Beginner issues with eBay chip amp

[fastfolkert]

The datasheet says Vmax is +/-50Vdc and operating voltage is between +/-10V and +/-40Vdc

A 28-0-28 transformer, with full wave rectifier results in (28*sqrt2)-(2*0.6V) = +/-38Vdc.
Although this would be (just....) within spec, at high mains voltage you might run into problems, as Michael stated correctly.

If you want to be safe use a 24-0-24 transformer or even an 18-0-18 like I suggested earlier.

YMMV

 

[Jcanderton]

Thanks everyone much appreciated

 

[Michael Bean]

Hi Jcanderton,

If you use a 25-0-25 volt (or less) transformer the worst case drops to around +/- 40 volts which will be safe.

Mike

 

[Jcanderton]

Thanks again, I've just bought a 25-0-25 one, will report back on just how bad my £10 ebay board is in a few days

 

[AndrewT]

a 230:28+28Vac transformer will over voltage a +-42Vdc when fed with 253Vac

A 240:27+27Vac should not over voltage a +-42Vdc when fed with 253Vac.

You need to look for the data of YOUR supply AND the transformer primary rating.

 

[Jcanderton]

Thanks Andrew. I've gone for a 25-0-25 so it should run a bit cooler

 

[fastfolkert]

It won't run cooler; it only guarantees that you're within spec of Vsupply of the amp.
Dissipated heat in your amp is constant at a given output power; Vsupply only determines the max. output power.

YMMV

 

[Jcanderton]

Thanks again everyone!

 

[AndrewT]

Thanks Andrew. I've gone for a 25-0-25 so it should run a bit cooler

calculate the maximum voltage that this transformer will apply to your chipamp in worst case conditions.

The Output voltage is
Vmains/Vprimary*Vsecondary*transformerRegulation

If your transformer is a 230:25+25 7% regulation and your supply is a nominal 220Vac with a maximum tolerance of +10% then the Maximum Output voltage will be
220*1.1/230*50*{1+0.07}= 56.29Vac
The rectified voltage after the smoothing capacitors will be
Vout*sqrt(2)-TwoDiodeDrops
The diodes can drop only 500mVdc when current is very low.
Worst case voltage for the example is
56.29*1.414 - 1V = 78.6V, i.e. +-39.3Vdc

Well below the 84V limit for a 3886.

You, as Builder, needs to calculate your worst case voltage.

 

[Jcanderton]

Quick update. I got the transformer and it sounds great! Probably wouldn't be anyone's first choice as a hifi amp, it's a bit noisy, but for what I wanted it for its ideal. Thanks again everyone

 

[AndrewT]

........... it's a bit noisy,..........

This symptom is probably sortable.

 

[Jcanderton]

I think it's probably the fact I don't have any volume control fitted currently, I'll try that next. Any other ideas welcome

I think my source, a tech 21 Fly Rig is a bit his sky, it's not actually a huge issue, not like I'm planning recording with it,nits just a gig backup

When I have more time I might try something a bit more sophisticated, but for now I have a working amp so I'm happy