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EJP

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Everything posted by EJP

  1. Sadly the silicon rings made exactly no difference either, which some around me predicted. Maybe adding some damping material under the top, which acts like a soundboard, might help, or changing to some variety of 12A?7, which were built for low microphone. But my customer and I are just about out of patience with this unit. Shame in some ways, because I quite like the circuit and the sound.
  2. Sadly all that made no difference. I corrected the LF response even further by adding 1uF below the 8k shunt feedback capacitor, to reduce the gain to unity at DC and from about 10Hz down, which is how I think it should really be, but that didn't help either. So I am presently considering the good old silicon rings around the 6SN7s. Curiously enough there is a brand of these called Cary ... I also rolled off the HF response by adding 22pf (or possibly 220pF pF, can't remember) in shunt at the input, 10K grid stoppers between the volume pots and the V1 grids (instead of the wires, easy enough), and 15pF across the 100K feedback resistor, which should all help with the AM reception while still keeping it reasonably wideband at 100KHz (was nearly 1MHz). AM break-in only happens at the customer site so I can't verify that at present. To document the gain arithmetic above: the gain of the grounded-grid stage is given by 100/4.7 ~= 21, divided by 2 because of sharing the cathode resistor with V1A, which halves the current and therefore the plate voltage swing, so about 10.5, or ~20.5dB. This is A in the general formula for gain: A/(1+beta*A), which is used when where A is not large enough to ignore, so taking beta as 8/100 this yields closed-loop gain of ~5.7 or 15.18dB. This is still a little high for the customer's equipment, so I actually increased the 8K resistor to 10K, which yields gain of 5.14 or 14.24dB. All this needs to be corrected further for the two cathode-follower stages, one inside the feedback loop: I will update this post for that.
  3. Well the gain would be 13.5 if the 6SN7 grounded-grid section had infinite open-loop gain, but it doesn't. It has OL gain of about 10, given by Ra=100k/Rk=4k7, divided by 2 because of sharing Rk with the prior cathode-follower stage. So the actual gain is about 14dB. (The construction notes say 12dB which is not correct on calculation or measurement.) However ... the actual problem seems to be that this unit has 6dB more gain from 10Hz downwards, for a closed-loop LF gain of 20dB, to which at present I am attributing the microphonics and acoustic feedback. Fortunately the cause is obvious , and the solution pleasingly simple. The feedback coupling capacitor is far too small at 0.1uF, and the solution is remove it and instead connect the 100k feedback resistor to the output side of the 2u2F output coupling capacitor. This basically flattens the LF response and also has the pleasing side effect of bringing that capacitor inside the feedback loop. i will be trying this tomorrow and will report results.
  4. I second most of this, except the potentiometer change, which doesn't seem to do anything useful. Major omIssion to me is reducing the gain, which is far too high at 13.5, and alone is responsible for much of the microphonics and instability. I'm planning to change this to about 5, by changing the feedback resistor to 33K. This might exceed the stability margin, so I might have to shunt it with some picofarads.
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