[Patrick Turner]

On Sep 30, 9:58*am, flipper wrote:
On Wed, 29 Sep 2010 23:00:49 +0100, Ian Bell
wrote:

I have been thinking a bit about Patrick's suggestion of using something like an EL84 in an SE
headphones amp. I was intrigued that he had built one using a 'cheap' output transformer so I have
looked at specs from various suppliers of such transformers. These transformers typically have a
primarily inductance of around 10 Henries (at 20 or 40mA) for a nominal 5K match. Now my transformer
rule of thumb is you want the primary inductance to be twice the plate load at the lowest frequency
of interest (say 20Hz). So for 5K ohms and 20Hz the inductance needs to be getting on for 40 Henries
so is not the bass response of these transformers going to suffer somewhat??

Cheers

Ian

The short answer is yes.

In practice it depends on design and what you mean by 'suffer'.

Open loop the OPT response is what it is. Closed loop is more
complicated because FB will attempt to keep the low end response up;
but saturation is saturation so it can't make full power bandwidth. On
the other hand, 'music' is not a full power monotone and 'average
program level', including bass, is usually at least 8-10dB down even
for compressed 'rock'. So if you put in enough NFB and can muster 1W
of bass from an otherwise 10 watt amp it will 'sound' about right.

The other 'suffer' is you use up the FB dBs in compensating for the
output drop so they're not there to reduce distortion. But, then, the
human ear is not terribly sensitive to low frequency distortion.

Well, the LF harmonic distortion is rarely the problem. But the
intermodulation distortion is a big problem if it is excessive and
caused by an OPT saturating and then all higher F will sound muddied
up.

Patrick Turner.



The net result is you can get bass response that 'sounds' surprisingly
better than what 'just specs' would suggest.