2x LM3886 In Parallel - How To Add DC Offset Adjustment

[audioguru]

The temperature of a loaded transformer indicates its max current.

 

[ParkingLotLust]

How many degrees above ambient? I cant think of any way to use lower and lower resistor values at wattages that high. I think Ill end up building it and monitoring the temperature, and if it gets too hot Ill pick up a bigger toroid.

 

[Nigel Goodwin]

Measure the physical size of the transformer, and compare that to manufacturers specifications, it's a good indication of the wattage.

 

[ParkingLotLust]

I emailed the company with the markings on the side, hopefully they get back to me. The problem I found with trying to compare with whats on the site is that they only list a few toroids per page, and there are ~28 pages that I would have to go through and click each one and check the dimensions. Also, the toroid I have has multiple (4, 2 of which are CT) secondaries and there isnt a single power toroid listed with more than one pair of secondaries.

Worst case scenario, their head office is within bus distance of where I live so I can always head there aswell. I guess its time to play the waiting game.

 

[Nigel Goodwin]

Don't count windings - just measure the size of it - what diameter, and how thick.

 

[ParkingLotLust]

3.5" in diameter by 1.25" thick
so
890mm in diameter by 320mm thick

EDIT yeah their site is pretty useless; the dimensions are the same for every transformer. For example:
Height: 117.5 mm
Width: 250.8 mm
Length: 250.8 mm

for an 80VA and a 1000VA transformer. The only thing different is the weight, and I dont have a scale handy to measure the transformer.

 

[mneary]

the dimensions are the same for every transformer. For example:
Height: 117.5 mm
Width: 250.8 mm
Length: 250.8 mm.

[edit] The dimensions and weight on the web page are probably for the shipping container. The weight is 0.50 kg more than the pdf. [/edit]

"Show Product Specifications" and get the manufacturer's pdfs for 80VA and 1000VA and you get very different numbers.

[edit again] You appear to have a huge transformer. It might weigh 5-10 kg which you can estimate on a bathroom scale. [/again]

 

[Mr RB]

3.5" in diameter by 1.25" thick
...

That's an 80 watt one. They are 90mm dia x 37mm thick. Almost exactly matches your size.

 

[Speakerguy]

It has multiple secondaries though. So hard to tell the VA of the windings he is using?

Don't worry, if it's not enough the amp will just clip. You'll hear it, it sounds terrible. For 7.50 it is worth a shot. These LM3886 chips have all sorts of protection built in, so you are very very unlikely to damage it.

 

[ParkingLotLust]

Thats what I figured, speakerguy. I also found another local store that seems to sell a lot more powerful toroids, including some with dual secondaries, and higher amperage ratings.

 

[ParkingLotLust]

Got a reply back from the manufacturer. Here are the specs:

Primary: 117VAC (BLK/BLK)
Secondary 1: 7.8VAC @ 0.83A (RED/RED)
Secondary 2: 9.2VAC @ 2.48A (ORN/ORN)
Secondary 3: 8.3VAC-0-8.3VAC @ 1.2A (VIO/GRY/VIO)
Secondary 4: 18.5VAC-0-18.5VAC @ 1.18A (BRN/WHT/BRN)

I think the secondary I was after has too low of an amperage rating

 

[mneary]

You can probably draw 1.4 times the rating (60VA vs. rated 43VA) from Secondary 4 if you draw nothing from the others. And, if you find another identical transformer to put in parallel, you could end up with about 120VA total.

Rationale: The total rating of the transformer is based on its core and the total I^2*R heating of each winding. I'm suggesting that 1.4 times the current will produce double the heating, hence my suggestion that you can get 60VA from one winding of the 80VA transformer. (I'm ignoring the primary heating.)

If you're hooking two transformers together that you aren't totally certain are identical, use separate rectifiers and join them at the DC connection.

 

[ParkingLotLust]

I can get another one of these transformers no problem, they had a bunch when I was looking. So I can just parallel the two primaries and the secondary that I need? How do I tell if theyre in phase? The primary has two black leads (no separate marks, but there is a keyed connector), and the secondary has 2 browns and a white.

 

[mneary]

If it's not inconvenient I would use two diode bridges.

Otherwise, hook the black wires in parallel, then hook the white center taps together. Each pair of brown wires that read zero volts between them are in phase.

 

[ParkingLotLust]

What if I wired the primaries in parallel, and used one transformer for the +ve rail and one for the -ve rail? The original configuration uses a bridge rectifier for each rail - Id assume that since theyre identical transformers using one for each rail will provide double the current?

 

[mneary]

One transformer for each rail isn't bad in principle, but in your case it isn't practical.

You cannot draw all of the current for each rail from half of each Secondary 4. The I^2*R loss would be too high.

The full Secondary 3 is only 16.6V, giving you enough current, but only ((16.6*1.4)-2)=21.5V rails.

If it were my amplifier, I would hook the center taps together for 0V and use two bridge rectifiers, connecting + to + and - to - for the rails. The rails would be ((18.5*1.4)-1)=24.9. Note there's only one diode this way.

 

[ParkingLotLust]

Im not 100% sure I follow you so I whipped up this schem. Is this what you meant?

**broken link removed**

 

[Mr RB]

Will you get full power with that transformer??

(from LM3886 page)-
• 68W cont. avg. output power into 4 at VCC = ±28V
• 38W cont. avg. output power into 8 at VCC = ±28V
• 50W cont. avg. output power into 8 at VCC = ±35V
• 135W instantaneous peak output power capability
• Signal-to-Noise Ratio >= 92dB
• An input mute function
• Output protection from a short to ground or to the supplies via internal current limiting circuitry
• Output over-voltage protection against transients from inductive loads
• Supply under-voltage protection, not allowing internal biasing to occur when |VEE| + |VCC| <= 12V, thus eliminating turn-on and turn-off transients
• 11-lead TO-220 package
• Wide supply range 20V - 94V

 

[mneary]

Will you get full power with that transformer??

Short answer: No.

He'll get maybe 55W (page 9 on data sheet TL/H/11833-10) instead of 68W.

He can get more power with the 18M22 or 18N22 transformer from the other local shop. Price listed is $50 or $56 instead of $15.

 

[mneary]

Im not 100% sure I follow you so I whipped up this schem. Is this what you meant?

Yes, exactly.