Siemens W295b Output Impedance

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StaticRick

Member
Joined
May 1, 2012
Messages
24
Location
Columbus
I've got a pair of Siemens W295b EQs that I'm finally getting around to racking.  I've read that these sound even better when you match the output impedance to what they were originally designed for.  The problem is that I'm not sure what that value is.  I read somewhere that they were designed for 600 Ohm, but the schematic I have scrounged up online seems to show 300 Ohm.

The output portion of the schematic is attached.  Am I reading that correctly?

Someone with more knowledge of these units might know if there was a "standard" output impedance for which the Sitral stuff was designed.



 

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Let me ask this another way:

Am I correct in reading the schematic as stating that 300 Ohms is the design/expected output impedance? 
It's entirely possible that the 300 Ohm value means something else that I'm not getting.
 
How about this?

Attach modern 10K load. Try 1K, 620r, 330r, 220r resistors across the line. Listen. Which sounds "better"?
 
Brian Roth said:
In addition, poke a square wave through the unit and look at the output with a scope as you try various output loads.

Bri

Thanks, that's what I'm going to do.  Not only will it help answer my question, but I may learn something more in the process  :eek:
 
Hi,

I have serviced and racked a number of these modules. It was so long a ago (10 years+) that I did a pair without output loads (must be switchable in case you go into something that is already 600 ohm in - 1176 is a common example - then you are not double loading) at first they came up edgy.

Put 620 ohm resistors on the outputs for feed into standard 10K balanced line in, and the sound went aaahhhhh. You know that aaahhhhh.

My reasoning for staying with 600 ohm is that it would be a change to load them 330 ohm into a 10K line in, then you would be at 300. Then switch the loads out and go into a 600 ohm input (most all non bridging inputs are 600 ohm, not 300) there would be a change in response between the two conditions. I haven't looked at a schematic in a long time, now I just service them, but it does appear to be a 300 ohm out.

Try it, if the response goes wilt, or the distortion goes way up and the headroom goes way down then you know 300 is too low. Try 600, see what you like. Heck try 1K, but that may get a little pushy up top.

Sweeps are educational. Square waves work too. Both is best.

Also 300 would reduce the output headroom more than 600, and increase the insertion loss slightly as well. I would have to check on the effect of that. 600 did sound good to everyone, and I have stayed with it.

I would have to dig, so not anytime soon, but pretty sure I have AP sweeps from my System One of W295Bs without load (100K AP following line in load only) and loaded 600 ohm. The 100K (10K is a little more dampened but not very much at all) plots showed significant HF rise (ringing from underterminated output TX) if I recall correctly.

1176 Rev F's also need, need a 600 ohm load when going into 10K ins as they ring like a church bell otherwise. The Class A ones are pretty well dampened into either 10K or 600 and are basically load independent, as are Class A Neve's as long as the output network is present and intact. Later TO129 output TX Neves are not. That would be 1091, 1093, 1095, 31105, 33415, 33114/115 etc. They must be racked correctly to sound good. There is an internal load resistor on the output connection, but it is sometimes missing. Those are some common examples of where problems are seen with this.

Something tells me leaving out the needed loads when going into 10K inputs with the above gear is one of the main reasons that caused it to fall from favor compared to say 15 years ago when I went full time into studio equipment as a business. Then it was less common to see loading errors in studios. All of the above Neve modules sound excellent, as does the Rev F, when correctly loaded. You can shape the HF rise to your taste, just don't go below 600 generally speaking, on most gear. 

I do this kind of work all the time, just really want people to get the most out of modules or equipment they may have. I see improperly loaded racked gear and also see double terminations in setups constantly.

Knowledge and control of impedance is a key to good audio. And if audio isn't as good as it can be it becomes bad pretty quickly. This kind of thing can really shortchange a vintage or old style piece.

Thanks,

James Rowell
www.ssevintageaudio.com
 
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