A Tale of Three LM1875 amps - Page 2 - diyAudio
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Old 27th February 2008, 10:16 PM   #11
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Quote:
Originally posted by Zero One
I am not sure why the larger voltage tranny sounds so much better, however the amp if run for full range applications does need a supply that doesn't sag under load, this is independent of the voltage, so my question is was the 18v one substantial enough for the job. There is not doubt that once the voltage sags the highs get muffled and the bass muddled.
Wow. WOW!! That's a beautiful speaker.

On the transformer? This is so weird! The champ power supply is a tiny little 36 va, 36vct (1 amp total 18+18VAC), Canadian made, with 4 diodes and 4 caps (4700uf per rail). The losers were. . . everything else, especially if larger.
I tried the larger supplies, 28vct 4A (muffled) and 36vct 4A (squeaky), with a simple supply, a smubberized supply, extra caps, bypass caps, and/or enough resistors to start looking like a christmas tree, and the el-cheapo mini supply still beat them on quality sound. ??????????

Okay, there was one larger goodie. A big 30 year old 55vct (28+28VAC) with the Brian GT power supply (using 4 MR860 diodes). Now that was cleaner. It was too hot. The dynamics were too fierce--Xmax all the time. One orchestra hit, and one had better have already been in the bathroom. Its kind of like the old Memorex commercial--if one could add some temporarily airborne cats. Perhaps if I owned 16 ohm speakers?

Well, you know my habit of heading, top speed, in whatever direction works, so I wound up with 36vct 1 amp monoblocks. They have sufficient headroom for dynamics at ear-botching volume levels. The problem (a nice one), is that it sounds so good that I'd like to turn it up more.
But, I do have to question why its working so well.

On 12v rails, have you tried B+P? I always loved the sound of bridged amps, but that would sound like 24v, so perhaps bridged + parallel?
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Old 27th February 2008, 10:21 PM   #12
BWRX is offline BWRX  United States
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This doesn't have to do with amplifiers, but I thought placing a speaker at the center of a circular baffle was the WORST case for diffraction loss - or does that not apply to OBs because the rear wave cancels out the front wave at the boundary? It's hard to see from your pictures but it appears that the rear of the speaker is inside some sort of enclosure. Is there a thread about these speakers in the full range or loudspeaker forums?
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Old 27th February 2008, 11:00 PM   #13
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Hi Brian! Isn't that disc a mechanical baffle step compensator?
Hi ZeroOne! Is that pipe 1/8th wave voight? EDIT: or is it a 1/4th wave?
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Old 27th February 2008, 11:16 PM   #14
BWRX is offline BWRX  United States
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No, it's just a baffle. The larger the baffle the lower the frequency at which the response drops off. You can compensate for the baffle step by using a shelving filter. See here for example: http://www.trueaudio.com/st_diff1.htm
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Old 27th February 2008, 11:44 PM   #15
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Hi BWRX

The speaker is mounted on a circle of ply, this is not the actual baffle, rather think of it as a mounting baffle which then goes inside a much larger baffle but without touching it, the real baffle is around 40 inches wide and and 36inches high but folded. the driver does not sit at the centre of this baffle at all, but is offset. The final baffles are under construction as we speak, in fact I am just about to sand the 2nd one for the gloss coats. Having the driver magnet mounted rather than baffle mounted makes for a whole different level of clarity, as the baffle is not coupled and therefore doesn't colour the sound. The little circular sub baffle does however add a nice bit of lower mid presence.

As an aside the design means I can swap out drivers for a different sound very quickly, any size up to 8 inches.

This really needs a whole separate thread but I am holding off till finished, in fact this is a complete system, here is a pic of most of the casework I am building before I sealed and sanded it.

In the end I will build a web page on the whole system but in the meantime a sneak peak.

Oh and my amps are built into the speaker cabs too.

Daniel I have not tried the bridged but I am definitely going to do this for the final system, it should give around 24watts.......plenty for big dynamics. I do expect it will sound the best of the lot but I have been tinkering a lot with trying to work out the perfect point to point layout and shielding arrangements. So soon.
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Old 28th February 2008, 12:14 AM   #16
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Quote:
Originally posted by Zero One
Daniel I have not tried the bridged but I am definitely going to do this for the final system, it should give around 24watts.......plenty for big dynamics. I do expect it will sound the best of the lot but I have been tinkering a lot with trying to work out the perfect point to point layout and shielding arrangements. So soon.
I'd like to mention three things.

First, I just love the idea of a bridge LM1875, because this would let me voice it to "center stage" instead of the soundfield ahead of the driver. More power, more soundfield. Oh yeah!! Please share.

And, second, please don't be disappointed if bridged runs optimal at a different voltage. It will probably like 15vdc or 16vdc, but that's just a guess. Maybe 3 smaller batteries per rail makes voltage options easier?

To heatsink inside a speaker enclosure, I usually zoom through the cabinet with a doorknob bit (big round hole), and then Liquid Nail + screw the heatsink onto the cabinet (either side is fine). That's a siliconized construction adhesive, which is also a thermal compound, so the amp runs ice cold. I like to put it up with the drivers, so people think its a mystery speaker. LOL!
Oh yeah! I almost forgot to mention. For non-isolated LM1875, its good to shoot any exposed part of the heatsink (not where the chip goes) with clear lacquer to prevent shocks.
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Old 28th February 2008, 12:14 AM   #17
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whoops...forgot to attach the pic of casework.
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Old 28th February 2008, 12:33 AM   #18
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Daniel
Here is a pic of the casework for 1 of the amps, this goes inside the speaker baffle. You will notice that there are 3 heatsinks, 2 for the bridged amp module and one for a purpose built tweeter amp, which will run different gain, input caps etc.

It will be painted gloss black to match the rear of the baffle.
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Old 28th February 2008, 03:39 AM   #19
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Quote:
Originally posted by Zero One
Daniel
Here is a pic of the casework for 1 of the amps, this goes inside the speaker baffle. You will notice that there are 3 heatsinks, 2 for the bridged amp module and one for a purpose built tweeter amp, which will run different gain, input caps etc.

It will be painted gloss black to match the rear of the baffle.
EDIT: Gloss black, with all that pretty woodwork, paint is almost a crime.

Ah, but what do the bicycle tires do? Is that a shop secret?
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Old 28th February 2008, 04:50 AM   #20
ttan98 is offline ttan98  Australia
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Quote:
Originally posted by Zero One
whoops...forgot to attach the pic of casework.
You are pretty handy with woodwork!
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