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This is just an area I can blab on about stuff without it using up bandwidth in the threads Hopefully someone will find it interesting or useful.

Tony.
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The problem with wanting to get the best performance

Posted 14th November 2009 at 09:28 AM by wintermute
Updated 25th November 2009 at 11:15 AM by wintermute (added o category misc)

Well for me anyway I think I must be borderline obsessive compulsive! It happens every time I start a project (or even before I start it). I spend hours and hours and hours deliberating on whether this would be better or that would be better. Honestly in some ways I wish I'd never become "aware" of alternatives to the kits that were available from the local electronics shops Dick Smith and Jaycar

I got the jfets for my B1 a few weeks ago. I finally started matching them earlier this week, I hadn't started because I was obsessing over the fact I needed a 100 ohm precision resistor and a transistor socket. I hadn't got those because I wanted to order all the other bits as well and only pay for one lot of shipping. But do you think I could make a decision on what parts to get?

First off I obsessed over the resistors (yeah I know).. wanted vishay RN60D's but local sources are 5 - 10 times the price of mouser, problem with mouser is unless I buy $200 AU worth of stuff they slug me $49 AU for shipping... decided in the end to get some ordinary metal films from RS components... then I had to decide what to do for the PS... I want to build a cap less (on input and output) B1, because it is looking like I'll have 6 of them (and 12 10uf Polypropylene caps will be a tad on the expensive side)... so powersuply is necessary.. LM337/LM317 (oh too noisy) Shunt? apparently I'll need one for every B1..... and it all looks too complicated....

Finally sense prevails (I think) afterall Nelson suggests a wall wart!!! and I decide I'll just build a simple LM317/337 based regulated supply (probably with Fred Dieckmann's suggested mod) and put some big caps on like Nelson did with the original B1 (except one for each rail). So I go looking for caps.. Now I can't decide whether to go with 3300uF Nichicon FG's or 10,000uF Nichicon KG's... arrggghhhh... I'm eyeing off percy audio's range (will probably get some for my PS for my playmaster too, they need to be 80V) then I stumble across PartsConnexion, and that they have them too (at significantly less cost)... only problem is they don't have the KG's yet....

after all this deliberating I decide I better draw up a block diagram to work out how many B1's I'll actually be building, and I come up with potentially 7!! I'm thinking I need 14 caps for the PS smoothing (two for each B1)... then I realise that if I have separate power-supply runs for each B1 (assuming I put them on separate boards) I'm probably going to have a massive earth loop problem if I have both signal and ground connecting each B1 to the next (with filter in between)... So I start thinking, can I just get away with one big board with common power for all B1's in the circuit and one set of caps... that sounds like a compromise and we couldn't have that... maybe I could just use the hot signal between the boards... but then the signal return currents are going back down the power supply route.. arggghhhh...

Sometimes I wonder how I ever get anything started, little own finished!!

So what is the problem with wanting to get the best performance? I think it should be obvious if you have made it this far There is a real risk you will never actually get anything at all because you will die trying to decide what will give you that performance

I was going to say I hope someone can relate to this, but that would be cruel, hopefully you can't and can just have a laugh about what a hopless case I am

Oh and by the way, in the end I just got some alligator clips and clipped them onto my multimeter leads and the legs of the jfets... worked a treat. I had a 64.9 ohm 0.1% resistor, and I measured both current directly and voltage drop over the resistor, and found that I got exactly the same result for the current. It seems my trusty old Metex meter is more accurate than I thought (and equally so on current or voltage).. It gave me a reading of 64.9 Ohms for the resistor too!

I've managed to grade roughly half of the 100 jfets so far.. at the moment just into separate bags for ranges of .5ma. Hopefully I won't decide I need 12 jfets all matched within .1ma


Tony.
Posted in misc
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  1. Old Comment
    Greg Erskine's Avatar
    Hahaha Tony,

    We must have been twins separated at birth. This story is nearly identical to mine except I haven't done my matching yet. I'll have to send my 150 fets over to you to match for me.

    I have recently setup a microprocessor to match my LDRs. It can collect 100 times the data for me to ponder over endlessly. I have even written a perl script to compare the data...but I haven't built anything.

    Oh, I have half a PCB design for this buffer.

    Greg
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    Posted 3rd July 2010 at 10:34 PM by Greg Erskine Greg Erskine is offline
  2. Old Comment
    wintermute's Avatar
    hehehe Greg, when I saw your comment on the baby Aksa thread, I figured you would relate to this 100 times the data and perl scripts to analyse!! I shudder to think!! It's taken me a couple of months of fiddling with the crossover sim in speakerworkshop to come to a "final" decision as to the MK II version of my crossover, and I only had five measurements!!!! on axis response for woofer/tweeter impeadance for each, and on axis for the existing crossover.

    I have actually progressed a little, I actually bought all the parts, except for the opamps for my crossover. but I still have a few parts to get for the PS, and have to work out the layouts (that is what has really pulled me up, I just have to stop procrastinating and do it.

    I ended up getting prp resistors (not cheap at 35c each, but won't break the bank either). I still have to work out the layout though for both the PS and the synergy. Still probably going to do it on verro board (if it isn't too hard) and I decided on doing everything on the one board (except the power supply).

    Oh and I think I have finalised the parts for the passive crossover too, so time to get cracking on working out what bits I want to get for the B Aksa cause I'll get them at the same time...

    Not sure if you are interested in a "passive active" crossover but if my synergy goes well and you are interested in it (since you have half the required stuff done already for the B1 Side of it you might want to have a crack at it assuming you would build something with opamps in it of course!!). PCB design is not something I've ventured into yet!!

    Tony.
    permalink
    Posted 4th July 2010 at 07:11 AM by wintermute wintermute is offline
 
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