[ACMP investiupgradifications] All things PREAMP

[0dbfs]

Just tested the acmp81 (+transistor mods) with onboard stock toroid and PSU
and both inductors replaced with Carhills+OEP shields. No other changes.
Tested in my rack with other units nearby and powered on.

The mid-hi and mid-lo EQ bands are now both noise free.

At IP and OP gain set for unity, EQ In, Mid- Hi (1.5KHz) and Mid-Lo (220Hz) IN and with max +ve gain,
mains harmonics are <90dBu (50Hz) and <105dBu (100Hz and on).

Overall noise is around -87dBu which is basically my converters.

The whole unit is now what I consider a very good performer in terms of unity gain noise floor.

Basically the EQ is now not adding any extra mains noise.

Alexc,

Sounds like the smoking gun. It's really very good to have a confirmed solution for the biggest and most frustrating problem with the 81's (at least from my perspective).

I (for one) fully appreciate your troubleshooting and documentation methodology that you have followed and shared.

Most excellent.

Thanks for sharing!

Cheers,
jonathan

 

[okgb]

I going this route because it is cheaper to start
and certainly won't hurt [ maybe the pocket book ]
BUT
even Cinemag recomended changing the pwr xfmr first

that said i would think the cinemag inductors to be an improvement
in the very least over anything chinese and that's my bet .
of course all the psu mods also won't hurt either
[ perhaps more so in long term reliability ]

I'd think replacing the i/o xfmrs to be an improvement as well ,
but would it be worth it ? it's not one of the problems right now

Alex , does your unit radiate noise into units place above or below it in the rack ?

 

[alexc]

Thanks - I hope it helps others. Certainly helped my units!
Difference compared to stock is like day and night.

I haven't so far noticed any excessive noise radiated by the acmp-81 into other units.
I'll do some more checks and post back.

Replacing components may well help reliability - power traffo, psu, audio traffos and so on.

There are several PSU caps that are low voltage rated (24V, 35V rated) and also the one on the output
amp board (25V rated on 24V supply). And the phantom power pops pretty bad too when switched.

I guess everyone will decide how far they want to go in improving these units. 
I wanted to find the minimum changes to get the audio working right.

The Carnhill inductors are nothing fancy, so I expect the benefits of changing should also translate to any
reasonable quality, shielded part.

The great news is that the acmp-81's sound pretty good! Definitely worth the effort.

For clean guitars, this would be my goto solid state preamp for sure.




 

 

[maxwall]

Alexc,

What mains voltage are you running your tests with 115Vac , 220Vac, or 240Vac ?

this may make a difference because less amps on the secondary if above 115Vac mains

 

[alexc]

I'm running at 240VAC.

If you mean that because I'm running the 230VAC rated transformer primary
at 240VAC, then yes that would result in a somewhat higher unloaded secondary voltage

That may or may not result in a lower current draw on the transformer secondary.
I'm not sure. I don't think so. I'd have to check more detail on how regulators operate.

In any case, it would be a fairly small amount which I don't think would translate into
much difference in the radiated secondary field.

 

[alexc]

Bias current in the 3055 turns out to be 55mA. A little low but whatever - it works.

That is - input a signal and turn up gain until the onset of clipping at output.
Adjust the 3055 bias pot such that the signal clips symmetrically.

55mA is the current at that point on my unit.
Max 3055 current with wild clipping is 80mA. Current with no signal is 40mA.

 

[0dbfs]

I going this route because it is cheaper to start
and certainly won't hurt [ maybe the pocket book ]
BUT
even Cinemag recomended changing the pwr xfmr first

that said i would think the cinemag inductors to be an improvement
in the very least over anything chinese and that's my bet .
of course all the psu mods also won't hurt either
[ perhaps more so in long term reliability ]

I'd think replacing the i/o xfmrs to be an improvement as well ,
but would it be worth it ? it's not one of the problems right now

Alex , does your unit radiate noise into units place above or below it in the rack ?

Hey Greg,

I do plan to upgrade my 4 81's with Steve's kit down the road.

I'm looking forward to hearing about the cinemag inductors though when they come through. I'm thinking that replacing those will be the most notable improvement and I will then be able to use the EQ's. I'll put in an order as soon as they are available either directly or via a Black Market thread if it goes that route.

Cheers,
j

 

[maxwall]

In any case, it would be a fairly small amount which I don't think would translate into
much difference in the radiated secondary field.

If there is even the slightest bit of relevence to this I can't be sure. However, there seems to be a exception in the test data between Steve Hogans results and anyone troubleshooting in the international community as yourself (240vac). nothing has been concluded by these two operating conditions and any discrepacies resulting from it. I don't recall Steve Hogan running his test results with a 240Vac primary or even mentioning it. I'm not totally sure if this will skew the resulting test data any. It seems if 115Vac is being used the secondary field on the transformer would be stronger and yield different noise figures. If the current was greater on the secondary do to a 115Vac mains powering then it would seem likely the secondary field would also be reactively different in some significant way. Is it possible ?

I have to keep in mind that these transformers were constructed in a country where 115Vac is non-existent and it is diffiucult to know what amount of quality control , if any, was performed with a lower mains voltage.
Currently , it is easy to assume very little quality control went into these power transformers even with 240Vac as the mains voltage. So anything goes at this juncture.

Alexc,
can you draw up a simplistic illustration on how your measuring the 55ma of current on the 3055 power reg. I'd like to familiarize the connections , are you inserting test leads into the output path (emitter) of the 3055 and monitoring current draw while adjusting the trim pot ? or measuring current drop across the 3055 (emitter,base) or (Emitter , collector) etc ...?

 

[alexc]

I think the voltage and current pulled by the circuit from the secondary is the same regardless of
whether the primary is operated at 115VAC or 230VAC.

So the radiated field from the secondary should be very similar.
The primary current, being higher, may well radiate significantly more.

However, without explicitly testing, I can only speculate.

To measure the current in the 2N3055 driver transistor you have to insert an ammeter in series
with the collector. I could it because I had already cut the wiring in my psu mod unit.
Lifting the wire to pin 3 (which is at _24V) on the primary of the output transformer is probably the
easiest way. I'll try that and let you know with pics.

 

[clintrubber]

Hi,

Don't know if these have been added to the to-do/attention-points/known-bugs lists yet ?

* [safety] when in 230V-zone, be sure to replace 250mA fuse for 100mA fuse
    (after you've set the voltage-selector to 230V & before switching it all on for the first time)

* [cosmetics/ACMP81] silkscreening 'bell' symbol incomplete for high band: it's just '-  -' (is this indeed a common thing ?)

* [cosmetics/ACMP81]  green LEDs in switches quite a lot brighter than the rest


Bye,

  Peter

 

[clintrubber]

To measure the current in the 2N3055 driver transistor you have to insert an ammeter in series
with the collector. I could it because I had already cut the wiring in my psu mod unit.
Lifting the wire to pin 3 (which is at _24V) on the primary of the output transformer is probably the
easiest way. I'll try that and let you know with pics.

FWIW: an easier alternative which will get you about the same number will be measuring the voltage across the 47 Ohms power resistor & using Ohms law.
The current through the resistor will be a little bit different from the current through the 2N3055 but not by much, and since this is a roughly-so-many-mA's ballpark adjustment...

Regards,

  Peter

 

[clintrubber]

Hi,


All those bugfixes aside, did anyone get a warm feeling already from the sound of these preamps ?

As they were meant to recreate those famous boxes of back then, what's your opinion ? Goal met ? And to which extend?

That's obviously most easy to answer for those that have access to the real deal, but people might have been comparing them
also already to the various micpre-only builds with proven signal-transformers (either originals or 'valid copies') as well.

BTW
I realize the '81 might be considered the most 'uncomparable' since it originally didn't exist with the class-A output topology,
but OK, switching off the EQ for now makes it look like a preamp-only '73 or '84.   


Bye,

  Peter

 

[Steve Hogan]

Are we still moving forward, Mr Hogan?

I just spent over an hour composing a reply to this question. However, I lost the whole post because I got logged out when I was working due to my only signing on for 60 minutes.  I should learn my lesson and never compose directly on the Board software, because if you lose it you get to start all over again since I haven't figured out a way to retrieve it from cyberspace. So here it is from scratch again:

Sorry for the long delay since my last post.  I have had to fit the R&D on the preamps around getting work done for my regular clients at the same time.  The new Molex terminals and housings for the new transformers shipped to me on Monday.  The new transformer cores have been wound, annealed, coated and are ready to wind.  The Magnet wire that had to be ordered arrived Monday.  I needed to make some final measurements to the prototype transformer in circuit in order to make final adjustments to the design before we start winding the production run.

Yesterday I tested the new transformer in an 84 with a rebuilt/recapped power supply board.  I ran the AC line voltage up and down to confirm the exact voltage where the regulators drop out of regulation and to confirm correct voltages at nominal 120/240 line voltage and at maximum high line voltage of 132/264 Vac.

As a result of my experiments on the '84, I decided to adjust the phantom winding up about 1 Volt, and to adjust the Bipolar 12 Volt windings down a bit, since they didn't lose regulation until 90 Volts, which is overkill.  The 24 Volt winding lost it at 100 Vac, which is fine for 60 Hz since at 50 Hz it will go higher, but still under my design goal of 105/210V minimum.

I then tried the prototype transformer in a modified '81 power supply driving an otherwise unmodified '81.
The changes to the phantom and bipolar 12 windings were the same.  The 81 has two 24V windings, however.  One winding powers only the output stage card.  The other winding/regulator runs all the rest of the 81 circuitry. The heavily loaded regulator gave up at 107 Volts, so I knew that I would have to raise the voltage on that winding.  I was concerned, however, that the extra current demand might be caused by oscillating Q4,Q5 transistors.  The transistors were quite warm, so I decided to shotgun replace all of them along with the Baker clamp diodes with my recommended replacements.

After replacement, the current demand seemed about the same, so I adjusted the voltage up on that winding.  I then spent several hours generating the CAD drawings for production of the two transformers.

I should now explain that I will be modifying the colors of the transfomer lead wires somewhat.  I hope nobody will have a problem with that, but the obscure primary color coding of the Chinese transformer makes no sense at all to me, and I am the one who has to wire up the new transformer to the selector switch.

Part numbers on the replacement transformers are TSS-Mp73/84  and TSS-Mp81.

New color code =
Primary 1 = Black, White
Primary 2 = Black/Wht, White/Blk
Phantom Secondary = Blue, Blue
24 Volt Secondary = Brown, Brown
Bipolar 12V Secondary = Orange, Yellow(CT), Orange
Additional 24V Secondary on TSS-Mp81 only = Violet, Violet
Faraday Shield = Green/Yel

The leads will break out of the transformer in such a way that everything dresses in nicely, and the windings line up with their respective connectors on the PS PCB.

I will now write a second post with all the rest of the stuff that I lost.


 

 

[jdbakker]

I just spent over an hour composing a reply to this question. However, I lost the whole post because I got logged out when I was working due to my only signing on for 60 minutes.  I should learn my lesson and never compose directly on the Board software, because if you lose it you get to start all over again since I haven't figured out a way to retrieve it from cyberspace.

Yeah, it's no fun at all when that happens. I've gotten in the habit of hitting Select All - Copy (Ctrl-A / Ctrl-C) before posting or previewing messages. At first I was afraid that I'd forget to do it at exactly the wrong time, but I've found that I'm now doing it without even thinking about it.

JDB.
[very interested in the fixes, and eagerly awaiting the final DIY transformer-plus-parts package, although I'm wary of the shipping cost of such (relatively) heavy items to Europe]

 

[Steve Hogan]

Power Transformer stray field at 240V vs 115V

. . . there seems to be a exception in the test data between Steve Hogans results and anyone troubleshooting in the international community as yourself (240vac). nothing has been concluded by these two operating conditions and any discrepacies resulting from it. I don't recall Steve Hogan running his test results with a 240Vac primary or even mentioning it. I'm not totally sure if this will skew the resulting test data any. It seems if 115Vac is being used the secondary field on the transformer would be stronger and yield different noise figures. If the current was greater on the secondary do to a 115Vac mains powering then it would seem likely the secondary field would also be reactively different in some significant way. Is it possible ?

In terms of stray, unwanted magnetic field being radiated from a power transformer, one of the most important variables is flux density in the core, with lower flux density resulting in less tendency for the flux to leave the core since it is far from saturation. For 115 (120V) use, the primary windings are connected in parallel, and for 230 (240V) use they are connected in series.  Although there is twice the voltage, there is also twice the number of turns, so the flux density will be exactly the same (at the same frequency).

Another issue that effects flux density is the frequency of the Mains.  At 50 Hz the flux density of any given transformer will be 20% higher than that same winding driven at 60 Hz.  This means that there will be more stray field at 50 Hz than at 60 Hz.  When I design transformers for 50/60 Hz, I design it for 50 Hz and the 60 Hz operation will be better.  The TSS-Mp73/84 and TSS-Mp81 power transformers are designed with a 50 Hz flux density of 10.54 kGauss, which is quite low.  It is even less at 60 Hz.  Another thing that effects stray field is the consistency and geometry of the primary winding.

There is a difference between 50 and 60 Hz in the way the filter caps in the power supply charge.  There will be more PP ripple at 50 Hz than 60 Hz due to more discharge time between waveform peaks.  For that reason, if you want the regulator to maintaing regulation down to 105/210 at 50 Hz, the 60 Hz drop out Voltage must be lower.

I forgot to mention that in addition to the magnetic shield band built into the new replacement transformer, I am also going to experiment with the effect of a copper band around the perimeter of the toroidal transformer.  In theory the thick copper band makes a shorted turn that intercepts only the stray flux and lowers it by loading it with a short.  I will report later on whether or not it makes any difference.

 

[okgb]

And how was the hum & noise from the frequency bands with inductors ?

tia regards Greg

 

[Steve Hogan]

And how was the hum & noise from the frequency bands with inductors ?

I didn't measure that yesterday.  The last time I measured that I believe I was getting about a 20 dB reduction over the stock transformer.  I will be investigating the need for cans very shortly, and that will include a careful study of that.

 

[Steve Hogan]

I will now write a second post with all the rest of the stuff that I lost.

Q4, Q5 oscillations, 81 EQ board, clipping indicator, and audio coupling cap issues

I pulled all the '81 EQ cards and shotgun replaced Q4 and Q5 and D3 with my recommended replacements in my primary test-bed '81 (Thanks Crazy-doc).

I didn't check for oscillations before replacement, since I was on deadline to finalize the transformer winding voltages, so I will have to observe the problem on one of the other '81s here for repair/upgrade, since I have no desire to replace the new parts with the old. I have a couple of questions from those who have already tackled this problem:

1. Is the oscillation problem consistent and repeatable?
2. Is it a low frequency (motorboating-type) oscillation or a high frequency oscillation?
3. Anyone put a scope on it so I'm sure that I am identifying and solving the problem thoroughly and completely?

I have the following observations about the EQ cards:

1. Due to the long, daisy-chained power supply and signal busses on the 81 preamp, and the presence of a 47 Ohm series resistor isolating each EQ PCB from the 24V Buss, I believe it would be a good thing to up the value of the 100uF/35V PS decoupling cap to 470uF/35V.  Since I purchased lots of them for the Power supply mods, I figure, what's four more?. This will give the individual EQ sections a 5 times bigger reservoir of electrons from which to draw locally, and may reduce any power supply interaction between sections.

2. There is no high-frequency, or for that matter, low frequency local power supply bypassing on the +/-12V rails for the TL072 opamps on the EQ PCB's.  There is an overabundance of 0.1uF caps on the power supply boards, where they do almost no good, due to the large electrolytics ensuring regulator stability.  The 0.1uF caps on the power supply are totally ineffective in bypassing the opamps in the circuitry due to the inductance of the many inches of trace/wires that are between the power supply and the opamps.  I tacked 0.1uF power supply decoupling caps to the bottom of each EQ card close to the opamps.  There are a couple of close-by pads that can be used for that purpose.

3.  As I mentioned before, the clip indicators employ a half-wave rectifier so that they only correctly indicate on Sine waves.  Asymmetrical waveforms (music or speech) may or may not indicate correctly depending on the shape of the waveform, since the peak light only sees the positive half of the waveform.
The peak circuitry is present in the clone preamps, but not on the Neve circuitry.  The input Z of the clip indicator circuit is 100K on the EQ cards, which should not cause any issues.  On the preamp card, however, the input Z is 10K due to 1R62 being 10K instead of 100K.  This somewhat low impedance acts as an additional load to the 22uF preamp output cap (1C13) which also is loaded by 51K (1R26) and either the input Z of the output amplifier (5K) or the much higher input Z of the "Filter" first EQ section, depending on the position of the EQ in/out switch.  For that reason, I suggest that IC13 be replace with a 100uF cap.  You can use one of the 100uF caps removed from Power supply decoupling duty that got replaced in item 1 above.  I would also recommend changing 1R62 to 100K like all the rest of the clip lights.

Whether or not to increase the size of the coupling capacitors in certain locations of the ACMP preamps is a dilemma.
In these preamps, there are coupling caps between each EQ stage that roll off the low frequencies a little bit more as the signal passes through each section. The result of these successive rolloffs is significant low and low mid phase shift.  A 20 Hz square wave will have huge tilt.  If I were modifying these for one of my regular clients who wanted "better audio", I would probably significantly increase the size of the coupling caps over what Neve (and the clone) used.  The resulting sonics would be a subjectively less compressed sound, and a smoother, more extended bottom, and better low-mid resolution.  In that way, it would affect the Neve-original "color", which may not be wanted since these preamps are "color" preamps. As a matter of fact, almost every British-designed piece of gear that I have ever seen uses coupling capacitors that are way smaller than one should use if you care about maintaining waveform fidelity to frequencies lower than 100 Hz.

  So unless I receive instructions from someone wanting more extended bottom end on their particular preamp, I am planning to restrict my capacitor changes to those capacitors that have become low-frequency "bottlenecks" due to the ACMP clone implementation of the Neve-like circuitry.  As an example, the very low impedance 5K load of the class A output stage was never in the 81.  Neither was the 10K load from the clipping indicator circuitry.  As a result, this 22uF (1C13) will be too small to deliver the same low frequency response that was in the Neve. This one in my opinion should really be changed because it compromizes the sound of the preamp when the EQ is bypassed.

For those of you who have sent me your preamps already and are patiently waiting for me to return them, I hope you will understand that I want to do as complete a fix as possible the first time, so I am working hard to have a complete list of fixes so you are happy.  Thanks for your patience.

 

[Paul G]

Thanks for the update, Steve.

How many of these coupling caps are there in a 73?  Would it be possible to mod this is such a way to switch between the different cap sizes thereby having a pre with two different sounds?  Maybe I'm way off here....

;D

 

[Steve Hogan]

How many of these coupling caps are there in a 73?  Would it be possible to mod this is such a way to switch between the different cap sizes thereby having a pre with two different sounds?  Maybe I'm way off here....

There are quite a few caps that could be upsized in the 73 if you wanted to improve the bottom end and midrange.
A quick check revealed the same 22uF cap driving the 5K load of the output line amp and a 10K input resistor to the clip light circuitry.  Changing the 10K to 100K and the 22uF to 100uF is a good idea.

With regard to making the cap size switchable ... anything is possible -- take crazy-doc's gain switch modifications.  However, I cannot see that switching them would be very practical.  To be honest, I've never had a customer request that I put the small caps back in after I made them bigger. Of course, YMMV.