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Old 14th November 2009, 08:41 PM   #1
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Default No center tap Help

Situation: remodeling an older QSC-1400 amplifier. The transformer, chassis, switches, connectors and circuit breaker are "ruggedized" and worth using, but there is no secondary center tap(s), just two secondaries of ~ 55 VAC, each (as wired for the two 120 VAC primaries).

I know it can be done ... maybe just use the powerline ground to chassis as the "center" and signal & DC power ground, but for audio equipment, I believe this would just leave to door open for more "hum" and other noise.

I guess I could reconnect the primaries (in series) for pseudo-240 VAC and then connect the secondaries in series for the desired ~55 / 0 / ~55. ...

What to do? Got recommendations? ...

Last edited by FastEddy; 14th November 2009 at 08:49 PM.
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Old 15th November 2009, 02:07 PM   #2
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It's common practice to use a full wave bridge and reference negative (or negative of one side and positive of the other on a +- supply to earth or common ground. This is indeed about as quiet as anything you can do. I would think it really depends on what you are building and it's power supply requirements.
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Old 16th November 2009, 11:00 PM   #3
Minion is offline Minion  Canada
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the normal way to get a center tap (and only way I know short of rewinding the transformer) is to put the secondaries in series and use the 2 wires that are connected together as your center tap , it will work the same as a regular center tapped transformer .....


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Old 16th November 2009, 11:09 PM   #4
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Thanks guys ... I'm going to use the primaries in series from 120 VAC ... to get ~55 / 0 / ~55 from the series secondaries ... seems the easiest without throwing out the transformer and replacing ... IU'll just have to live with being unable to use this in Europe.
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Old 17th November 2009, 02:51 AM   #5
Bigun is offline Bigun  Canada
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I'm not sure I understand the question - is it a case of wanting to get split rails (+ and -) from a single winding (with no CT) ?
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Old 17th November 2009, 03:44 AM   #6
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yes, split rails ...

The transformer has two sets of windings, two primaries and two secondaries. When the primaries are parallel, they give ~55 VAC through each secondary, so putting the secondaries in series would give me twice the rail voltage needed for the amp. However putting the primaries in series and powering with 120 VAC and the secondaries in series I can get ~55 / 0 / ~55 and use the central connection as the center tap as is common practice ... as you obviously know ... but I can't use the amp in Europe where the wall power is about ~240 VAC.

I hate to waste a perfectly good chassis, transformer and all the parts (used ~US$50) ... so I'm planning on DC rail voltage of about + 76 / 0 / -76 VDC to power an older version of a set of amp modules like these: http://www.aussieamplifiers.com/nxv500.htm (prototype ~US$200 each)and using a power supply filter board like this: http://www.aussieamplifiers.com/psu.htm (used US$50) ... should = around 300 watts each into 8 ohms on modest heat sinks. (The old QSC has a very healthy transformer of > 900 VA, but the fan had to go, so by "derating" it a bit, hopefully ... it should live a long time. ... I believe a worth balanced input amp with decent power for a garageband PA = low cost as the total "all in" is less than US$600.

Last edited by FastEddy; 17th November 2009 at 04:04 AM.
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Old 17th November 2009, 04:45 PM   #7
Bigun is offline Bigun  Canada
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Well you can get split rails without a Centre tap although ripple is a little higher as a result you can probably deal with that if you want to.

Here's how it works (attached):
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File Type: gif psu.gif (12.7 KB, 346 views)
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Old 17th November 2009, 05:02 PM   #8
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Quote:
Originally Posted by Bigun View Post
Well you can get split rails without a Centre tap although ripple is a little higher as a result you can probably deal with that if you want to.

Here's how it works (attached):
Wow, Man! Thanks a lot! I knew there was a relatively simple way to do it ...

As long as the guts of the thing are open to experiment, I'll give it a go.

BTW: this would be a half wave diode bridge, so I'll use fast, extra duty diodes and double up on capacitance.
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Old 17th November 2009, 05:40 PM   #9
Bigun is offline Bigun  Canada
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Hum is your only risk as far as I can see. Most amplifiers have very good PSRR for their power hungry output stages. The issue is the front end of the amplifier. You should focus your effort on the front end, perhaps isolating the rails to the front end from the output stage by using a diode, resistor and capacitor to ground.
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Old 17th November 2009, 05:40 PM   #10
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Just a heads up: that's a full wave doubler and not much good for high current applications (such as a power amp).
You would be better off using both secondaries together in series, giving 55-0-55.
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