58604022   
   From: ruffrecords@yahoo.com   
      
   Patrick Turner wrote:   
   > On Oct 1, 7:58 am, Ian Bell  wrote:   
   >> 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?   
   >   
   > Its all very well to consider a "SET OP transformer with 10H and a 16   
   > ohm secondary" but you have not mentioned the primary load nor type of   
   > device driving the primary load.   
   >   
   > If you have a single EL84 in SE class A1 triode then consider a   
   > typical op point of Ea = +300Vdc and Ia = 30mAdc, and Ra = 2k0 approx.   
   > Total RL + Rw for maximum PO = ( Ea / Ia ) - ( 2 x Ra ) =   
   > ( 300/0.03 ) - 4k0 = 6k0 and let us say Rw total is 10% of the totalo   
   > RL so actual RL = 6k0 = 600R = 5k4.   
   > Max PO = RL x ( Idc squared ) / 2 = 2.43W, and anode load Va = 0.707 x   
   > 5,400 x 0.03 = 114.5Vrms.   
   > Your OPT has a 5k4 : 16 ohm ratio which gives ZR = 337.5:1 so the Ra   
   > appears at the output as 2k0 / 337.5, plus the total Rw / 337.5 =   
   > 5.96R + 1.77 = 7.73R.   
   >   
   > This is a not very good outcome unless we add at least about 12dB GNFB   
   > from speaker connection to the driver tube cathode to reduce Rout to   
   > about 2 ohms, giving a DF of 8 with 16 ohm load.   
   >   
   > The response at low levels is given as the pole formed by Lp in   
   > parallel to RA, which is Ra + RL in parallel. RDH4 spells it out.   
   > Anyway, assuming the load is resistive at low F where LF cut off   
   > occurs, then RA = 6,000 // 2k0 = 1k5, and 10H has XL = 1k5 at 23.9Hz,   
   > not too bad. If the load around cut off was very high the low triode   
   > Ra manages to keep the source R low so the Fco remains low.   
   >   
   > If the sec load is higher than 16 ohms, say 32 ohms or 320 ohms, the   
   > triode Ra remians the main dominant source resistance and Fco will   
   > remain fairly low. My example presumes anode RL with 16 ohm sec =   
   > total of 6k0, and this is 3 x Ra and maybe you get 4% THD at 2 Watts.   
   > With anode RL = high as possible, ie, CCS, then THD will be about 1%   
   > at the maximum Vo but because there is no Ia change there is no PO, so   
   > THD/IMD will vary between about 4% and maybe 2% at maximum *useful*   
   > Vos for RL above 6k0.   
   >   
   > But should you use ther EL84 in pentode with Ea = 250V and Ia = 40mA,   
   > Ra = maybe 50k0 and then the Fco is MUCH higher and more dominated by   
   > total RL which may be 0.9 x 250/0.04 = 5k6, if in fact there is a an R   
   > load present at Fco. With 10H and RA = approx 5k0, Fco = 79.6Hz.   
   > The lower the Rsource the lower is the distortions caused by the damn   
   > iron.   
   > EL84 THD in SE mode at near clipping without any NFB anywhere is   
   > usually well over 10%, and fulla odd order H.   
   >   
   > Triodes are the gold standard for driving OPTs but one may completely   
   > overcome the shortcomings of pentodes by using NFB in the form of   
   > global, or local CFB or maybe UL plus global.   
   >   
   >   
   > When I made a pair od SET amps for a customer a couple of years ago I   
   > used 2 x 845 in parallel to get PO max = 60W.   
   > The noise was only barely discernible with head held against a   
   > speaker, and noise was estimated at 0.25mV.This illustrates that one   
   > may use any power tube one wishes to use for either driving a speaker   
   > *or phones*. With a luxury of a 60W output max to 5 ohms, or 17.3Vrms,   
   > one might only want say 3Vrms max at the phones of 32 ohms which is   
   > 280mW. The R divider might have R across phones output = 2R2, and feed   
   > R of 10R, so that noise is then reduced to 0.25 x ( 2.2/12.2 )   
   > 0.045mV, and if the average phones level was 0.2Vrms, then unweighted   
   > SNR = 20 log 0.000045/0.2 which I guess = -72dB approx and possibly   
   > acceptable, because the background noise of the recording venue is   
   > likely to be higher if nothing else is.   
   >   
   > So a single EL34 for phones would be OK, or a 1/2 of 6AS7, and so on,   
   > providing you think it through. PP use of 6SN7 was common with a 20k:   
   > 16 OPT.   
   >   
   > If one wants to drive down to 8 ohms without an OPT, I'd suggest a   
   > single power mosfet in source follower mode which may be driven by CR   
   > coupling from EL84 in triode. The triode needs a B+ of say +350V and a   
      
   [continued in next message]   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)