interfaces that made me uncomfortable. That said I was running these Medallion cases with said amp and the panels and over the course of a few days came to a point during a jam session with these that I smelled the "burn". Whoa nelly! Never had that happen but I started to sniff around and sure enough the "hot" acrid smell was coming from the right channel "C" mod interface. I shut the whole rig down. felt around the bass transformer and the wire wound power resistors that the stators hook to and sure as $hipt they were warm bordering on hot. Discharged and opened up stuck my nostrils in and I knew I had heated up the bass tranny. did not look different but the left side was slightly warm too just no smell. i began to wonder what I'd done and remembered that the HK amp ran a little warmer too with these "C" Medallions in the mix. Did not think anything of it till the burn smell. Definitely not the amp and no damage to it. I immediately rigged up the non-Medallion units I have been using and sure enough things were normal. Plenty of volume less heat from the amp and still only driving one panel sounding pretty darn good. Found out later from another Acoustat enthusiast the some have claimed that the original interfaces play louder than Medallions. Hmmmm... so what went on here? Did the amp go into high frequency oscillation maybe and start to heat things up due to the capacitors and general architecture of the Medallion boxes being different???? Can't tell. What is unclear is that there may be an issue of Medallion bass transformers being less robust and that is my question. The only picture of a "cooked" Acoustat transformer that I have ever seen is on Russ Knotts ESL repair site and it is a picture of a seriously damaged Medallion bass transformer. Do these have a propensity to heat up and arc or something. I have never felt the resistors or transformer emit heat from the original non-Medallion Mk 121A interfaces even when arcing the panels. Never arced a panel to the point of discharge either. The Medallions I had exhibited a kind of "blatty" sounding bass distortion which in retrospect could have been core saturation.

I have another pair of Medallion "C" mod that are the large style like the 121's from a Model 3 or 4. There will be a time I will hook them up and see what they sound like but I am concerned that the amp will hurt those as well. Something tells me the original Mk121A is more robust in the windings than the factory Medallion units but hey what do I know?
Anyone having anymore insight on this I would appreciate the dialog.
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Old 6th March 2011, 03:49 AM   #72
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Sounds to me that your amp may be unstable into the load.

Only a scope will tell you for sure...

Or else you were clipping the carp out of it, or both.

The "bass" xfmr is actually full range. Iirc, it sees the full range of signal.

Both xfmrs overlap several octaves in their coverage.

Andy would know, but I suspect that the xfmrs are the same between these units and the 121 interfaces.

You want to pick an amp that is stable into what is a capacitive load.
Many are not.
You can not tell by specs or a big heatsink.

_-_-bear

PS. I think it doesn't matter if the replacement for the electrolytic flies on its leads or not, it's so much better than the electrolytic you won't care one way or the other. Don't get too fine with this, especially if ur running them with 99% of all bipolar power amps. (the last part is just my personal opinion)
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_-_-bear
http://www.bearlabs.com -- Btw, I don't actually know anything, FYI -- [...2SJ74 Toshiba bogus asian parts - beware! ]
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Old 6th March 2011, 04:36 AM   #73
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Bear, were you able to find any pics of your hinged setup? I might give that a try. What I'm thinking of doing is attaching two panels together via piano hinge, and making four of these two-panel modules, two for each channel, side by side. Or maybe try to attach four panels together with three hinges. Another option is to do what you did, but with my 1+1 & double it up to a 3+3. I suppose could also try the same with my 2+2 but I would be stuck with the existing 2+2 angles, and 8 panels would be pushing it with my single pair of servo amps.
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Old 6th March 2011, 04:13 PM   #74
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Quote:
Originally Posted by JonasKarud View Post
Andy, do you know the distance between stator wires and membrane, wire to wire distance and the thickness of the wires?
Hello JonasKarud,

I don't know if all Acoustat panels used the same wire and spacers, but the 2 models I measured (1+1s, Spectra 11) were the same.

Spacers were 0.15" thick.
Wire was 24 gauge stranded with rather thick insulation with OD = 0.08"
Based on this, the D/S spacing would be 0.07".

But, the Acoustat factory tour document states that:
"...the solvent action of the methylene chloride causes the tightly stretched wires to sink slightly into the plastic louver..."

So, the actual D/S spacing will be a bit more then 0.07".
I would estimate a D/S spacing of about 0.09" based on visual inspection of the amount of wire sinking.

Perhaps Andy will be able to give you a more exact number.

Last edited by bolserst; 6th March 2011 at 04:28 PM.
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Old 6th March 2011, 07:46 PM   #75
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Thanks, bolserst!

The reason for asking is that I have been doing some experiments with different wire to adjacent wire spacing, and haven't been able to verify that much increase in output with smaller spacing between wires.

Please note that I'm talking about wire c-c spacing, not wire/ membrane spacing. ( which, in this experiment was constant at 1.5 mm)

I experimented with 2 mm piano wire with .5 mm PVC coating, and up to
7 mm c-c spacing there was only 1-2 dB less output compared to 5 mm c-c.
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Old 6th March 2011, 08:28 PM   #76
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Quote:
Originally Posted by JonasKarud View Post
Thanks, bolserst!

The reason for asking is that I have been doing some experiments with different wire to adjacent wire spacing, and haven't been able to verify that much increase in output with smaller spacing between wires.

Please note that I'm talking about wire c-c spacing, not wire/ membrane spacing. ( which, in this experiment was constant at 1.5 mm)

I experimented with 2 mm piano wire with .5 mm PVC coating, and up to
7 mm c-c spacing there was only 1-2 dB less output compared to 5 mm c-c.
Those results sound reasonable.

With D/S = 1.7mm, I compared test panels built using 22gauge solid wire with 1/64" PVC insulation, OD = 0.06".
First panel with 4.5 wires per inch = c-c spacing of 5.6mm. (73% open area)
Second panel with 9 wires per inch = c-c spacing of 2.8mm. (45% open area)
Third panel with 11 wires per inch = c-c spacing of 2.8mm. (34% open area)
I chose the 34% open area as the final comparison based on the Philips paper showing this to be about optimum as far as efficiency goes.

I measured 2.5dB - 2.7dB increase from first to second panels in the 1kHz - 5kHz range I was comparing.
I saw <0.5dB increase from second to third panel.

I believe it is constructed with 3 wires per cube section = 6 wires per inch = c-c spacing of 4.2mm = 52% open area.
I should have the c-c spacing for the Acoustat's written down somewhere.
Will see if I can track it down for you later this evening.

Last edited by bolserst; 6th March 2011 at 08:38 PM.
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Old 6th March 2011, 08:57 PM   #77
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Hello everyone,I just wanted pop in and welcome Andy to the site as well.
As the Acoustat design is what inspired my own designs.

Way back in the 80's i lived in naples,fl and wanted to get a set of acoustats, but nobody in my area would (or could) get a set to have a listen too, even though I had already decided to buy a pair, but not sight unseen.

So I got a hold of the factory and they said I could go out there and check them out ,but I got sidetracked and it never happened.

By the time I got back to it, the company had vanished!
So I ended up with a set Infinity SM-150 (Incredable speaker in the day) and it was at this time I had discovered Roger Sanders articles in speaker builder.

So ,Welcome ,Andy, to the forum! jer.
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Old 7th March 2011, 08:57 PM   #78
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Quote:
Originally Posted by speedracer5 View Post
I have an interesting question. Puzzling really.
I just acquired a pair of 1+1 type narrow "C" mod like charlesp210 pictured above but with the back mounted large cap. I have gotten into the habit of wiring up just one single 9" panel and though it is a bit "challenged" it sounds quite good and loud considering. I have used a stock non-Medallion set of interfaces from a set of Model 3 along with a high current Harman Kardon power amp model PA-2400. At 170wpc it works quite well and I never though the amp could do any damage. Well when I received the smaller case sized 1+1 Medallion units it was a nifty coincidence that you could bolt a couple "sticks" to the interface and they were just the right width to screw in a loose panel. Now I have had no instance with the non-Medallion interfaces that made me uncomfortable. That said I was running these Medallion cases with said amp and the panels and over the course of a few days came to a point during a jam session with these that I smelled the "burn". Whoa nelly! Never had that happen but I started to sniff around and sure enough the "hot" acrid smell was coming from the right channel "C" mod interface. I shut the whole rig down. felt around the bass transformer and the wire wound power resistors that the stators hook to and sure as $hipt they were warm bordering on hot. Discharged and opened up stuck my nostrils in and I knew I had heated up the bass tranny. did not look different but the left side was slightly warm too just no smell. i began to wonder what I'd done and remembered that the HK amp ran a little warmer too with these "C" Medallions in the mix. Did not think anything of it till the burn smell. Definitely not the amp and no damage to it. I immediately rigged up the non-Medallion units I have been using and sure enough things were normal. Plenty of volume less heat from the amp and still only driving one panel sounding pretty darn good. Found out later from another Acoustat enthusiast the some have claimed that the original interfaces play louder than Medallions. Hmmmm... so what went on here? Did the amp go into high frequency oscillation maybe and start to heat things up due to the capacitors and general architecture of the Medallion boxes being different???? Can't tell. What is unclear is that there may be an issue of Medallion bass transformers being less robust and that is my question. The only picture of a "cooked" Acoustat transformer that I have ever seen is on Russ Knotts ESL repair site and it is a picture of a seriously damaged Medallion bass transformer. Do these have a propensity to heat up and arc or something. I have never felt the resistors or transformer emit heat from the original non-Medallion Mk 121A interfaces even when arcing the panels. Never arced a panel to the point of discharge either. The Medallions I had exhibited a kind of "blatty" sounding bass distortion which in retrospect could have been core saturation.

I have another pair of Medallion "C" mod that are the large style like the 121's from a Model 3 or 4. There will be a time I will hook them up and see what they sound like but I am concerned that the amp will hurt those as well. Something tells me the original Mk121A is more robust in the windings than the factory Medallion units but hey what do I know?
Anyone having anymore insight on this I would appreciate the dialog.
There is nothing inherently 'less robust' about the Medallion transformers. In fact, due to the use of teflon wires and Nomex insulation, Medallions are typically LESS prone to arcing-over. But still not indestructible! In all fairness, I should say that the later-production Medallions were more robust than the early production, but the early ones were still better than the pre-Medallions

There should be no electrical differences in the case-style of the interfaces - they are the same electrical circuit in a different box.

In your particular case of having problems with the Medallions, you may have one of more transformers that are sub-par for some reason (overdriving by the previous owner, manufacturing defect?). Or there may be some slight electrical differences that gives your amp a problem that you did not experience with non-medallions. I have talked to thousands of Acoustat owners, and I do not recall any problems with the Medallions being harder to drive. In fact, the C-MOD version is probably a little easier to drive at higher frequencies.

I do not recall any reports of the Medallions playing any more or less loud than the pre-Medallions. If you notice a difference in volume, you may have a variation in, or a problem with, the high voltage bias supply.

Do keep in mind that to get appreciable volume from only one panel, you must drive the interface very hard. In general, the smaller the Acoustat, the more robust the amplifier must be, due to the need to drive it harder. That's just one of those funny-backwards facts o' life.

Do write again if I can be of further assistance.
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Old 7th March 2011, 09:44 PM   #79
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Thanks Andy!
Yes you are right about the amp. The HK PA2400 seems quite at ease with the single panel. I got a mint condition Krell KSA200S this summer and when we hitched it to the interfaces with a single 9" panel the bias ramped up (Plateau bias model) and it shut down. Tried it twice! The Krell did not like it at all.

I was keen on checking the transformers by hand for heat/warmth about when I started fooling with Acoustats about a year ago. I was aware that the core could saturate but on the original Mk 121 I never felt any heat from the resistors or trannys.

Andy are you saying that the capacitors in the audio path are layed out the in same fashion in the "c" mod units as the original?

Thanks again! Let me mull this over a bit. I'm sure I'll have other questions.
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Old 8th March 2011, 02:42 AM   #80
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Default Capacitor value changes in "C" version

I've just been thinking about the 3 capacitors in the "C" version Medallion interface as compared with previous interfaces. The total capacitance in the "C" version is 57.01uF (47+10+.01). The previous versions used a 220uF capacitor, or 220+10+0.1 for a total of 230.01uF in the "B" version.

I'm been unable to find actual schematic of "C" version (my owners manual shows the pre-medallion version even) and I'm wondering if these changes were made without changing the network somehow to achieve the same relative high-pass effect.

I have heard the low frequency transformer runs as full range as it can in both versions. So unless other changes were made, simply reducing the capacitance from 220 or 230.01 to 57.01 is going to have some effect on the overall frequency response because the HF transformer will be contributing less at it's lower end. A loss of output in the midrange, for example, around 2khz might be expected. So given the ability to do so (which I'm thinking about adding, during my current still-in-progress capacitor modification) changing the capacitor to suit room, taste, etc., might be desireable.

On the other hand, if you have the pre-C version, you might get more high frequency headroom (and less HF transformer melting) by changing to the "C" version capacitor values. I also thought I had heard or read here that one of the motivations for the "C" modification was to increase high frequency headroom.

Now I'm wondering if that didn't have some affect on the frequency response also, and perhaps one that might not be preferable to all listeners.
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