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Guitar Amps...

Posted 15th December 2010 at 04:38 PM by chris661

Now then (does that make sense?).

Today, I played a small gig at a college (6th form) talent show. Maybe 100 people or so in the audience.

So, some background... I bought an old, faulty guitar amp off ebay. A resistor (1/2 watt one) had burned, as well as one of the output stage transistors. This had got hot enough to short one of the rails to the speaker output. This killed off the vintage Celestion driver in there. Lovely. Next up, the power supply was under-done. The mains transformer had been replaced before I'd got it (odd screws used to hold it in), and the old 2,200uF caps failed to meet half that value. The rectifier was a small chip, stood on the circuit board.

So, using a spare Eminence driver (modified Alpha 10 - felt dustcap, looser suspension), I fired it up, hoping there was some life in it. Life, there was. Lots of volts DC on the output, with a 100Hz buzz on top of that. Needless to say, I switched it off again.
Replaced...
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D1080 MkII 08 actives - crossover schematic

Posted 15th December 2010 at 02:28 AM by abraxalito
Updated 5th March 2012 at 02:54 AM by abraxalito

Here's a somewhat simplified schematic I entered to model the response curves in LTSpice. The volume control isn't shown (as LTSpice doesn't have the symbol) but happens before the first opamp. I've fudged up symbols for the bass and treble controls - they're simply variable resistance in the HF and LF feeds to the power amp board. LT1057 isn't actually used in this unit, its just the nearest library part to the TL084

<edit> I've added the response curves now - look and weep
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D1080 MkII 08 actives - under the hood

Posted 14th December 2010 at 11:46 AM by abraxalito
Updated 5th March 2012 at 02:55 AM by abraxalito

I've been spending my free time these past couple of days reverse engineering these speakers, as a prelude to devising the mods. The first stage in doing that is to photograph the PCBs both sides and have both images up on the screen at once, the underside is flipped horizontally so its as though I'm looking through the board. I then annotate the components by hand with the mouse drawing tool. Sometimes I'll write the values in this way too, other times just paste text for the value next to the component. Most of the time though I'm too lazy to add any annotation.

Here's the result for the volume/tone control PCB - one of two PCBs in the design. There are two quad opamps, TL084s - they're marked up in magenta. Interestingly there's no tone control circuitry as such, the bass and treble controls just act as volume to the woofer and tweeter respectively! I've never seen tone controls implemented in this way before...
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From W Mclean

Posted 12th December 2010 at 02:37 AM by wmclean

I have an old pair of MB Quartz 850S speakers that i gave to my daughter. One of the speakers has a blown tweeter. Where can i buy a replacement tweeter ? I believe there 1" titanium dome tweeters. Thank you for any help in advance.
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Old

How low can you go in active speakers?

Posted 11th December 2010 at 11:54 PM by abraxalito
Updated 5th March 2012 at 02:57 AM by abraxalito

On one of my regular pilgrimages to the computer plaza a couple of days ago, I noticed these little puppies. The price apparently was a shade over $80 the pair so I was indeed very curious. A quick peek around the back showed they were not using a standard speaker cable, rather a custom 4-way umbilical between the two boxes. Whenever I see this, my heart misses a beat because what floats my boat is genuine active speakers, not just powered ones which still sport passive crossovers.

Click the image to open in full size.

Not wishing to ask to disassemble the powered unit on the spot (and doubtless be turned down) I went back home to do some online research. I was pleased to discover these were indeed true actives, and what's more they're powered by TDA8947 chip amps. I'd not come across this part before but its cool at least on paper because its 4 channels in one package - they're using two...
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Old

Subwoofers...

Posted 11th December 2010 at 07:53 PM by chris661

Well, here we are again (unless, of course, you're here by mistake).

After much contemplation, I've decided that my subwoofer needs upgrading. Currently, it's a pair of MCM DVC woofers (you will probably have seen me going on and on about them), which, while good for the price, aren't that good compared to my Fostexes (which, now fully broken in, are fantastic). They're given ~100w between them, with a Linkwitz Transform to get them down low. Good for normal listening levels, but quickly run out of displacement below 40Hz at anything about normal listening levels.

So, a new bass system is in order. I've always wanted to try a Tapped Horn, so I've chosen a design originally intended for an Exodus 6.5" midbass, that's shown to work pretty well with the Tang Band 6.5" subwoofer... All the way down to 25Hz. Those will be fed by an amp1-b from 41Hz, with a +/-34V supply. Should be more than 100w per side. Ca suffice, especially for a bedroom system ...
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Old

First test post to blog - general audio gear

Posted 11th December 2010 at 01:48 AM by googlyone

Couple of projects of mine - not sure if the photos will load OK.

First is a pair of 25Hz horn subs I used to have - used 18inch p.audio drivers, placed facing one another along a wall to form part of the horn and achieve adequate mouth area for the 25Hz cutoff.

Efficiency was well over 100dB/w/m.

Second photo is one of my digital crossovers I built, 4 way AD1940 based audio DSP with an AD1939 CODEC as A/D and D/A. Works a treat, and makes setting speakers up a snip, as can program in delays, HPF / LPF, parametric etc.

Also included my subwoofer couch - I built this as a challenge from a colleague of mine. The carcass of the couch forms an 800 litre enclosure, the ports exit at the back of the headrest three of them, and I used four fifteen inch drivers. It is tuned to 19Hz, and achieves a -3dB point below 20Hz.
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Old

krell

Posted 6th December 2010 at 03:10 AM by ihaveKRELLparts

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Old

Software

Posted 29th November 2010 at 07:36 PM by klewis
Updated 5th January 2011 at 10:25 PM by klewis

Pcb software

Sprint-Layout

Audio analysis software;

Virtins MI http://www.virtins.com/
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Old

Opamps for dc servo

Posted 29th November 2010 at 02:57 PM by klewis

For servo duty, there are some newer TI offerings which look worthy. That is assuming that lowish noise and bias currents are desirable (they would be to me if I were seeking out a servo amp). OPAX141, OPAX376 and OPAX209. The latter is bipolar but has lower noise than any JFET I've seen coupled with commendably low input bias current for a bipolar.
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