SPL GoldMike picking up radio

[saint gillis]

  I have an SPL GoldMike dual mike preamp picking up radio, on booth channels, and in different locations (always picking up radio wherever we are).
  The input cable seems to act like an antenna. I didn't read anything on internet about users having this problem so I guess something is wrong on the device, the input stage seems quite simple, should I check bad ground connections for the input XLRs and the chassis?

Schematic : https://groupdiy.com/index.php?action=dlattach;topic=45607.0;attach=9192

 

[RuudNL]

I had this before with another preamp.
In my case it helped when I added two 100uH inductors in series with pin 2 and 3 of the XLR input. (Or in the cable connector.)
(This does not affect the audio performance.)
Maybe it is worth a try...

 

[JohnRoberts]

+1... picking up radio (aka rectification) happens when electronics can't slew fast enough to follow signals (like radio).

Extra input filtering can't hurt.

JR

 

[analogguru]

Here is the correct way how to deal with RF-interference (from the Audio Handbook 1976).  Don't skip the last point:

 

[Ricardus]

I see ferrite beads showing up in a lot of the builds I've done lately. Hopefully they're helping. 

 

[Brian Roth]

If the schematic shown in the first posting is correct, it shows pin 1 of the input XLR connected to audio "0V" rather than chassis as it should be.

Bri

 

[saint gillis]

Good point Brian, IIRC input XLR s pins1 are connected to the point on the PCB going to chassis and earth plug, but I'll check this out and if it's not the case I'll tie them to the chassis. Otherwise I'll restore the solders of the very first input stage. And if none of this works I'll try an LC filter, but many copies of this preamp have been sold without anyone complaining so I guess it should work without it. Thank you all for your help.

 

[Brian Roth]

Proper design mandates that pin 1 gets IMMEDIATELY "nailed" to chassis with the shortest-possible wiring, vs. a wire meandering through the internals of the preamp.  This was all documented 20+ years ago by folks like Neil Munvy (who introduced the term "pin 1 problem" into the discussion).

Bri

 

[CJ]

is there a Radio Free Europe transmitter in the area becuz those things run at 50,000 watts like XETRA,  maybe somebody broke into the transmitter shack with a diesel generator,

 

[saint gillis]

I've tried everything on this preamp, it has a ground loop breaker that is not indicated on the schematic : 100n & 10r in parallel through a switch that connects or disconnects to the chassis (strange usually it is more a hard link to the chassis OR the cap/resistor network) . So I made a hard connection between the loop breaker and the chassis, connected XLR's in & out pins 1 to a close chassis point. It didnt change anything for this heavy radio reception. The SSM2017 had been replaced by 2019's, I've changed them to that1510, same radio thing, so I ended with a 20µH 1n network at the inputs, it reduced massively this radio reception (it it still a bit audible at very high gain).

But now I have another strange issue on channel 1 only : when the -20dB pas is on, there is a huge white noise, and when the metal armature of the gain potentiometers doesn't touch the chassis, the problem disappears, I'm quite puzzled...

 

[guy_4]

Hello saint gillis,
A few years ago I had a client who experienced the same problem as you with his Gold Mike pre.
I modded it with a Jensen 115K Mic transformer on the input.
Problem solved, no HF anymore
And the client is very happy as he can now use the GM with his Ribbon mics, he was desperate to not have enough gain before...
Best,
Guy

 

[saint gillis]

Hi Guy, thanks for your feedback.
The gain is not really an issue, the goldmike seems pretty generous in dBs (specs say 72dBs and I wouldn't be surprised if it's true).

I finally found something that seemed to solve almost all the problems I had with this unit : I've grounded the unused pin of the 48V switch, so the inputs are tied to a ground reference via the 6K8 resistors. I know it seems a bit esoteric but it works.

 

[lovebuzzz2]

But now I have another strange issue on channel 1 only : when the -20dB pas is on, there is a huge white noise, and when the metal armature of the gain potentiometers doesn't touch the chassis, the problem disappears, I'm quite puzzled...

Hello saint gillis, I started having exact problem with white noise after transformer died and I changed it to a new one... It's very puzzling and confusing...
Did this white noise and cracking go away after you grounded Phantom switch? I understand it was almost 4 years ago, but maybe you still remember?
Thank you!

 

[saint gillis]

Hey, I remember it was a pain
I would advise : try to ground the phantom switch first, then check for good chassis connection of XLRs' pin 1, then if you have radio or things like that add LC networks at the input..
Give us a feedback !

 

[lovebuzzz2]

Hey, I remember it was a pain
I would advise : try to ground the phantom switch first, then check for good chassis connection of XLRs' pin 1, then if you have radio or things like that add LC networks at the input..
Give us a feedback !

Oh, great thanks for a quick answer, I will try all these steps and report back!
It's weird this noise happens only in one channel (left one), I also noticed it goes away if gain is at MAX or close to it, got me thinking some woodo is involved)

 

[saint gillis]

you might have to check the SSM2017 or the components around it..

 

[saint gillis]

Or the gain pot

 

[Matt Syson]

There is no voodoo involved, it is a cheap and badly made unit using 'common parts' and they ignored many of the rules of good EMC and interference suppression design. being able to work in high RF environments is what sets professional gear apart from the cheap toys that copy the 'expensive' designs circuit wise but leave out the many very subtle measures that are necessary to keeo interference out. So a Goldmike and other cheap gear WILL work but you cannot Guarantee it will be noise free in all situations. 80% of the time, probably , but proper designs factor in extreme environments.

 

[saint gillis]

The MKII has been REALLY improved !
You are right about the MKI, but I must say it sounds very good when it works

 

[lovebuzzz2]

Thanks for all the tips, saint gillis! Grounding of phantom switch helped, and I also discovered that grounding of one of those 100/63 caps in MICPRE section does the trick as well. Also, without any grounding manipulations the noise would disappear if phantom was engaged.

And then I actually realized I was chasing my own tail all this time, because I was testing this GoldMike with nothing plugged into Input, but as I plugged microphone into XLR input - noise just disappeared. So, basically problem is non existent when actually using the preamp.. Which means I'm afraid Matt is absolutely right - there are some shortcuts in design causing this strange behavior.

I guess SPL was aware of this particular "not a bug, but feature" of MKI, because I also have another GoldMike MKI, but it is a later revision of it and it doesn't exhibit same behavior on channel 1.

Thanks again everyone!