[ACMP investiupgradifications] All things PREAMP

[floxe]

if you double the voltage (115 to 230), you need to half the current (250ma to 125ma.)

sounds quite logical now that i think about it  :

i'll have a look which fuses are used in the power supplies for my acm-1200 and acm-310 and will then go fuse-shopping

thanks,
flo

 

[Steve Hogan]

Just a further note on the QC on this group buy stuff. While just now checking the value on the fuse in the ACM900 mic power supply (T160mA), I noticed the fuse holder wasn't inserted flush to the receptacle. When I took it out, the cause was immediately apparent - the prongs that hold the fuse were bent over to one side, and the fuse had been wedged in there to the side.

It's getting more apparent that we got what we paid for, but it's good we can correct most of it.

I just added checking the fuse to my list of things to check for when upgrading the ACMP preamps with the new power transformer, etc.  The list does seem to get longer and longer.
Just an update .. my Schottky Baker clamp diodes for the '81s arrived today, and I placed the order for the Molex terminals and housings for the new transformer connections to the power supply PCB.  I will be using premium Molex terminals and housings to make the best connections to the male pins on the PCB.  I will be using the polyester housings instead of nylon, and I will be using the Molex Phosphor Bronze (springier and higher conductivity than the cheaper brass) Trifurcon terminals which have three points of contact to the square header pins instead of only one.

Molex Trifurcon Terminal

It seems appropriate to use premium connectors on the premium transformer.
BTW, final pricing is now in on the transformer which has, in addition to its low stray field design, a magnetic shield and a Faraday electrostatic shield.  The replacement '73/'84 tranformer will be $75 and the '81 version, which has an additional 24V secondary will be $80.

 

[Purusha]

Are these transformers OK for 220V (EURO) also?

 

[Steve Hogan]

Are these transformers OK for 220V (EURO) also?

My replacement transformers are rated for 50/60 Hz with dual primaries that will connect to the same voltage selector switch as the original transformers. Although the nominal voltages on my transformer are 120/240Vac they are designed to operate correctly in the (modified) ACMP power supply PCB's from 105/210Vac (-12.5%) to 132/264Vac (+10%).  Lower than 105/210Vac may cause the regulators in the power supply to lose regulation and pass on ripple, but most power companies nowadays run Voltages high for minimum power loss in the distribution lines. Power at my wall is almost always exactly 120Vac. Of course, YMMV where you live.

If anyone has power less than 220Vac on a regular basis, you should let me know now, so I can make adjustments in the design before the transformers are wound.  To make the transformers maintain regulation at lower voltages, I make the secondary voltages higher.  That means that under normal operating conditions of 120/240Vac, the regulators have to dissipate more heat to throw away the higher unregulated voltage. It also means that replacement of the original input filter caps with higher voltage rated caps is a must.  It's a balancing act, and I generally find that if the regulators keep working at 105/210Vac, that covers almost any realistic low-line condition.  I predict that other studio gear will already be losing it at that low a line voltage.

 

[Purusha]

Thanks! 

 

[Paul G]

Thanks for the update, Steve.

;D

 

[crazydoc]

This is the probably final version of my gain switch mod. The trace at position 6 is cut, and the SPST switch reconnects the trace after the gain switch connection to the adjacent positions is broken. (The extra hole in the arm was a mistake - the screw didn't have enough excursion to contact the switch, and it was easier to drill another hole than find a longer screw.)

Refer to http://www.groupdiy.com/index.php?topic=27791.520 post#539.

 

[maxwall]

Crazydoc,

Have you thought of using a low voltage photo optical sensor switch with flag arm paired with a miniature relay, used in many laser printers. Takes the mechanical idea to the next level. no parts come into physical contact.

 

[crazydoc]

Crazydoc,

Have you thought of using a low voltage photo optical sensor switch with flag arm paired with a miniature relay, used in many laser printers. Takes the mechanical idea to the next level. no parts come into physical contact.

That sounds like a really good idea - I'll look into it. Thanks.

 

[redddog]

Are we still moving forward, Mr Hogan?

 

[okgb]

Would love to hear the results of the upgraded
pwr transformer by itself

 

[redddog]

Seriously. Seems like we hit a brick wall here.

 

[rodabod]

Seriously. Seems like we hit a brick wall here.

In what way?

 

[redddog]

The flow of info.

 

[rodabod]

I think we've got plenty. Steve's done his work and will follow up with any offerings of service I'm sure.

Other than that, we seem to know most of the issues, and people can report back when they tackle them. I'll be sure to do so when I get mine soon.

 

[clintrubber]

Let's have a placeholder here already for the bewildering amount of threads that might arise from the TNC-groupbuy

...

And I propose likewise for the ACM-mics:

http://www.groupdiy.com/index.php?topic=334580

...

Cheers,

 Peter

Looks like we lost the TnC/ACM-mic thread !   

Removal of older stuff only in The BlackMarket as I understood, or could it be related to the latest software move ? Does that link needs to be upgraded ?

Bye,

  Peter

 

[okgb]

I would think steve has other gigs

clint i thought i saw the mic thread a page or two down  , not that far

otherwise i'm waiting to recieve the inductors and check them in circuit
with orgi pwr xfmr [ Alex reports good results shielding them as well ]

 

[crazydoc]

Let's have a placeholder here already for the bewildering amount of threads that might arise from the TNC-groupbuy

...

And I propose likewise for the ACM-mics:

http://www.groupdiy.com/index.php?topic=334580

...

Cheers,

 Peter

Looks like we lost the TnC/ACM-mic thread !   

Removal of older stuff only in The BlackMarket as I understood, or could it be related to the latest software move ? Does that link needs to be upgraded ?

Bye,

  Peter

This one? http://www.groupdiy.com/index.php?topic=27793.80

 

[clintrubber]

Thanks guys !

(I've updated the link to the Mic-thread on the first page of this thread)

Bye,

  Peter

 

[alexc]

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.


Another thing of interest : +24VDC current draw is 150mA.

The 2n3055 is biased very low - around 40mA.
Compared to around 80mA that  I normally use.
So I think these need to be rebiased. I'll do that and post the actual value I find.