Will I need a transformer for this DIY Amplifier?

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kizm0

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Joined 2009
http://cgi.ebay.com/NEW-LM3886TF-2X...57abf660&_trksid=p3286.c0.m14&_trkparms=65:12|66%3A2|39%3A1|72%3A1234|293%3A1|294%3A50

This ad seems very deceiving to me. They say DIY, but the item shown in the photos is pre-made. They also don't say whether or not you need a transformer.

It might be a DIY kit that has been assembled already. I don't know.

What do you guys think? Will I need a transformer for the PSU of this?
 
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Puffin

Member
Joined 2006
The kit is for a stereo amp (2 power amp boards) and a power supply board.

You will need to supply a transformer. A twin secondary 18vac would do. You will need to know how to wire the transformer to the mains and the power supply board and house the amplifier in a case so that no-one gets electrocuted. Do you have the skills?
 
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AndrewT

R.I.P.
Joined 2004
Hi,
the PCBs look the same as some I bought from another seller.
I had my doubts about whether all the components were genuine or fakes.

The assembled PCBs play fairly well into 8ohm speakers using either 15+15Vac or 25+25Vac transformers.
I cannot vouch for the pre-assembled versions.

Be careful with dissipation of the TF chips. They need bigger heat sinks and are expensive to cool on 4ohm loads.

You need
chassis
heatsink/s
hardware
transformer

The budget for these four items is likely to be two to ten times the $40 of the PSU and 2amps.
 
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odysseybmx414

Member
Joined 2005
Hi. 28v secondaries will be too high. Your chips will likely fry eventually. 28v will be 28 * 1.41 = 39.48v. +-40v is too much, with mains fluctuation your chips will explode. Those kits are fine. www.customaluminumboxes.com will create a custom encosure for you at an amazing price. just give them dimensions and general details. You should get an 18-0-18 VAC or 18-0 18-0 VAC transformer, rated at at least 200va to supply both of those chips. At (18*1.41=25.38VDC) your chips won't get very hot, affording you a heatsink that won't take up your whole chassis.
 
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Puffin

Member
Joined 2006
odysseybmx414 said:
Hi. 28v secondaries will be too high. Your chips will likely fry eventually. 28v will be 28 * 1.41 = 39.48v. +-40v is too much, with mains fluctuation your chips will explode. Those kits are fine. www.customaluminumboxes.com will create a custom encosure for you at an amazing price. just give them dimensions and general details. You should get an 18-0-18 VAC or 18-0 18-0 VAC transformer, rated at at least 200va to supply both of those chips. At (18*1.41=25.38VDC) your chips won't get very hot, affording you a heatsink that won't take up your whole chassis.
Click to expand...


I use twin 30vac trannys with my LM3875 monblocks. No problem.
 
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AndrewT

R.I.P.
Joined 2004
the rated input voltage to rated output voltage ratio is critical if you want to run at close to the National maximum voltage specification.

I reckon a 240:28Vac 300VA transformer (6% regulation) feeding a pair of 3886 chipamps will be just above the +-42Vdc maximum quiescent specification. But when significant power is drawn from either or both amplifiers then the supply will drop below the +-42Vdc spec maximum.

I recommended 240:28Vac a long time ago as the maximum to supply an 8ohm loaded pair of chipamps and got shouted down as far too big a supply voltage.

Measure what you have and monitor temperatures, but if you choose to run above National's specification maxima then the risks and consequences are wholly of your own making.
 
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pacificblue

Member
Joined 2008
The trouble with the 2x30 V transformer is that it is recommended in National's AN-1192, but people tend to overlook the fact that it is only recommended in combination with 8 Ohm speakers and with profound heatsinking. As long as an amplifier is not permanently connected to a dedicated driver in an active speaker or similar set-up, you can not be sure that it will never be connected to a 4 Ohm load. Just as little as you can be sure that nobody will ever turn the volume up, while your amplifier is connected to a 4 Ohm load, when it should not be.
 
guitvinny

guitvinny

Member
Joined 2005
It's not so hard, you have basically 3 things to check :

- Primary winding, is it for 120 or 240 V
- Secondary winding, in your case 18 - 0 - 18
- VA rating, which is *grossly* the power the transformer can handle

The 2nd one from Newark looks very nice, the first one can only deliver 4 VA... its going to fry.
 
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