5e628ee3   
   From: ruffrecords@yahoo.com   
      
   Patrick Turner wrote:   
   > On Sep 19, 7:22 pm, Ian Bell  wrote:   
   >> Patrick Turner wrote:   
   >>> On Sep 18, 8:39 am, Ian Bell    wrote:   
   >>>> flipper wrote:   
   >>>>> On Fri, 17 Sep 2010 10:48:50 +0100, Ian Bell   
   >>>>> wrote:   
   >>   
   >>>>>> flipper wrote:   
   >>>>>>> On Thu, 16 Sep 2010 21:58:37 +0100, Ian Bell   
   >>>>>>> wrote:   
   >>   
   >>> Snip for brevity.   
   >>   
   >>>> Exactly the same as at the grid - about 2H at -30dB or worse. If I short   
   the input resistor 2H at   
   >>>> the grid drops below -70dB and at the anode falls to -40dB which is about   
   what I would expect. So I   
   >>>> was expecting -40dB 2H at the anode and got a lot higher so traced back   
   to the grid and found it was   
   >>>> the same but that it wnet away with near zero source impedance.   
   >>   
   >>> Your variations of Dn with input series resistance for 12AX7 is   
   >>> interesting, and not many ppl would be wise to it.   
   >>   
   >>> But what of other tubes?   
   >>   
   >>> In all my tests for line level preamps using 12AU7, 6SN7, and my   
   >>> favourite, 6CG7, the Dn you mention as a result of high input source   
   >>> resistance never seemed to occur. What if you change from 12AX7 to   
   >>> 12AU7? The 'AU7 should work in your circuit with minor load&biasing   
   >>> changes. I recall measuring a 6CG7 line stage with 100k volume control   
   >>> before a CC gain tube using 47k anode load then direct coupled CF to   
   >>> output and at typical levels to drive a power amp at normal listening   
   >>> levels Dn was always less than 0.02%, or about -74dB, which I thought   
   >>> would always be inaudible, and completely overwhelmed by the Dn of the   
   >>> power amp and speakers.   
   >>   
   >> I am tempted to use one of my 6CG7 mu followers as a driver but that would   
   only have a gain of about   
   >> 20. To get the 102V pp drive mentioned in my last post would require an   
   input of 5.1V pp or about   
   >> 2.5V peak - my 5mA 6CG7 mu followers bias at close to -5V so it should   
   avoid grid current. I may   
   >> then need another gain stage to achieve a suitable sensitivity and of   
   course the input signal would   
   >> become even greater if I use NFB to reduce the white distortion. Of course   
   teh 6CG7 mu follower only   
   >> achieve about 0.4% distortion at 20V rms output itself and I really need   
   about twice this.   
   >>   
   >> I am tempted to try changing the windings of the transformer to the 6:1 tap   
   to lower the drive   
   >> requirements.   
   >>   
   >> Cheers   
   >>   
   >> Ian   
   >   
   > The use of a cascade circuit with 12AU7 and normal high ratio OPT with   
   > trioded EL84 seemed to work for me to give enough gain and allow some   
   > global NFB.   
      
   Cascade should give enough gain but adds to the zeros in the loop so NFB   
   stability is compromised -   
   not a route I want to take unless I absolutely have to.   
      
   > I gave two lots of L&R outputs with different levels and with a DACT   
   > source select switch and attenuator. He says it makes a  mighty fine   
   > preamp for his power amps. The EL84 with 5K load has a damping factor   
   > of 2 without NFB. Gain between EL84 and sec is about unity, like a   
   > follower, but the load match is much better than having say 600 ohms   
   > driven by a follower.   
   > I didn't seem to ever need to have high signal voltages anywhere.   
   > Certainly never 102VP-P.   
   > The guy runs Sennheiser phones which I found rather good, and I just   
   > used some El-cheapo crap phones which were less sensitive for the   
   > testing phase.   
   >   
      
   Indeed, one of the really big questions is just how load does the client want   
   the phones to be -   
   that and the sensitivity of the phones makes a big difference to the output   
   requirements. As a rule,   
   100mW into almost any headphone will blow your ears off and at least 10mW is   
   needed to give a decent   
   level even in the most sensitive phones. That's a 10dB difference in power   
   that translates in to a   
   voltage range of about 3 to 1.   
      
   For this client the worst case is probably his 100R phones that will take 3V   
   rms. That means with   
   the 12:1 tap the primary must supply 102V pp into 14K or so. Alternatively   
   with the 6:1 tap we need   
   51V pp into 3600R for about 90mW in the phones. The 541V pp is less of a   
   problem than the 3600R load.   
      
   His other phones are 34R types.  For 90mW these need 1.75V on the secondary   
   which at 12:1 is about   
   60V pp- again not too bad, this time into 4896R.   
      
   So I think that where I need to head. The problem still remains how to get the   
   60V pp swing without   
   unnecessary first stage distortion.   
      
   Cheers   
      
   Ian   
   > Patrick Turner.   
      
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