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Old 6th August 2013, 05:29 AM   #1
PB2 is offline PB2  United States
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Default NHT 3.3 Recap and Clone

I was contacted by PM by a fellow who is cloning the NHT3.3. He purchased drivers and a crossover from 1995 and he asks if he should recap the old electrolytics. He also asks if he should compensate for ESR if he uses film caps in place of the NPEs.
My opinion is that yes you should certainly recap the NPEs in the system. I will go though the crossover schematic and suggest what I would do. Often if there is resistance in series with the cap a film can directly replace an NPE without compensating for ESR. Here is a link to the crossover schematic shown toward the bottom of the page:
Biamping the NHT M3.3

You might also find this link to SY's mods to this system of interest:
NHT 3.3 Modifications

The most straightforward thing to do is simply to replace all the electrolytic caps with new electrolytics. I would use Bennics or ERSE, since I've found them to age well and they have low dissipation factor (another way of expressing ESR):
Bennic Capacitors
Note that values below 150 uF have a DF of 5% and those above are 10%.

These ERSE caps are 5% tolerance and are probably a better choice:
NPE, 12uf
I've never seen mention as to which of these are already film, if they are then
there is no need to replace them:
C1 200uF use a pair of 100uF in parallel for lower DF, use .5R in series if you go to film.
C2 92.5uF use 40+31+22 NPEs for 93uF total, use .25R in series if you go to film.
C3 88uF use 65+22 NPEs for 87 total, use .25R in series if you go to film.
C4 71 uF use 31+40 NPEs, use .25R in series if you go to film or increase R11.
C5 11.8 uF use a 12uF NPE, or 12uF film ESR not critical due to R1
C6 31 uF use 31 uF NPE, use .25R in series if you go to film.
C7 3.3 uF use 3.3 film ESR not critical due to R3
C8 6uF use a 6 uF film ESR not critical but if tweeter too hot increase R9 to .47R or more
C9 11.8 uF use a 12uF film ESR not critical due to R6 and R7
C10 .33uF already film? No need to replace.

Please let us know how your clones sound.

I've been considering a clone myself for many, many years now and I have the woofers
and tweeters. Anyone know of a source for the SEAS MP12VC H453 upper mid? Jack
Hidley is sold out:


I had been planning to buy 2 or 4 but never got around to it. Anyone know who bought
up Hidley's remaining stock?
Pete Basel

Last edited by PB2; 6th August 2013 at 05:59 PM.
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Old 23rd January 2015, 01:44 PM   #2
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Default COPYING NHT 3.3

Thanks Pete. I'm finally done with the 3.3. I did replace all the electrolytic capacitors since the crossover was dated 1995. I took a reading of these '' old caps '' and I was quite surprised that they were still within 5 to 10% their nominal values so that I could not really hear a difference in sound... (maybe it's only me ). Then for the sake of clarifying doubts on how '' better film caps '' would make an improvement, I swapped them for Clarity caps ( ESA ). To me, Ken Kantor, Jack Hidley (and others forumers ) were so right !! Even though so called high end film caps, I believe it's a waste of money and time. Then, I decided I would push the experience further. I copied the original crossovers from the scratch. I used my Clarity caps, had all the steel laminated core customized to their original values ( so that I would get the same DCR ). Same for resistors and electrolytic caps. To summarize, in all three cases, I found no difference in sound quality of the speakers. ( again maybe it's only me...) They are now hooked up as front speakers in my HT with 2.9 ( also copied ) as rear and AC-2 as center. These speakers really sing, the 3.3 being favorite though. Special thanks to Ken Kantor, Jack Hidley and Peter ( PB2 ) for their precious help. P.S. : I'm not an engeneer nor a technician, only a passionate. I've only copied these great speakers. So, be indulgent if I can't answer questions that are too technical.
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Old 23rd January 2015, 05:41 PM   #3
Pallas is offline Pallas  Pakistan
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FYI there's someone on eBay selling reproduction foam strips for these speakers. No relation, no experience with the product. Just mentioning because yours don't have them.
Serious Audio Blog
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Old 23rd January 2015, 08:19 PM   #4
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Thanks for the info but I already have a sheet of that open cell foam bought at part express( if my memory serves me well ) about 2 years ago on NHT support recommendation. I just went on their site and I no longer see that product. Not sure if I'm gonna put them on. I read that you don't get that much perceptible difference in sound...
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Old Yesterday, 04:21 PM   #5
PB2 is offline PB2  United States
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I agree with you about the film caps, the only reason why I mentioned them is that most people want to use them. I do try to use low cost films just so that I don't have to recap them in another 20 years.
I still believe that it is good that you replaced those electrolytics, what if one became leaky in 3 or 5 years and you lost a midrange or tweeter?
Look at this PSB Stratus Gold crossover with charred PC traces, no drivers were blown and I think that the caps that failed were in a shunt path so they just stressed the amplifier and not the drivers:
PSB Stratus gold X-over help
Click the image to open in full size.
Probably about the same age as your crossovers.

I know the Stratus Gold well and those caps even had a resistor in series that would have partially limited the peak current - still obviously, they failed.

I've seen reports of caps in SPICA TC-50s that exploded with capacitor ribbons inside the box.

I see Ken over on AK telling people _NOT_ to recap, he just likes to be a contrarian and it is very bad advise in my opinion.

Nice work by the way! VERY NICE!

I'd enjoy hearing more about your construction project, or anything more about them.
How did you stuff them?
Pete Basel

Last edited by PB2; Yesterday at 04:40 PM.
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