[ACMP investiupgradifications] All things PREAMP

[rodabod]

I noticed the shift in input impedance of the resistor ladder too as it is switched. It is slightly unfortunate, but then again, the Neve values shifted too. Perhaps not quite as much though.

In case it interests you, here is a response graph of the stock TNC mic channel versus one which I adjusted the transformer networks for:

Mic transformers can be a bit more tricky since some modern mics present as little as 50 Ohms which usually means more ringing. What I've done in that instance above is set the response to fall closely to the original transformers.

 

[Steve Hogan]

I noticed the shift in input impedance of the resistor ladder too as it is switched. It is slightly unfortunate, but then again, the Neve values shifted too. Perhaps not quite as much though.

Mic transformers can be a bit more tricky since some modern mics present as little as 50 Ohms which usually means more ringing. What I've done in that instance above is set the response to fall closely to the original transformers.

It turns out that the higher the gain of the amplifier, the lower the input impedance. The first preamp stage has 30 dB of gain and has lower input impedance than the 28 dB second stage.  By backing off the first stage gain to 29 and increasing the second stage to 29 when operated in mic mode, it might be possible to better balance the load on the mic input secondary. Right now I am trying to figure out where to put the "off" position, to distribute the gain/attenuation in an optimum way.  From a user point of view making all the steps 6.67 dB apart will make the smallest change.  There is no position, however, with that gain structure that has the transformer directly connected to the second stage, as there was in the original popping topology.

In my twin servo preamp and other stand alone custom preamps I use a "Schoeps" switch which switches a pair of 60.4 Ohms in front of the Mic transformer primary. This builds out the low Z microphones like Schoeps and TLM 170's to about 150 Ohms so the tranformer response stays correct.

 

[clintrubber]

This thread remains growing w.r.t. valuable info, very nice!

Then this:

Has the topic of transistor types been brought up yet ?

The BC184 NPNs are there, but for the BC214 (PNP) the BC557 is substituted, which per datasheet has poorer noise performance. Might not matter that much, but since it has been reported that engaging the EQ increases the noisefloor one might be better safe than sorry.

There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

Bye,

  Peter

 

[rodabod]

There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

I did look into this briefly, and since I did have a bag of NOS Motorolas, I switched these in place of the original ST models. Interestingly, when re-measuring, I noticed a drop in THD by about a factor of five.... I'm not sure why this would be the case, but I didn't set the bias before or after swapping, although I was only testing the unit at zero-level.

 

[clintrubber]

There's also of course the 2N3055-thing, but might be a topic of its own & personal preference
(Motorola, RCA, NOS, etc etc...)

I did look into this briefly, and since I did have a bag of NOS Motorolas, I switched these in place of the original ST models. Interestingly, when re-measuring, I noticed a drop in THD by about a factor of five.... I'm not sure why this would be the case, but I didn't set the bias before or after swapping, although I was only testing the unit at zero-level.

I forgot which were 'the best' (...) but have some old RCAs that I probably use when the s'iron is going.
Factor of five, that's a lot ! From where to where ?
One would obviously expect THD to remain about the same when swapping devices & setting for same current again.

Bye,

  Peter

 

[rodabod]

Factor of five, that's a lot ! From where to where ?

Just checked my test reports; from "modded" to "modded plus 2N3055" I get a drop from 0.01% to 0.006%. That's a factor of two. I think I was previously comparing earlier figures (these preamps have now existed in various different stages, each with their own test results).

What makes the THD measurement a little more tricky is the PSU noise (not just from the unit itself but picked up from other racks) which interferes with the figures.

I'll maybe measure this again with another box. Interestingly, the PCB looked a little scorched where the heatsink is mounted before I even touched the circuits. It's either that, or somehow a dye is leaking from the heatsink onto the board (maybe from de-fluxer?)

 

[clintrubber]

Interestingly, the PCB looked a little scorched where the heatsink is mounted before I even touched the circuits.

Haha, no stone remains unturned.

We might need to groupbuy a few of these, if only for the title, OK, actually only for the title:

http://books.google.co.uk/books?id=bGEbLt1C168C&printsec=frontcover&dq=fun+voiding+warranty#PPR2,M1

8)



W.r.t. the THD, did you look at the spectrum ? Say sending 1 kHz & rotating the trimpot & see how harmonics go up & down ?

I realize we might not necessarily want to set for minimum THD, to 'nice-up' things.


Bye,

  Peter

 

[rodabod]

We might need to groupbuy a few of these, if only for the title, OK, actually only for the title:

http://books.google.co.uk/books?id=bGEbLt1C168C&printsec=frontcover&dq=fun+voiding+warranty#PPR2,M1

Hah!

W.r.t. the THD, did you look at the spectrum ? Say sending 1 kHz & rotating the trimpot & see how harmonics go up & down ?

I didn't actually. The thing is, the output sections on these preamps are like power amps. I'm only ever going to be driving around zero-level nominal, maybe +8dBu on peaks with my current soundcard. Certainly for me at least, the majority of the character comes from the preamp section, whether with the original transformers or with Carnhills.

 

[clintrubber]

W.r.t. the THD, did you look at the spectrum ? Say sending 1 kHz & rotating the trimpot & see how harmonics go up & down ?

I didn't actually. The thing is, the output sections on these preamps are like power amps. I'm only ever going to be driving around zero-level nominal, maybe +8dBu on peaks with my current soundcard. Certainly for me at least, the majority of the character comes from the preamp section, whether with the original transformers or with Carnhills.

I've read various thoughts on this, it's the iron, it's the pre, it's the tants, it's the Marconi-knob, it's the last section etc, but who knows, I guess in the end it'll be the sum of all contributions.

My plan is to combine this amp with a variable post-output-TX-attenuator, so to drive the output-stage & then level down to usable levels again.

Bye,

  Peter

 

[crazydoc]

Here's quite a bonus I found today while working on my 73. There is a metal ring that has been inserted on the preamp board during manufacture, coupling one of the input transformers with capacitors C1 and C2. In effect it makes this component group into a flux capacitor, bringing the current and voltage back into phase.

As an added bonus, when it is magnetically coupled with the 3 (yes, three) washers inserted between the 48V phantom switch and its standoff,  it allows me to travel the space-tone continuum with the touch of a dial, with a sound that is rich and intimate, spacious and articulate. It combines the eq boards through algebraic phasing with a high-power transformer drive to create an indispensable acoustical window.

 

[wmtunate]

I wish I could confirm your findings, but I haven't yet had the balls to get my '81s up to 88mph.

 

[okgb]

Some of the BEST QC ,

i didn't get that with mine , can you do up a bom for
that flux cap ?


anyone [ fwiw ] hear from chance recently about
the supposed warrenty on pres & mics ?

Well i mean curious if he's actually pretending to
or in the totally walk away phase

 

[redddog]

Warrantee. You some kind of funny boy?

 

[Baltimore]

anyone [ fwiw ] hear from chance recently about
the supposed warrenty on pres & mics ?

Well, I recieved 4 84s instead of 81s and I've been trying to get him to send me the right pres since January. He can't even pull that off...I realize that this is somewhat of a blessing in disguise, as my '84s are at least somewhat useable out of the box.  But i would love that extra eq on the 81s. Anyone wanna trade?

 

[Steve Hogan]

Here's quite a bonus I found today while working on my 73. There is a metal ring that has been inserted on the preamp board during manufacture, coupling one of the input transformers with capacitors C1 and C2. ...

Wow, a washer. Thats a nice surprise  , possible cap explosion waiting to happen.
and people have the nerve to tell me I'm not politically correct in bashing China quality on a case by case basis.
looks like it was planted by a rebellious employee.

Well call me blind and stupid.  I must confess that I thought crazydoc had started to live up to his namesake with his futuristic waxing poetic about preamps.  Funny bit, but I didn't get the point until electrochronic explained what I should be looking for in crazydoc's picture.  That piece of metal does not look like something that will add to long-term or even short-term satisfaction.

To this day I still haven't figured out what was going on with the front panel knob photos a while back in this thread.

Because the mounting tabs for the input transformers are not flush with their pin side bottoms, the factory has used washers to fill the spaces between the PCB and the tabs before hand-soldering in the transformers.  Obviously one of the washers got away from the assembler in this case and because it got stuck under the caps, didn't just fall out when the board was turned over.  Looks like the inspector (ha) missed it like I did.

 

[crazydoc]

Sorry about that, Steve.  ;D

My poetry was ripped off and modified from the CAD mic site:

http://www.cadmics.com/M179.php

I've long admired their hyperbolic mic descriptions.

 

[Spendor]

Hi there - just received my ACMP84. Fired up OK and prelim test seems OK  - quiet and clean. Has the jump on the gain of course so very keen for a solution there. I went looking for decaying resistors (looked OK) and found 5R11 on the mid EQ board missing. Could someone confirm whether this is universal or just missing on my board? if i do need to fix it what is the value? Thanks!

 

[okgb]

the knobs were a guise for the real improvement
felt marker over the logo

 

[rodabod]

found 5R11 on the mid EQ board missing.

From what I can see, 5R11 shouldn't be necessary. It shunts one of the inductors and was omitted.

 

[TB-AV]

"I believe that a more practical choice for resistor replacements would be Roederstein MK-2 resistors. I wouldn't wholesale replace the resistors, though, just the ones that show signs of physical deterioration and the ones that must be changed to fix the gain switch pop, etc."

Thanks for the guidance on the resistor replacement. Anyone know a good source for these resistors. I can't seem to find them.

Tom B.