SE Headphones Amp
On Sep 30, 8:00*am, 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
Most old radio OPS designed for a 6V6 operating as a beam tetrode with
no NFB have low Lp because they are designed to give from only 100Hz
upwards as the little speaker used just can't cope with any below
100Hz.
You are right about the 40H. If there is that much L and Ra was 2k2
with EL84 in triode and the RL was a lot higher or not even present,
then response will be -3dB when XLp = Ra or at 8.75Hz, so Lp could be
16H to give -3dB at 22Hz. FB can flatten the resonse further. The real
other problem is that if Lp is low, it is often because the core Afe
size is too small and there are not enough P turns to prevent core
saturation, ie, Bdc + Bac exceeding about 1.6 Tesla at an F that is
too high.
In other words, we want Fsat to be low as possible at the 1kHz
clipping signal voltage.
If one designs for Fsat at below 20Hz then the size of a 5 watt rated
SE OPT can be 4 times heavier than some silly bean counter designed
crap from an old radio. Some such old radios or old hi-fi sets do have
just enough Lp and freedom from saturation at too high an F but they
should be carefully measured before use.
Most old radi OPT just have one sec section wound over the top of a
single primary which gives a poor HF response.
But even if Fsat s at say 50Hz for full Vo, for phone use the Vo may
only be 1/4 full Vo max and Fsat will then be 12.5Hz which s
acceptable.
Patrick Turner.
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