Warm Audio WA-84

[Emmathom]

Polystyrene has a 240°C melting point, and is susceptible to solvents. Try out on a styrofoam cup
As these caps are often not potted, or use metallized foil, but use metal foil, alternate with polystyrene film, liquids may enter between the layers, the cap may have kicked the bucket.

I've ordered C0G/NP0 caps to replace all polystyren. Will desolder them, clean the board and then solder the ceramic ones...
470pF, 5pF and 15pF are present in the WA84. I've also ordered 1uF and 4,7uF in Nichicon Fine Gold to try instead of those tantalums

 

[wafelki]

In the meantime you can desolder jest the 5pf one... I think those Nichicons are meant to block DC in the audio patch but thats ok If you want to "calm down" the sound just change the C5 cap. This way the transistor will "receive electrons" not as fast as from tantalum cap, causing high frequencies to be "slower". For C8 I think the tantalum is better because it filters "faster". All my Tbone SC450's are made this way, but hell, what do I know Without the 5pf cap the mic should be just louder. Try it on the snare if you can

 

[Emmathom]

In the meantime you can desolder jest the 5pf one... I think those Nichicons are meant to block DC in the audio patch but thats ok If you want to "calm down" the sound just change the C5 cap. This way the transistor will "receive electrons" not as fast as from tantalum cap, causing high frequencies to be "slower". For C8 I think the tantalum is better because it filters "faster". All my Tbone SC450's are made this way, but hell, what do I know

C7 & C8 are Wima MKP 100nF to filter the PSU so I will let them as they are.
For the 5pF (C2) I have ordered C0G/NP0
For 15pF (C9) N0G/NP0 too (but this cap is only present on the WA84 pad)
C5 & C6 are filter/stocking caps > tantalum now but I will put Nichicon Fine Gold
C3 is the R source cap > tantalum now but I will put Nichicon Fine Gold
C4 is the coupling cap to OPT > tantalum now but I will put Nichicon Fine Gold

... and the Jfet is J112

To resume what's intalled now (WA84)
- tantalums for C3 - C4 - C5 - C6
- wima MKP for C7 - C8
- polystyren for C1 - C2 - C9 (C9 is not present on the original KM84 schem. but present for pad in the WA84)

 

[wafelki]

I would not touch C6 tantalum, its fine there. As for 1uf output - the tantalum will sound "stiffer" and more edgy, electrolytic soft and round, and film would be the most neutral

 

[Emmathom]

I would not touch C6 tantalum, its fine there. As for 1uf output - the tantalum will sound "stiffer" and more edgy, electrolytic soft and round, and film would be the most neutral

1uF film would never fit in this body anyway...
I will start by removing the polystyren's, clean the board (and try to take the coating off) and replace them with C0G/NP0... Then I will go further if needed and will leave C6 as long as it's ok.
Thank you for your advice

 

[wafelki]

1uF film would never fit in this body anyway...
I will start by removing the polystyren's, clean the board (and try to take the coating off) and replace them with C0G/NP0... Then I will go further if needed and will leave C6 as long as it's ok.
Thank you for your advice

Don't forget the sh*tty lead-free solder... You need to add some good old PB, suck it off, clean, add again... repeat...

Sure, I did quite a few R87's which are very similar so... been there Most of them ended up with another jfets btw...

 

[Emmathom]

Don't forget the sh*tty lead-free solder... You need to add some good old PB, suck it off, clean, add again... repeat...

I only use silver+lead solder (6% silver can't remember exactly the %). Iron set to 300° ~ 330° and fast soldering on delicate components

Sure, I did quite a few R87's which are very similar so... been there Most of them ended up with another jfets btw...

J113 in place of J112 maybe ?

 

[wafelki]

Seems ok. Some different types of solder mix ugly. Its not bad to use sanding paper on the leads of hi-z parts. NTE 312 and 2SK208 mostly.

 

[s2udio]

Quote
"I will start by removing the polystyrene's"

Polystyrene's are used because of the high tolerance to temprature drift, better than ceramics.
Just my 2 cents worth.

 

[jp8]

Quote
"I will start by removing the polystyrene's"

Polystyrene's are used because of the high tolerance to temprature drift, better than ceramics.
Just my 2 cents worth.

Nothing beats C0G/NP0 ceramics. The drift you are referring to, is related to Class 2 ceramics (X7R, X5R etc.). These should not be used in the audio signal path and are what give ceramic capacitors their bad name. But this applies definitely NOT to C0G/NP0. Lots of information available on the net. Links available here on my website.

 

[Emmathom]

Some report...

I've changed the polystyren capacitors for C0G and wired all components in front (470pF + 1GΩ + 4,7pF + 15pF) "en l'air" so components not touching the board... but the J112 has suffered from desoldering - soldering (I tried first a BF245) and its gate's leg broke so I changed for a J113

The mics works fine now : it gives +1,5dB more output level compare to the original one, but I've got some very low "rumble" that I don't have with original mic. @jp8 told me that the 4,7pF I've installed in place of the original 3,9pF (well the schematic says 4pF...) could be the issue (and also the J113 instead of J112 could affect FR). He will kindly send me two C0G 3,9pF and I'll check with this value.
When things will be ok with this mic I'll do the same mods exactly on the second one so to have a real pair.

To be continued...

 

[jp8]

Some report...

I've changed the polystyren capacitors for C0G and wired all components in front (470pF + 1GΩ + 4,7pF + 15pF) "en l'air" so components not touching the board... but the J112 has suffered from desoldering - soldering (I tried first a BF245) and its gate's leg broke so I changed for a J113

The mics works fine now : it gives +1,5dB more output level compare to the original one, but I've got some very low "rumble" that I don't have with original mic. @jp8 told me that the 4,7pF I've installed in place of the original 3,9pF (well the schematic says 4pF...) could be the issue (and also the J113 instead of J112 could affect FR). He will kindly send me two C0G 3,9pF and I'll check with this value.
When things will be ok with this mic I'll do the same mods exactly on the second one so to have a real pair.

To be continued...

I haven't heard a recording of the rumble @Emmathom refers to, but theoretically, low-end peaking could occur when the output impedance of the JFET is lowered by more feedback, which is the case when increasing the feedback capacitor. At least, the low-end will be extended. Output impedance is also affected by the JFET properties. I've explained this on my website here. Below is a copy of the normalized frequency response charts where the low-end response is plotted for various feedback capacitor values (ignore the high-end drop-off, which is caused by a capacitor connected to the secondary side of the OPT). As can be seen, low-end FR is very sensitive to the feedback capacitor. Now imagine what happens if C2 is further increased to 4.7pF and add to that the higher Cgd of the J113 compared to the BF245. Then FR will extend to very low frequencies that can be experienced as rumble.

I'm curious if reducing C2 to 3.9pF will completely cure the rumble issue. But at second thought, it might be necessary to change the JFET also back to the original BF245, or reduce C2 further down to maybe 1pF or 2.2pF or so due to the higher Cgd of the J113.

 

[Emmathom]

I'm curious if reducing C2 to 3.9pF will completely cure the rumble issue. But at second thought, it might be necessary to change the JFET also back to the original BF245, or reduce C2 further down to maybe 1pF or 2.2pF or so due to the higher Cgd of the J113.

The original Jfet installed was a J112 but its gate's leg broke during mods... and I don't have spare J112 but a lot of J113 so that's why I put one...
I note that 3,9pF may be still to much and should already chase a lower value (Mica or C0G)
Thanks Jan