JW
Well-known member
Just wanted to share some observations that others might find useful. These are just my findings on my speakers in my room with a couple of mics and an a/b switch.
Background: I had a full 16 channel mixer at one point that I had fixed up nicely, and had about 8 channels left over that I had acquired with the intent to build out to 24 ehhh. anyway, sold that 16 channel board and now I have built up an 8X2 with spares and a Power One power supply.
So, first some observations when recently working on these channels:
First, I prefer the original "noisy" 2SC1681 transistors to the Motorola MPSA18 I tried as replacements. In this test I was switching mic output cables using an SM7 from one channel with MPSA18's and the other with the originals. (same input level on the input switch, with faders maxed on both channels. Channels are idential othewise, recapped with Nichicon PW caps and Wima polyester replacing the tantalum capacitors) The original (2SC1681's) sounded more wide and real to my ears. The Motorola was possibly a tiny bit more clear but a little duller in comparison. I tried this with an SM7 first. Then with an Electro Voice RE10, and consistently heard the same thing between channels. I like the original transistors and regret replacing them on 4 of the 8 channels. Hmmm. I still have most of them. Haven't decided what to do in this regard. If you're contemplating switching these things out to get lower noise, well, maybe don't or at least do it with the intention to try out a bunch of replacements, not just one like me.
Second ,I rigged up a switch to switch between a 1000uF cap and a 330uF cap for C37, the output coupling cap before going to the Tamura 600:600 transformer, wired as a direct out identical to most of the current preamp rackjobs out there. This was an interesting experiment. I had only read of one instance when someone recommended upwards of 1000uF cap here. http://groupdiy.com/index.php?topic=14588.0 So I was interested to try this out, as it had been recommended on tapeop or other places a few times to have a 270,330,470 etc. cap in place of the original 33uF.
So, I was switching between a 1000uF and a 330uF at this spot whilst playing the low string open on a Fender Jazz bass (DI'd this time into the PM1000, and I think the 1000uF cap seems preferable. (perhaps) The 330uF may be a bit more punchy in the low mids. The 1000uF though just seems more right and less hyped maybe, extended down a little? This was a subtle test that I'll probably try again. I might have been fooling myself.
Third, I rigged up my a/b switch to compare caps for C1 and then I did the same for C18.
C1 is the input cap. C18 is right after the fader. For C1, I was switching between a WIMA MKS2 3.3uF, a Wima MKM4 3.3uF cap, an Epcos 3.3uF polyester, and the original tantalum 3.3uF. The MKS sounds very similar to the Epcos, with the MKM seeming wider possibly (seemingly more clear down lower in the frequency spectrum?) but less punchy than the MKS series. The tantalum seemed thin and harsh much like they're accused to be. I think the MKM was possibly the best, with the MKS being not too bad, just more punchy and less wide seemingly. Why should there be such a difference in these cap positions, I don't know enough to explain, but it seems very clear to me. More dramatic than switching between different sizes of the C37 cap described above.
For C18, the observations were nearly identical to C1. These tests were done using the Electrovoice RE 10 on my voice, throwing the switch back and forth, a/bing between two caps at a time.
Of course, then I changed the mic to an SM7 and the results were reversed between the MKM series capacitors and the MKS series, where MKS sounded wider and more articulate and the MKM seemed more distant, though still with a seemingly more extended low end.
So, these effects are possibly negligible, and can be overcome or seem redundant in regards to what mic you're using?
I think, since the MKM series seems to be discontinued, (I don't know where I got this cap) I might try to do a couple channels (of the 8) with MKP series in spots C1 ane C18. The MKP's are metallized polypropylene, while the MKM was metallized "mixed dialectric" So, anyway, I'll leave 6 channels with the MKS caps that are already in there, at C1 and C18 and then maybe make two "special" channels that are better for singing into mics like the RE10. But my point is, it sounds to me like there is enough difference in these spots to experiment, and it can VERY probably sound better with something other than the tants that were there.
Things to try:
I'm gonna try changing the output transistors before C37 to 182 NPN? as I've heard this will allow for more current and less THD when driving the 600 ohm output transformer. But I'm pretty skeptical at this point about changing out transistors. Maybe the designers really knew what the hell they were doing. But we'll see.
I'm contemplating digging up the old 2CS1681 transistors and maybe putting them back in if I can find them. Hmmmm. I'm on the fence about this. Probably a lot of stress being taken out and then being put back in.
Questions:
I'm curious to hear any new discussion about the size of C37. Was I tricking my ears? Is 330uF enough to get the full frequency response and lower the phase shift out of the listenable range and drive a 600ohm transformer? That 1000uF cap was pretty big. Hmmmm
Also, C18, the cap after the fader is a .47uF cap (on the schematic.) I wonder why I ended up with a 3.3uF at this spot? Perhaps the channel had a 3.3uF tant originally that I was going off of (different from the schematic)
Anyway, I'm wondering if this cap (C18) needs to be this high. I can probably replace with a nice polypro and have a preferable sound. And the lower the capacitance the cheaper they are. Hmmm.
Also, has anybody had any problems with grounded the shield on the output to the chassis? I'm using 1/4" TRS output connectors, so they're not isolated from the chassis.
Background: I had a full 16 channel mixer at one point that I had fixed up nicely, and had about 8 channels left over that I had acquired with the intent to build out to 24 ehhh. anyway, sold that 16 channel board and now I have built up an 8X2 with spares and a Power One power supply.
So, first some observations when recently working on these channels:
First, I prefer the original "noisy" 2SC1681 transistors to the Motorola MPSA18 I tried as replacements. In this test I was switching mic output cables using an SM7 from one channel with MPSA18's and the other with the originals. (same input level on the input switch, with faders maxed on both channels. Channels are idential othewise, recapped with Nichicon PW caps and Wima polyester replacing the tantalum capacitors) The original (2SC1681's) sounded more wide and real to my ears. The Motorola was possibly a tiny bit more clear but a little duller in comparison. I tried this with an SM7 first. Then with an Electro Voice RE10, and consistently heard the same thing between channels. I like the original transistors and regret replacing them on 4 of the 8 channels. Hmmm. I still have most of them. Haven't decided what to do in this regard. If you're contemplating switching these things out to get lower noise, well, maybe don't or at least do it with the intention to try out a bunch of replacements, not just one like me.
Second ,I rigged up a switch to switch between a 1000uF cap and a 330uF cap for C37, the output coupling cap before going to the Tamura 600:600 transformer, wired as a direct out identical to most of the current preamp rackjobs out there. This was an interesting experiment. I had only read of one instance when someone recommended upwards of 1000uF cap here. http://groupdiy.com/index.php?topic=14588.0 So I was interested to try this out, as it had been recommended on tapeop or other places a few times to have a 270,330,470 etc. cap in place of the original 33uF.
So, I was switching between a 1000uF and a 330uF at this spot whilst playing the low string open on a Fender Jazz bass (DI'd this time into the PM1000, and I think the 1000uF cap seems preferable. (perhaps) The 330uF may be a bit more punchy in the low mids. The 1000uF though just seems more right and less hyped maybe, extended down a little? This was a subtle test that I'll probably try again. I might have been fooling myself.
Third, I rigged up my a/b switch to compare caps for C1 and then I did the same for C18.
C1 is the input cap. C18 is right after the fader. For C1, I was switching between a WIMA MKS2 3.3uF, a Wima MKM4 3.3uF cap, an Epcos 3.3uF polyester, and the original tantalum 3.3uF. The MKS sounds very similar to the Epcos, with the MKM seeming wider possibly (seemingly more clear down lower in the frequency spectrum?) but less punchy than the MKS series. The tantalum seemed thin and harsh much like they're accused to be. I think the MKM was possibly the best, with the MKS being not too bad, just more punchy and less wide seemingly. Why should there be such a difference in these cap positions, I don't know enough to explain, but it seems very clear to me. More dramatic than switching between different sizes of the C37 cap described above.
For C18, the observations were nearly identical to C1. These tests were done using the Electrovoice RE 10 on my voice, throwing the switch back and forth, a/bing between two caps at a time.
Of course, then I changed the mic to an SM7 and the results were reversed between the MKM series capacitors and the MKS series, where MKS sounded wider and more articulate and the MKM seemed more distant, though still with a seemingly more extended low end.
So, these effects are possibly negligible, and can be overcome or seem redundant in regards to what mic you're using?
I think, since the MKM series seems to be discontinued, (I don't know where I got this cap) I might try to do a couple channels (of the 8) with MKP series in spots C1 ane C18. The MKP's are metallized polypropylene, while the MKM was metallized "mixed dialectric" So, anyway, I'll leave 6 channels with the MKS caps that are already in there, at C1 and C18 and then maybe make two "special" channels that are better for singing into mics like the RE10. But my point is, it sounds to me like there is enough difference in these spots to experiment, and it can VERY probably sound better with something other than the tants that were there.
Things to try:
I'm gonna try changing the output transistors before C37 to 182 NPN? as I've heard this will allow for more current and less THD when driving the 600 ohm output transformer. But I'm pretty skeptical at this point about changing out transistors. Maybe the designers really knew what the hell they were doing. But we'll see.
I'm contemplating digging up the old 2CS1681 transistors and maybe putting them back in if I can find them. Hmmmm. I'm on the fence about this. Probably a lot of stress being taken out and then being put back in.
Questions:
I'm curious to hear any new discussion about the size of C37. Was I tricking my ears? Is 330uF enough to get the full frequency response and lower the phase shift out of the listenable range and drive a 600ohm transformer? That 1000uF cap was pretty big. Hmmmm
Also, C18, the cap after the fader is a .47uF cap (on the schematic.) I wonder why I ended up with a 3.3uF at this spot? Perhaps the channel had a 3.3uF tant originally that I was going off of (different from the schematic)
Anyway, I'm wondering if this cap (C18) needs to be this high. I can probably replace with a nice polypro and have a preferable sound. And the lower the capacitance the cheaper they are. Hmmm.
Also, has anybody had any problems with grounded the shield on the output to the chassis? I'm using 1/4" TRS output connectors, so they're not isolated from the chassis.
