[ACMP investiupgradifications] All things PREAMP

[esiu]

Folks: I'm thinking of 'consolidating' some of the info in regards to fix/mods...etc. on TnC stuff (on a web page or something).  I want to gauge if there's interest in this effort (use the info, or help compile/validate info).

 

[crazydoc]

I'm sure many of us would be interested in helping with this. There's a guy at Gearslutz (maybe it's you?) that's interested in doing this too - you should get together to avoid duplication of efforts.

http://www.gearslutz.com/board/3911056-post1785.html

It would be nice if TNC were to take an interest in this too.  :

 

[esiu]

I'm sure many of us would be interested in helping with this. There's a guy at Gearslutz (maybe it's you?) that's interested in doing this too - you should get together to avoid duplication of efforts.

http://www.gearslutz.com/board/3911056-post1785.html

It would be nice if TNC were to take an interest in this too.  :

Yeah that's me - I just cross-posted on GS/HR/PSW/PP today ^_^

slaveern...

 

[okgb]

I'm sure tnc wouldn't mind if this just takes care of itself
so other than your time what could be bad about consoldating the info ,
for anyone who hasn't followed the tech thread,
and good  the different grps info gets together

sure , thanks

 

[recordinghacks]

Thanks to Chance, here are the ACMP-84 schematics (archived together with the ACMP-81; I'll add the 73 when I find them):

http://recordinghacks.com/tnc-audio-preamps/

UPDATE: I plugged in an SM57 and recorded some clips of room sound. Two of my four ACMP-84s are pretty quiet. One has buzz. The other was DOA (hoping it's a loose cable inside). I put a "quiet" clip and a "buzz" clip at the URL above. The quiet units are quiet even with the EQ engaged; the humming unit hums with or without EQ. And yet even the quiet units are a lot noisier than my DAW's pre's, so I'm interested in cap/inductor/transformer/shielding/grounding upgrades.

--
matt.

 

[mitsos]

just to say I got my other 84 opened up today, and it was the same as the other one. Hum/buzz, then flipped the toroid and it got way quieter. Usable, yet I will try to find a 100% solution.

 

[Steve Hogan]

I was just made aware by a friend of the Group Buy Clone Preamps and the enormous amount of frustration the purchasers are having with hum and buzz in their units.

When I was VP Engineering at Jensen Transformers I designed all the JT-series audio transformers and some EQ inductors as well. I make my living today modifying and upgrading pro audio gear -- mostly vintage, but I also design inductors and power supply transformers, both toroids and conventionally laminated for a California transformer company that speciallizes in small to medium run custom transformers.  We have designed and manufacture many power transformers for audio manufacturers.  I will disclaim right now that I am not up to speed with everything that has been tried to fix the hum in these preamps, but I want to add some technical expertise to the analysis of the problem.

The Chinese toroid is likely generating significant magnetic field, which is inducing hum into the rest of the circuitry.  We have taken apart some Chinese toroids and found out that the core in those units, which is supposed to be a single continuous strip of grain-oriented silicon steel, was actually made from multiple scraps welded together.  The dreadful QC of some Chinese mfgs is the reason we still make everything here in the US, including winding our own cores.  The clone preamp toroids appear to vary widely in quality, but they all appear to induce hum due to excessive stray magnetic field.

It is a myth that a toroidal transformer automatically generates less stray magnetic field than a conventionally laminated transformer.  The amount of stray field is much more related to the way the transformer is designed.  For Audio use, one wants the power transformer to have minimal stray field, not only to prevent induced hum in the equipment it powers, but in order to prevent the power transformer from infecting neighboring devices with induced hum.  It should be a "friendly neighbor" for the units racked above and below it.   It appears that these Group Buy preamps suffer from both afflictions.

The place I would start in fixing these units would be to reduce the stray field inside the box by retrofitting a new power transformer specifically designed to have low leakage flux. An "audio grade" low stray field design requires more turns and/or a larger core than a general purpose (cheaply made) power transformer.  One manufacturer of lunch box type amplifiers commented to me that the transformer we made for his rack was quieter than his laboratory bench supply! A good design, properly made can have VERY low stray field.

Regarding the inductors:  A properly designed and executed pot core inductor is self-shielding and can be very consistent in inductance, resistance and suseptability to pickup of hum. I don't know what is going on with the Chinese inductors in these units, but if the DCR is low compared to the old Neve units, then they are using fewer turns of fat wire (cheap) on a who-knows-what core.  The inductance can be controlled quite well by purchasing high quality precision-gapped cores.  They should be able to be made 5% tolerance without too much difficulty.  The DCR built into the coil will effect its Q.  Low DCR makes for less damping, so ringing and resonance problems might occur.

Regarding EQ hum pickup: I have not personally seen the PCB layout of these preamps, but I would bet (if I were a betting man) that there are significant loop areas in the layout of the PCB. An example might be a ground trace that goes around the circumference of the pcb. Such a loop acts like a transformer secondary in the presence of stray field from the power transformer, and generates hum like crazy.
Sometimes the problem can be solved by a strategic trace cut or two, but other times, the PCB must be laid out by someone who understands these things.

Regarding pops from switches:
Has anyone checked out the switches to see if they are make-before-break types, especially in the frequency selection area?  If they are the wrong type for the circuit, that could be a show-stopper due to needing to replace the switch.

I am exploring the possibility of making available for sale a properly-designed replacement power transformer with low stray field.  Perhaps I might be able to offer inductors, as well, if there looks like there's some interest.  I was originally thinking of offering to mod these units for a flat fee, but realizing the various and severe QC problems that the buyers have encountered, it seems that just replacing the power transformer would be just the beginning to really make these things professional.

Please let me know if anyone out there would be interested in a new, better power transformer for their unit. It seems that if these preamps could be improved to have professional hum levels, that one would still be ahead in value for dollar spent.  A new transformer may make an unuseable device a working tool in one's bag of tricks.  Maybe a "group buy" for a new power transformer is the way to make these boxes work correctly.

Steve Hogan
The Sound Steward

 

[Bluzzi]

Yes I think there would be enormous interest in these. Anyone that has built a NEVE clone will agree that they are more sensitive to stray magnetic field pickup. I'm sure if the price is right many of us would use them for all their DIY.

Jim

 

[tarnationsauce]

FYI The 81's transformer is different than the one in the 73 & 84.
I took a look at them and evidently they are custom transformers specifically for these preamps (looking at the model numbers).

(info off transformer labels)
Transformer model numbers "Mp81" and "Mp73/84". Brand: YTDZ Mianyang Yingtian Electron Co. LTD  http://www.ytdz.cn


Mp81
50/60Hz
0--115        0--115
15--0--15   0--26
0--26         0--45

Mp73/84
50/60Hz
0--115        0--115
15--0--15 
0--26        0--45

It looks like the only difference is the Mp81 has an extra 26v tap. I see no VA ratings.

 

[rodabod]

We have taken apart some Chinese toroids and found out that the core in those units, which is supposed to be a single continuous strip of grain-oriented silicon steel, was actually made from multiple scraps welded together. 

Eek!

The DCR built into the coil will effect its Q.  Low DCR makes for less damping, so ringing and resonance problems might occur.

I didn't consider this point before, yes.

Thanks for your input, Steve. That was helpful. I'm sure there would be many cutomers interested in what you might be able to offer.

 

[gyraf]

We have taken apart some Chinese toroids and found out that the core in those units, which is supposed to be a single continuous strip of grain-oriented silicon steel, was actually made from multiple scraps welded together. 

Eek!

Don't eek that - think of it as an air-gapped transformer. Less sensitive to mains DC offsets also..



Jakob E.

 

[alexc]

Regarding the low DCR and lower damping - that's a good point.

But when I look at the frequency sweep in the mid bands with the inductor I see very broad curves.
5KHz or so of peaking bandwidth - way broader than I expected.

And not that much boost - around 8dB only at max (specs say 18dB)
HiQ doesn't do much that I can see.

The freq switches are more the 'pop before squeal' variety.

 

[Steve Hogan]

FYI The 81's transformer is different than the one in the 73 & 84.
I took a look at them and evidently they are custom transformers specifically for these preamps (looking at the model numbers).

(info off transformer labels)
Transformer model numbers "Mp81" and "Mp73/84". Brand: YTDZ Mianyang Yingtian Electron Co. LTD  http://www.ytdz.cn


Mp81
50/60Hz
0--115        0--115
15--0--15   0--26
0--26         0--45

Mp73/84
50/60Hz
0--115        0--115
15--0--15   
0--26         0--45

It looks like the only difference is the Mp81 has an extra 26v tap. I see no VA ratings.

Thanks for the heads up.  I could design 2 different transformers or maybe just have one and wire the 2nd 0--26 Volt secondary in parallel with the first when used in the 73/84 clones.

For me to design a replacement transformer, it would be helpful for someone to make some accurate measurements of the existing transformers for me as follows:
1. The overall height.  Since this is a 1 RU box, I also need to know the maximum height I can make the replacement transformer.
2. The overall diameter.  The pics show quite a bit of space around the transformer, so I presume a larger OD would be fine.
3. The DCR of each winding, including separate measurements of each primary. These resistances must be measured with the transformer disconnected from the circuitry.  (easy if the trafo is connectorized).
4. The open circuit (disconnected, no-load) voltage of each winding when the primary voltage is held at 115 Volts with a Variac or whatever.
5. Mounting details -- does it have an epoxy center or does it just use rubber washers and a round disk to hold it in place?

I would make the replacement transformers 50/60 Hz. (Low flux density at 50 Hz requires 20% more turns or core than 60 Hz, but the resulting 60 Hz operation has even lower stray field)
I would think that a 120V/240V nominal primary would make more sense nowadays since utilities are trying to hold voltages closer to 120 than 115 to lower losses.  With more reserve, the transformer will have better regulation than the original(s).  I would design it for +/- 10% (108/216 to 132/264 Volt) operation.  If we really want to get carried away, a faraday (copper foil) shield between primary and secondaries will prevent capacitive coupling of AC line noise to the circuitry and the chassis.

In the published schematics did I see capacitors intentionally wired from the IEC input connector line and neutral to chassis?  EEK!  What a way to make more Pin 1 problems with excessive AC chassis currents. The faraday shield is a much better approach.

 

[rodabod]

Can anyone see why they bothered with the two 26V secondaries? They both seem to be providing +24V after regulation.

I was also wondering if it might be possible to use an off-the-shelf 120+120:15+15+15+15 for those who are not able to buy from the USA. Off the top of my head, that would give 15+15 for the bipolar 12V rails, 30V for the 24V reg. (heatsink the reg.), and 45V for the phantom power.

 

[Steve Hogan]

Can anyone see why they bothered with the two 26V secondaries? They both seem to be providing +24V after regulation.

I was also wondering if it might be possible to use an off-the-shelf 120+120:15+15+15+15 for those who are not able to buy from the USA. Off the top of my head, that would give 15+15 for the bipolar 12V rails, 30V for the 24V reg. (heatsink the reg.), and 45V for the phantom power.

"Off-the-shelf" power transformers are never low-flux density, because they must compete price-wise with all the other off-the-shelf power transformers.  They are all, therefore, made with the smallest cores and the least amount of wire that will result in published specifications. Some manufacturers are better than others.  Some of the off-the shelf toroids in plastic PC mount cases have appalling, totally unuseable stray field. Since we already know that the EQ inductors and maybe the PCB layout is very sensitive to stray field, I doubt if an off-the-shelf replacement transformer will help much.

That's why a custom-designed low stray-field design is necessary to really solve the problems in this preamp correctly.
The only other good alternative is to remote the transformer and power supply regulator boards from the box and pass the regulated DC to the preamp via a cable.  (Just remoting the power transformer means having long cables with high-current rectifier pulses outside the box to infect other wiring).

 

[okgb]

Would be interested in seeing some
" how much for how many " numbers

Cinemag is looking into the inductors ,
but curious to see what you can come up with

nice to " see " you again steve

 

[crazydoc]

FYI The 81's transformer is different than the one in the 73 & 84.
I took a look at them and evidently they are custom transformers specifically for these preamps (looking at the model numbers).

(info off transformer labels)
Transformer model numbers "Mp81" and "Mp73/84". Brand: YTDZ Mianyang Yingtian Electron Co. LTD  http://www.ytdz.cn


Mp81
50/60Hz
0--115        0--115
15--0--15   0--26
0--26         0--45

Mp73/84
50/60Hz
0--115        0--115
15--0--15   
0--26         0--45

It looks like the only difference is the Mp81 has an extra 26v tap. I see no VA ratings.

Thanks for the heads up.  I could design 2 different transformers or maybe just have one and wire the 2nd 0--26 Volt secondary in parallel with the first when used in the 73/84 clones.

For me to design a replacement transformer, it would be helpful for someone to make some accurate measurements of the existing transformers for me as follows:
1. The overall height.  Since this is a 1 RU box, I also need to know the maximum height I can make the replacement transformer.
2. The overall diameter.  The pics show quite a bit of space around the transformer, so I presume a larger OD would be fine.
3. The DCR of each winding, including separate measurements of each primary. These resistances must be measured with the transformer disconnected from the circuitry.  (easy if the trafo is connectorized).
4. The open circuit (disconnected, no-load) voltage of each winding when the primary voltage is held at 115 Volts with a Variac or whatever.
5. Mounting details -- does it have an epoxy center or does it just use rubber washers and a round disk to hold it in place?

I would make the replacement transformers 50/60 Hz. (Low flux density at 50 Hz requires 20% more turns or core than 60 Hz, but the resulting 60 Hz operation has even lower stray field)
I would think that a 120V/240V nominal primary would make more sense nowadays since utilities are trying to hold voltages closer to 120 than 115 to lower losses.  With more reserve, the transformer will have better regulation than the original(s).  I would design it for +/- 10% (108/216 to 132/264 Volt) operation.  If we really want to get carried away, a faraday (copper foil) shield between primary and secondaries will prevent capacitive coupling of AC line noise to the circuitry and the chassis.

In the published schematics did I see capacitors intentionally wired from the IEC input connector line and neutral to chassis?  EEK!  What a way to make more Pin 1 problems with excessive AC chassis currents. The faraday shield is a much better approach.

1) The current ratings are on the schematics (? accuracy)

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.

 

[crazydoc]

Here's the pics:

 

[crazydoc]

On a different note, I've been trying to deal with the gain switch noise between positions 6 and 7. Its not just a pop - it's a low frequency oscillation if you hold the switch between those positions (not for too long though - you'll fry the 3055's emitter resistor.) It looks like this on the scope (at the junction of 1R3, 1R14, and 1C1 - that is, the preamp input) - about 0.5v PP and 2 Hz. It is present in the mic or line input position - you just don't hear it (much) when switching through it in the line mode. Of course it is present on the output of the stage too, just hugely amplified.

The switch is make-before-break, so in essence, the preamp's input and output are connected together through R40, 41, 42 and 43 at this point, apparently turning the preamp first stage into an oscillator.

I tried substituting a 470uF cap for 1C1 in an attempt to lengthen the time constant for the oscillator, and allowing the switch to pass through that point before the first wave could be produced, but this didn't work - it only changed the relative duration of the negative and positive excursions of the wave, without changing the frequency.

So I've currently cut the trace between the two poles of the switch, making this an off position. I'd like to be able to have this an active position, if anybody has any ideas how to prevent the oscillation. I'm afraid my knowledge of electronics theory (Ohm's Law) just isn't enough to get me through this. 

 

[mitsos]

Steve, thanks for joining the fray! I'd be interested in a new toroid. Worst case scenario, it makes no difference.. but I can still use it in another project.

As far as making one toroid or two (for the extra winding in the 81) I would vote to make only one (the 81) if we'd benefit from economies of scale.  Or maybe make a single higher current 26V secondary that could be run in parallel to both inputs on the PSU for the 81??? Any inherent problems with that?

Inductors.. would you be able to match the footprint? 

Thanks again for offering to help! 


and Crazydoc: Thanks for the find.. Hopefully one of the Neve gurus around here can shed some light.