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kanifee 20th April 2008 03:59 PM

transformer for my chip amp?
Hi all, just finished construction of my amp and im looking for a suitable transformer for it, i found these but just looking for some clarification which would be better for me?

Primary 230Vac
Secondary: 2 x 18v @ 6.25A
Regulation: 7%


Primary 230Vac
Secondary: 2 x 25v @ 4.50A
Regulation: 7%

or this

Primary 230Vac
Secondary: 2 x 35v @ 3.21A
Regulation: 7%

And if im just well off the mark and none of these are suitable what should i be looking for?

heres some snaps as i cant help my self :D

dfdye 20th April 2008 04:59 PM

Looks nice so far!

You should be fine with either the 2*18V or 2* 25V options. They should rectify out to 25V and 35V respectively, which are within the recommendations for the chip. The 35V secondaries, on the other hand, would produce ~50V rails, which would be greater than what the chip was designed to take, so the third option is right out!

The amperage ratings for both of the first two seem sufficient, so I wouldn't worry about either. The potential concern would be that the +/- 35V rails would cause the chip to produce more heat than the +/-25V rails, so make sure your heat sink is adequate if you use the larger rail voltages.

kanifee 20th April 2008 05:38 PM

would i gain anything by going with the larger of the 2, better stability under load? i only ask because the price of the 2 are identical so not really a concern.

danielwritesbac 20th April 2008 06:58 PM

2 Attachment(s)
The attached chart might explain it.

EDIT: Multiply the individual AC rails voltage by 1.45 to get an approximate DC voltage (rectification to DC makes that difference).

EDIT2: For example, the 25+25vac transformer makes approximately 36vdc rails, which, according to the chart, is a centerpoint tolerance for "most" speakers.

Minion 20th April 2008 07:36 PM

It depends on what Impedance Load you are going to drive...

If you are driveing 4 ohm load then the 2x18v would be better and if driveing 8 ohm loads the 2x25v one would be better....


dfdye 20th April 2008 08:16 PM


Originally posted by kanifee
. . . the price of the 2 are identical so not really a concern.
Not surprising since they would have the same amount of copper in each. :)

You will get marginal power gains with the higher rail voltages, and has been mentioned, if you know you will be driving 8ohm loads exclusively, there is no reason not to go with the larger of the two except for heat. Having the larger rails will result in more heat at the same music power, so be sure to double check that you can dissipate enough heat to adequately use the +/-35V rails if you do go that route.

danielwritesbac 21st April 2008 07:48 AM

Shown in the photos above, the option for the NFB cap (shown installed) may be set to the "off" position (see manual), which may sound nicer (see manual); however, do remember to check your DC offset before connecting speakers.

kanifee 21st April 2008 09:55 AM

Yeh i opted for the NFB resistor in the off position, i juist need the Transformer first.

AndrewT 21st April 2008 10:35 AM

Hi Kan,
4 to 8 ohm speakers have become very common in the UK.

If your amp is designed to cope with this load for now or some time in the future then go for a lower voltage transformer.
18Vac or 20Vac, >=150VA

danielwritesbac 21st April 2008 12:00 PM

This might simplify it. With 8 ohm speakers, the 2x 18vac transformer option would give you a 25 watt amplifier.
However, the 2 x 25vac transformer option will run your amplifier at near its expected power output, albeit needing larger heatsinks.

Something like 2x 22vac transformer might also be a good option.

Next up, what sort of preamp have you selected to match up with your power amp kit?

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