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Old 3rd January 2007, 01:06 PM   #31
CarlosT is offline CarlosT  United States
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The idea was to create a cabinet that resembled an old Fender Champ tweed guitar amp that kids could plug in their iPod into and play music. This was an old school look and sound with one driver, no tweeter or crossover, etc. Going stereo would complicate the cabinet build and dimensions and would take away from the "old school" look.

I initially bought the AudioSector LM3875 kit but this is a non-inverting design so supposedly not well suited to combining channels. The inverting design lends itself better to combining channels. If you look at the picture of the "test mule", you can see that I'm cramming everything into a 9" x 5" x 2" Hammond aluminum box so no room for buffer preamp or regulators or any other fancy schmancy stuff. As it is, I got an Avel Lindberg 90VA tranny, an AudioSector rectifier board and a TechDIY amp board and a heat sink all in the little box

I'll post pics soon of the completed project.
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Old 3rd January 2007, 07:50 PM   #32
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Hi Carlos,

what a grand idea. Maybe you could try an old table radio look as well and use the lit up dial for the volume control.

I am most impressed, hope this takes off well.
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Old 3rd January 2007, 10:36 PM   #33
CarlosT is offline CarlosT  United States
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We have already successfuly delivered 2 "TweedybirdOnes" for Xmas...kids love them so far
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Old 9th January 2007, 06:58 PM   #34
fosforo is offline fosforo  Italy
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Maybe it could be a stupid question but should it be fine to use 2 x 100VA - 18VAC instead of 1 200VA 18-0-18 Vac?
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Old 9th January 2007, 10:18 PM   #35
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Quote:
Originally posted by fosforo
Maybe it could be a stupid question but should it be fine to use 2 x 100VA - 18VAC instead of 1 200VA 18-0-18 Vac?
Yes...that should be fine. The larger transformer likely has
better regulation, but nothing to worry about.

Cheers,
Dennis
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Old 10th January 2007, 05:52 AM   #36
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20+20 vac transformer
= 40 vac * 1.414
= 56.56 vdc / 2 for center tapped psu output
= 28.28 vdc rectified per rail - 0.8v diode drop
= 27.5 vdc per rail

gainclones at peak output in normal configurations are like 50% effecient...
(find the graph in the data)

so 63w * 2 = 126w, plus headroom cuz reality is mean.

so like, a 200VA 20+20vac transformer would be perfect

(i just bought two 400VA toroids to run pairs of 3886 amps =)
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Old 10th January 2007, 12:05 PM   #37
fosforo is offline fosforo  Italy
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Quote:
Originally posted by Dennis Hui


Yes...that should be fine. The larger transformer likely has
better regulation, but nothing to worry about.

Cheers,
Dennis
Yes, sure, but if the total power is calculated by a sum of each power from each seconday then, a transformer with a 18-0-18 200VA isn't the same as two transformer 18VAC - 100 VA?
Obviously we are just cosidering a theoric power calculation
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Old 10th January 2007, 04:05 PM   #38
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if you parallel the 18+18vac 100va secondary coils on one transformer, you have a single 18vac 100va part. each 18vac half of the secondary coils is actually 50va.

do this to the secondaries of both 100va transformers, put the parallel coils of each transformer in series, creating a center tapped part.

now you have 100va at 18vac + 100va at 18vac

which is basically what 200va 18+18vac toroid is. due to the series/parallel nature of the part you made, id think the combined coils should have about the same inductive and resistive properties as a single secondary coil (half of a center tapped secondary).

so yes, theoretically, mathamatically, its the same. (unless you wanna do all the math for the EM fields from the copper thru the core...)

realistically, its not, but who knows if 2x100va is actually better or worse than 1x200va.

the 100va transformers themselves are prob smaller than the 200va transformers (tho i highly doubt half the volume), so your may be giving yourself more options when making an enclosure for your system. (i remember reading that toroids should be mounted close with their center axis perpendicular to each other, so as not to couple EMI or something.)
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Old 11th January 2007, 02:31 PM   #39
CarlosT is offline CarlosT  United States
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I got a buncha 100VA 25+25 trannies and I'm wondering what to do. 100VA is really not big enough to drive a pair of GC channels but two of these trannies...each one driving its own recitifier board and GC channel could be pretty cool...
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