What toroid transformer to use

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chevy2410

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Joined 2005
#1
What toriod transformer to use

Hi all,

I'm new to gainclone chipamp and I was wondering what toriod transformer to use with briangt's stereo version of the Lm 3886? I've searched until I was blue in the face with no results.

Also I was wondering what wpc can I expect from this amp?


Thanks,
Chevy
 
#2
first you need to know what setup you want:

- 1 trafo or 2 trafo ( 1 for each channel )
- you will use only 8 Ohm loudspeakers,
or only 4 Ohm LSP
or a version that can drive both 4/8 Ohm good
- if you will go for max power = max voltage and current
or go for safety margin and good hifi ( lower distortion )


with only 1 trafo, you need a bigger = more VA
with 8 Ohm LSP you need more voltage
with 4 Ohm LSP you need less voltage but more current ( Ampere )


Guys like carlosfm, Peter Daniel, BrianGT and Nuuk
will give you details
or give you links to best threads here in Chip Amps forum


Nuuk has one of the best informative gainclone website
in the web:
Decibel Dungeon
http://myweb.tiscali.co.uk/nuukspot/decdun/gaincloneindex.html


Good luck!
🙂
 
#4
I will be using stereo version Lm3886 and 1 transformer. The loadspeakers I'm using are 6 Ohm. I don't think I want to max out on the transformer, but rather stay in the middle.

BTW does anyone know of a good place to buy a transformer in Canada?


Thanks,
Chevy
 
#5
I'm building a stereo (shared torroid) 3886 Brian Gt kit for use with 4 Ohm speakers. I got a dual 18V secondary, 225VA Hammond transformer. With 6 Ohm you might want to go dual 22v and from what I've read, I don't think you should go much less than 200VA for a shared supply.

Max
 
#7
hi chevy

I have both a plitron and hammond trafo. You can get a hammond in about a week; just search for hammond and look up the local suppliers here; check them out on their website. Both mine are 225va dual sec.which has been enough but I would recommend 300va, better to have some reserve. Plitron have a good reputation, look nice, are potted in the center, more compact for the same va as hammond. Plitron has more voltage options and are priced perhaps better. They ship with ups so you can add another $30 from Toronto. They custom make so you have to wait for 5 weeks for delivery.

cheers
doggy🙂
 
#11
Plitron is ordered directly from the plitron website. Best way is to decide what vac and va you want; the coding system is at their website; phone to place order. A 400va trafo. is about $90. +$30 ups.

cheers
doggy🙂
 
#13
Chevy,

If you mean the 350va job with all those windings, you may have a problem.

The Brian GT kit requires a dual secondary transformer and the Parts Express one says "16.2-0-16.2". That would imply 3 wires out, not the four that are needed. I'm not sure if a torroid could be centre-tapped, so I don't know what's going on there.

Also, 16.2v seems a touch on the low side for 6 ohm speakers.

I would see about getting a Hammond one locally if I were you.

Check here for the closest dealer:

http://www.hammondmfg.com/cregion.htm#British Columbia

Either the #182G18 (dual 18V) or #182P22 (dual 22V) both are 225VA.

Oh, and the Hammonds are made in Canada, if that matters to you.

Max
 
#14
Hi,
the dual secondary (V,0 + V,0) toroid and the centre tapped (V,0,V)toroid will perform exactly the same.
Either of these can be wired up to give a plus & minus supply with ground reference.

The advantage of the dual secondary is the option to use dual rectifiers. This loses an extra 1.4V in return for less heat load, less current load and less voltage stress on each rectifier.

Dual secondary single rectifier +V,0,-V supply or dual rectifier +V,0 & 0,-V.
Centre tapped single rectifier +V,0,-V.

If building a stereo amp then a 4 secondary transformer would be even better followed by full dual mono transformers.
 
#16
AndrewT said:
Hi,
the dual secondary (V,0 + V,0) toroid and the centre tapped (V,0,V)toroid will perform exactly the same.
Either of these can be wired up to give a plus & minus supply with ground reference.

The advantage of the dual secondary is the option to use dual rectifiers. This loses an extra 1.4V in return for less heat load, less current load and less voltage stress on each rectifier.

Dual secondary single rectifier +V,0,-V supply or dual rectifier +V,0 & 0,-V.
Centre tapped single rectifier +V,0,-V.

If building a stereo amp then a 4 secondary transformer would be even better followed by full dual mono transformers.
Click to expand...

The Brian GT is setup with dual rectifiers.

chevy2410 said:


No I was looking at THIS ONE.


Thanks,
Chevy
Click to expand...

That one is just fine. When I looked before, I did a search for "transformer" at the Parts Express site and that one didn't show. I guess their search function kind of sucks then.

Max
 
#18
Here's the info from the Brian GT instructions:

"For the transformer, a transformer with dual secondaries producing between 18vac and 25vac is recommended. One DIYer has used a 2x30vac transformer without issues, but 2x25vac should work best for a wide variety of loads. One can choose a variety of VA ratings and rail voltages. Keep in mind that after rectification, the rail voltages are somewhat higher than the non-rectified AC secondary rating of the transformer. The secondary voltage averages to 1.4 * the AC voltage, minus diode losses. The transformer regulation is also a factor, dependent on the size and regulation characteristic of the transformer in question. Suffice it to say that a commonly used 18V transformer results in approximately 25V rectified, while a common 22V supply produces around 34V rectified. Referencing the National datasheet LM3886.pdf, you can find the practical maximum rail voltage rating for the average impedance of your speaker. The Output Power vs. Supply Voltage chart is a good indication of the maximum rail voltages for a given speaker that you will design to. When looking over the curves a speaker with a nominal impedance of 4 ohms tend toward 25V rails with a reasonable margin of safety, while voltages above 35V are still well within the range for 8 ohm speakers. This shows that transformers with 18-22V secondaries are well within reason for many common commercial and DIY speakers. A transformer with 25V transformer secondaries can also be successfully with less of a safety factor. The VA ratings on transformers is also a consideration. Within reason, a larger transformer has more constant regulation under load, but this chip operates very successfully without extremely large transformers. Many have successfully used 160VA transformers, while the 220VA range seems to be adequate for almost all stereo implementations, not straining the transformer. The price point between 220VA and 330VA, however might lead one to purchase the larger of the two. Anything above this could be considered frivolous for a stereo pair, unless one happens to be on the shelf or in a surplus vendors stock. Don’t be tempted to buy an extremely large transformer such as used for Class-A applications as it is simply not required. The fuse required will be dictated by the size of the transformer, due to the inrush current when power is first applied. A 2 amp slo-blo typically works fine for transformers around 220VA or less, while a 3 amp slo-blo fuse might be required if using a larger transformer. "

Max
 
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