Calibration of W295b

GroupDIY Audio Forum

Help Support GroupDIY Audio Forum:

This site may earn a commission from merchant affiliate links, including eBay, Amazon, and others.

dogears

Well-known member
Joined
Nov 15, 2017
Messages
548
Location
Texas
I did a thorough forum search, but I don't see any responses here. Apologies if this has been answered before.

In the output block  of the W295b, there is a feedback network with a capacitor, a fixed resistor and two trimpots.

It seems the purpose of the 5k R29 is to set the overall gain of the circuit (+/- around 5dB I expect). I don't understand the purpose of R31 though.  I assume this is for some kind of calibration. Can anyone explain this to me?
 

Attachments

  • w295b fb.gif
    w295b fb.gif
    88.1 KB · Views: 55
Thanks PRR, makes perfect sense. With the absence of other info, how would you set the bias? Drive it to clip and make it symmetrical?
 
Note that if you drive it into clipping, it might throw off the DC bias (such as because the input transistor base sinks current and charges up a coupling cap). At the factory they probably just had a specific voltage that they set it to.
 
That leaves me wondering why it’s a trimmer at all. The other stages have fixed DC outputs, but they do vary.

I’m also looking at the input transformer. The  specs says >2k impedance input. Glancing at the circuit it’ll be somewhere around 5.5k. I talked to both Jensen and Cinemag and neither seemed to think they have an ideal pairing for this.  What vintage transformers would be used in a similar fashion? 
 
Back
Top