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amp advice for the 104ab

 
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iggy
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Joined: 06 Dec 2013
Posts: 5

PostPosted: Sun Dec 15, 2013 1:05 pm    Post subject: amp advice for the 104ab Reply with quote

a little back-story, important re costs.
this year has been somewhat hard, i have recently quit my job to be closer to family and receive psychiatric care for years of holding past trauma in to myself.

sorry, i feel i have to get that out of the way, apparently its also important healing-wise.

ANYWAY! with that in mind its important to note that if it might be out of my price range, then sadly it is. id love to buy all sorts of fancy things, but right now i cant Smile

on to the speakers. they are older than i am, passed down from my father and when my arcam 7r took a fit one day and to my utter horror managed somehow to blow out one of the 104s drivers, my first reaction was 'dad is going to kill me'. i didn't have the courage to speak to him about this until my eventual collapse some 2 months later.

good news. i managed to procure a matched pair of sp1039s, and at the same time a pair of 104ab crossovers, which will also be upgraded with new components, likely from one of the wonderful schematics to be found on this board. luckily although i currently do not remember much about electronics, i am sober now for the first time in 15 years and my brain is in overdrive. ive been soaking up information on relevant subjects like there is no tomorrow.

to that end, and realising that a fixed arcam 7r is going to be no match whatsoever for these speakers once they are unleashed in their upgraded form, i am faced with 2 options.

1. arcam 8ps are going for silly low prices on ebay. i am wondering if it possible to run 2 of them as mono-blocks with the repaired 7r as a pre-amp. the only info i have found is regarding bi-wireable speakers, as i'm sure you're all aware, this does not apply to the 104. is it possible to run the left and right outputs in some sort of bridged mode with the 7rs stereo output supplied as mono to the l/r inputs of both the 8ps?

2. screw it, full on project mode, its more expensive, but its WAY WAY more fun. a pair of quad 405 clones. i have seen several interesting designs, not least of which is an interesting design from www.vicol-audio.ro i feel more comfortable buying one of their pcb designs rather than these ebay designs from china. i have heard horror stories. vicols documentation is very well suited to my own knowledge, ie theirs is good and i have little Very Happy

i believe its fully possible to make a pair of suitable amps whilst still keeping under budget through judicious use of ebay and my own uncanny sober 6th sense of finding cheap donor parts for the more expensive bits.
HOWEVER i am aware that the most expensive part would likely be an oscilloscope, used for checking the circuits being built for faults.

as it stands, and having spent the last 3 days researching, there is currently very little available in an affordable price range.
my options here are as follows;

1. use the PCs soundcard, with some adequate over-voltage protection in between.

2. one of these cheap n cheerful usb oscilloscopes that i have seen dotted about on ebay and amazon like 1 of the following
[URL="http://www.amazon.co.uk/Hantek-Digital-Storage-Oscilloscope-Portable/dp/B00EDFQ3EU/ref=sr_1_11?ie=UTF8&qid=1387108073&sr=8-11&keywords=oscilloscope"]this one[/URL] or [URL="http://www.amazon.co.uk/SainSmart-DDS-140-Virtual-Oscilloscope-Black/dp/B00FXH8HNM/ref=sr_1_10?ie=UTF8&qid=1387108073&sr=8-10&keywords=oscilloscope"]this one[/URL]

bear in mind at the moment i don't need it to do anything too fancy. i just need to be sure that my electronic work isn't going to blow anything up Very Happy

be assured that my electronics knowledge will, by the time i get around to building my own amps, be up to at least better than average. my brain has been asleep for a very long time and it is hungry

of course, option 1 is the cheaper, i believe if i can use the 8ps as mono-blocks, a pair is going to set me back the princely sum of around 100 pounds, with just a little luck. (please dont go searching ebay, i hasnt enough moneys to outbid you :p)

i reckon if i pull the most expensive bits (transformers etc) and root around for other donor parts, then i can keep the 2nd option project to around the 250/300 mark. bear in mind this is the absolute upper limits of my monetary capabilities atm.

my mother bought my father these speakers as an anniversary gift. i grew up with them singing music to me, and listening to the howl of the f1 motors on a sunday through them, she is sadly no longer with us, and it is really important to me to get these speakers singing again, for myself and my father, in the best possible manner, as befits these amazing sounding animals. to this day i havent heard anything that beats them for less than a good few grand.
obviously i am leaning more toward option 2. i love the arcam sound but i have also heard that quads method is particularly well suited to getting the best out of the 104s, and i love nothing better than a good project.

thanks for your help, apologies if this is in the wrong forum, and merry christmas, everyone.

stewart.


more thoughts; it would be worth getting the 8p's if i can do what i want to do with them anyway, then the speakers can be tested, upgraded and tested again, and once the diy units are complete, sold on. ill probably make some/most of the money back.
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audiolabtower
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Joined: 06 Jan 2009
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PostPosted: Tue Dec 17, 2013 12:43 pm    Post subject: Reply with quote

I used to build my amps, nowadays I don't think it is worth it if cost is your no 1 priority compared to say the personal satisfaction. By the time you have a good mains transformer and power supply caps, and particularly a good looking case drilled etc you have spent more than a good amp off eprey. I used to have oscilloscopes at work but you don't really need one, checking dc offsets and running on an old speaker to test first is just as good IMHO if using a good design. Nowadays your pc is a signal generator if needed.

I think your biggest DIY upgrade would be to bi-wire the 104aBs and either run the tweeters from the integrated amp (probably enough) and use a power amp for the 1039s, or just use the integrated as a preamp with 2 x 8ps wired one for left channel woofer/tweeter and one for right. (I can't remember but don't think the 7r had pre/power links to remove).

So the DIY would be to remove the Kef plastic speaker terminals and fashion something with 4 terminals, cut the crossover tracks to wire tweeter crossover to one set and woofer crossover to the other.

The bi-amping gives a subjective impression of a much bigger, smoother power amp, since the cymbal crashes are now not on top of the bass drum thumps in terms of the current supply, and works best IMHO on BBC style low colouration, pin point image speakers like the 104aBs. Just another idea, but one with a lot of bang for the buck I believe since the 104aBs are very inefficient and need power to sing.
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iggy
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Joined: 06 Dec 2013
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PostPosted: Tue Dec 17, 2013 7:15 pm    Post subject: Reply with quote

i always liked playing with lego as a kid Wink

building my own amps, well, i have a fair bit of time to spare before im ready to get back to work, and i just love building my own stuff Smile

the 7r is dead at the moment as well, and im making do with a little set of nakamichi satellite speakers. theyre not too bad, but ive decided theyll do while i get this done right.

the speakers are going to get stripped down and redressed in a nice deep laquered piano black for a start, with the gaps in the ... bitumen sheet? filled in with some floor resin.
ill thin some down and treat the forward facing exposed chipboard with some thinned resin as well, get a nice seal on that stuff, i think thats definitely a good idea.

Quote:
Kef plastic speaker terminals and fashion something with 4 terminals
brilliant idea, a nice bit of aluminium machined to fit the hole would likely do the trick Very Happy

Quote:
cut the crossover tracks to wire tweeter crossover to one set and woofer crossover to the other


purty much like this then vs

the original pulled from the ab mkII thread.

i think ive settled on making my own amps. i can do 1 at a time and run the first bi-amped with the 7r, then once the 2nd is complete, retire the 7r to pre-amp duties. ive been pricing the bits and bobs i need and i can do this under budget, especially if a dedicated scope wont be required :0

building my own is going to be so much more fun and educational than just buying a couple of amps off'n ebay.

i might even rip the 7r out of its shell and make it look a bit more up to date while im at it. i can get nice thick bits of ally pretty cheap from a family member, so, sorted Smile

thanks very much for your help, i was on a bit of a downturn today and your post got my mind going again and just picked me right up. merry christmas to all Smile ive no doubt ill be picking brains again before long

edit: turns out its the tweeter that likes more power out of the 2, 100w vs 80w for the drivers Surprised
Now im going to start playing with this spice thingy and see whats what. time to learn electronics Very Happy
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speakerguru
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Joined: 18 Nov 2005
Posts: 954
Location: Green Hut, Tovil

PostPosted: Wed Dec 18, 2013 9:53 am    Post subject: Reply with quote

iggy wrote:

edit: turns out its the tweeter that likes more power out of the 2, 100w vs 80w for the drivers Surprised

When we designed the KM1, we had eight Quad 405 boards to apportion. It ended up as one for each of the 4 woofer and 2 mids and the last 2 in bridge mode for the tweeter. Not that the tweeter needed or could handle the available long term power from 2 x 405 in bridge mode, but the available voltage swing needed to prevent clipping was much more than you could imagine.
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iggy
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Joined: 06 Dec 2013
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PostPosted: Wed Dec 18, 2013 7:47 pm    Post subject: Reply with quote

it took me a few minutes to work out if your words said good or bad things about this idea Very Happy

Had to look up voltage swing, and am learning lots of new things every day, havent really used the grey matter fora wee while Very Happy

also 'we'! the 104s are older than i am, and, now approaching 40 years of age(!!!!), were a gift from my father. theyre getting some tlc one way or another.

any advice regarding splitting the crossover? is this a good idea or should i just leave it? currently im assuming theres a wattage ratio devider that should be observed at least. need to do me sums. i was also thinking about just upscaling the circuits to provide the full 100/80 but thats 360watts all in which just seems insane.
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audiolabtower
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PostPosted: Wed Dec 18, 2013 10:11 pm    Post subject: Reply with quote

iggy wrote:
im assuming theres a wattage ratio devider that should be observed at least.


The crossover is a parallel circuit, ie the woofer section and tweeter section are both connected to the speaker input. Thus you can separate the tweeter section by isolating one end of an inductor from woofer negative in, and one end of a capacitor (actually 2 caps connected together in the aB circuit) from woofer positive in and run these to a separate amp through another set of speaker leads.
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