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Old 18th April 2004, 09:18 AM   #1
Dazzzla is offline Dazzzla  Australia
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Default Removing excess voltage from amplifier supply?

I have 4 * 50 watt amplifiers and a couple of 35-35v 4.2A transformers sitting around collecting dust. The amplifiers only require +- 35vdc @ 1.5A, the transformers after rectification and filtering provide about +-50vdc @ 3A. Would it be possible to use these transformers to run the amplifiers by using 13-14 rectifier diodes (Vf of 1.1v) in series with each of the +ve and - ve just after the bridge rectifier to get rid of the +-15vdc?
I know this is wasteful but I can't justify buying new transformers or amplifiers.

Thanks in advance.
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Old 18th April 2004, 12:38 PM   #2
lucpes is offline lucpes  Romania
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Could be feasible, but you'd need to buy T0220 diodes (or similar format) and put them on a heatsink - ugh... or use 3 25A 400V bridge rectifiers per rail wired in series after the main BR(also heatsinked).

Best would be to try and carefully unwound the trafo secondaries until you get 26V ac secondaries to have something like +/- 38V dc after rectification.
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Old 18th April 2004, 01:48 PM   #3
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Default Re: Removing excess voltage from amplifier supply?

Quote:
Originally posted by Dazzzla
Would it be possible to use these transformers to run the amplifiers by using 13-14 rectifier diodes (Vf of 1.1v) in series with each of the +ve and - ve just after the bridge rectifier to get rid of the +-15vdc?
I have the same problem.

It would work, but put a resistor across each rail to draw ~50-100mA because the diodes will not drop as much at very low current levels. Or you could just use a big Zener diode in series with each rail. Or or you could make a simple pair of series regulators using an N&P channel mosfet wired as source followers with the gates fed with about 4-5 volts greater than what you want out of the sources. Regulation won't be wonderful, but probably better than the diode string setup. Again, a bit of pre-load would be nice; say 50mA.
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Old 18th April 2004, 03:13 PM   #4
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You may use a smaller transformer connected in autotransformer configuration to lower the input voltage on your tranformer
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Old 18th April 2004, 03:51 PM   #5
Dazzzla is offline Dazzzla  Australia
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I think trying to find a zener diode with enough wattage mite be a problem but you got me thinking when you mentioned series regulators, what about using current boosting with a tip2955 and 2n3055 from standard 7805 and 7905 regulators with their reference set at 30V which would give me +-35v regulated.
It could even allow me to use smaller filter capacitors.
Any thoughts?
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Old 18th April 2004, 04:18 PM   #6
lucpes is offline lucpes  Romania
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The 7805/7905 sollution would be current limited, and that is before they get cooked...
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Old 18th April 2004, 04:58 PM   #7
sreten is offline sreten  United Kingdom
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Current boosted regulators are the obvious option.

You need only modest capacitance preregulators and post
regulators very modest capacitance.

sreten.
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Old 18th April 2004, 05:09 PM   #8
fab is offline fab  Canada
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Quote:
Originally posted by Dazzzla
I think trying to find a zener diode with enough wattage mite be a problem but you got me thinking when you mentioned series regulators, what about using current boosting with a tip2955 and 2n3055 from standard 7805 and 7905 regulators with their reference set at 30V which would give me +-35v regulated.
It could even allow me to use smaller filter capacitors.
Any thoughts?

If you are talking about a circuit "similar" to this one, remember that you will have to dissipate "15V x Max_current" in the 2955/3055 transistors. For 8 ohms load during continuous sinus signal at 50W, it gives 3.5 A, thus more than 50 W to dissipate for positive supply and the same for negative supply! So a big heat sink will be required.

There is no magic, using diodes or current boosting regulator, you will have a loss of power to dissipate using your actual too high voltage transfo. However, current boosting regulator approach provides at least a theoritical improvement on the sound of the amp. Refer to http://home.mira.net/~gnb/audio/a100.html for a high voltage power regulator use.
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Old 18th April 2004, 06:12 PM   #9
Eva is offline Eva  Spain
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This solution doesn't require extra parts, altough it forces all channels to be driven from the same supply

- If you have 120V mains [120 to 35+35 transformers] :

Option 1 : Take first transformer and wire the primary in series with a secondary to get a 155V to 120V autotransformer. Use this autotransformer to reduce mains from 120V to 93V [120V applied to 155V side]. Use these 93V to power the second transformer and you will get about +-38V after rectification

Option 2 : Take first transformer and wire both secondaries in series, then wire them in series with the primary to get a 190V to 120V autotransformer. Use this autotransformer to reduce mains from 120V to 76V. Use these 76V to power the second transformer and you will get about +-31V after rectification


- If you have 230V mains [230 to 35+35 transformers] :

Take first transformer and wire both secondaries in series, then wire them in series with the primary to get a 300V to 230V autotransformer. Use this autotransformer to reduce mains from 230V to 176V. Use these 176V to power the second transformer and you will get about +-38V after rectification


Remember that two windings connected in series may sum or cancel their voltages depending on relative polarities betwen them, so you will need to find the right polarities to make the autotransformer work properly
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Old 18th April 2004, 07:04 PM   #10
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If I may get back at the original question, I doubt you get 50VDC. Try it out, wire the xformer up with a rectifier and filter cap and draw 1.5 amps. You probably won't get much more than 40VDC. Those amps might be perfectly happy with that, even put out a trifle more power.

If it really is too high, wind a couple of windings and put them in series with the primary (opposite phase, you may have to try the connection that gives lower secondaries). Make sure you insulate it properly of course.

M
But, first make sure you really have a problem before trying to solve it.

Jan Didden
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