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Old 12th February 2019, 03:18 AM   #11
rayma is offline rayma  United States
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Quote:
Originally Posted by gigigirl View Post
Thanks for that guys.
All you really need is to add a 2x18V transformer, not two power supplies.
The board already has rectifiers and capacitors.
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Old 16th February 2019, 04:04 AM   #12
tribute is offline tribute  Australia
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Join Date: Oct 2017
Default Still not quite there

Gentlemen, thanks for the advise. I pooped around to a friend's house this morning as he advised he had a trans that may do the job, (as Rayma advised all I needed was a trans with 18-0-18 V).
Unfortunately the trans was only 15-0-15 v, but when connected up it gave a output DC voltage of +20/-20.
So I am now not sure if I buy an 18-0-18 V trans it will get me the extra DC volts I am looking for.
Sorry I do not understand the hole thing, the add says it cannot boost, but I can get an extra 5 volts from a 15-0-15 input. Is the information for this P/S just completely wrong or am I still missing something.

Very confused, Cheers
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Old 16th February 2019, 04:13 AM   #13
Michael Bean is offline Michael Bean  United States
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Location: Minneapolis, Minnesota, USA
Quote:
Originally Posted by rayma View Post
All you really need is to add a 2x18V transformer, not two power supplies.
The board already has rectifiers and capacitors.
He said he needs +/- 28 volts. A 24 VAC (X2) transformer should do the trick.

Mike
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Old 16th February 2019, 10:06 AM   #14
jonners is offline jonners  United Kingdom
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@tribute

Multiplying the transformer AC voltage by 1.4 will give you the approximate rectified DC volts. However if you are running the transformer at less than its maximum specified current the voltages will be somewhat higher than nominal.
I suggest you find and read a basic guide on audio power supplies.
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Old 16th February 2019, 01:01 PM   #15
FauxFrench is offline FauxFrench  France
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Join Date: Dec 2017
Location: The mountains, calm and quiet.
Quote:
Originally Posted by tribute View Post
Gentlemen, thanks for the advise. I pooped around to a friend's house this morning as he advised he had a trans that may do the job, (as Rayma advised all I needed was a trans with 18-0-18 V).
Unfortunately the trans was only 15-0-15 v, but when connected up it gave a output DC voltage of +20/-20.
So I am now not sure if I buy an 18-0-18 V trans it will get me the extra DC volts I am looking for.
Sorry I do not understand the hole thing, the add says it cannot boost, but I can get an extra 5 volts from a 15-0-15 input. Is the information for this P/S just completely wrong or am I still missing something.

Very confused, Cheers
Hi tribute,

Mike and jonners have given you correct information but it can be difficult to get the overview.
You want +/- 28V at the output of the voltage regulator board for your phono amp.
The voltage regulator board is based on LM317/337 that need a minimum of 2-3V drop from input to output in order to regulate. Thus, we need a minimum of 31Vdc at the input of the regulator board. The diode voltage drop is included and the input voltage to the regulator board may be higher than 31V (but not below 31V) because the regulator ICs will just take-up that extra voltage drop.
If we try to calculate the situation with a 2x24Vac (nominal) transformer as suggested by Mike, we do as jonners says and multiply with 1.4 (more precisely 1.416) and arrive at a rectified voltage of 34Vdc. 34Vdc is higher that the 31Vdc we need as a minimum so all is fine. When you make such a system and measure the input voltage to the regulators you will have a little more voltage because, as jonners says, the 24Vac are at full (nominal) loading of the transformer and with full voltage drop from winding impedances. You will probably measure some 36Vdc, which is still fine because the (series) regulators will just include that further tension in their regulation such that you still have 28Vdc at the output.

Above I do not speak of both the positive and negative voltages which you will evidently need because the calculation is the same for both voltage circuits.

So, as suggested by Mike, a transformer with 24Vac-0-24Vac or two 24Vac secondary windings is just what you need.
Assuming your phono amp is not very current hungry, you will need a transformer power of around 20VA.

For a start we discussed stacking two 30Vdc SMPS as input to the regulator board. That would result in the regulators (almost) saturating (and not working as active regulators for which 31Vdc is needed) but you would still get around 28V output because a saturated regulator has a voltage drop close to 2V.

I hope this explanation helps on your confusion.
Enjoy the last part of the summer - up here on the northern part of the globe we are waiting for the sun!

Last edited by FauxFrench; 16th February 2019 at 01:16 PM.
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Old Today, 03:35 AM   #16
canvas is offline canvas  Taiwan
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The board takes AC-input and you were sending DC? That's fine. You need to input at least +33V/-33V DC into this board to get the desired voltage.
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