[ACMP investiupgradifications] All things PREAMP

[stereokillah]

yes thank you Alexc i've readen all your post the 40 pages in fact.

but you would to say , even after changed the transistor the stay but  less after if i change the inductor to carnhill inductor the noise is reduced but stay again more than 80 db ?

I think to change the output transfo to carnhill 1148 but i don't the sepcification of the chineese one.

i'm a older newbie i lurn.

 

[0dbfs]

Hey mono-maker,

My understanding from alexc's work is that using the carnhill mu-metal trafo-can you can basically eliminate the mains based inductor hum without replacing the original inductors themselves.

That means that at a minimum one would need three mod's to make these channelstrips useable with the full intended functionality:

- transistor swap.
- mu-metal can for the two low-mid eq bands.
- gain stage mod to prevent the pop if desired.

From there Steve has provided kits and/or services to upgrade other parts of the units which provide better signal path and longevity improvements to the unit.

Alexc, please correct me if I am wrong about the carnhill cans and original inductors.

Btw, I have carnhill cans ordered and on the way and will be able to confirm alexc's findings soon. All indications seem to point in the positive direction for that as far as I can tell right now. I don't expect to add any new information to the current pool of knowledge.

Cheers
J

 

[crazydoc]

I hate to beat a dead horse here, but for those with continued doubts about the EQ fix - FIRST CHANGE THE TRANSISTORS!

This fixes that horrible noise you hear when you switch on the EQ pushbutton switch, even though all the filters are switched off. I haven't heard of any other real fix for this problem.

You're not going to even hear the inductor noise (which is what you'll hear when you switch each EQ filter on and through its frequency settings) until this is done.

 

[crazydoc]

BTW, Steve Hogan is alive and well - just buried in other work to keep his head above water. He says he will post this week with pics of the new transformer in place.

 

[alexc]

>but you would to say , even after changed the transistor the stay but  less after if i change the inductor
>to carnhill inductor the noise is reduced but stay again more than 80 db

Stereokillah - Firstly, changing transistors removes the first 30dB or so of noise.
Then, when you engage mid-lo and max boost at 50Hz-300Hz, you will see an extra 22dB or so appear at mains-harmonics.
Replacing inductors reduced this by about 12dB or so.
Adding shields to the replacement inductors reduced it another 8db or so.
Similarly in the mid-hi band but to about 1/2 as bad overall to begin with.

These numbers are repeatable to around 3-5dB in general, depending on proximity of unit to other powered on units.
I note that your '80dB' may or may not be the same as my '80dB'.


>My understanding from alexc's work is that using the carnhill mu-metal trafo-can you can basically eliminate the
>mains based inductor hum without replacing the original inductors themselves.

0dBfs  - what you say is correct, except I use Carnhill inductors + OEP mu-metal transformer shielding cans
           and I would suggest 'reduce' rather than 'eliminate' when using stock inductors + shields.

I very strongly recommend replacing inductors with a shielded, higher quality part to get
the best performance with respect to the system mains-harmonics noise.

If that is not possible, then try shielding stock ones.

Crazydoc - I don't understand. Change the transistors, you say? Will that help? ;D

 

[okgb]

Hey Alex [or anyone ] have the pin out handy for the 81 inductors ?
the cinemags arrived today , and when i get up tomorrow i'll look at
installing them  [ using the old cup or not  ]
they seem to have model numbers on them , that's promising

 

[stereokillah]

ok fine and soory i would to say -80 db

i 've oredered  BC327-16 and BC337-16 plus diodes bat86 like Steve Hogan cause i don't find BC441 and BC461 with -6 mentioned
will oredered carnhill .  vtb9048  .  vtb9047 and output transfo 1048
So i can't find too the shielded from oep on farnell web site.

 

[stereokillah]

i find this can perhaps will be fine

http://fr.farnell.com/jsp/search/productdetail.jsp?sku=4446720

 

[crazydoc]

i find this can perhaps will be fine

http://fr.farnell.com/jsp/search/productdetail.jsp?sku=4446720

I see two possible problems. One is it is probably too large to fit on the board. The second is that it is made of fer-blanc électrolytique - electrolytic tinplate. This may not be the equivalent of mumetal in its RFI/EMI rejection.

This can from Farnell was suggested earlier in the thread, which is probably of more appropriate size and material:
http://uk.farnell.com/oep-oxford-electrical-products/a262can/screening-can-transformer/dp/1172345?crosssellid=1172345&crosssell=true

 

[stereokillah]

thank you Crazydoc, all RFI/EMI rejection in farnell france was in aluminium. i thought it was bad for this solution.
For sure  this are smaller

 

[okgb]

Cinemag inductors installed ,
seem inconclusive at the moment to stop the Mains related hum ,
so far without measurment gear just a mic in hand
i used cable ties for the moment and extended the leads

with all gain at zero switching eq in does increase hiss ,
the inductor bands seem to be a little noiser closer
to the lowest freq settings depending on gain  ,
[ but in a broadband way not hum ]
will try sheilding them next , this is just me with a 57 mic talking
into the unit . [ and in a room where you have to twist the mic
like a gtr pickup to get minmum noise  ]

I used the 2n2907 & 2222A transistors but they do seem to
get fairly warm , anybody been running those for a while  ?

 

[stereokillah]

what is the method to change  the inductor to the carnhill .  vtb9048  .  vtb9047
is it the same link on the place, sorry i don't know how to say that in english.
i need to do some modification ?
Resistor or other

Thank you

 

[okgb]

take the old inductors out , make sure you match
the taps to the inductance of both
[ the chinese seems to start on the pin marked 2
on top of the white cup , use this as your lowest
starting tap of new inductor going clockwise looking
from the top ]

Anyone want to see the Cinemags check back
to the previous page . they don't have the mu metal
cases , anyone know a good / cheap source ?

 

[stereokillah]

Thank you okgb,
it was the night i did'nt see the number on the chinese inductor .
with you help i could put it easily.

 

[clintrubber]

with all gain at zero switching eq in does increase hiss ,

Hi,

'Hiss' as in 'noise' or 'hum' ?

Apart from 'the really wrong ones' you already addressed, IIRC there were some transistors that could be changed to lower-noise, see somewhere earlier (...) in this thread. This shouldn't matter much with most gain already pre-EQ, but just in case...

Bye,

  Peter

 

[okgb]

What i am experiencing is a system type noise
more hiss and less hum unless i have the lo mid on the 220 hz
although there is an almost ground like hum that lessens if
i cut at the 220 and this goes away totally with eq switched out

this is consistent with either the Chinese inductors or the cinemag
which likely indicates shielding needed , if not something done about
the power xfmr  , It may also be unrealistic amount of gain but i guess
they won't get used for Narration .

Again , anybody using thier's with the 2n2907 & 2n 2222a transistors ?
mine seem to through allot of heat , you can feel it as soon as you take
the cover off , i'm wondering about these in the long term .

 

[0dbfs]

I have found that shielding the original chineve inductors with the oep mu-metal cans reduces the mains hum to a useable level.

I have also experienced the hiss with the eq engaged. No specific fix for that yet that I am aware of although rotating and nulling the toroid affects the quality of that hiss.

I still think that locating the toroid outside the unit will make it work best.

FWIW, I have an api 3124 where the 4th preamp has some mains induced hum. That 4th pre is located right next to the toroid.

Cheers,
J

 

[crazydoc]

Again , anybody using thier's with the 2n2907 & 2n 2222a transistors ?
mine seem to through allot of heat , you can feel it as soon as you take
the cover off , i'm wondering about these in the long term .

I have them in mine, and have not noticed any heat issues (nor were the original transistors overheating.) I haven't specificaly checked their temperatures, though I'll do so this next week if I get a chance.

I too have noticed increased hiss when the eq is switched in at high gain. I figured this was just due to the design, but am now wondering if the 2N2222 and 2N2907 could be contributing? Maybe I'll try different substitutions to see if there's any audible difference.

 

[stereokillah]

sorry have read all the 40page but my english comprehension is poor.
and don't fine or don't remember me solution for the pop when you make the eq ON.
on my side transistor change is very good , i've change c11 and c16 like said  on the thread and change the iec ground to make it on the screw hole in the right side of the rack.
and i win 1to 2 db.
now the signal is about -64 db when eq is on all gain  of max (eq in and out) i'm waiting for changed the inductor and sielded it.

thank for all your help.

 

[stereokillah]

other thing, when i touch the switch (without the knobs) of the mid low frequency and the hi low a big signal appears.
but it's not the case o n the other switch.