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"Upgrading" an old integrated amplifier - Power supply advise
"Upgrading" an old integrated amplifier - Power supply advise
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Old 4th October 2019, 03:32 PM   #11
SVI2004A is offline SVI2004A  New Zealand
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"Upgrading" an old integrated amplifier - Power supply advise
Just bang in STK4913, bigger xfmr n resivoir caps

LM series is American produced monolithic and though streets ahead of class D is lacklustre to the serious STK chips
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Old 4th October 2019, 03:48 PM   #12
MadCarburetor is offline MadCarburetor  India
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Originally Posted by FauxFrench View Post
If you mean using the D and F power supply outputs for the LM3886 amplifier, it is clearly NO GOOD IDEA. I have problems reading the component values of R1 and R2 but when you use drop resistors and shunt zeners it is because you only have a very limited output current. It may be sufficient current for the phono preamp but it will be clearly insufficient for the LM3886 amplifier that will just pull the voltage down.
The +/- 42V outputs (A and B) will have to be used for the LM3886 amplifier but you can use some drop diodes before the big decoupling capacitors if you like less voltage. However, your +/- 42V is with no loading at all. With loading from the LM3886 amplifier, I guess you will be close to +/- 40V idle. Playing music on the amplifier will cause the voltages to drop another 2V.
Also for the big decoupling capacitors I have problems reading the value. I get the impression that the value is four digit and starting with "3". The total decoupling capacitance on each rail should be in the order of 10000uF. So, you should probably add some 4700uF (50V) on each supply rail.
Thanks for your advise.

If I understood correctly, the diodes can be connected in series where "A" and "B" outputs are marked on the schematic, is this right?

R1 and R2 are 6800 Ohms.
The big caps are 3900uF.

I also just picked up this cheap STK4141II board to try out in the Sansui with this power supply, until I get the time to build the LM3886s


Quote:
Originally Posted by SVI2004A View Post
Just bang in STK4913, bigger xfmr n resivoir caps

LM series is American produced monolithic and though streets ahead of class D is lacklustre to the serious STK chips

Maybe I will just stick with the STK if it sounds good! I noticed that there is a lot of praise online for LM3886, but not much information on the STKs.
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Last edited by MadCarburetor; 4th October 2019 at 03:54 PM.
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Old 4th October 2019, 04:21 PM   #13
FauxFrench is offline FauxFrench  France
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No, you put drop-diodes in series between the "+"-terminal of the bridge rectifier and the big cap for the positive rail. Similarly, between the "-"-terminal of the bridge rectifier and the big cap for the negative rail.
The idea is that they drop the voltage some 0.7V per diode, but as a bi-effect they will also add a little to the output impedance of the power supply. If you put the diodes as described above, they will only add impedance at low frequencies (and DC) and not disturb your higher frequency transients.
If you just add them at the "A" and "B" outputs, the diodes add impedance in the whole frequency range and may affect your transients.

3900uF, you better add 4700uF on both rails.

Last edited by FauxFrench; 4th October 2019 at 04:23 PM.
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Old 6th October 2019, 04:56 PM   #14
MadCarburetor is offline MadCarburetor  India
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Quote:
Originally Posted by FauxFrench View Post
No, you put drop-diodes in series between the "+"-terminal of the bridge rectifier and the big cap for the positive rail. Similarly, between the "-"-terminal of the bridge rectifier and the big cap for the negative rail.
The idea is that they drop the voltage some 0.7V per diode, but as a bi-effect they will also add a little to the output impedance of the power supply. If you put the diodes as described above, they will only add impedance at low frequencies (and DC) and not disturb your higher frequency transients.
If you just add them at the "A" and "B" outputs, the diodes add impedance in the whole frequency range and may affect your transients.

3900uF, you better add 4700uF on both rails.
Thanks for clearing that up.

I have just ordered some larger caps to add on the rails.

Had a question about the STK board. I compared the circuit to the STK chip data-sheet, it is almost identical, but the data-sheet for the chip shows the negative sides of C5 and C6 connected to pins 1 & 18 respectively. However, on my board it is the other way around (positive sides are connected to the pins). Is this an error on the board or does the polarity not matter in this case?
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Old 6th October 2019, 05:49 PM   #15
TBTL is offline TBTL  Germany
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It's OK. C5 and C6 block dc. If the source that is connected to the inputs has no dc-offset, there will be zero voltage across C5 and C6 and their orientations do not matter. Most sources have no dc-offset on their outputs.

If a source has a dc offset, it usually is positive. Then the + of the capacitor should go towards the input.

Did you buy the STK chip through a reputable seller? Some versions are being counterfeit. A counterfeit STK chip poses a risk to your loudspeakers. Reputable sellers might have some new old stock.

Last edited by TBTL; 6th October 2019 at 06:03 PM.
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Old 6th October 2019, 06:42 PM   #16
MadCarburetor is offline MadCarburetor  India
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Originally Posted by TBTL View Post
It's OK. C5 and C6 block dc. If the source that is connected to the inputs has no dc-offset, there will be zero voltage across C5 and C6 and their orientations do not matter. Most sources have no dc-offset on their outputs.

If a source has a dc offset, it usually is positive. Then the + of the capacitor should go towards the input.

Did you buy the STK chip through a reputable seller? Some versions are being counterfeit. A counterfeit STK chip poses a risk to your loudspeakers. Reputable sellers might have some new old stock.
I bought the board off Amazon. It's definitely of dubious origin so I only plan to use it with my cheapo test speakers until I have the time to build a proper amp out of the LM3886s or a genuine STK ic... What is a good online source for genuine STKs?
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Old 8th October 2019, 12:48 PM   #17
Kay Pirinha is offline Kay Pirinha  Germany
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Quote:
Originally Posted by SVI2004A View Post
Just bang in STK4913, bigger xfmr n resivoir caps

LM series is American produced monolithic and though streets ahead of class D is lacklustre to the serious STK chips

Who is/was the manufacturer of those STK's? I assume it was Sanyo? As they had to close their semiconductor division due to the 2004 tsunami, there aren't no genuine Sanyo STK devices available any more. And yes, Mouser doesn't list the STK4843 nor the 4913.
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