c65af149   
   From: info@turneraudio.com.au   
      
   On 11 Nov, 11:32, "Alex Pogossov"  wrote:   
   SNIP   
      
   Alex typrd....   
      
   > Also I think that stray capacitance noise will not be so much of a   
   > problem -- because it leaks only in 60% of the OPT, which has 36% of   
   > impedance. Besides, GNFB will kill the residue of noise if any.   
      
   I've just done more careful measuring of the noise in the Reformed   
   Jardis amps.   
      
   I did mention I thought the Jardis Chokes had L = only 80mH, roughly.   
   That was based on measuring 100Hz Vr hum across C2 470uF and across   
   choke. The 100Hz attenuation factor of the LC then is VC/VL and this   
   was initially 0.6V/0.05V = 1/12, and XL is about 12 x XC = 12 x 3.4r   
   at 100Hz = 41r then dividing by 628 gives 0.064H, ie, 64mH, even after   
   I set the air gap closer to get the minimum Vr with 0.15Adc flow. But   
   the trap everyone is prone to is that the Vac across a second C in a   
   CLC filter usually has lots of noise content below 100Hz, hence just   
   measuring across the C2 tells you a lie. So, I use a 10:1 resistance   
   divider with 3 x 100k and 1 x 33k so that Vdc is reduced fy factor of   
   1/10 to reduce the VDC measuring to +43Vdc in this case, and then I   
   have a 0.22uF across the 300k, so that you have a filter reduces DC   
   and LF but gives no attenuation at 100Hz, as it is -3dB at around   
   24Hz. This means nearly all the LF noise on B+ rail dissapears, and by   
   switching the CRO to max sensitivity and usung the scale I have taped   
   alongside the screen and calibarted in mV rms, I can read low voltages   
   even with a few mV of trace wobble due to LF noise.   
   I found 100Hz noise = 4mV, so attenuation factor of the CLC was better   
   than I thought at 0.6/0.004 = 150, so XL - 150 x 3.4 = 510r, so L =   
   0.81H, and that's about right considering Rw = 15r, and the size of   
   the choke, and the way it responded to correct air gap sizing.   
   Then I found another Jardis caused source of noise. Jardis put the IEC   
   chassis plug facing down into chassis and beside the PT. There is a 3   
   connection double cap on IEC plud with wires to A, E and N and they's   
   have no effect at LF. Then they had a 50mm green-yellow lead to one of   
   the 6mm bolts holding PT to stainless steel chassis. Now I had to stop   
   this amp humming by placing 22r between chassis and 0V rail. But today   
   I measure 0.47Vac across the 22r, and from PT bolt to chassis nearby,   
   so some serious I flow is generated within very short distances of   
   iron material, but hey, I could expect that no? bolts are subject to   
   stray fields from the large PT. So I reach for junk wire box, find   
   green-yellow wire, connect Earth terminal to a bolt holding a clamp   
   for 470uF cap. I get that done and remeasure. Bingo!, only 3mV across   
   22r, although yeah, the PT bolt is still having some voltage induced   
   in it. The 0V is connected to chassis via 22r at 300mm away at amp   
   input end, I find all is well. My ancient olf Fluke hand held meter   
   probably has dirt inside, and it gives a 2mV reading on Vac even with   
   leads twised together and propds shorted and away from magnetic   
   fields. Anyway, I can't measure noise at Vo terminals with meter, even   
   with no GNFB, But my best probe for my CRO with short earth lead to 0V   
   and short lead from amp 0V to CRO 0V shows 0.5mV without NFB, and   
   artifacts are various H of mains. This increase a fair bit without   
   shorting input to 0V with RCA shorting plug. OK, 120k input R does   
   make a bit of noise. With GNFB, the noise becomes almost a dead flat   
   line but I estimate 0.2mV. Earlier I noticed some trace wobble with a   
   regular timing, which telle me someone is switching something on/off   
   on mains thus making B+ bounce a bit with main F being probably 8Hz,   
   which is the Fo for 0.8H plus 470uF. Not much I can do about that, but   
   hey, a coupla mV of noise below 10Hz doesn't matter. So. I am happy   
   with result with regard for noise.   
   OLG response is 20Hz to 16kHz, no GNFB, -3dB.   
   With 9dB GNFB, CLG response with input CR filter shunted = 6Hz to   
   56kHz, 5 ohm load, 0.5dB peak at 8Hz, before steep roll off below 5Hz.   
   With HPF connected, LF pole is 8Hz, -8dB but dead flat from 10Hz   
   upwards. The shelving means GNFB is reduced to 4.5dB, ( halved ) at   
   15Hz and 20kKz with lessening FB applied outside this band. Sounds   
   great, no weird behaviours, all looks well. waether fine, level   
   horizon, will land for dinner soon.   
   Then to spend a night on working up ther page for the Net about the   
   mods, so that if anyone wants a good SET recipe, they can have it.   
   Patrick Turner.   
      
   --- SoupGate-Win32 v1.05   
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