33853a52   
   From: ruffrecords@yahoo.com   
      
   Patrick Turner wrote:   
   > 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.   
      
   Which is an interesting point. Say we have a SET OP transformer with 10H and a   
   16 ohm secondary.   
   One watt into 16 ohms need 4V rms whcih is plenty to drive the phones I am   
   intersted in. So suppose   
   I connect a 32 ohm headphone to this 16 ohm winding and it requires no more   
   than 2V rms (for 125mW)   
   I would expect lower distortion because of the lower level and higher   
   reflected ac load. What would   
   happen to the bass response with a 10H primary?   
      
   Cheers   
      
   Ian   
   >   
   > Patrick Turner.   
   >   
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