Bass rumble on old German EQ's (Filtek/BFE)

[Alistair]

Hey guys,

I've got a pair of BFE/Filtek MK5b EQ's from an old East German console. Great EQ's, but one of them has always had an issue with the low frequencies. All sections are bypassable, so I've just been using it on sources that don't need much bass work done, but it's a PITA and I'd really like to have them both working 100% so I can use them on stereo sources etc.

Essentially, when I flip the low end EQ out of bypass and into the circuit, there is a big oomph, and then a constant crackle. It still passes signal, but it's pretty distorted and seems to cut out a lot of the low end.

I've opened up the EQ's to have a look, and nothing obvious- no blown caps etc. I've also retouched all solder points in case of cold solder.
My thinking would be that considering the HPF, LPF, Hi and Mid sections all work perfectly, that it has to be something in the bass section. From what it looks like, there are two pots that mount directly to the circuit board (frequency select, and dB cut/boost), a wide/out/narrow bypass switch, a whole lot of resistors (I presume the settings on the frequency select), and about 8 caps. There is also a single IC.

Any ideas where to trouble shoot or what to replace? I'd rather not replace every part searching for the crackle. It may have to go into the shop for a fix up, but I've just made a couple of big purchases so can't afford to fix it for a while unless I can do it myself.

[Futureman]

Whats the IC for the bass section?

I'd start with replacing that, or if it's the same as another section, swapping it over to the working one.

(Assuming it's in an IC socket, and easy to remove... but even if it isn't, I'd prob still swap it over)

Regards
Mike

[Alistair]

It's a NE5532N- apparently a little dual op amp. It's not socketed, but it's easy enough to get in and work on the EQ. I'll give it a try and see what happens. Thanks Mike!

[rob]

does it use tantalum capacitors?
These fail with those exact symptoms that you have

[Alistair]

I didn't see any Tantalum's but I'll have another look.

I cannibalized a NE5532 from a different spot in the circuit and have removed all the hum associated with the bass- there is still a bit of crackle on loud signals though. Sadly, I've also lost all the Bass EQ! I wonder if I read the circuit wrong and have actually removed the IC that was controlling the bass.

I'll get some new NE5532 and replace them all in that EQ, and then hunt for Tantalums. I'll let you know what happens.

[Alistair]

2 months later, and I still can't find any sources for NE553N chips. The only places that have them are discontinued electronics stores in the USA, that want $15 per IC at a minimum orders of 100 pieces! Are there any good (preferably Aus) sources for small quantities? I only really want 2 plus a couple of spares incase anything happens in the future.

[rob]

hang on

what sort of IC are you after? A 5532 or a 553? I don't think there is any such thing as a 553
If it is a 5532 these are pretty common.Aztronics sell them.

[Alistair]

Sorry Rob, typo. Yep it should be a 5532n. I've just checked the Aztronics site, and they seem to have a 5532AP, which I believe is a premium low noise version of the chip so I'll get in contact with them (and read a couple of datasheets to make sure they are compatible). Thanks for the heads up!

I've also just grabbed a Trimax A55 from Julian- he suggested you're the man to make it a happy preamp, so I'll shoot you an email once I've got my hands on it.

[rob]

sure 5532A is just a guaranteed noise version of a 5532

you could put just about any dual opamp in there to prove a point. A LM833 would be a fair substitute

[Thirteen]

Wes components in Sydney shows the NE5532P in it's catalog for $2.75 each.

I have one NE5532N left in my parts drawer and a pair of NE5532P's.

[Alistair]

Thanks for all your help guys- followed up a source from here (Aztronics) and got some IC's. Sadly not a happy ending, I've replaced the IC's for the Bass section and no difference whatsoever. I'll have a hunt for Tantalum caps- certainly none near the IC's, but it's a bit of messy double-sided-board-shoved-in-a-tiny-box so I'm not convinced I haven't missed something obvious....

[rob]

I had an active ribbon mic in the workshop today with a low freq rumble / crackle.
I replaced all the likely candidates but in the end it turned out to be a zener diode.

I had a crackling 1073 module in last week, it was a BC184 transistor

Tracking down noise problems can be very difficult and often it is a matter of substituting parts until you get lucky. Having a store room full of spare parts is an advantageous starting point!

[Linear]

well i'm no expert, but i've had enough noisy things to get some kind of system going.

firstly, you need a scope and the schematic.

if it's crackly at the output, connect the scope to the outputs until you have that noise showing up on the screen. feed some tone through if needed.

then, you can either work forwards from the input, connecting the scope to each section of the circuit. the signal should be nice and clean until you hit the suspect part. or you work backwards from the output, until you get to the part that is causing the rumble.

this has worked for me in the past. i guess you'd only do this if you could operate a scope and could read a schematic.

Chris

[rob]

And the noisy part isn't inside a feedback loop, causing the noise to be seen in lots of places. But a scope is THE handiest tool in fault finding

[Alistair]

But buying a scope doesn't sound nearly as fun as buying more parts to finish of DIY projects!
Thanks for all the help guys- at the moment I think I'm going to replace the caps in the bass section of the circuit (I'm fairly sure its not the resistors as they are frequency specific- it needs to be something in the circuit the whole time the bass is engaged), and then it will have to go into the too hard basket until it goes off to the tech. You may get an email Rob!

[Alastair Reynolds]

Another nice little trick for tracking noisy active components, i.e. semiconductors, that I use a lot is a can of freezer spray with a fine tube, like you get on WD-40 etc. Cooling a suspected noisy transistor or IC can (sometimes) reduce the noise dramatically for a short period until the component returns to normal operating temperature and the noise returns, similarly , heating them with a fine jet of hot air from a surface mount desoldering gun or a hairdryer can increase the noise, thus helping identify the part which is on the way out. Capacitors, as Rob says,replace any tantalums. They age , they go short circuit, they get noisy, other small electolytic capacitors often die too. 20 Years ago when I was working on a lot of Quad gear , most of the Quad 33 Pre-amps coming in were noisy as old metal Can BC108 transistors died and electros dried out. 90% of faults were fixed by changing all BC108 for BC549C low noise transistors and replacing all the electolytic caps with new high grade ones. Cost in parts about $15 and an hour to swap them out. result.. quiet preamps.
Silicon semiconductors actually have a high failure rate in their first few seconds of use then anything that survives initial power is usually good for at least 10 years, after that the the failure rate starts to climb rapidly. This is known as "The Bathtub Curve". As for 5532 op amps of any variety, there are a myriad of better, quieter ICs you can drop in it's place like the LM833(As Rob said) , and not adversly affect performance , if anything it will improve. And make sure you get a schematic.

[Alistair]

Thanks Alastair, some good tricks in there. I think I'll follow everyone's advice and replace all of the caps in the bass section of the circuit just to be safe. Frustratingly they seem to use a brand of capacitors (probably another East German/behind the wall job) that don't give full specs. Eg, the Tantalums just say 1 35v or 4.7 35v, so I'm just trying to figure out if they are 1uf or pF or otherwise! I'm thinking the colour of the actual caps might be the multiplier (some are blue, some are green).
At least I'm learning something!

[Alastair Reynolds]

For tantalum they'll be uF,
PF would be silver mica or ceramic generally, tantalum tends not to be much smaller than a 0.47uF. If it is a single body colour then Colour coding is generally different manufacturers I have green, blue, yellow and brown. If colour codes are used on older tants they are bands or dots on a background colour.
Cheers
Alastair

[Alistair]

Thanks Alastair, that's the gut feeling I had. I figured if they were 4.7pF for example, if nothing else they wouldn't have a polarity marking on them.
I've seen newer tantalums that are yellow, brown and I think orange, but never the blue or green.

Thanks again, hopefully this time they will get up and running!

[Thirteen]

I find a simple capacitive decoupled audio probe to be invaluable to trace low noise in audio circuits. An oscilloscope, particularly a digital scope often won't show noise well if it is combined with signal and it is at low levels, whereas an audio probe through an amp and speaker allows your ears, which are far more attuned to noise anomalies within a signal than a scope to do the job.

[Alistair]

Success! Tantalums replaced and distortion/noise gone.
Rob, you got it straight off the bat- I should have listened to you rather than chasing up the IC's, although it's good to know I've got a couple of spares now in case.

I forgot how much I like these as a stereo pair- I've been using them individually as I couldn't use the bass at all on the faulty one. Super punchy and aggressive, but not too harsh. Doesn't suit everything, but when they work they are great.

Thanks everyone, I owe you all a beer!

[rob]

yep!