Old school Push pull audio amplifier transformer flux managment

 jla...@highlandsniptechnology.com wrote:
===============================
> 
> >I have a switched mode power supply background and an looking at something like this for an old school audio amp design. 
> > 
> >https://www.circuitstoday.com/push-pull-amplifier
>
> Nice example of thermal runaway. And random transformer phasing. And 
> distortion. 
> 

  **  Pinocchio is not a real boy. 


....  Phil 

On Saturday, December 11, 2021 at 4:23:59 PM UTC-6, palli...@gmail.com wrote:
> dmcous...@gmail.com wrote: 
> ========================
> > I have a switched mode power supply background and an looking at something like this for an old school audio amp design. 
> > 
> > https://www.circuitstoday.com/push-pull-amplifier 
> > 
> > For SMPS pushpull we always used current mode control to assure the flux in the core does not walk all the way into saturation. 
> >
> ** P-P audio amplifiers are nothing like SMPS.
> > Looking at these designs, none monitor or measure the current to assure the flux does not walk.
> ** Simply not an issue.
> > Many of them are biased class AB so there is always a small DC current through the windings. 
> > if they are not perfectly balanced, that would > cause the core to bias to one side.
> ** P-P audio output transformers with many E and I shaped laminations create a distributed air gap that stops this. 
> 
> Core balance can become an issue if a toroidal type is used - which is quite rare in audio amps.
> > I'm looking at adding a DC balancing circut with opamps and a reference to assure the split in bias current is as close to perfect as possible.
> ** Not necessary.
> > in short, for you guys that have made a few of these back in the day, what did you do to assure the flux in 
> > the output transformer did not wander to the saturation zones due to transistor mismatch or DC bias mismatch?
> ** Match the Hfes of the output pair. 
> 
> The point you are missing is that transformer core saturation is only an issue at the lowest operating frequencies - generally under 50Hz for a high quality design. Higher frequencies are barely affected by a bit of magnetic imbalance. 
> Cos they swing the core so little. 
> 
> 
> 
> ...... Phil

Makes since.   Thanks Phil.

On Saturday, December 11, 2021 at 11:37:36 AM UTC-8, bitrex wrote:

> Did they try to match the transistors on that topology, like tubes? 
For sure, they did.

And to replace finals on a Phase Linear 400, that means buying a matched pair 
of 2SD555 transistors, which is rather pricey (if one fails, do NOT reuse its mate...
you have been warned).

The Witless Troll =  whit3rd wrote:
=======================
> , bitrex wrote: 
> 
> > Did they try to match the transistors on that topology, like tubes?
>
> For sure, they did. 
> 
> And to replace finals on a Phase Linear 400, that means buying a matched pair 
> of 2SD555 transistors, 
>

** The PL400  mk1 and 2 used  6 output transistors, needing to be Vbe & type matched in groups of 3. 

> which is rather pricey (if one fails, do NOT reuse its mate...  you have been warned).

Toshiba 2SD555s were never installed in PL400s by the makers - but were offered as replacements.


.....  Phil