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Old 23rd April 2004, 02:39 AM   #1
SSV is offline SSV  United States
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Default Missing Ground?

How can I "star" ground my amp if I'm using a transformer with only 2 secondary leads? Can I use one of the AC secondaries before the rectifier bridge?

Thanks much!
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Old 23rd April 2004, 05:11 AM   #2
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You'll need to use one each of the secondary leads to form
the center tap, which goes to ground. Be sure not to use
both leads of one winding or you'll have a short circuit. Helps
if you have an ohmmeter to check, or the transformer is
marked or comes with a diagram identifying the leads.

The only sizable toroid I have has red, black, green and white
leads; I used the green and white leads as center tap, and
the remaining two go to the bridge rectifier + and -
terminals. Center tap usually goes to the grounded sides
of the capacitors, and thence to star ground.

I don't think phase really matters, but someone will sound
off if it does.

When in doubt, ask. We've made all those mistakes too.

--Damon, been there, burned that
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Old 23rd April 2004, 06:02 AM   #3
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Default Re: Missing Ground?

Quote:
Originally posted by SSV
How can I "star" ground my amp if I'm using a transformer with only 2 secondary leads? Can I use one of the AC secondaries before the rectifier bridge?

Thanks much!
If you only have two secondary wires (one winding) you have to have an amp designed for single voltage. That is the first.

AC out from the transformer + rectifier bridge + smoothing caps = your power supply = two wires, + and - .

Star ground is simple even for single supply amps. Just take "-" from the supply, speaker ground and signal ground (from the input) and tie them together, in this order also. Otherwise you can get hum.
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Old 23rd April 2004, 07:33 PM   #4
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Old 23rd April 2004, 08:13 PM   #5
joensd is offline joensd  Germany
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If you wanna use that transformer which you probably have on hand you could also use the "one way symmetric" shown in this link.
As it is only one way rectifying the power supply ripply will be higher but as most chips (like LM38XX) have high PSRR you migh get away with it easily.
But I have to say I never tried. Anybody?

If you still have to buy the transformer certainly go for one with two secondary windings.
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Old 23rd April 2004, 11:14 PM   #6
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Quote:
Originally posted by joensd
If you wanna use that transformer which you probably have on hand you could also use the "one way symmetric" shown in this link.
As it is only one way rectifying the power supply ripply will be higher but as most chips (like LM38XX) have high PSRR you migh get away with it easily.
But I have to say I never tried. Anybody?
That splitting circuit is not recommended for the 3886 or any power amp. It's only useful for low current projects.

If you want to use such a transformer you should implement the single supply circuit, as on the datasheet. It has a large capacitor at the output.


Carlos
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Old 24th April 2004, 01:05 AM   #7
joensd is offline joensd  Germany
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So what´s the reason for that.
You could easily take a transformer that is up to the task.
So the power supply ripple/droop will be too significant?
As said I never tried it but thanks for the correction.
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Old 24th April 2004, 11:24 AM   #8
joensd is offline joensd  Germany
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Simulation or simple calculations tell me I´d need about 2,2*more capacitance with the one-way symmetric power supply than with a normal two way rectifier.
So it certainly is stupid to go one-way if you don´t have the transformer AND the appropriate amount of caps around.

Still the "usual" power supply for gainclones used here is 2*1000uF which gives a substantial ripple of several volts at full output.
To get about the same result with one way you could simply use 2*2200uF.

Remember this is theoretical and the sound result will not be predictable but would you have thought you get a nice sound with 2*1000uF and so several volts ripple?

If somebody is still curious I´ll go and wire it up.

Any comments are welcome.

Regards
Jens
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