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seeyou 6th October 2012 07:31 PM

SPL EQ Magix studio mic preamp
 
Hello!
I started a hobby modification on my SPL EQ Magix preamp. It is a microphone preamp made in Germany in the mid 90ies. It has a mic preamp with an SSM2019, a full parametric eq and an "spl patented" vitalizer circuitry. I don't have a schematic but I thought I could give it a try.
Its main problem was the hiss and the ugly distortion at loud input gain/clipping.
When I opened the case, I discovered a lot of cheapy parts inside. Plus, around 14 ICs of the type TL071, TL072 and TL074 placed on sockets. Doing a little research, I replaced them with the ..."safe" OPA134, 2134 & 4134 and got rid of the sockets. After that, recording through my Digidesign 96 interface showed that there was about 15db less noise and i also noticed a huge improvement in overall sound. Next I replaced the SSM2019 with a THAT1510. Slight improvement as well.
Then I normalized the recorded signal in my recording software (gave it an 80db boost) and listened to it. There was a slight tone in the signal at about 13KHz and a hum at 50Hz. I am positive that the tone&hum come from the preamp, I don't get them from my audio interface. I guessed that the op-amps were oscillating, so I soldered .1uF capacitors (ceramic multilayer) from +/- of each opamp to ground. The ~13KHz tone went away. And finally, I got stuck with the 50Hz hum.

The 50Hz hum comes from the power supply.
The power supply consists of a 2x15V transformer, rectification and LM317/337 regulators, caps and 2x potentiometers for 15V adjustment. The newly replaced OPAs draw way too much power so the toroidal transformer gets REALLY hot. I also noticed a drop from +/- 15 V to 12,5V when there is a loud input signal present. So I ordered a new transformer, replacing the 2x15V 15VA with a 2x15V 50VA. It fits the housing, everybody is happy.
Since I am replacing parts in the power supply, my question is wether to change other parts as well. I have already changed all el caps with Panasonic FMs, should I change rectifier diodes as well (I bet SPL didn't put the best diodes in there in the first place:) )? Should I also change the LM337/317 ?? Without having proper test equipment or a schematic, what would you suggest? Don't forget, this is a hobby project, please feel free to comment!
Thanks in advance :)

The Space Egg Corp 7th October 2012 04:57 AM

Hi SeeYou

Nice work on the SPL !

I have a couple of SPL units as well.
The 'Optimizer' & 'Vitalizer'.
They are however, line-level units; and are well behaved noise wise.

I did pop the hood on the Optimizer a while back; just out of interest.
I have not seen the inside of one of their mic-pre's though.
Do you have a pic with it 'topless' !!!

I have built & refurbished microphones & a number of pre-amps; and generaly waste as much of my time as possible messing with studio & audio gear.

I guess you replaced the SPL transformer with a 'Toroidal' ( donut type ).
The square 'I E' types can give off much larger magnetic fields.

Was hum present BEFORE the transformer swap ?

Sometimes with the real 'low-level' signals from mics & also TT's; strategicly placed metal shielding, earthed to the chassis, can be all that's needed to kill hum.

Is it also possible you might have a 'ground-loop' or any dodgey cables ?
The amount of wrongly wired XLR cables out there is just untrue !

I wouldn't bother replacing the 317 regs; but you could replace the rectifier diodes with 'Schottky diodes' which are cheap & easy to find.
Less possible RF emissions & generaly better performance due to zero-switching time.

If you try shielding the transformer; leave adequate space for ventilation.
For first tries; nothing fancy needed.
A flattened Coke-can bent to shape is often just as effective as fancy mu-metal & the like.
A steel can as opposed to aluminium, can disperse & scatter a stray magnetic field away from any trouble spots as well.
Obviously taking care to fix well in possition & insulate if needed.

If it isn't a ground-loop or dodgey cable; It's most likely a shielding or grounding problem in the unit.
I believe the SPL probably also has a 'ground-lift' switch somewhere ( ? ).
Also did you reconnect the chassis earth after the refurb ? ( I hope so ! ).

Hope that might help some.

Cheers

Si.

Good luck !

The only humming we want; is to the melody !!!

The Space Egg Corp 7th October 2012 05:01 AM

Just another thought.

Did you change the location of the transformer inside the chassis for it to fit.

If it was moved close to any mic-input transformer ( if the unit has one ) that is VERY BAD !

Just a thought.

Post a pic of the SPL's guts.

Si.

seeyou 7th October 2012 03:53 PM

Hello Space Egg!
I haven't put the transformers inside the case yet! I just ordered them on thursday and they will be ready on monday-tuesday. Tomorrow I'll go to the studio and make some pictures for you! thanks for the advice.
When you say "shottky diodes", is there a brand I should consider as good? I mainly buy parts from rs!

The Space Egg Corp 7th October 2012 09:45 PM

Oh Mr Schottky we love you...The 'perfect diode'...& the ribbon mic too !
 
Hi See You

SPL may have fitted Schottky's. :note:
But somehow I doubt it too. :grumpy:
Would have cost them an extra $0.01c :eek:

If the rectifier is a 'solid-block' with 4 legs, it isn't Schottky.
Check to see if you can read the code# on the individual diodes ( if fitted ).
Then just check RS search, to see if it's silicon or Schottky.

A popular Schottky diode is the 1N5822
This is a 3A 40volt type; about 1 euro for 5of.
The only 'downside' to 1N5822, is it has slightly thicker leads than the 1 Amp types.
May need PCB holes widening a bit.
It is also marginaly bigger at 5.6 dia x 9.5 long...
...as opposed to the 1A type at 2.7 dia x 5.2 long.
Are you pushed for space ?
It's always a downer, when the parts don't fit ! :mad:
The slightly smaller 1A type is 1N5819

You can pretty much use any Schottky diodes that will fit.
The only difference in spec. ( apart from V & A ) is the forward Voltage drop...
...which in your application is not important. :D

Perhaps get some small heatsinks for the 317's if they don't already have some.
Never a bad move ! :)

Bit worrying that you say the Voltage drops to 12.5v on loud signals !!!
That REALLY should NOT be happening !!!.....EVER !!!

Also...

If you haven't already done so...
...clean the XLR sockets.
Especialy the mic input one.
It's amazing how much cigarette ash, gaffer-tape sticky, bubble-gum etc...gets in 'em.
You may well be ASTOUNDED :eek: at a further lowering of background noise.
A pipe cleaner soaked in meths, Zippo fuel or just plain old Stolli, cuts through the crud !

If you're tuning up your mic chain generaly...
...not a bad idea to make your own mic-cables.
Shop ones are 99.9% of the time; TOTAL TRASH.

A pair of nice gold-pinned Neutrik XLR plug/skt connectors from RS is a MUST.

Having used loads of different mic-cables over the years...
...my WAY favorite...is a recent purchase from CPC in the UK.
I have to say also, this is the cheapest mic-cable I have ever bought.
The only thing it compares to as far as I have used, is the old VanDen Hul D109 MKII ( I think it's called ).
Very good cable...
...but if you have that kind of dough to spend...
...you may as well just hire George Martin to do the recording for you !!! :rolleyes:

Cheap & Chinese this cable may be...
...but well worth getting 50 metres of it it is.
THICK !!! silver plated cores; ZERO resistance.
Not just 1 shield & cheap conductive-plastic...
...but SOLID foil covering as well.
Pretty thick at 9mm dia...
...a mobile vocalist on-stage may develop bigger muscles, dragging it around...
...but you can slam it in a fire-door a couple of hundred times; and it'll be fine.

No I don't work for the company that sells it; but have the order # somewhere if your interested.
The point is THIS IS GOOD CABLE.
1 euro per metre ( ish ), bargain !!!
Great for all Hi-Fi, guitars, backs of 'hummy & buzzy' 19" racks, near SCR lighting-rigs, cell-phones & nuclear-detonations...
...& any thing else that might add NOISE to your precious :note::note::note: 's

Cheers

:D
Si.

Mr Schottky invented the ribbon-mic as well.
Obviously a music lover............................................. ............:note::note::note: :p :note::note::note:

The Space Egg Corp 7th October 2012 11:42 PM

Hi See You

There are more significant upgrades you can do to this unit.
May take it into the SUPER-LEAGUE !!!

Get HIGH QUALITY close-ups of the area around the mic input XLR.
Look for a small transformer OR a pair of different to Pana FM looking capacitors.
If you see these capacitors, note down #'s if visible.

Also look for an extra power regulator near by.
This is for the mics 48 Volt phantom power.
This maybe marked TIP121 ( a popular choice here ).
Also lurking in this area may be more black diodes, with silver bands.
Mr Schottky...we need you again !!!
Note down #'s if visible.

There may also in this area, near the input XLR, be a PAIR of resistors...that may well look different to others on the board...could be around 6.8 K Ohm...photograph & note #'s if visible.

Looked for schematic...can't find also...

" We Don't Need No STINKIN' SCHEMATICS !!! ".....( movie quote ? )

PLEASE...
...High quality close-ups !!!
Shadow free...Shake free...

NO iPhones...etc...!!!

" GOT THE MESSAGE ??? " - Joe.....Reservoir Dogs

Cheers

:D
Si.

We look & see...See You.

See Y'................................................ .....................................

seeyou 9th October 2012 09:29 AM

10 Attachment(s)
Here are some pictures! Yesterday I swapped the toroidal and got some schottky diodes from the local market. The transformer is also a local product, it has good reputation. I also put an mu-metal coat on it! The transformer is keeping low temperature now, but the weird thing is the following:
I have 2x SPL's. I did the same thing on both. Changed same caps, same opamps, same transformers etc. But the one SPL has 3db more noise than the other one..
There are 2 potentiometers near the LM337/317. Since the input voltage of the opamps is marked at +/- 15 V, I adjusted the potentiometers for the LMs to give exactly +/- 15V. for the one SPL, this worked flawslessly. For the second one, as soon as I reach +/-15V, I can hear a buzz. The SPL seems to like the a maximum of +/- 14V. It is the same unit as the one having 3db louder noise floor. I am suspecting the two large can capacitors of the power supply. They are the only ones I didn't change on both units. Could the LMs be faulty? Could the regulating potentiometer be the case?

Any ideas?? :)

seeyou 9th October 2012 10:39 AM

I'll make more close ups today!

seeyou 9th October 2012 10:43 AM

by the way, I use pricey Belden multicore cables with Neutrik gold plated XLRs. Have double checked everything. It is not a matter of the cables. I have already swapped them, re-solderd them and measured them etc. It is not a matter of microphone either. I am doing my tests using microphone as well as Digidesign's line in/out. The SPLs have a mic/line option. Same thing, nothing changes.

The Space Egg Corp 9th October 2012 12:42 PM

Hi See You

Thanks for the pics !
I know I asked for good quality; but...
...can you resize the next ones on your computer ?
I have a valve powered Pentium & the planets worst internet conn.
1024 x 768 re-size is ideal.
Thanks.

My SPL parametric has the blue transformer also.
I used to use that type on boards sometimes.
They do get hot.
Although the spec indicates they are good up to 60 deg C...
...that seems NUTS to me !
I think you're better off with the new big-dounut.
Much better regulation of power, before the circuit.

As for the big power-caps...
...I've NEVER EVER seen components like this fail in modern low-voltage units like this.
60's valve ones always busted...
...but I don't think these are your problem here.

I doubt also that the regulator is busted either.
It might be that where you live you have particularly LOW mains-power level...
...or the transformer simply can't supply enough volts under-load to the regulator.
If the problem persists after you fit the new dounut one...
...I would then try fitting a 'low-dropout spec' pair of regulators.
You can test the function of the old diodes after you de-solder them and fit the Schottky's.

The Schottky's will drop slightly less Voltage than the old diodes...
...so the problem MAY disapear.
It does sound a bit marginal however if this is the case.
If you fit the Schottky's BEFORE you pull the blue-transformer...
...and the problem goes away...
...it pretty much proves the transformer was the problem...
...and this problem shouldn't happen again.
Those blues are gut-less !

How many mains in Volts do you have where you are ?
Here mine go DOWN quite a bit, especialy in the evening & in the winter, even more !
Too many electric heaters !!!...
...Damn cold in ol' Blightey !!!

See You later See You.

Si.

Homebrew cables...
...NICE !


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