Need part number for a transformer

[bluegti]

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.

 

[CarlosT]

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/showdetl.cfm?&Partnumber=122-625

Or step up one

http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=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

 

[CarlosT]

And that's what a helpful post looks like for the rest of you jades

 

[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?

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.

 

[Greg Erskine]

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

 

[bluegti]

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_transformers/rectifier_transformers/rectifier_transformers_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/showdetl.cfm?&Partnumber=122-620

Thanks again.

 

[CarlosT]

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.

 

[CarlosT]

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...

 

[CarlosT]

...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.

 

[CarlosT]

...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

 

[CarlosT]

Here's the link to the handy Overture calculation file. If you have Excel, you can open it and plug in some values in the blue fields for secondary DC voltage and you can see the power output and heatsink requirements.

http://www.national.com/appinfo/audio/files/Overture_Design_Guide15.xls

 

[Greg Erskine]

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

Hi CarlosT,

What is the measured output of the secondaries?
What is your mains voltage?
What is the regulation figure of your little 90VA toriod? I should be roughly 10% or 12%.

Here's a quote from the manufacturuer I use.

"The output voltage tolerance is +/- 5%, conforming to BS3535 and CEE15. All voltages are quoted at FULL LOAD. Add the appropriate regulation figure to obtain the off load voltage."

regards

 

[CarlosT]

Too many questions for my little mind. How about if I measure all these things in about a week when I put together amp # 3?

The little beauties I got must have been lovingly wound in England by gorgeous Brit chicks because the regulation is only 7%

Specs

Primaries 115V + 115V

Secondaries 20V + 20V

Nominal Load VA - 90VA

Regulation - 7%

Temperature Rise EC - 40

Iron Loss W - 1.00

Copper Loss W - 6.50

Don't ask me what all this means...just tell me they're good specs

 

[Eric_B_C]

How about this.

I ordered the same kit. With 25v+25v and 330va can I get 50 watt per channel out of this setup?

 

[Greg Erskine]

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:

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/showdetl.cfm?&Partnumber=122-620

Thanks again.

hi bluegti

18VAC = 18 volts alternating current
160VA = 160 volt amp

If you see VDC that's "volts direct current". AC changes to DC after rectification and filtering.

If I read the table correctly, 718.222 is a 22-0-22 190VAC toriod. That's what I would buy. I use 22-0-22 225VA toriods on my GC. I have used 18-0-18 160VA and even 12-0-12 300VA with little difference to the sound.

22-0-22 = one 22 volt secondary and one minus 22 volt secondary.

BTW: Smaller toriods like CarlosT's will work OK as well. Smaller toriods tend to run wamer.

Also, don't get to worried about specs. What happens is over time in forums like this is the "recommended" or "optimum" changes depending on who's active on the forum. Over the years, all these otpions have been tried and have worked well. I don't remember any houses buring down.

regards

 

[Greg Erskine]

Too many questions for my little mind. How about if I measure all these things in about a week when I put together amp # 3?

The little beauties I got must have been lovingly wound in England by gorgeous Brit chicks because the regulation is only 7%

Specs

Primaries 115V + 115V

Secondaries 20V + 20V

Nominal Load VA - 90VA

Regulation - 7%

Temperature Rise EC - 40

Iron Loss W - 1.00

Copper Loss W - 6.50

Don't ask me what all this means...just tell me they're good specs

The regulation does seem good for a small toriod.

I think the Iron Loss and Copper Loss make up the regulation percentage. Temperature Rise is the increase in temperature when operated at full load.

regards

 

[falcott]

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

Was this measured under load? I've measured 41V out of a 25V transformer before putting the load on it. It settles down once loaded up...