Transformer Coloring Box Again

[apzx]

.

 

[ruffrecords]

Looks like a good start. I would replace the bridged T with a 10K pot and series capacitor to the op amp and fit the latter with it own bias resistors. On the other hand you could be adding 20dB of gain to the input signal so it might be an idea to add a T attenuator on the output to restore the level to sensible levels. Your basic setup is almost exactly how I build my 'tube tone boxes' but for them the idea is to drive the tube harder but it does also drive the output transformer harder (but I use a VTB2291 which can take it).

Rupert Neve's 'Silk' circuit I believe adds a little dc current to the output transformer. This shifts the magnetic bias so the transformer clips unevenly which tend to create odd harmonics.

Cheers

Ian

 

[apzx]

.

 

[ruffrecords]

I have never tried adding dc to a transformer either but I have seen the Neve silk schematic on line somewhere so that might be a good starting point.

Cheers

Ian

 

[apzx]

.

 

[ruffrecords]

Thinking on this some more, it really doesn't inherently make sense to apply a DC bias voltage to the transformer. At least to my mind because an inductor should just pass that to ground. Would it not make more sense to actually bias the primary of the transformer with a small DC biasing current from a simple constant current source?

Running a constant current through the transformer should saturate the core more readily, and be easier to control. Unless I am way out in left-field with my thinking here.

Edit - Actually, having done a bit more reading on things (granted in the context of power transformers and such), it seems that applying a DC offset would be the correct action here rather than a current source.

This is an area of transformer theory I am still not 100% clear about but a dc current flowing in the winding will cause a static flux in the core. Because the flux has direction, just like current has direction, this static flux acts like a dc offset so one half of the waveform the core saturates at a lower level than the other half. CJ knows all about this so maybe he will chime in. So I think the current source idea is the way to go. As I said, I am sure I have seen it in the Neve schematics so I would suggest you have a look at how they did it.

Cheers

ian

 

[apzx]

.

 

[ruffrecords]

Thinking is good for you.

Cheers

Ian

 

[apzx]

Well I placed the order to Edcor yesterday. So, in a couple of months I ought to have the transformers on hand to test the ideas for biasing the transformer. Gives me some time at least to contemplate some ideas. Right now I have AC coupled the transformer and from what I've been able to surmise adding a DC bias is not as easy as I thought it'd be. So, I'll just have to head scratch and try some different things. I figure as long as I am a bit careful about the amount of current I let flow I shouldn't blow up the transformer.

 

[ruffrecords]

The formula for calculating the flux in a transformer from the number of turns, the core size, its material and the applied voltage has a 1/f term which is why transformers saturate more easily at low frequencies. But this means that at zero frequency i.e. dc, the flux would be infinite. The confusing thing for me is that the flux is related to the magnetizing for by the BH curve (B is the flus and H the magnetising force) and H is measured in ampler turns so maybe this is the formula you use for dc. I don't know. Some aspects of transformers are still a mystery to me.

Cheers

Ian

 

[abbey road d enfer]

The formula for calculating the flux in a transformer from the number of turns, the core size, its material and the applied voltage has a 1/f term which is why transformers saturate more easily at low frequencies. But this means that at zero frequency i.e. dc, the flux would be infinite.

That would be if the total DCr of the circuit was zero. The magnetizing current at DC is the simple result of U/R.

 

[apzx]

.

 

[abbey road d enfer]

Actually, the magnetizing current isn't that simple (I've done a lot of reading today haha). But it really does boil down to Ohm's Law with some additions namely in the form of non-ideal properties of the inductors in use, and also looks really familiar to the equation for finding the current in a LR series circuit. I did find a derivation for the saturation current that requires knowing the saturation limit of the core material, how long the winding is, the magnetic permeability of the core material, and the number of turns.

You're making things more complicated than need to.
We're talking about DC current. Inductors just do not existe at DC (as well as capacitors.)

 

[skipwave]

To calculate the amount of current that will flow through the primary winding, as you say it’s simply Ohm’s Law. The DCR of the primary winding as the divisor of the volts. V/R = A

The impact of any change exact amount of current flowing on the flux and therefore asymmetry of clipping does not seem useful to calculate. Try it with a variable voltage source, simple pot as voltage divider will do, and twist the knob until it a sounds rad.

I’ve been wanting to do this for awhile, particularly because all the vintage Triad transformers I have specify a DC current limit in the primary, which I just don’t know whether it was because builders were making small signal Class-A stages to maybe reduce part count or something else.

 

[Michael Tibes]

I have revised my idea a bit based on your suggestions and a bit more thought on my part.

A small detail: I'd reconsider S2. The way you designed might cause a short moment of 'no connection' when you flip the switch, leaving the opamp without feedback which might result in a big pop or whatever noise. Maybe always keep R10 and only switch R7 (needs t be recalculated) parallel to it?

Are there any practical experiences with these concepts yet or has it only been a theoretical discussion so far?

Michael

 

[SWAN808]

Which discrete op amp are you thinking about using? That will impact the sound to a degree. I have the DIY Colour CTX Mk1 and Mk2 and the LTL Mass Driver (which uses an Edcor)...they are similar but the difference is in the way they start to clip...the discrete clip sooner and nicer...before that they are very similar...

 

[apzx]

.