Update from Steve Hogan
To say that I am overdue for posting on this forum is the understatement of the year, and probably last year, too.
After ordering your preamps from Chance, waiting a long time to get them, finding out they had problems, getting excited about my mods, and following my almost daily posts as I dug into these preamps to solve their issues, all of a sudden I dropped off the map as if I had fallen into a black-hole.
I must take full responsibility for my lack of communication, for which I owe an apology to the entire group and especially to those who entrusted me with your preamps and deposits. There is no excuse for me not making the time, no matter how slammed, to let you all know what is going on with me and your parts kits and preamp repairs. I optimistically thought my next post would contain another piece of the puzzle -- another problem solved, but as time went on I was also embarrassed that so much time had passed -- no excuse, just trying to understand my own psyche.
Another element of my pysche is that I have not yet gotten a handle on is that of being overly optimistic of just how much I can do in a given time frame. I seem have a real knack of getting myself into projects that start small and grow into huge. An example is this ACMP project. A friend told me that there was a group buy of preamps that needed a new power transformer. When I got into it, however, it turned out that a new power transformer was just a small part of the problems that needed addressed. Another example is the studio install that was supposed to be 2 days, but ended up requiring rework of every connector, and a complete rebuild of the console's master module and power supply. I spent the first 14 days or 2010 getting that project done, and it put me 10 days behind on other things, including finishing up the work on the ACMP preamps.
Here's what happened when I entered the "cone of silence". My biggest client from which I derived about 80% of my monthy income suddenly fell on hard times, and as a result so did I. I was faced with finding several new clients and new business in a real hurry. The situation put me in a hole that I am still digging out from. My need to generate immediate, substantial income made me feel that I had no choice but to put the preamp R&D aside until the crisis had passed. The problem is that I am still having to work "fast cash" jobs to keep up with the bills and I have been working as many hours every day as I could and just barely making it.
You might ask "Why not start shipping out the ACMP repairs and parts kits then, to help with the income deficit?" The answer is that although a substantial amount of work had already been done to resolve the problems with the preamps, the R&D was not finished, and I felt that I was unable to spend required additiional time on it right then, due to my need to work on projects that would result in immediate cash. When this all came down, I had already spent all the deposit money on the first production run of new power transformers and other parts, so that money was unavailable to support the needed continuing R&D.
So Let me bring you up to date:
At this time the following R&D is complete and parts are available to implement the mods.
1. New Power Transformer, correct mounting hardware, Nomex insulator to prevent accidental mounting bolt contact with lid.
2. Mods to Power Supply PCB, including Fast-Soft Rectifiers, Power Transformer damping network, new electrolytic caps with proper voltage ratings and values, Current Regulator diode for phantom regulator,new resistors for 317 regulators. There is a tiny bit of work needed to finalize the Phantom regulator zener topology and pass transistor.
3. '81 replacement transistors on EQ cards.
4. '81 schottky Baker clamp diodes on EQ cards
5. '81 0.1uF PS decoupling caps for bipolar 15V on EQ cards
6. '81 larger electrolytics on eq cards.
7. Phantom feed circuit changes: matched PRP resistors, 200 Ohm feed resistor, 220uF/63V cap, green wire and ground lug to add to phantom switch to discharge phantom circuit when "off"
8. Reworked some of the resistor values on the line level preamp card.
The following R&D is started but not yet finished:
1. Finalizing all the changes needed to the preamp card to rework gain structure, fix pop, terminate transformers. Because the ACMP preamps used a different switching arrangement than the Neve preamps, they ended up with fewer gain steps and the big "POP" due to temporary positive feedback between ranges. The ACMP preamps also split the gain between first and second preamp stages differently then the Neve method. My solution for the ACMP was to add an "OFF' position to the gain switch and distribute the gain equally over the rest of the range. However, this ends up in an awkward distribution of gains between the first and second stage that I have not yet resolved.
I have already spice modelled the entire preamp (both sections) and found that the input impedance that loads the mic transformer, and consequently the microphone, changes with the gain of the preamp. The higher the gain, the lower the input Z.
I still need to find an optimized set of gains to minimize the mic loading and give the best S/N ratio.
Once the gains are finalized, then the input impedance will be finalized, and a proper RC network can be found to tame the overshoot and ringing on both input transformers. I have already determined that they are capable of quite excellent transient response with the right RC network applied. There are a couple of Electrolytic caps that should be made larger as well, since they have heavier loads on them then the caps in a real Neve.
2. What to do with the inductors --
So far I know that the inductors are picking up hum not only magnetically, but electrostatically as well. Waving my hand over the coils can increase or decrease hum pickup. Because the inductors are epoxied onto the ceramic pin bases instead of being secured by metal clips to a grounded circuit PCB trace, it turns out that the cores are not grounded, so they offer no electrostatic shielding. figuring out how to ground them helps with the hum. Also, the large diameter of the pins makes for a large loop area in the PCB which tends to pick up magnetically. It may take a mu-metal shield to kill the problem dead, but
it may be that just grounding the core along with the new power transformer will do the job. For those who have installed the Carnhill inductors, note that they use the metal spring-clips which ground the cores. It will be impossible to remove the Chinese coils from their ceramic bases without destroying them so I am thinking that a wire attached to a screw through the core might work if one doesn't tighten it down too hard.
I have even obtained original winding data and BOM for original Neve cores as part of my R&D, but it is unlikely to prove useful unless I end up making new inductors.
I pulled the ACMP preamp project off the back burner in December and worked on it for a couple of days. I started taking pictures of all the mods that I have already figured out, and purchased and kitted additional parts. I have only been able to do a few hours of work on them this month, but I have been able to order some additional parts and do some kitting.
the Bottom Line is I am still alive and kicking and have not given up on the ACMP preamp mods. I am working hard to resolve this situation and deliver everyone's stuff. Although it is too late to be timely, I hope that the improved performance of the modified gear will mitigate your frustration.
A few forum members have contacted me regarding my progress/situation/timetable. I always answer my phone. I answer my emails. If you didn't get a response to an email you sent me, I almost certainly didn't see your important email among the clutter of my inbox. Please feel free to contact me directly so I can work out a solution for your situation.
For those who ordered kits, one solution is to send you what I have now now and what I have later later.
Frank F., for example, ordered 3 transformer kits for his 81's I have a box packed with all the parts for his units ready to go, but so far he has opted to have me keep adding to the box additional parts to make a more complete kit. The pics are necessary to know where to add all the protection diodes to the back side of the power supply PCB, etc.and I am still working on them.
The preamps here for repair are different, in that they really need the Preamp Pop fixed as well as the rest of the mods. I can install all the fixes that are already done, but I still haven't worked out the details for the front end yet.
Unfortunately, I cannot change the past. I can only do better from here on. I hope to repair the damage to my reputation by coming through for all of you. I will be able to do more work on these in February, and my goal is to finish them up by the end of that month.
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