DIY Transistor Amp Using NVA Boards

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Cressy Snr
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#286 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Finally did something with the little Mission speakers that I have been meaning to do since I got them.
I replaced the cheapo 4.7uF electrolytic tweeter cap with a Jantzen Standard blue metallised poly cap of the same value. Another veil lifted and all that jazz. Didn’t realise the ring tweeter fitted to the mkII version (Mission 731i) was quite so fast and smooth until I fitted the Jantzen caps to the crossover. These thirty year old Missions are excellent little budget speakers.
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Niiiice.
“Idiots make it into every company. They tend to interview well.”

Bonnie Garmus (2022) Lessons in Chemistry.
Cressy Snr
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#287 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

Cabs might look like the usual budget fare with fancy plastic front, disguising chipboard baffle:
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But inside it’s a different story:
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Bass/mid driver is mounted on four posts connected to rear ABS plastic tray, which means that the driver is somewhat decoupled from the chipboard carcass. The tweeter is mounted on the ABS front baffle, so that it does not share any mountings with the bass driver, so decoupling it also.
I would have found difficulty in working out how to get it apart, had not an ebay seller been selling a pair of the earlier 731 cabs without drivers.

Interesting (well I think so.)
“Idiots make it into every company. They tend to interview well.”

Bonnie Garmus (2022) Lessons in Chemistry.
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IslandPink
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#288 Re: DIY Transistor Amp Using NVA Boards

Post by IslandPink »

It is interesting - haven't seen anything like that before. I assume there's some sort of gasket all the way round ?
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Cressy Snr
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#289 Re: DIY Transistor Amp Using NVA Boards

Post by Cressy Snr »

IslandPink wrote: Sat Jun 11, 2022 2:03 pm It is interesting - haven't seen anything like that before. I assume there's some sort of gasket all the way round ?
Yep, there’s a rubber gasket around the baffle plate edge, then another neoprene gasket to seal the woofer against the rear of the baffle.
“Idiots make it into every company. They tend to interview well.”

Bonnie Garmus (2022) Lessons in Chemistry.
Ant
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#290 Re: DIY Transistor Amp Using NVA Boards

Post by Ant »

Got the a20 to work with the fanes, modded the p20 with a shunted tocos pot and a potential divider on the output of of to pull it right down from the 2v of a cd player to about 400mv.

It is now useable, pulls about 15w out of the wall instead of 250 ish w that the f5 pulls out of the wall so will cost less to run it.

I prefer the f5/b1, but the a20/p20 is perfectly adequete and perfectly listenable for a little amp now its had the feedback level altered.

Doing my bit for the electric bill
Also starring Rex Hamilton as Abraham Lincoln

www.bte-designs.weebly.com
Ant
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Posts: 1959
Joined: Mon Jul 31, 2017 6:45 pm
Location: Yorkshire

#291 Re: DIY Transistor Amp Using NVA Boards

Post by Ant »

Just seen that you can buy a potential divider shoved in a phono plug to do the same job from rothwell audio
About 30 quid or so for a plug in solution
Also starring Rex Hamilton as Abraham Lincoln

www.bte-designs.weebly.com
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