What is the correct impedance for Marantz Imperial 6 phenolic tweeters?

[Jens Sinding]

I recently found a pair of Imperial 6G’s for a very reasonable price (about 40$). The tweeters had suffered the fingers of destruction and were essentially missing the dust cap, so I ordered the supposedly drop in replacement, the GRS PRT-8 phenolic tweeter.
However, the impedance of the old tweeters measure at 4.7 Ohm, whereas the GRS measure at 7.8 Ohm (it is specified as an 8 Ohm unit).
In direct comparison the old tweeters sound better, so I suspect that I may have the wrong units.
All things being equal, the old tweeters should be louder, or are other factors in play?
Does anyone have any experience with this replacement, and what was the impedance of your old tweeters?
The crossover is rather interesting with a level switch for the tweeter.
In neutral, there is a 3 ohm resistor in series, and a 8 ohm resistor in parallel with the tweeter.
In high, the 3 ohm resistor is placed in parallel with a 1 ohm resistor, reducing the resistance in series with the tweeter an thus increasing the volume..
In low, an extra 6 ohm resistor is placed in parallel with the tweeter, thus allowing more energy to bypass the tweeter.
I have considered removing the 3 ohm resistor, or increasing the 8 ohm resistor, to adjust for the tweeter impedance, but I think that it will be better to get some advice from wiser karmists first

 

[Lavane]

8 ohm. Nice score on the 6's! I'm guessing you bought the ones Parts Express sells? The reproduction phenolic ring tweeters they sell aren't quite as good as the originals. Your measurement is close enough they will work just fine though. Another even cheaper replacement that works very well is the 8 ohm Goldwood paper cone tweeter Parts Express sells. I've tried them with my Imperial 6's just to see if I could tell the difference, I couldn't. Original used tweeters for the 6's pop up on ebay pretty regularly. I had a damaged tweeter in one of mine and found one on ebay pretty fast. Speakers won't test exactly at the ohms rating they are supposed to be, so don't worry about it. Just run the ones you bought until you find some originals. The tweeter for the Imperial 5's are the same also.

 

[Lavane]

Oh, and don't mess with the crossover, you will regret it. Just replace that old wax electrolytic capacitor with a good polypropylene film cap and keep looking for original tweeters. Those reproduction tweeters only go to 15khz, that may be part of the problem. The originals are supposed to go to 18khz. It could be that the old capacitor has drifted and hanged the crossover point.

 

[painttoad]

8 ohm. I'm guessing you bought the ones Parts Express sells? The reproduction phenolic ring tweeters they sell aren't quite as good as the originals. Your measurement is close enough they will work just fine though. Another even cheaper replacement that works very well is the 8 ohm Goldwood paper cone tweeter Parts Express sells. I've tried them with my Imperial 6's just to see if I could tell the difference, I couldn't. Original used tweeters for the 6's pop up on ebay pretty regularly. I had a damaged tweeter in one of mine and found one on ebay pretty fast. Speakers won't test exactly at the ohms rating they are supposed to be, so don't worry about it. Just run the ones you bought until you find some originals. The tweeter for the Imperial 5's are the same also.

goldwood is such a dirty, dirty word.

i had one experience with goldwood years ago..long story short: i told the tech "i thought goldwood makes cheap crap" and he replied "not these" and i took them home and they sounded like ass. luckily they took them back and gave me a refund.

i will never even consider goldwood again.

 

[Lavane]

goldwood is such a dirty, dirty word.

i had one experience with goldwood years ago..long story short: i told the tech "i thought goldwood makes cheap crap" and he replied "not these" and i took them home and they sounded like ass. luckily they took them back and gave me a refund.

i will never even consider goldwood again.

Same tweeter? It's the only Goldwood driver I'm familiar with. The response was right and it sounded pretty much the same as the phenolic ring tweeter. I know these are used by others for some Pioneer and other brand speakers that had phenolic ring tweeters with good results.

 

[painttoad]

Same tweeter? It's the only Goldwood driver I'm familiar with. The response was right and it sounded pretty much the same as the phenolic ring tweeter. I know these are used by others for some Pioneer and other brand speakers that had phenolic ring tweeters with good results.

no. it was a 6.5" woofer. i just shudder at the name.

when a woofer with failing surrounds sounds better than the replacement, there's a problem. it was some infinity bookshelves ss2001 or 2 or something. ended up with some discontinued audax aerogel woofers. i wish those were still available for $25 each!

 

[the skipper]

You didn't measure the impedance. You measured the DC resistance, There's a difference. DC resistance will be lower than the rated impedance.

 

[Jens Sinding]

Thanks guys, that’s very good information.
I was not aware of the exact difference between DC resistance and impedance, though I was aware that impedance changes with frequency.
I have replaced the wax caps, though they measure fine using my capacitance meter. I used Dayton PP caps.

the woofer surrounds are a bit neeeh, but they actually sound very good.

also changed the binding posts to something that won’t kill your amp if you fumble slightly

 

[Lavane]

Nice, looking good. I just used a 6.8uf poly cap. It's well withing the tolerance of the original 7uf cap. The rubber surrounds used on several of the Imperial models can benefit from turning the woofer 180°. They can sag over time. I did this with mine.

 

[onwardjames]

I have championed these overperforming and humble beauties since 2004 or so. Check my post history.

Those of us in the know have always loved the easygoing champs of pleasant sound, the Marantz Imperial 6. The 5's are also good.

You'll see many of AK's most respected members giving these a high recommendation.

Hell of a score, my friend, and Lavane's first post pretty much says it all.

 

[Lavane]

They just play everything well and even sound good at low volumes. Not the best speakers ever made, but damn easy to listen to and get a lot right.

 

[Jens Sinding]

I have championed these overperforming and humble beauties since 2004 or so. Check my post history.

Those of us in the know have always loved the easygoing champs of pleasant sound, the Marantz Imperial 6. The 5's are also good.

You'll see many of AK's most respected members giving these a high recommendation.

Hell of a score, my friend, and Lavane's first post pretty much says it all.

Hi Onwardjames
I’ve had great pleasure reading your posts regarding the Imperial 6, and they certainly don’t disappoint
Best regards
Jens

 

[Jens Sinding]

Nice, looking good. I just used a 6.8uf poly cap. It's well withing the tolerance of the original 7uf cap. The rubber surrounds used on several of the Imperial models can benefit from turning the woofer 180°. They can sag over time. I did this with mine.

Hi Lavane
Mine have doped cloth surrounds, but they have certainly sagged so 180 it is. Regarding the caps, you are right that a 6.8 uf would be fine, but the 0.2 uf was cheap, works as a better quality bypass cap, and with a bit of duck tape makes the assembly fit the bracket for the old wax capacitor.

 

[Lavane]

Hi Lavane
Mine have doped cloth surrounds, but they have certainly sagged so 180 it is. Regarding the caps, you are right that a 6.8 uf would be fine, but the 0.2 uf was cheap, works as a better quality bypass cap, and with a bit of duck tape makes the assembly fit the bracket for the old wax capacitor.

Ya, I noticed a few dimples in that picture of the one woofer. It can get worse with the 12" woofers in the 7's. Just steer clear of the 7's by the way. Its a bad market driven design.

 

[Jens Sinding]

Oh, and don't mess with the crossover, you will regret it. Just replace that old wax electrolytic capacitor with a good polypropylene film cap and keep looking for original tweeters. Those reproduction tweeters only go to 15khz, that may be part of the problem. The originals are supposed to go to 18khz. It could be that the old capacitor has drifted and hanged the crossover point.

Hi Lavane
I will try the Goldwood tweeters if I can get hold of them at a reasonable price.
I’m Danish, so my best source seems to be Soundimports from the Netherlands. I’m also considering doing something with some surplus Phillips silk dome tweeters that seem a good match, but nothing irreversible

 

[Jens Sinding]

I’m considering whether to put a few braces in the cabinets, and maybe a layer of bitumen damping mats.
Do you have any experience with this?
It’s a cheap and easy modification, if it’s worthwhile.
I rescued the woofer dust caps with the vacuum cleaner and wallpaper glue, and also had to patch a few spots on the membranes causing the dark spots. A very active child with nail scissors?
Here are some before pictures.

 

[Lavane]

Hi Lavane
I will try the Goldwood tweeters if I can get hold of them at a reasonable price.
I’m Danish, so my best source seems to be Soundimports from the Netherlands. I’m also considering doing something with some surplus Phillips silk dome tweeters that seem a good match, but nothing irreversible

Ya, they probably aren't worth the added shipping

 

[Lavane]

Being a ported design, the batting they used in the back is all you need. If it needs replacing just try and find some batting the same thickness. I don't think added bracing would hurt anything. It may keep the cabinet from any ringing. I have a dented dust cap on one of my woofers. A vacume cleaner didn't work. I may have to try something really sticky. That tweeter dust cap is pretty bad. If the tweeter still works, the bad dust cap shouldn't effect the sound though.

 

[Jens Sinding]

Mine had been worked on using the needle method
Audiofriends have a video on YouTube, describing how to use the wallpaper paste. It worked a treat, and the mass increase should be insignificant. I used the flat nozzle and my fingers to prevent it latching on to the dust cap. Just search for “dented paper dust cap”. By applying the paste you temporarily soften the paper and allow the vacuum to pull it into shape.
In my case the dust caps had gone soft due to the pincushion approach to dent repair, which is not good for the mid range, so the paste also restored their technical function.
The repairs I did on the membrane are quite visible, but those holes needed to be closed. The repairs on the dust caps darkened the color noticeably, so that’s a cosmetic trade off.
The old tweeters are kept safe, but the dust caps are totally destroyed, as in ripped to shreds.
I’ve considered whether to just remove the residue, or remove it and glue in a patch made from the dome of a dead tweeter. For the time being I think I’ll just leave them.

 

[Jens Sinding]

Being a ported design, the batting they used in the back is all you need. If it needs replacing just try and find some batting the same thickness. I don't think added bracing would hurt anything. It may keep the cabinet from any ringing. I have a dented dust cap on one of my woofers. A vacume cleaner didn't work. I may have to try something really sticky. That tweeter dust cap is pretty bad. If the tweeter still works, the bad dust cap shouldn't effect the sound though.

The batting is fine, and I don’t intend to add any dampening, but only to dampen any resonance from the cabinet walls. The added mass, and “soft” nature, of the 0.7 mm bitumen anti resonance plates/mats should lower the resonance frequency and reduce the amplitude.
I will listen to them as is for now