Negative voltage from Center tapped vs Dual transformer

Thread Starter

kriksis

Joined Feb 23, 2016
21
Hello,

I am in middle of building Gainclone amp. It's based on two LM3886 op amps. Power requirement is +25v and -25v. This is minor problem, that I cannot figure out myself and there probably a thread here somewhere, but I failed to find direct answer.

Center tapped transformers are expensive and hard to get here, where I am from. Thus I'm interested in getting both - positive and negative voltages from dual rail transformer. I have attached a picture, that I found on internet (unfortunately I don't know the author). Could someone please comment, if this would provide the outcome I desire?

Additionally - thing that confuses me, is that each (positive and negative) side has it's own ground. (Here comes a amateur question) - should I treat these separate grounds somehow special? Or should I just wind them together and make common ground as usually?
 

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ericgibbs

Joined Jan 29, 2010
9,302
hi k,
The transformer you have posted has two isolated secondary windings are you able to purchase a transformer of this type.?
If not, you could use two separate mains transformers to give + and - supplies.
E
 

Thread Starter

kriksis

Joined Feb 23, 2016
21
Hi,

I thought of this as option, but two separate transformers cost about the same as one with two secondary windings. I have some old ones laying around, but I believe they could fail to offer the needed power needs.

Regards
 

Thread Starter

kriksis

Joined Feb 23, 2016
21
Yes 1R is 1 ohm.

Thanks. One more question about this circuit, - if I understand it right, then if the transformer gives 18vac, then after bridge and caps, the output will be something around 25vdc? Can I rely on that or will it drop when load is added?
Datasheet for LM3886 tells me, that range for power supply has to be within 20v - 80v. That's why I'm asking.

Thank you in advance!
 

Dodgydave

Joined Jun 22, 2012
8,671
Your output voltage will drop under load possibly to 20v or less, i would use a transformer with a voltage of 22-30Vac, then it should be in the range you need, or use voltage regulators for precision.
 

crutschow

Joined Mar 14, 2008
24,055
Note that you should derate the transformer RMS current rating at least 40% as compared to the needed capacitor-rectifier DC current output due to the high peak RMS current pulses the capacitor-rectifier draws from the transformer.
 

Thread Starter

kriksis

Joined Feb 23, 2016
21
Thank you for advice. I think I will go with joining the two transformers into one.

I am keeper and re-user, so I have all sorts of salvaged transformers. If I take two different transformers?
This would probably mean secondary voltages would differ slightly (within range of ~2v) and power rating as well. This would end bad, wouldn't it?
 

Thread Starter

kriksis

Joined Feb 23, 2016
21
I have recently found several DIY projects where people are making these Gainclones which all have two LM3886 op-amps. None of them actually tell, what transformer needs to be used aside from one author claiming that it should be at least 120 VA.
Based on amps datasheet power dissipation is 125 w, so I can assume, that I need at least 106.25 VA per amp and totally forget what he suggested?
So either, they are lucky that the transformers are still functioning (if of course they are) or without driving the amps to full capacity they just never hit that peak where it starts to overheat.
 

tcmtech

Joined Nov 4, 2013
2,868
Very few audio amplifiers are ever working continuously anywhere near their maximum wattage rating so that's why they can get by with a slightly lower primary transformer VA rating than their output wattage rating would suggest they would need.

Add in the fact that if a transformer is rated for 100 VA it doesn't instantaneously burn up if a 101 VA load gets applied to it and that's how a lower VA rated transformer can live for years without burning up even in a hard run audio amplifier rig.

In the power supply end the peak demands only make up a small percentage of the average power usage which overall is well below the transformers continuous capacity.
 

Thread Starter

kriksis

Joined Feb 23, 2016
21
Add in the fact that if a transformer is rated for 100 VA it doesn't instantaneously burn up if a 101 VA load gets applied to it and that's how a lower VA rated transformer can live for years without burning up even in a hard run audio amplifier rig.

In the power supply end the peak demands only make up a small percentage of the average power usage which overall is well below the transformers continuous capacity.
So can I provide supply based on average power usage instead of peak usage? If so, then for 4ohm speaker outputs, I'd need only 57.8 VA per apm, thus the originally suggested transformer of 120VA could actually be used?
*sort of glad I tried to overshoot rather than go under. :)
 

crutschow

Joined Mar 14, 2008
24,055
According to this, the rule-of-thumb for the average audio power to a speaker for a very loud music listening level (85dB or similar to a loud movie theater ) in a medium size (3000 cu ft) would be between 50W to 80W.
Although that may sound like a large range of power it only represents a 2dB difference in perceived sound intensity, which is barely audible.
If you listen at a more sedate level (but still fairly loud) of half that perceived sound level or 75dB (a normal piano level is 60-70-dB) then the average amplifier power would be reduced to only 5W to 8W.
 

Thread Starter

kriksis

Joined Feb 23, 2016
21
Yesterday I disassembled old (sort of) radio and got transformer, that seems to be massifve. Problem is that I cannot find any official datasheet for it. After searching rtp2n3b013 I found the picture with the same transformer (attached), but all the info is in Polish. Despising that I got a hint, that it should contain 2x23vac and 2x40vac secondaries.
Problem, is that before this I haven't had to determine the windings, without any color reference. As I understand, then there should be pairs of terminals with continuity, correct? After a bit of googling, I found that the secondaries can be in phases and in such case, separate secondaries have to be determined by resistances in between. Can someone maybe explain this please? In some sort of simplified version.
 

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Dodgydave

Joined Jun 22, 2012
8,671
Start out by measuring each pair of outputs, mark them up with the voltage,

For phasing, put the same pair of voltage windings in series and if correctly wired will give you twice the voltage, if incorrectly wired will be near to zero.
 

tcmtech

Joined Nov 4, 2013
2,868
So can I provide supply based on average power usage instead of peak usage? If so, then for 4ohm speaker outputs, I'd need only 57.8 VA per apm, thus the originally suggested transformer of 120VA could actually be used?
Yes the odds are it will work just fine. To be honest for most average listening applications a 60 VA would probably still handle things well.

Build it and find out. At worst you find it can't handle your listening preferences and runs too hot so you then have to get a bigger one. If not life is good and you did a lot of worrying over nothing.
 
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