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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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