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Another inductor hum hack I've done - raise the inductor up on stilts so it is centered in the shielding cylinder. (This is on the 73.) This is moderately helpful in further decreasing the level - the shield can only extend to the top of the 1U box, of course. Mounting it horizontally as I did in the ACMP81 is probably marginally more effective.

inductor-stilts.jpg


In any case, the hum on either unit is now only heard in the line-in position, with the output gain of the pre maxed and the input gain of the listening/recording device maxed. It is inaudible at any microphone setting, as the baseline noise of the preamp's gain stage is far greater than any inductor induced noise. I wish there were a way to decrease this: I've substituted several types of low noise transistors without any notable improvement.

To summarize, I've gotten my units functional by (in this order):

1) Swapping the Q4 and Q5 transistors to stop the oscillation (ACMP81)

2) Rotating the toroid power transformer to the position of least hum in the mid-eq inductors

3) Shielding the toroid and the inductors

4) Cutting the trace between the two poles of the 6th position of the input gain switch (not an optimal solution - loses a gain setting)

This has cost me about $30 per unit for 4 units ($12 for transistors and $100 for mumetal, though there is enough shielding left over to do a few more.) My only remaining issue is just the baseline preamp hash/noise at high mic gain, which shouldn't be a problem unless I'm recording a very soft voice/guitar with a ribbon mic. Thanks for everybody's help, energy and good will in working on this, with more to come as necessary.

Hopefully, Steve Hogan's entry into this will give some better options for those who want to pay a little more (and wait a little longer.  :))
 
swapped transisters in one of my 81's , warning
if you get the info off the internet double check the ebc pinout
an DON'T heat em up too much , they do lift easy
[ i pushed against one of mine to break the solder connection ]
my local active store had some 2n2222 & 2n2907's that were in
metal cans marked audio / VHF Amp  don't imagine they sound any better but it cleared up most of the problem
thanks Z

also noticed that the pcbs are dated 2007 and although most of the
soldering had a slightly dull look , the inductors pins were shiny & fresh
[ it would seem  ]
 
Can anyone think why they bothered to use a different transformer/supply for the '81 compared to the rest?
 
rodabod said:
Can anyone think why they bothered to use a different transformer/supply for the '81 compared to the rest?

I know you've asked this before and no one's responded. I can't think of a good reason either, but: why would they design a PS with extra parts/expense if there were no reason. Maybe with the 81, there were even more oscillation/hum/noise issues with the output amp being on the same supply as the eq boards (or at least they thought so.)

Anyway, a good question in light of possibly designing another transformer for the power supplies - simpler and cheaper to have only one 26V secondary. If I get the time (and the nerve), maybe I'll short the two supplies together and see what happens.
 
Hi,
Help needed.
I just received 2 ACMP 73s
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.
I'm doubting the wiring to the power selector switch.
Could someone have a look inside a working 73 and tell me the cable colors connected to the selector switch.

Looking at both of mine from the front of the unit the four connections are:

                    BLACK        YELLOW+GREEN
                    RED             PURPLE + RED

Cheers,
peteb
 
One of my 81 units has all +24V supplied from a single regulator  on series connected secondary windings via the JLM Powerstation PSU board.
30VA transformer with 24VAC-0-24VAC secondary to +48V, +24V, +15V, -15V  all located on-board.

I haven't noted  any problems at all so far (small T220 heatsinks on each of the regulators), after several weeks of usage.

I didn't notice any noise performance benefit either. (No additional shielding on the power transformer. Used a 'reasonable' quality chinese model)

Mounting externally reduced the noise from the inductor boards in the 1KHz-6KHz region by about 6dBu (from -89dBu to -95dBu components)
and made no impact on the mains supply harmonic noise resulting from the mid-lo inductor,
which now seemed to be picking up noise from surrounding rack units (as shown by switching surrounding units on/off and measuring)
 
Alex.. that kinda sucks.. I hoped a toroid swap would significantly reduce the noise... Have you used the trafo you are using now without problems?  Maybe someone has a really nice quality trafo they could test?  I know Altran makes high quality toroids, but I don't know if they would have a 24-0-24 stock (so as not to pay setup fees)... I was going to try that myself, but hearing your results, not sure it would help much...

Maybe there are other factors as Steve Hogan mentioned..
 
crazydoc said:
rodabod said:
Can anyone think why they bothered to use a different transformer/supply for the '81 compared to the rest?

I know you've asked this before and no one's responded. I can't think of a good reason either, but: why would they design a PS with extra parts/expense if there were no reason. Maybe with the 81, there were even more oscillation/hum/noise issues with the output amp being on the same supply as the eq boards (or at least they thought so.)

Anyway, a good question in light of possibly designing another transformer for the power supplies - simpler and cheaper to have only one 26V secondary. If I get the time (and the nerve), maybe I'll short the two supplies together and see what happens.

Don't short the outputs of the 2 supplies together.  You won't be happy with the resulting damage.  See my notes for
modifications below.

Having now spent several hours with the help of Mylithra, Matt at recordinghacks.com, and others here,  I now have more info and already have a preliminary replacement power transformer design.

I'm not sure why the manufacturer of the 81 clone used two separate supplies, but it seems clear that the one supply has enough current to drive the mic/line amplifiers, and the second regulated supply powers the EQ section.  The separate regulated supplies will offer some isolation.  If I make a transformer with two 25 volt secondaries, then they can be wired separately for the 81 and in parallel for the other units.  Or the winding can just be left unused.  Anyone have both an 81 and a 73/84 to see if there is much of a size difference between the two transformers?

The size of my preliminary design is 3.625 to 3.8" OD x 1.4 to 1.5" high. This large size allows EXTREMELY low stray field, even lower than my usual low stray-field designs.  I want the transformer to fit in the original space.  I know it will fit in the 84 chassis, so I would appreciate it if those of you with 73 and 81's can measure to see if that would fit in place of the old one.

Here are some potential modifications that I recommend from quickly studying the schematics of the 81 clone.
(sorry, I don't have an answer to the gain switch problem yet)

1. 81 Schematic error:  Filter caps C2 and C6 are upside down. The + terminal must go to ground on the negative supply.

2. C19 (100uF/63V) main filter cap in 48 V supply should be 100 Volts. At high line the Voltage here can exceed 63 Volts, and an exploding cap inside your preamp is not a pretty sight.

3. The LM317 regulators do not have the required protection diodes. Add 2 each 1N400x rectifiers, one in parallel with R2 and another in parallel with R4. Cathode (banded end) connected to the output and the anode side connected to the LM317 adjustment terminal.   Add a third 1N400x protection diode Cathode (banded) end to Input of LM317 regulator and Anode end to output of regulator.  Add a fourth 1N400x protection diode to the other regulator.

4. Changing C11 and C16 (10uF/35V) to 100uF/35V will reduce the HF noise of the LM 317.

5. Phantom power circuitry:
    A. 1R58 and 1R59 (6.8K) must be matched within 0.4% to meet DIN specs for Phantom.  Absolute value is not critical, check the resistors with an Ohmmeter and if they aren't matched, replace them with a matched set.  Mismatched resistors will add Low frequency THD to Mic Preamp input transformers and microphone output transformers.
    B.  1R60 (1K) is too big for use with 48Volts.  High current demand, modern mics will be starved.  Replace with 100 Ohms for stock supply, or if you get my new transformer, you will be able to get 52 Volts from the 48 V supply and you can use 200 Ohms here.
     C.  1C23 (100uF/63V) would benefit by being increased to 220uF/63 V, especially since 1R60 must be lowered in value.
     D.  Switch contact 4 of K2 "48V" switch should be connected to the same ground point as the bottom of 1C23.
and not left floating.  This discharges 1C23.  Without grounding the contact, the phantom stays on the XLR until the cap is discharged by a plugged-in microphone.

6. RF bypass caps C15, C16, C17, C18 (1000 pF) represent significant capacitive loads on any microphone or line level device.  At 33pF per foot of good mic cable, this represents the capacitance of an additional 30 ft of mic cable.  I have not checked to see if the original Neve circuits had these caps, but if they did they should be of good quality and well matched.  Otherwise I would get rid of them and let the input tranformers deal with the RF.

7. Warning!!  Half-wave rectifiers are used in peak lights and metering.  The metering and peak lights do not employ full-wave rectification of the waveform. When  monitoring highly assymetrical waveforms like male speech,for example, the meters will respond quite differently depending on the absolute polarity of the signal. In one polarity the meters/clip lights may light up like crazy, and in the other polarity they may hardly register.  Be forwarned that the clip lights and meters may be lying to you in regard to levels, unless you end up modifying the precision rectifier circuitry to full-wave.

That's all for a quick glimpse.

Hope it helps.

 
By the way, has anyone put a scope (AC coupling, line triggered) on the regulated DC output voltages to see if the power supply regulators are putting out low-ripple power?  A bad PCB layout on the power supply can easily increase the ripple 20 dB or more over a proper one by allowing the sense terminals of the regulators to be attached to a ground trace carrying the high-current rectifier pulses from the filter caps.  Since the Neve circuitry is single-ended 24 Volt, a low-ripple supply here is a must.
 
peteb said:
Hi,
Help needed.
I just received 2 ACMP 73s
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.
I'm doubting the wiring to the power selector switch.
Could someone have a look inside a working 73 and tell me the cable colors connected to the selector switch.

Looking at both of mine from the front of the unit the four connections are:

                     BLACK         YELLOW+GREEN
                     RED             PURPLE + RED

Cheers,
peteb
My working unit is the same, with two unconnected lugs to the left of the black and red wire connections. Switch set to 115.
 
peteb said:
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.

Is 100mA large enough?  There's probably more than 100mA of inrush current on these things.  Maybe try a slo-blo fuse?
 
Steve Hogan said:
Don't short the outputs of the 2 supplies together.  You won't be happy with the resulting damage.  See my notes for
modifications below.
Why not? I'm maybe too dense to find the explanation below?  ;D

The size of my preliminary design is 3.625 to 3.8" OD x 1.4 to 1.5" high. This large size allows EXTREMELY low stray field, even lower than my usual low stray-field designs.  I want the transformer to fit in the original space.  I know it will fit in the 84 chassis, so I would appreciate it if those of you with 73 and 81's can measure to see if that would fit in place of the old one.

As I posted about a page ago:

crazydoc said:
2) Transformer dimensions (to 1/16 inch): diameter 2 1/2 inches, height 1 inch

3) The box's inside depth is a tad over 1 5/8 inches

4) The transformer is a doughnut, mounted with rubber washers above and below, and a steel disc above.

5) There is a dimple into the bottom of the box for the mounting hole, so this might make remounting the transformer in a different location slightly problematic. There is 1/4 inch of space between the transformer and the wires coming off the mains connector (if you bend them a little,) so the transformer could be 3 inches diameter in its current location.
I'll post some pics in a bit. Hope this helps.
There's not enough room in the 81 for that size, without remounting it using a different hole location - then you have to deal with the raised dimple from the old location.

Thanks for your time and effort in this - it's a great help.
 
crazydoc said:
peteb said:
Hi,
Help needed.
I just received 2 ACMP 73s
Set both to 230v (Australia), put in a 100mA fuse turned each one on. They both blew the fuses.
I'm doubting the wiring to the power selector switch.
Could someone have a look inside a working 73 and tell me the cable colors connected to the selector switch.

Looking at both of mine from the front of the unit the four connections are:

                     BLACK         YELLOW+GREEN
                     RED             PURPLE + RED

Cheers,
peteb
My working unit is the same, with two unconnected lugs to the left of the black and red wire connections. Switch set to 115.

Thanks Crazydoc. Are the two unconnected lugs bridged?
Cheers,
peteb

wmtunate, I'll try a couple of 125mA fuses. The operational manual that came with the units says 100mA but a pdf I've come across on the net says 125mA.
Cheers,
peteb
 

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