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What is this opamp doing in this power supply schematic?
What is this opamp doing in this power supply schematic?
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Old 26th June 2020, 01:25 AM   #1
Hofmann is offline Hofmann  Netherlands
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Default What is this opamp doing in this power supply schematic?

I have this pcb that I got off ebay for a few bucks.
I'm building a preamp and I need a -15/+15v supply for it.
The schematic they included in the listing has slightly different resistor values compared to the ones on the pcb because it is for a fixed output. This is actually how I plan on implementing it. The board also allows for an adjustable output using trimpots.

Only thing that has me scratching my head is the LF353 opamp. What is the function of this? Does it keep the outputs equally symmetric?
Can I safely leave this out of the design? Or is it recommended I leave it in?
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Old 26th June 2020, 01:51 AM   #2
bimo is offline bimo  Indonesia
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I think the op-amp for noise reduction. Elvee used 2 transistors instead of op-amp.
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Old 26th June 2020, 02:04 AM   #3
nigelwright7557 is offline nigelwright7557  United Kingdom
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any noise on the output of regulator is inverted into regulator to cancel it out.
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Old 26th June 2020, 09:21 AM   #4
Elvee is offline Elvee  Belgium
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Yes, it is an opamp denoiser; it is also wired for a correction factor of ~30dB.

The advantage of an opamp is more convenient coupling capacitors values, the disadvantage is that the noise will be higher: you need a very good opamp to match a very cheap transistor like the BC337, and no opamp will match a transistor like the ZTX851.
But the circuit should work OK, and improve the PSRR and the output Z
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Old 26th June 2020, 09:36 AM   #5
Vovk Z is offline Vovk Z  Ukraine
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I may wrong but calculating the output voltage remains the same:
(2k2/200R +1)*1.25 =15V.
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Old 26th June 2020, 12:27 PM   #6
keilau is offline keilau  United States
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Quote:
Originally Posted by Hofmann View Post
I have this pcb that I got off ebay for a few bucks.
I'm building a preamp and I need a -15/+15v supply for it.
The schematic they included in the listing has slightly different resistor values compared to the ones on the pcb because it is for a fixed output. This is actually how I plan on implementing it. The board also allows for an adjustable output using trimpots.

Only thing that has me scratching my head is the LF353 opamp. What is the function of this? Does it keep the outputs equally symmetric?
Can I safely leave this out of the design? Or is it recommended I leave it in?
It is a dual LDO regulator with an opamp de-noiser. At $10.80 fully assembled, it is one of the best PSU board deal on e-bay.

The LF353 a good choice for its function. It is on socket and can be esaily changed if so desired.
AC-DC LM317 LM337 LF353 Servo Rectifier Filter Power Supply Module Adjustable M | eBay

But it is NOT a 1.5A output board. The heatsink should be fine for a class A line level or headphone amp biased at 200mA or so, but not much more.

Last edited by keilau; 26th June 2020 at 12:45 PM.
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Old 26th June 2020, 12:35 PM   #7
keilau is offline keilau  United States
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Quote:
Originally Posted by Vovk Z View Post
I may (be) wrong but calculating the output voltage remains the same:
(2k2/200R +1)*1.25 =15V.
The 2x5K trimmers give the board pretty good range for output voltage adjustment. Set the trimers at 2K2ohm if you want 15 volts outputs.

Last edited by keilau; 26th June 2020 at 12:46 PM.
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Old 27th June 2020, 12:38 PM   #8
Hofmann is offline Hofmann  Netherlands
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My local electronics component seller didn't have the LF353 in stock. It appears to be an obsolete opamp. Can I replace it for a NE5532?
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Old 27th June 2020, 03:10 PM   #9
Elvee is offline Elvee  Belgium
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There are many equivalents, TLO72, 82 and many others.

The 5532 could work, and would have a better noise performance, but there is a risk of instability. The compensation cap might need to be adjusted
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Old 27th June 2020, 09:30 PM   #10
PRR is offline PRR  United States
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What is this opamp doing in this power supply schematic?
LF353 was commercially replaced by similar but cheaper TL072, which your store sure should have. The '353 is still available, and costs only pennies more, but nearly all the early 1970s stuff that started on LF353 moved over to TL072, often as a drop-in, often without noting this on the docs.

'5532 "should work" and maybe less hiss but probably too little difference to tell.
 

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