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DUAL Merus MA12070 fully balanced Amplifier pictures inside

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The big ones. 2200 uF right besides both the MA12070's. According the datasheet the E indicates that it is rated at 25V.
Must be, as there is no 2200uF at 35 or 50V in the datasheet.

See datasheet: https://www.mouser.de/datasheet/2/315/ABA0000C1247-1144552.pdf

Did we read the same datasheet? It's the opposite of what you said. There is no 2200uF with 25V, but there is a 2200uF with 35V!

That means the big caps are 35V just as the other smaller ones.
But there are very small 25V rated ones in the middle of the board, towards the backpanel, supplying the 2x7.5V circuit leading to the 15V DC out on the back and probably the NRCJ volume controller. But maybe those get stabilized by another module before?
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Several 35v here : Sabaj A20a modification... worth the effort?
(never found a lytic rated under 50v sounding as well as 50v and over, even for 3.3v rail...)

Thats an interesting remark. Has this to do with higher dV/dT at higher voltages like you see specified for film/foil caps?

I mostly have 50V caps. But the old Philips dry aluminum (blue with the red) caps sounds still the best to me. You get of everything more. But these are only low voltages. The few I have still have are 16 till 25V.
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Ah, I see what is going wrong. Sorry, I linked to the FKS series. And indeed that series has a 2200uF / 35V, still the E stands for 25V not 35V. The FKS 35V has also 18mm instead of 16mm diameter.

The cap in the A20a is a FK. This is the correct datasheet: https://www.mouser.de/datasheet/2/315/ABA0000C1181-947564.pdf

No, wait a second. It can't be "FK" because they look different, having a black marking left side which is larger. It must be "FKS" even tho it states only "FK" on the cap, since in the datasheet the "FKS" ones also have "FK" only printed on them.
The only thing misleading is the "E" on them, which should be a "V". I will measure the size next time to go sure.

Im not sure if running a 25V cap@26.13V is much of a problem at all.
That's only ~4,4% should be within the tolerances of about any module.

edit: Seems okay do to, if I read this thread correctly: Is it okay to use 25v rated caps in 24v rail output of SMPS - Badcaps Forums
But since they all advise to go for 35V caps on a 24VBoard, I strongly assume Sabaj was sharing the same level of knowledge those guys have and went for 35V caps.
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"...Applied voltage does not influence life time too much if an applied voltage to the aluminum electrolytic capacitor is below 0.9x the rated voltage but can be a factor of sudden failure."

This is what I do not like...

Also this FK cap has an ESR of 0.035 and a ripple current of 1800mA.
A FM or FR of just 1000uF (through hole) has an ESR of just 0.016 and a ripple current of 3320 mA (50V rated). So half the ESR and almost double current.
These will fit in also. I guess this will be a good replacement.
Never had sudden failures with my A20a which I've been using since the end of june this year. Neither with ~23V(default setting) nor with ~26V on the Mornsun, nor with ~26.13V on the new Meanwell RPS-400-24.

I believe those smaller 25V caps downside of the board near the back panel are coming in line after that copper-wire-transformer on the left who is assumambly converting the 24/26V mains input to a 15V which is both going to the 15V back out and being filtered and later divided into two +/-7.5V paralleled in order to feed the NWRJ volume controller who needs those 2x7.5V. So those 25V caps are for a 15V line, not for the mains, which makes sense.
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Connected with XLR direct from the dac, the volume of the amp at maximum (0db with no music) and the ear glued to the ribbon tweeters, it is total silence.

impossible to say if the amp is open.

I am going to test the RCA input as some have mentioned that there is a slight hissing sound.
The A20a is transparent, detailed and true to the source. If the track has low bass, you will get low bass. If it has room-shaking bass you will get that. Seriously on bass heavy tracks I feel it's even close to being too much (woofers visibly bumping out of my speakers). The power is definately there, it's just not being shown with every track. I know that there are amps that give you heavy bass on any track regardless, but most of them especially cheaper ones are distorting the sound/EQing a "U" form, raising both ends of the frequency spectrum, that is probably why you sense a lack of "dynamics" with high frequencies too. You can put the A20a in "E3" mode, which is "dynamic bass" that will basically do the same. Or -best- use a subwoofer.
I finally conducted an operation on the A20a.

[x]Replaced Mornsun PSU with Meanwell RPS-400-24 (@~26V)
[x]Drilled a hole in the back panel, put a cable from the 5Vout of the Meanwell PSU through to feed my DAC from it.
[x]speaker-out-cables, speaker-out-panel and 24v-mains cables have been shielded additionally. (copper foil wrappage with rubber coating)
[x]Rubber feet replaced with much bigger rubber feet. (better ventilation underneath+better looks)

It's too early to draw a full conclusion, but it's very detailed now, probably more detailed than before. I am partially hearing new cues in known tracks and there is more detail/nouance in the bass, but do not overestimate this, because the DAC is running on the 5Vout of the Meanwell now (which is lower ripple than any other 5V adaptor except for linear ones). That probably contributes most to it. Unfortunetely I am also having a ground loop from this 5Vout making the speakers whine/sing <60% volume. Trying to find a solution to it. Other than that, it sounds great, very "clean", "deep" and "black".

[x]Next step is plugging the FAN-out to my DAC to see if there is improvement with the RPS-400 in theory being able to go up to 470W (instead of 290W w/o FAN). Both before hiccup.


instead of copper foil, can we use aluminum foil?
Don't insulate anything with copper as I did. Start with the ferrite ring to the AC cable short before the plug. That alone should improve this amp quite a lot. Probably even more than insulating the internal cables. If you feel like there is still room for improvement after that, I would either insulate the SPK-cables (internal) or twist them and put some small ferrite clips to it as daniboun suggested.