siemens w293 buzzing noise

[alexgg]

hi I recently got a stereo siemens w293 from ebay as untested

I got the tuchels wired up as per schematic (attached for w295 , w293 on kubarth is exactly the same I think)

the unit passes sound fine, no loss of signal, and the highpass filter is functioning.

1 problem with filter is one channel is passing higher frequencies,
at bypass 1 channel is flat frequency response,
the other has a low frequency hp filter working even in bypass,
this frequency gap in hp remains the same as the shared dial is turned.

I think maybe the shared frequency dial is ,misaligned in one channel,
it kind of looks like 2 mono units connected to a shared dial frontplate design

more worrying in a way is the relatively high noise floor across the frequency spectrum that sounds like buzzing when amplified

has anyone maybe any experience with these units + can offer some advise ?

 

[alexgg]

can anyone please suggest what might be wrong with this unit making a buzzing noise

its basically 2 mono w293's stuck together with same faceplate

the buzzing noise only occurs when plug to power supply (24v dc wallwart) is plugged in
before I even switch the power on at the plug socket, there is buildup of noise at high end
when I switch power on there is loud buzzing across the spectrum, it then quickly subsides to a lower level (-67dbfs = lower level )

when I switch off power but leave plug in socket the noise level drops to what I would expect from german unit like this then slowly buildups again until I actually remove the plug from socket.

I checked power connections to tuchel + seems correct 24v across
this psu split between both tuchels, should be correct for these units I think

any ideas ?

 

[ArnauTS]

You need to filter the psu and make sure that the grounds on unit and case are ok.
Hope it helps

 

[Harpo]

Whatever you might call 'ground'...,
you connected the +24...30V DC side of your sufficiently VA rated PSU to pin-13 and PSUs negative side/0V reference voltage to pin-2/3 ?
Your cable shield and case ground connect to pin-1 ?

 

[synthiaks]

Hello Alex,

The Siemens Sitral Modules that i have come across dont play well with Walwarts. As stated above the power source needs to be filtered to work without buzzing or other strange sidefx. I would go for a true regulated +24 PSU just to be on the safe side, using a crap PSU on highend outboard isnt going to make anybody happy

/Seb

 

[alexgg]

hi
thanks for the responses

yes to harpo's questions , I have 35w , 24v supply split between the 2 units
( I think these basically 2 mono w293 units stuck together)
24 v >> pin 13
0v / negative >> pin 2
cable shield >> pin 1

this the exact same type of power supply I have 2x filtek bke compressors running off , and another powering 2x rfz w735 eqs,
neither of these has any problems with it , but I must admit those were professionally racked for me.

u think siemens have such a noticeable problem ? I thought they had some kind of internal power regulation ?

I would honestly suspect my wiring more but I got these units off ebay + they were sold as untested, same vendor sold 2x w293 as tested perfect couple of weeks after so must have had means to test these, it might be the problem with 1 channel highpass higher than other, but might be a real buzzing issue going on, not sure what else to look at, I haven't opened them up properly yet.

one thing I notice, the noise is identical level across the spectrum between both units, left + right.

 

[alexgg]

another thing I noticed is that with the tuchels disconnected from w293 but io connected to audio test equipment (pc) and the power supply switched on, there is quite a ground hum present 50hz + all its harmonics, is that normal ?

the noise is much louder when the tuchels are connected to w293 + buzzing noise spread across the spectrum, sounds very like a noisy old synth buzzing.

(schematic attached)

 

[synthiaks]

Hello Alex,

I have a wallwart that i sometimes use for quick testing but For the Siemens sitral's it doesn't work. I always get Loud buzzing and Hum. If i use a Regulated supply the noise and hum is gone. My BFEs, RFZs And Neumann's all work with the wallwart Though.

So if you still haven't tried with a good PSU i suggest you do!

If the Modules are 100% original you have Electrolyt Caps in there from the 60s so my second advice would be a Recap, Only change the electrolyts though if the filter FQs works as they should.

/Seb

 

[alexgg]

hi synthiaks

thanks for reply , I only just checked the thread.

yep it seems siemens the problem , I bought another module w294 , assured to be perfect

again same problem

only other thing i could think of was directly connecting the ground connector to 0 voltage, pretty sure it does this anyway inside the unit but might be bad connection.
i cracked open my filtek rack unit btw + notice what looks like a long yellow axial capacitor set into the grounding connection wire from the wallwart ???


can anyone please suggest for me what is a decent power supply to use for these units then , or a kit maybe ?

i am in uk , using 220/240v

 

[synthiaks]

Hi,

I thought so. The Siemens are early 60s design and need a well filtered PSU

I bought a switch PSU from China that works fine. It has enough big caps to filter the output. I also built a straight regulated 24V PSU that also work well and is much smaller than the Switch PSU this however involves doing PCBs which might not be everyones cup of tea. Any regulated 24V PSU kit with a couple of ripple filter caps would do it! As long as you stay away from Wall warts ( Or try the MFOS regulated AC walwart kit)

However if this doesn't help the next thing to try is a full recap. It is possible that some caps that should filter out DC somewhere in the circuit ( like transistor bias) no longer work the way they should. The electrolytes are some 60 years old by now so even the high quality Siemens stuff dry out, this will change bass response. You will have a piece of gear that sounds much thinner than it should. But as I said before, ONLY change the elko's. The foil caps could be important for the sound of the EQ and probably are still OK.

Seb

 

[alexgg]

hi synthiaks

yeah thanks for the suggestion , I got a regulated switching psu with similar specs to a siemens / telefunken desk psu I think(2amp 48watt). this was actually very cheap (£7!!) and compact.

when I wired it up there seemed to be a noticeable drop in hum + hiss from the wallwart but looking at daw analyser it was still very high.

what I then did was tie up the audio output ground to pin 1 (with input ground) + also bridged pin 1 + pin 2 (connected to 0v). im not sure if connecting output ground helped but I think it was bridging pin 1 + 2 ( I thought it was doing this internally anyway), guess its set up for chassis grounding rather than plug + play, im not sure,  but . ...

now its passing super clear audio, these siemens modules are crazy quiet, line level audio coming through perfectly with no gain or attenuation + noise floor now indistinguishable from my dac (120db dynamic range)!

I still have 1 channel of the stereo HP which is set higher than the other but it now has 400 + 500hz options so maybe I leave it as an option ( I think its just set wrongly in the dual connector knob mechanism on front panel).
guess ill be rigging as single channel with high pass + low pass in series for single channel tracking.

anyway thanks again for the help.

alex

 

[synthiaks]

Hi Alexgg,

Well I'm happy i could help. The high pass difference could also have something to do with failing Caps but i have to see a schematic first to say where i would look.

The old Siemens modules are super quiet, but they also were fantastic designs. Those German engineer sure knew what they were doing.


/s