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From: Arfa Daily on 28 Jul 2010 09:11 Owner of local high-end hifi store that I do some service work for, bought these in with the surrounds rotted away on the bass drivers. Apparently, these things were 8 grand apiece 12 years ago when new. He managed to get some new cones / surrounds from the States somewhere for a reasonable price, and fitted them. One of the amps also needed a new level pot, so he put one of those in, too. When he repowered them, one worked perfectly, but the one that he had put the pot in, buzzed so loudly that it nearly destroyed his new cone work. He couldn't see anything that he had done wrong, so he sent them both to me. One double hernia later, I had them both up on the bench. They are signal-sensing for main power turn on, so first I had to work out which was the relay for this, and bridge it such that the amp could be powered via a variac. At about 40v input, the amps started working (separate amps for bass and mid / top drive) but the bass amp started motorboating violently. I spent a very long time checking caps in the power supply and bass amp. Interestingly, I found one with very poor ESR on the driver board, but replacement made no improvement, and then a 470uF that read very good ESR - in fact so good, that it rang alarm bells, so I whipped it out and put a good old analogue ohm meter across it. It was short circuit (near enough). Remember we were talking about just this a couple of days back in the thread about SMPS cap testing, where someone asked if an ESR meter showed a cap as good, could it still be bad, and someone else then commented that a straight ohms test should be carried out "for sanity" I think was the phrase. Well here we are with a good real life example. Anyway, that turned out to be a red herring too. I don't know what that cap did as I have no schematics, but a replacement had no effect on the instability problem. At this point, having already spent too long on the job, I phoned the store owner to tell him where we were at with the problem. He then went through what he had done again, and then casually tossed in that the voice coils on the bass units that he had re-coned, were unusual in that there were two of them per driver ... Now some real alarm bells started to sound in my head. I asked him if he meant that the drivers had four wires going to each of them, and he said yes. I hadn't actually seen the backs of the drivers, as they come out from the fronts of the cabs, and the backs are packed with damping material. I rang off, and pulled one out, and indeed, there were four terminals, two of which had a fairly heavy piece of twin connected to them, and the other two, had a piece of thin screened wire connected to them. Looking at the soldering, he had definitely had these wires off, so I removed the screened wire, and reversed it. That cured all the problems, so I have to conclude that the second 'voice coil' is in fact some kind of feedback winding, and when the cone went say forward, this winding, being reverse connected, was telling the amp that it had moved backwards, playing havoc with the stability. In many many years of servicing all sorts of hifi and audio amplification equipment, I don't ever recall having come across a bass driver with a feedback coil like this. Has anyone else on here, or anyone have any knowledge of the system, and what the advantages are claimed to be ? Arfa
From: Ron on 28 Jul 2010 09:29 On 28/07/2010 14:11, Arfa Daily wrote: > Owner of local high-end hifi store that I do some service work for, > bought these in with the surrounds rotted away on the bass drivers. > Apparently, these things were 8 grand apiece 12 years ago when new. He > managed to get some new cones / surrounds from the States somewhere for > a reasonable price, and fitted them. One of the amps also needed a new > level pot, so he put one of those in, too. When he repowered them, one > worked perfectly, but the one that he had put the pot in, buzzed so > loudly that it nearly destroyed his new cone work. He couldn't see > anything that he had done wrong, so he sent them both to me. One double > hernia later, I had them both up on the bench. They are signal-sensing > for main power turn on, so first I had to work out which was the relay > for this, and bridge it such that the amp could be powered via a variac. > At about 40v input, the amps started working (separate amps for bass and > mid / top drive) but the bass amp started motorboating violently. I > spent a very long time checking caps in the power supply and bass amp. > Interestingly, I found one with very poor ESR on the driver board, but > replacement made no improvement, and then a 470uF that read very good > ESR - in fact so good, that it rang alarm bells, so I whipped it out and > put a good old analogue ohm meter across it. It was short circuit (near > enough). Remember we were talking about just this a couple of days back > in the thread about SMPS cap testing, where someone asked if an ESR > meter showed a cap as good, could it still be bad, and someone else then > commented that a straight ohms test should be carried out "for sanity" I > think was the phrase. Well here we are with a good real life example. > Anyway, that turned out to be a red herring too. I don't know what that > cap did as I have no schematics, but a replacement had no effect on the > instability problem. > > At this point, having already spent too long on the job, I phoned the > store owner to tell him where we were at with the problem. He then went > through what he had done again, and then casually tossed in that the > voice coils on the bass units that he had re-coned, were unusual in that > there were two of them per driver ... > > Now some real alarm bells started to sound in my head. I asked him if he > meant that the drivers had four wires going to each of them, and he said > yes. I hadn't actually seen the backs of the drivers, as they come out > from the fronts of the cabs, and the backs are packed with damping > material. I rang off, and pulled one out, and indeed, there were four > terminals, two of which had a fairly heavy piece of twin connected to > them, and the other two, had a piece of thin screened wire connected to > them. Looking at the soldering, he had definitely had these wires off, > so I removed the screened wire, and reversed it. That cured all the > problems, so I have to conclude that the second 'voice coil' is in fact > some kind of feedback winding, and when the cone went say forward, this > winding, being reverse connected, was telling the amp that it had moved > backwards, playing havoc with the stability. > > In many many years of servicing all sorts of hifi and audio > amplification equipment, I don't ever recall having come across a bass > driver with a feedback coil like this. Has anyone else on here, or > anyone have any knowledge of the system, and what the advantages are > claimed to be ? > > Arfa Didn't Phillips do something like this back in the 70's? Motion feedback or some such daft name? Ron -- I am using the free version of SPAMfighter. We are a community of 7 million users fighting spam. SPAMfighter has removed 1580 of my spam emails to date. Get the free SPAMfighter here: http://www.spamfighter.com/len The Professional version does not have this message
From: whit3rd on 28 Jul 2010 11:37 On Jul 28, 6:29 am, Ron <r...(a)lunevalleyaudio.com> wrote: > On 28/07/2010 14:11, Arfa Daily wrote: > > > > > > > Owner of local high-end hifi store that I do some service work for, > > bought these in with the surrounds rotted away on the bass drivers. > > Apparently, these things were 8 grand apiece 12 years ago when new. He > > managed to get some new cones / surrounds from the States somewhere for > > a reasonable price, and fitted them. One of the amps also needed a new > > level pot, so he put one of those in, too. When he repowered them, one > > worked perfectly, but the one that he had put the pot in, buzzed so > > loudly that it nearly destroyed his new cone work. He couldn't see > > anything that he had done wrong, so he sent them both to me. One double > > hernia later, I had them both up on the bench. They are signal-sensing > > for main power turn on, so first I had to work out which was the relay > > for this, and bridge it such that the amp could be powered via a variac.. > > At about 40v input, the amps started working (separate amps for bass and > > mid / top drive) but the bass amp started motorboating violently. I > > spent a very long time checking caps in the power supply and bass amp. > > Interestingly, I found one with very poor ESR on the driver board, but > > replacement made no improvement, and then a 470uF that read very good > > ESR - in fact so good, that it rang alarm bells, so I whipped it out and > > put a good old analogue ohm meter across it. It was short circuit (near > > enough). Remember we were talking about just this a couple of days back > > in the thread about SMPS cap testing, where someone asked if an ESR > > meter showed a cap as good, could it still be bad, and someone else then > > commented that a straight ohms test should be carried out "for sanity" I > > think was the phrase. Well here we are with a good real life example. > > Anyway, that turned out to be a red herring too. I don't know what that > > cap did as I have no schematics, but a replacement had no effect on the > > instability problem. > > > At this point, having already spent too long on the job, I phoned the > > store owner to tell him where we were at with the problem. He then went > > through what he had done again, and then casually tossed in that the > > voice coils on the bass units that he had re-coned, were unusual in that > > there were two of them per driver ... > > > Now some real alarm bells started to sound in my head. I asked him if he > > meant that the drivers had four wires going to each of them, and he said > > yes. I hadn't actually seen the backs of the drivers, as they come out > > from the fronts of the cabs, and the backs are packed with damping > > material. I rang off, and pulled one out, and indeed, there were four > > terminals, two of which had a fairly heavy piece of twin connected to > > them, and the other two, had a piece of thin screened wire connected to > > them. Looking at the soldering, he had definitely had these wires off, > > so I removed the screened wire, and reversed it. That cured all the > > problems, so I have to conclude that the second 'voice coil' is in fact > > some kind of feedback winding, and when the cone went say forward, this > > winding, being reverse connected, was telling the amp that it had moved > > backwards, playing havoc with the stability. > > > In many many years of servicing all sorts of hifi and audio > > amplification equipment, I don't ever recall having come across a bass > > driver with a feedback coil like this. > Didn't Phillips do something like this back in the 70's? Motion feedback > or some such daft name? I think they called it 'David', it was a motion feedback woofer in a powered-speaker form factor. This kind of system can control the low frequency behavior of the cone very accurately. Mossbauer apparatus uses the same hardware for motion control with < .01% distortion, but that's a linear-motion measurement, no idea what the air movement effectiveness is.
From: Arfa Daily on 28 Jul 2010 12:50 > >> Didn't Phillips do something like this back in the 70's? Motion feedback >> or some such daft name? > > I think they called it 'David', it was a motion feedback woofer in a > powered-speaker > form factor. This kind of system can control the low frequency > behavior > of the cone very accurately. Mossbauer apparatus uses the same > hardware for motion control with < .01% distortion, but that's a > linear-motion > measurement, no idea what the air movement effectiveness is. > OK, both. Don't remember the Philips system, but then the company that I was working for back then, didn't do much with Philips. Still, that sounds like the sort of system. Those specs for the Mossbauer are impressive. I guess that's the sort of thing that it's going to be, given the claimed value of these things. I've just been listening to a CD through them. They sound nice, and very smooth with a very flat and stiff sounding bass. Quite easy on the old lug-holes, but I'm not sure that I would rate them as 16 grand more easy than my home-built EMI 13 x 8 cabs from the 70's ... :-) Arfa
From: Ron on 28 Jul 2010 15:10
On 28/07/2010 17:50, Arfa Daily wrote: > > >> >>> Didn't Phillips do something like this back in the 70's? Motion feedback >>> or some such daft name? >> >> I think they called it 'David', it was a motion feedback woofer in a >> powered-speaker >> form factor. This kind of system can control the low frequency >> behavior >> of the cone very accurately. Mossbauer apparatus uses the same >> hardware for motion control with < .01% distortion, but that's a >> linear-motion >> measurement, no idea what the air movement effectiveness is. >> > > OK, both. Don't remember the Philips system, but then the company that I > was working for back then, didn't do much with Philips. Still, that > sounds like the sort of system. Those specs for the Mossbauer are > impressive. I guess that's the sort of thing that it's going to be, > given the claimed value of these things. I've just been listening to a > CD through them. They sound nice, and very smooth with a very flat and > stiff sounding bass. Quite easy on the old lug-holes, but I'm not sure > that I would rate them as 16 grand more easy than my home-built EMI 13 x > 8 cabs from the 70's ... :-) > > > Arfa Ahh 13 x 8's did they have the whizzer cone in the centre? -- I am using the free version of SPAMfighter. We are a community of 7 million users fighting spam. SPAMfighter has removed 1580 of my spam emails to date. Get the free SPAMfighter here: http://www.spamfighter.com/len The Professional version does not have this message |