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Old 24th December 2006, 01:22 PM   #1
bluegti is offline bluegti  United States
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Default Need part number for a transformer

I ordered the LM3886 Amplifier Kit with "Snubberized" power supply from chipamp.com. I've have experience putting kits together before, but they always came with all the parts.

The instructions include the following guidelines for a transformer:
  • "a transformer with dual secondaries producing between 18vac and
    25vac is recommended."
  • "...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."
  • "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."

While I can follow directions and solder, I have no idea how to interpret the spec and find my own transformer. Can someone provide me with a website and part number for a transformer that will work.

Thanks in advance.
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Old 24th December 2006, 04:19 PM   #2
CarlosT is offline CarlosT  United States
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Contact Avel Lindberg and they'll set you up or look at any vendor like Parts Express or the guy on Ebay. The only important spec besides VA rating is that you want dual secondaries.

Let your fingers do the walking. Most transformer folks you talk to or email will be able to help.

Short of that here's a nice one


http://www.partsexpress.com/pe/showd...number=122-625

Or step up one

http://www.partsexpress.com/pe/showd...number=122-640

The recommendation about staying with the lower voltages is right. I experienced a voltage step up factor of 1.55 times the rated voltage of the transformer after rectification. If you took the above 25 volt transformers, you would be marginal/dangerous driving a low load like 4 ohm speakers with 39 volts (after rectification). If you're gonna drive regular speakers rated at 8 ohms, you should be cool with 25 volt trannies...just

This is why sometimes you're better off contacting the trannny manufacturer directly and getting the exact one you need. The pricing is comparable anyway to Part Express or the Ebay dude.

http://www.avellindberg.com/

Before fooling with trannies, you should try to become knowledgeable about them. Here's a nice little blurb to read

http://www.bcrn.com/Media/ToroidalPowerTransformers.pdf

and Hammond's blurb on hooking them up

http://www.hammondmfg.com/5CHook.htm
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Old 24th December 2006, 04:21 PM   #3
CarlosT is offline CarlosT  United States
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And that's what a helpful post looks like for the rest of you jades
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Old 25th December 2006, 03:34 PM   #4
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I'm in the same boat.

What is the difference between the two PE parts? I see one is 250va and the other is 330va. What will that get me at the end of the day?

I'm going to check at a local electronics store tomorrow but usually order from PE since they are only 100 miles away and I get the stuff the next day.
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Old 25th December 2006, 08:22 PM   #5
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Quote:
Originally posted by CarlosT
And that's what a helpful post looks like for the rest of you jades
That's called pay back. Others have helped you, you help others.

Your figure of 1.55, theoretically should be 1.4 (root 2) times the measured secondary output voltage less a couple of volts for the voltage drop across the diodes (4 diodes in the case of chipamp kit). This is correct for full wave bridge rectification (as supplied with the kit). Also, this DC voltage is only after rectification and filtering. Underload, this output will drop depending on the "regulation" of the toroid. Generally, bigger toroids have lower "regulation" factors, therefore lower voltage drop under load.

Hi bluegti,

After reading some of CarlosT's helpful links you should find that to original instructions was very clear.

Here's the specs for the LM3886

http://cache.national.com/ds/LM/LM3886.pdf

regards
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Old 26th December 2006, 03:48 PM   #6
bluegti is offline bluegti  United States
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The instructions may be clear if I understood what 18VAC means as well as 160VA, 220VA and 330VA. I just don't know what I'm looking at when I read specs of these things. Especially when I start looking at a page like this:

http://www.toroid.com/standard_trans...rmers_dual.htm

Just for my own education, can someone identify which transformer on that page is appropriate (if there is one) and which columns/numbers you are looking at to determine that it is.

Finally, I do have some concerns over the comments like, "... you would be marginal/dangerous driving a low load like 4 ohm ..." and "... you should be cool with 25 volt trannies...just." I would rather not be living on the edge of dangerous with small children in my house. Would the 18V model be "safer"?
http://www.partsexpress.com/pe/showd...number=122-620

Thanks again.
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Old 26th December 2006, 07:33 PM   #7
CarlosT is offline CarlosT  United States
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Quote:
Originally posted by Eric_B_C
I'm in the same boat.

What is the difference between the two PE parts? I see one is 250va and the other is 330va. What will that get me at the end of the day?...
The 330VA can deliver more power at the rated secondary voltage. For example a 330VA 25V tranny will be able to deliver 6.6 amps per secondary (330 / (25 + 25)). A 250VA tranny will put out 5 amps (250 / (25 + 25)).

For my single LM3875, I chose a 90VA 20V + 20V tranny...a very nice encapsulated Avel Lindberg one so the power would be 2.25 amps.

More power coupled with decent caps in the power supply board or amp board will allow for sudden demands like let's say a bass drum hit without distortion. I guess what Peter and Brian say is that there's a sweet spot for VA and then there's overkill in terms of packaging, weight, cost, etc.
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Old 26th December 2006, 07:34 PM   #8
CarlosT is offline CarlosT  United States
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BTW the nice encapsulated tranny from Avel Lindberg was made in England while their 100VA Y023 model was made in China. Does it matter? Prolly not...
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Old 26th December 2006, 07:42 PM   #9
CarlosT is offline CarlosT  United States
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Quote:
Originally posted by Greg Erskine
...Your figure of 1.55, theoretically should be 1.4 (root 2) times the measured secondary output voltage less a couple of volts for the voltage drop across the diodes (4 diodes in the case of chipamp kit). This is correct for full wave bridge rectification (as supplied with the kit)...

I was just pointing out my actual experience. My little 90VA 20V + 20V tranny put out 31V + 31V after rectification. Do the math...yep...a 1.55 step up factor

The funny thing is that I ordered 6 of these little beauties and each one tranny/rectifier board combo gave a slightly different DC output...like from 30.1V to 31V depending on which way the wind was blowing.

I kept using the 1.4 factor in my power calculations (in the National Semi Overture Excel file) and I guess I was surprised when the factor actually turned up to be 1.55...good thing I went with an 8 ohm driver.

I think that Brian's chipamp guide actually talks about this variability. If you're skirting with disaster in terms of using 4 ohm drivers and such, I'd plug in 1.6 just to be safe.
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Old 26th December 2006, 07:49 PM   #10
CarlosT is offline CarlosT  United States
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Quote:
Originally posted by bluegti
...Finally, I do have some concerns over the comments like, "... you would be marginal/dangerous driving a low load like 4 ohm ..." and "... you should be cool with 25 volt trannies...just." I would rather not be living on the edge of dangerous with small children in my house...
I'm sorry...that comment had nothing to do with kids and electrical safety. I was referring to burning out your chip. A 4 ohm speaker sucks some serious power out versus an 8 ohm speaker. If you do the power calculations, you'll see that high DC voltage and 4 ohm speakers will easily exceed the chip's capabilities and/or demand a huge heatsink to keep it from reaching the China Syndrome
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