[ARC5] Fwd: Smart People: 60Hz Transformers Faster?

[Scott Robinson]

Hi Brian,

I was the one who, while building audio amplifiers with no money as a 
teen-ager, used power transformers as PP output transformers. So:

1) I agree that the HF response was probably not great. I was using 
triodes, with their inherent low output impedances, in the output stage, 
which helped.

2) When discussing DC in the transformer, you say that's a problem with 
using such transformers for output duty. You're right, but I knew that; 
my circuit was, as stated, push-pull, with little or no net DC in the 
transformer primary. Actually, in the course of restoring various 
radios, in a two year period I found two radios with single ended output 
whose output transformers had been replaced with universal transformers 
designed for push-pull service.The result was very little bass, as the 
core was saturated and the magnetizing inductance was way too low, 
shunting out the bass.

3) As a designer of, among other things,  switchmode power supplies, I 
will say that none of them that I have seen use half-wave rectification. 
Diodes are cheaper than monster electrolytics.

Regards.

Scott


> ---------- Forwarded message ----------
> From: *Brian* <brianclarke01 at optusnet.com.au
> <mailto:brianclarke01 at optusnet.com.au>>
> Date: Sun, Nov 22, 2015 at 10:43 PM
> Subject: Re: [ARC5] Smart People: 60Hz Transformers Faster?
> To: Spr Imop <spr at earthlink.net <mailto:spr at earthlink.net>>, David
> Stinson <arc5 at ix.netcom.com <mailto:arc5 at ix.netcom.com>>
> Cc: ARC-5 List <arc5 at mailman.qth.net <mailto:arc5 at mailman.qth.net>>
>
>
> Hello Dave,
>
> In general, you can use a transformer at a higher frequency than that
> for which it was designed. But there is a limit. It is to do with the
> quality of the iron. As the frequency gets higher, some
> transformer-specified irons / steels will start absorbing much more
> energy than at the rated operating frequency. Running a transformer
> designed for 60 Hz at 800 Hz should be OK - but you will be carrying
> more iron than you need.
>
> People who claim that they have used power transformers as audio
> transformers are often old folks for whom presbycusis has set in and
> they can't hear that the high frequencies are severely distorted and /
> or attenuated. While the transformation ratio may be OK, the impedance
> matching will be all over the place. Another problem with using power
> transformers for audio work is that there is usually DC flowing, which
> alters the behaviour of the iron - the whole hysteresis curve gets
> shifted up, one side of the audio wave flat-tops and the transformer
> heats up. If there is enough heating the iron can be permanently damaged.
>
> You can also stuff a transformer by having net DC flowing when you use
> half-wave rectification - doesn't matter if it's at 60 or 800 Hz. Most
> computer SMPSUs rely on half-wave rectification - and electricity supply
> authorities get upset because of the net DC flow in their distribution
> transformers.
>
> 73 de Brian, VK2GCE.
>
> On Monday, November 23, 2015 4:47 AM, Scott opined:
>
> I used to use power transformers for pp output transformers and they
> worked fine. I expect 400 Hz to work fine, too.
>
> /Scott Robinson
>
>     On Nov 22, 2015, at 7:59 AM, David Stinson <arc5 at ix.netcom.com
>     <mailto:arc5 at ix.netcom.com>> wrote:
>
> <snip>>
>
>     But what about running a 60 Hz transformer at 400?
>     Saturation shouldn't be a problem, but what about hyste...hister...
>     yeah those loses.
>
> <snip>>
>
>     73 Dave S.
>
>
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