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Old 29th August 2014, 09:58 PM   #1
SonyFan is offline SonyFan  United States
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Default I shouldn't have to ask this question, but...

It's one of those things that will bug me if I don't. I am working on an amplifier that uses the same +/-55v rails for the power amp section and preamp section. I was planning on giving the preamp its own power supply to cut noise down. To do this, I wanted to give the preamp its own capacitor bank and current source, unregulated of course. Right now the +/-55v is created from a center tapped transformer, center tap grounded. Is there any reason I can't connect two rectifiers to this one secondary to separate the power amplifier and preamplifier DC supply? I don't think it will work but I can't seem to find a sure answer anywhere else on the net. I need another transformer right?

If I do need another transformer to give the preamp its own rectifier, is there a simpler way of separating the two capacitor banks so the power amp can't draw from the preamp caps and vice-versa? Perhaps connect the preamp capacitor positive to the rectifier by diode and place the load after this diode? Or would this also be a bad idea?
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Old 29th August 2014, 10:19 PM   #2
rayma is offline rayma  United States
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Originally Posted by SonyFan View Post
Is there any reason I can't connect two rectifiers to this one secondary to separate the power amplifier and preamplifier DC supply? I don't think it will work but I can't seem to find a sure answer anywhere else on the net. I need another transformer right?
If I do need another transformer to give the preamp its own rectifier, is there a simpler way of separating the two capacitor banks so the power amp can't draw from the preamp caps and vice-versa? Perhaps connect the preamp capacitor positive to the rectifier by diode and place the load after this diode? Or would this also be a bad idea?
A separate transformer is best, but if the amp and preamp are in the same enclosure, you might be able to work out the details to have low hum and noise. Most receivers use only one transformer, and they work well enough. The diode decoupling idea has been used by Audio Research in their tube power amps, between the output stage and driver stage, but they added lots of capacitance at the driver.

With two rectifiers on the same secondary, when the amp draws large current peaks, this will pull down the AC secondary voltage, and the preamp's rectifier will be cut off at those times. You'll need lots of capacitance after the preamp rectifier to hold up its output voltage well enough.

Last edited by rayma; 29th August 2014 at 10:27 PM.
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Old 30th August 2014, 03:10 AM   #3
SonyFan is offline SonyFan  United States
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Originally Posted by rayma View Post
A separate transformer is best, but if the amp and preamp are in the same enclosure, you might be able to work out the details to have low hum and noise. Most receivers use only one transformer, and they work well enough. The diode decoupling idea has been used by Audio Research in their tube power amps, between the output stage and driver stage, but they added lots of capacitance at the driver.

With two rectifiers on the same secondary, when the amp draws large current peaks, this will pull down the AC secondary voltage, and the preamp's rectifier will be cut off at those times. You'll need lots of capacitance after the preamp rectifier to hold up its output voltage well enough.
I figured that since rectifiers draw most of their current from the peaks of the AC waveform this would probably be bad practice even if it worked. So I take it take it the diode idea isn't unsafe? I may as well experiment with it. I will be ordering large caps for both supplies since they are unregulated and so, as I understand it, why not? Going from 8,200uf x2 shared per channel to 4,700uf x2 per preamp channel and 15,000uf x2 on the output. I should be more specific about the "preamp" board since it is the line stage (preamp, right?) and voltage driver.
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Old 30th August 2014, 03:35 AM   #4
rayma is offline rayma  United States
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I figured that since rectifiers draw most of their current from the peaks of the AC waveform this would probably be bad practice even if it worked. So I take it take it the diode idea isn't unsafe? I may as well experiment with it. I will be ordering large caps for both supplies since they are unregulated and so, as I understand it, why not? Going from 8,200uf x2 shared per channel to 4,700uf x2 per preamp channel and 15,000uf x2 on the output. I should be more specific about the "preamp" board since it is the line stage (preamp, right?) and voltage driver.
Bear in mind that the diodes can only handle a certain amount of peak current, which will be increased by the increased
capacitance. The safest device to use would be a 30A or larger diode bridge rectifier. These have 300A peak rating.
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Old 30th August 2014, 03:56 AM   #5
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Originally Posted by SonyFan View Post
It's one of those things that will bug me if I don't. I am working on an amplifier that uses the same +/-55v rails for the power amp section and preamp section. I was planning on giving the preamp its own power supply to cut noise down. To do this, I wanted to give the preamp its own capacitor bank and current source, unregulated of course. Right now the +/-55v is created from a center tapped transformer, center tap grounded. Is there any reason I can't connect two rectifiers to this one secondary to separate the power amplifier and preamplifier DC supply? I don't think it will work but I can't seem to find a sure answer anywhere else on the net. I need another transformer right?

If I do need another transformer to give the preamp its own rectifier, is there a simpler way of separating the two capacitor banks so the power amp can't draw from the preamp caps and vice-versa? Perhaps connect the preamp capacitor positive to the rectifier by diode and place the load after this diode? Or would this also be a bad idea?
Many higher power integrated amps have been built commercially and have only one transformer but nearly always have a separate winding for the preamp power. That's mainly because preamps run more tame power supply rails - split 12 or 15 because that is the common limits for the opamps.

To get the 55s down to 15 will need a special regulator to handle 40+ Volt drop and the heating will be a problem. Also, your 55 volt rails can exceed 65 depending on the power company. They don't do a great job of controlling line Voltage swings. Just things to keep in mind.

G
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Old 30th August 2014, 04:21 AM   #6
SonyFan is offline SonyFan  United States
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Originally Posted by stratus46 View Post
Many higher power integrated amps have been built commercially and have only one transformer but nearly always have a separate winding for the preamp power. That's mainly because preamps run more tame power supply rails - split 12 or 15 because that is the common limits for the opamps.

To get the 55s down to 15 will need a special regulator to handle 40+ Volt drop and the heating will be a problem. Also, your 55 volt rails can exceed 65 depending on the power company. They don't do a great job of controlling line Voltage swings. Just things to keep in mind.

G
I think this is a little more than a preamp, it is the output stage driver. I am not building this amp, it is a pioneer A-77X. I live in Florida and FPL provides us with 119.9v at 60.1hz bang on 24/7 unless the storms pop a transformer or something locally.
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Old 30th August 2014, 08:28 AM   #7
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I think this is a little more than a preamp, it is the output stage driver. I am not building this amp, it is a pioneer A-77X. I live in Florida and FPL provides us with 119.9v at 60.1hz bang on 24/7 unless the storms pop a transformer or something locally.
And the line Voltage doesn't drop when the neighbors A/C fires up? My lights flick a little when the next door A/C turns on. I get a much larger flick when my own A/C turns on. Of course things like this happen in southern California.

I'm pretty sure if you ran a chart recorder on your line Voltage there would be a little more spread throughout a 24 hr period. The point is not everyone has nice stable line Voltage and it must be accounted for in designs.

G
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Old 30th August 2014, 02:11 PM   #8
SonyFan is offline SonyFan  United States
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Originally Posted by stratus46 View Post
And the line Voltage doesn't drop when the neighbors A/C fires up? My lights flick a little when the next door A/C turns on. I get a much larger flick when my own A/C turns on. Of course things like this happen in southern California.

I'm pretty sure if you ran a chart recorder on your line Voltage there would be a little more spread throughout a 24 hr period. The point is not everyone has nice stable line Voltage and it must be accounted for in designs.

G
I have a new AC with a new start capacitor so there is no fluctuation when the AC turns on, my neighbors AC doesn't seem to do anything either. FPL does fairly well, besides the reliability in bad weather, but that is any above-ground system. This amp will run for a few seconds off of its caps right now so I assume you would need some pretty major fluctuation to cause any problems.
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Old 31st August 2014, 12:54 PM   #9
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Did anyone suggest an RCRCRC filter direct from the smoothing caps of the mains supply?

Select the R to drop voltage at the expected current draw.
Make the final C adequate for the peak current draw.
some suggest 2000uF per amp.
but that means only 100uF for a 50Apk of current.
I would put in 1mF (ten times as much)
The preceding caps could be sized at 1mF to 2mF per amp.
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Old 31st August 2014, 03:10 PM   #10
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Originally Posted by AndrewT View Post
Did anyone suggest an RCRCRC filter direct from the smoothing caps of the mains supply?

Select the R to drop voltage at the expected current draw.
Make the final C adequate for the peak current draw.
some suggest 2000uF per amp.
but that means only 100uF for a 50Apk of current.
I would put in 1mF (ten times as much)
The preceding caps could be sized at 1mF to 2mF per amp.
I believe a similar filter is already present on the board, made with some resistors and 2x 47uf 63v caps + 2x 100uf 63v caps per channel. I am thinking at this point that I will go with one rectifier per channel made up of Vishay fast diodes then use regular diodes to decouple each supply, four diodes total one for each pre and power amp channel. I will listen with various different stages decoupled and see what sounds best. Hopefully the benefits of isolating the supplies will outweigh the noise gained from the diodes.
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Last edited by SonyFan; 31st August 2014 at 03:13 PM. Reason: specificity
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