[ACMP investiupgradifications] All things PREAMP

[diyfanatic]

keep us posted.

ooohhh you can count on that!  ;D

 

[mitsos]

OK, finally got around to doing some mods on this thing.. Namely the PSU mods and some others suggested by other lab members. Accidentally shorted my phantom supply while testing it (OOPS). I tested the NPN in my DMM and it gave me 1000 hFE (pretty nice, but the datasheet says 250 MAX) so I figured it was done.  Replaced it with a BC550 and was back in the game. So DONT SHORT YOUR PHANTOM SUPPLY! 
One thing I was looking forward to was replacing the 2N3055 with a NOS moto and rebiasing properly.  But I'm having trouble with my generator and scope. If I hook them up directly, I can see the signal nicely, and when I hook up the generator to the preamp while it's hooked up to my DAW, I can see/hear the signal there, but when I hook up the scope to the preamp I cannot see the signal in my scope. Weird?  Set to mic in, and low impedance.

 

[diyfanatic]

the PSU mods

what do you plan to do, change the value of caps or change the psu?
I want to know cause I'm not sure what is the best thing to do!

I've started to test the unit...pretty windy!
so I've shielded the inductor and the power tx with Hotbaby's shield and something buged me.

when I take a look at some 81 pictures, inductors are on board 3 & 4.
I've got 3 inductors on board 2,3,4!! is that normal?

anyway, still some noise (only on mic, line is totaly quiet) and less bzzzzzzz when eq is on.
it's funny because when I engage the eq, the buzz is pretty loud and after a moment it's starting to fade to a not so anoying level (but still not good)

I've ordered the tansistors, they'll be here in couple of days. we'll see then!

cheers.
Jr.

 

[okgb]

As requested earlier can we keep the posts to
relevant  Q & A only ?
[ it took me hours to complie half of this thread ]

3rd inductor  ? i 'd guess that's gonna pick additional noise

 

[diyfanatic]

yep, I only made shield for 2 inductors, need one more...
but I can't understand why I have a different board than every pics a I saw!
made I've got a wrong unit with 2x board 3...
I'll try to take a pict this week end!

thanks again Okgb!

 

[mitsos]

fanatic:

PSU mods were posted by Steve Hogan way back.. like January or a bit later.  I swapped the rectifiers for UF400x (only fast ones I had around besides MUR860 which would require serious bending and I didn't feel like doing it). Swapped the 2x2200uF filter caps for 2x1000uF  63V, swapped the little 10uF for a 100uF and upped the post regulator cap to 470uF as it was the largest that would fit. All nice panasonic caps. This was in the 24V area.  
Then I swapped the first cap in the phantom circuit (I would have swapped the rest but I had no more that would fit, and didn't change the rest). I also changed the little ceramic bypass caps to 0.1uF Wima MKS or something.

I started to get hum again and had to turn the toroid again, now it's gone.

I also get some weird buzz with the lid off, put the lid on and still the same, but it mostly went away when I screwed the top lid screws.

Questions:
1. I sometimes (not always) get buzz when I put my hand close to the preamp board (with the lid closed). Anything come to mind as a cause?

2. Has anyone tried swapping the preamp transistors for MPS-A18s or similar?  Should be a bit cleaner than the BC184c...mine has entirely too much "haze."

fanatic, about the PSU board I think it might be possible to design a new board that fits the same holes using a standard off-the-shelf toroid (2x25 for example). A standard toroid won't be as nice quality as Stave Hogan's custom but it would be much cheaper, and better quality than the crap that's in there now...

 

[diyfanatic]

hey Mitos, thanks for the psu tips!

Farnell is very quick, transistors ordered yesterday afternoon, received today!! 

I'll try take some pictures of my 3 inductors unit and to do the swap this week end if I have the time...

I feel my unit is like the guy with 3 nipples!  ;D

cheers.
Jr.

 

[crazydoc]

2. Has anyone tried swapping the preamp transistors for MPS-A18s or similar?  Should be a bit cleaner than the BC184c...mine has entirely too much "haze."

Here's from my post#742 back in May:

I have not been happy with the noise in the preamp at high gain. I bought 100 each of the MPSA18, BC550 and more BC184C's, thinking I could hand select lowest noise transistors for the first preamp stage. I tried 3 or 4 of each, without any notable difference when listening, so I gave up on it. Do you think this or any other mod might be worth pursuing to lower the noise?

I gave up on these preamps for awhile (waiting for Steve Hogan's mods - still not forthcoming.) I now have one 81 and one 73 with all my own mods that certainly work adequately - no hum, buzz or gain switch blast, but there is still more "transistor hiss" than I would like.

 

[mitsos]

OK, I missed that.. Though I did search. I was thinking of swapping ALL the 184C for MPSA18.  I too don't like the hiss on my 84, but it's only at really high gains, so maybe I'm nitpicking.

 

[clintrubber]

[W.r.t. noise & ACMP81]Maybe somebody with RealRuperts can chime in here ? I don't have any, but can bell imagine the originals aren't that noiseless either.

W.r.t. swapping BJTs, note that apart from the 'power-devices' in the DOA's to cure the bigger problem,
there are indeed a few earlier on in those stages that can be swapped for reasons of noise. BC557 IIRC, could be BC2*** or BC560C, as discussed on one of the earlier pages of this thread.

Bye,

 Peter[/W.r.t. noise & ACMP81]

 

[mitsos]

Those are PNPs ???  If you mean BC550C, I read a post where SteveH mentioned swapping the first transistor for an MPSA18 or BC550C (bit noisier), but I didn't think anyone had swapped all of them.  No real neves close by here, but I too am sure they are not noise-free. I'm basing my question on what "modern adaptations" of the neve design are using, GR and others. From net searches, I believe it's the MPSA18, maybe others as well?  I don't care for "authenticity," was more curious as to possible improvements. Anyway, was going to get a few to test... pretty cheap at mouser.

When you say power devices, which are you referring to?  There are 3 BC184C transistors per stage.

 

[clintrubber]

Ah, oops, remark was ACMP81-specific, I'll adapt.

 

[diyfanatic]

Hi guys!

I've done the mod pop, the shielding and the transistor swap on the 81.
- eq engaged, no more buzzzzzzzzzzzzzz! and almost no hum 
- I find the reason why I have 3 inductors...I've 2 board 4!!!!  ??? fµck...maybe I can change some cap value to match the board 3 but the inductor is anoying
- like mitos and crazydoc (could be a cartoon title) I still have some hiss at higher gain...but I'm pretty happy with this.

next, output tx swap and some psu mod!!!

 

[clintrubber]

- I find the reason why I have 3 inductors...I've 2 board 4!!!!  ??? fµck...

This is bad...,  quality control reaching new levels... poor situation !  Wait, no collectors item !  :

 

[mitsos]

keeps getting better and better....

 

[diyfanatic]

5 bucks to see the freak...it's collector!!  ;D

 

[mitsos]

solved my little buzzz issue. I posted that it mostly went away when I screwed the top on, but it would happen with a mic plugged (or dummy load) whenever I would go near the gain knob. If I touched the mic/xlr/some part of the case with my other hand, it would be silent.
Last night I had an idea.... Too much paint (fancy that!). Basically there is paint between the faceplate and the main case (where they bolt together), so the faceplate itself is isolated from the rest of the (grounded) case. Scrape that paint off (use a knife, paint is pretty thick, one thing our friends did right!) and it's gone.  Be careful bolting it back on though, I stripped one of those crappy pressnut thingies. No biggie, it's sturdy enough for now, but I will find a nut to put on the back of it... eventually.

 

[rodabod]

solved my little buzzz issue. I posted that it mostly went away when I screwed the top on, but it would happen with a mic plugged (or dummy load) whenever I would go near the gain knob. If I touched the mic/xlr/some part of the case with my other hand, it would be silent....Last night I had an idea.... Too much paint (fancy that!).

Thanks for that. I have a similar issue still, so I may try what you did there.

 

[mitsos]

hope that doesit for ya. post back

 

[okgb]

I would think he may have more luractive business
perhaps he thought he could slip this inbetween
bigger gigs but now is busy but with time & money invested
[ thus lack of refund ] needs to see it through
kind of sounded like someone was whispering in his ear

btw i had complied the first 17 pages of the thread
[ but this is before he started posting ideas ] we
should get the rest saved before anything happens to the info

And for those so inclined Cinemag can make the inductors
[ i got prototypes ] but it was inconclusive to whether this
solved any problems - at least at North American voltages
But Good inductors are available