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.
I know this is wasteful but I can't justify buying new transformers or amplifiers.
Thanks in advance.
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.
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.
I have the same problem.Dazzzla said:
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.
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?
It could even allow me to use smaller filter capacitors.
Any thoughts?
Dazzzla said:
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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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
- 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
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
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
The transformers are iron core so winding extra turns may be to hard.
I seem to recall reading somewhere that auotransformers for mains voltage are illegal in Australia, better not use this method just encase.
Please correct me if I'm wrong.
I think I'll try this way using Tip2955 and Tip3055 which both have a total power dissipation of 90W.
Thanks for all your help.
By winding turns in reverve on the primary, you effectivly reduce the magnetic field. Its nearly the same as removing the the primary turns, turn for turn . In an ideal world you mite have say 400 turns on the primary and by winding 400 turns in the reverse direction you could null the secondary voltage (0vac), you would probably create quite alot of heat and destroy the transformer. Fire and death!
So I wouldn't advise doing this!
Mains voltage is DANGEROUS
Powering consumer equipment directly from an autotransformer may be forbidden due to the absence of mains isolation, but in the solution I proposed the output is taken from the second transformer so it's perfectly isolated from mains. The purpose of the first transformer is solely to step down the mains voltage seen by the primary of the second transformer
EDIT:
Dazzla : Sorry to say you are wrong. Adding turns to the primary in reverse direction actually increases secondary voltages and also increases magnetic field at the risk of core saturation. In the other hand, adding turns to the primary in the same direction reduces secondary voltages and also reduces magnetic field so core saturation will never happen and core losses will be lower
EDIT:
Dazzla : Sorry to say you are wrong. Adding turns to the primary in reverse direction actually increases secondary voltages and also increases magnetic field at the risk of core saturation. In the other hand, adding turns to the primary in the same direction reduces secondary voltages and also reduces magnetic field so core saturation will never happen and core losses will be lower
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bigeyes If you wind extra turns for the primary you must NOT reverse.
Transformer is a ratio device, voltage is proportional to the number of turns.
If you add severals turn to a winding ( not reverse for substraction, but in the same direction ) you will get a winding with higher voltage.
so if you add severals turns consecutive to the secondary you will get a higher voltage at the output winding.
so if you add severals reverse turns to the secondary you will get a lower voltage at the output winding.
If you add severals consecutive turns to the primary the primary will have a higher nominal voltage but when connected to a standard voltage power line the output will drop
If you add severals reverse turns to the primary the primary will have a lower nominal voltage and when connected to a standard voltage power line the output will raise, and also the core will saturate and that may run into overcurrent and overheat
')bigeyes If you wind extra turns for the primary you must NOT reverse.
Transformer is a ratio device, voltage is proportional to the number of turns.
If you add severals turn to a winding ( not reverse for substraction, but in the same direction ) you will get a winding with higher voltage.
so if you add severals turns consecutive to the secondary you will get a higher voltage at the output winding.
so if you add severals reverse turns to the secondary you will get a lower voltage at the output winding.
If you add severals consecutive turns to the primary the primary will have a higher nominal voltage but when connected to a standard voltage power line the output will drop
If you add severals reverse turns to the primary the primary will have a lower nominal voltage and when connected to a standard voltage power line the output will raise, and also the core will saturate and that may run into overcurrent and overheat
Dazzzla said:
Adding to what I indicated on that, remember that a typical 8 ohms speaker has a varying impedance with frequency over the audio band. It can be easily 4 ohms at specific frequencies. Of course, this high amount of current only appears during big transients. I would consider parallelling the 2955/3055 with another one to be on the safe side. It seems a lot of components but this fully regulated supply (including power stage) will most likely improve the sound performance (it is present in extremely expensive hi-fi power amps like Classé Omega, Halcro dm series).
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