[ProAudio] Transformer color revisited -- DC injection

[Bill Whitlock]

First, you'll get a lot more "effect" from el-cheapo transformers (smaller cores) rather than better ones like the 111's.
Second, the chances of any significant remanent magnetization depends on many things. Higher DC current increases risk, of course, but technically the width of the BH loop of the core material also has a huge effect.  In general, steel has the widest and high-nickel alloys (like Permalloy) have the narrowest. But then you may like the sound of a magnetized core. It will have stronger even-order harmonics than odd, which normally predominate.
Cores can be demagnetized by driving them into saturation with AC and then slowly reducing the AC amplitude to zero (over a period of 15 to 30 seconds). Cores will, of course, saturate with the least voltage at very low frequencies, say 10 or 20 Hz.  Only one winding need be driven and you can tell you've reached saturation by looking for flat-topping of the voltage output at any other winding. Always check to be sure the driving source isn't clipping when you think you've reached the saturation point.
Bill WhitlockAES Life Fellow(from my new home in Chino, CA)

Sent from the all new AOL app for Android 
 
  On Sat, Jul 22, 2023 at 6:50 AM, Bob Katz<bobkatz at digido.com> wrote:    
Hi everyone. Hope you're still around!
 
I'm finally getting to finding time to build my "ultimate transformer color box". 
 
 
Back in January, 2022, Bill Whitlock wrote:
 "Yes, increasing series R on the drive side will exaggerate magnetic non-linearities of the core material (i.e., THD).  If you want to shift the mix of odd to even order harmonics, add a few mA of DC on the primary side (say 0 to 30 VDC through a 10 k-ohm resistor directly to the primary side." 
My box contains a pair of WE111 and a pair of UTC A-20. Two sets of transformers with distinctly different saturation characteristics. It uses a couple of Wayne Kirkwood mastering insert relay boards to optionally insert a variable series resistor, and to switch in one or the other transformer.
 
Attached is a schematic for those who are interested. Simple stuff, but fun!
 
 
I wasn't planning on having a DC injection circuit, then it occurs to me I have a spare pair of relays. I'm trying to keep the box totally passive, so the question is how to handle the DC injection. We don't know what the source will be as it could be from any of my mastering gear, which is typically an active balanced output. With relays I could insert a large series capacitor to keep the DC injection from back feeding into the balanced source. block the DC injection from contaminating the source, plus the variable DC injection into a transformer primary. How does that idea sound to you?
 

 
 
Next question: Should I worry about the DC injection circuit permanently magnetizing the core of a transformer?
 

 
 
Best wishes,
 

 
 

 
 
Bob
 -- 
 

If you want good sound on your album, come to
Bob Katz 407-831-0233 DIGITAL DOMAIN MASTERING STUDIO
Author: Mastering Audio
Digital Domain Website

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