Re-Jigging the jig: Speaker testing device for Arta, Speaker Workshop & HOLMImpulse - Page 2 - diyAudio
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Old 23rd February 2008, 05:05 PM   #11
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Well, the OPA548 amp module is up and running, (is it small enough I wonder? ) now I just have to wire up the voltage dividers and switching.
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Old 24th February 2008, 01:14 AM   #12
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Quote:
Originally posted by daatkins
Haven't we all been busy!

The attached pic is my take on the all-in-one Speaker Workshop jig. It contains an L165V chip amp, my own design mic pre-amp and all the signal switching of the Wallin jig but compressed onto two rotary switches. I've also added a spectrum analyser function for measuring the frequency response of active crossovers and the like.
David.

Hi David,
Very Nice! The rotary switches are the best choice or extra functionality. I should have considered it (next time ).

The spectrum analyzer is PC based?


Here is a shot of the guts of mine from behind. The amp, preamp and jig are all on separate boards, connected together via terminal blocks. Makes for a bit of a snakes nest, but gives more flexibility in squeezing everything in.
The LM3886 amp is attached to the bottom aluminum plate, which serves as the heatsink.
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Old 24th February 2008, 01:21 PM   #13
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Ok, time to take it apart again and rewire it properly so the sense resistor actually connects into the speaker line...
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Old 24th February 2008, 01:44 PM   #14
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Quote:
Originally posted by gootee


This all sounds good.

One thought that just occurred to me: A higher chipamp gain than 11 V/V might give better performance. If so, you could just put a resistive divider before the input, and keep the overall gain wherever you want it.

Hi Tom,
I don't think there's a drop in performance with the gain at 11. The minimum (for stability) is 10. I did some squarewave testing on the completed amp (with a 4 ohm coax speaker as load) and it runs beautifully.

Here's an up close shot of the amp board. notice how the leads from the LM3886 are surface mount on the top and bottom of the board.
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Old 24th February 2008, 02:12 PM   #15
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Quote:
Originally posted by pinkmouse
Well, the OPA548 amp module is up and running, (is it small enough I wonder? ) now I just have to wire up the voltage dividers and switching.

Quote:
Originally posted by pinkmouse
Ok, time to take it apart again and rewire it properly so the sense resistor actually connects into the speaker line...
Hi Al,
Any specific reason why you used the OPA548? I haven't looked closely at the specs, but it's about $11 from Digikey here, almost twice the cost of the LM3886. I had some spares (with short leads - cut off of my active amp boards), so no cost for me really. How's the performance of the OPA stack up to the LM? Or is it more like my situation - as in "this is what I have"

I'm impressed with the LM3886 though, not only sound quality wise: I etched the board, looked it over then populated it. Looked it over again and fired it up on the +/-18 V supply. It didn't sound right, too low and distorted. It turns out there was a tiny copper bridge from the output to the negative rail.
This abuse , coupled with the previous abuse I heaped upon it in the active amp board fiasco, tells me these are tough little customers. With that said, I doubt it would have survived a higher rail voltage, say +/-35. I need to break out the old magnifying glass. .

A full frontal view, nude. I wanted to make it so that everything was attached to the sub chassis, and the cover would slide on over it all. Complicates construction a bit, but makes it easier to put together and work on in the future. There's an abundance of recycled parts, gleaned from old receivers and other low dollar "finds".
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Old 24th February 2008, 02:41 PM   #16
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Yup, the LMs are pretty robust. I used the OPA a lot when I was in full on gaincloning mode and liked it, though I have plenty of the other usual suspects, but in multiples of two so thought they were worth saving for stereo applications.
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Old 24th February 2008, 02:42 PM   #17
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And the setup in action:
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Old 24th February 2008, 03:19 PM   #18
anatech is offline anatech  Canada
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Hi John, Al,
I am wondering about one thing. Why make the chassis so small? It makes the pickup of noise from the supply that much more likely. It's also more difficult to add to or service.

I understand that one uses what one has on hand, but wouldn't a case large enough to slide under another item be easier to deal with? Also, it wouldn't tend to skid around when connecting to it.

A nice, flat toroid (can you believe I'm recommending s toroid???) may allow a larger, flatter case. One you could place your laptop on top of and even provide for extra cooling for the laptop. Then you have just saved on bench space on top of this.

Just an option for future.

-Chris
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Old 24th February 2008, 04:46 PM   #19
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Quote:
Originally posted by pinkmouse
And the setup in action:
What sound card are you using? I had an old Thinkpad 780ed that I tried before with an Audigy PCMCIA card, but it wouldn't work - processor wasn't fast enough (PII, 266MHz). I gave up on that and switched to a desktop, but I'm having issues with that too (see below)


Quote:
Originally posted by anatech
Hi John, Al,
I am wondering about one thing. Why make the chassis so small? It makes the pickup of noise from the supply that much more likely. It's also more difficult to add to or service.

Hi Chris,
I considered a wider case, but I thought a smaller, narrower one would be neater and more mobile. This would need to be moved every once in a while. Another problem with putting the unit under something else is cable access - if you need to change something, it needs to be moved. This is unless you put everything in the front panel, or use spliced cables for quick removal.
To do it again, I'd make it even smaller - narrower but a little deeper.

As for noise, I used a shielded transformer, grounded to the case. All of the small signal runs are shielded cable. This whole unit, with the mic preamp inside, is ultra quiet, certainly more than quiet enough for the purpose.

I'm now having computer problems. The computer I have been using is an 8 year old P3 1Ghz. Integrated audio has one functional channel, and the Soundblaster card that I originally put in there 6 years ago is burned out (I did it - overloaded it using RMAA to test an amp).
Now the replacement soundcard, Audigy 2 SE, was working ok for a while but with some glitches. Random restarts, BSOD, freeze ups. A driver issue, most likely. I bring the computer up here for internet access (none in my "lab") to find some drivers.
I discover the chipset, Via VT82C686A has an audio problem, that was never resolved by Via. In the installation troubleshooter for the new Audigy, it specifically singles out this chipset.

Long story short, I'm looking for another socket 370 motherboard. I see several on Ebay, a particularly nice on is the Intel D815EEA.

My unit is fully functonal and works like a charm. I had it up here connected to my real computer and everything looks good.
Here's my (not working) lab setup:
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Old 24th February 2008, 04:59 PM   #20
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Quote:
Originally posted by MJL21193


I'm looking for another socket 370 motherboard. I see several on Ebay, a particularly nice on is the Intel D815EEA.
Hi John,

I hope you are aware that a motherboard with Intel 815 chipset can only address up to 512MB memory. It may not be an issue for you but maybe worth mentioning.
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