Quote:
Originally Posted by Robert Gribnau
Good to see you restarted your project.
About the Dansette schematic. If I'm not mistaken, both positive and (more) negative feedback are applied in it.
The negative feedback ...... (than it has no connection with R4 and R11 anymore).
Be carefull not to try a much lower value for R5 (470K). The first and second triode section are dc-coupled so R5 more or less sets the voltage at the grid of the second triode, which sets the voltage at the cathode of the second triode (that voltage automatically settles a tiny bit higher than the voltage on the grid). By choosing R5 high (as to create a larg enough voltage drop over it), the voltage at the grid and the cathode of the second triode will stay low enough to respect the maximum cathode to heater voltage of the ECl86/PCL86, which is 100 V.
Of course the quality of your amplifier will depend mostly on the quality of your OPT, like others already wrote. But I think that the schematic you chose is a good one.
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I reviewed this paragraph once more and remembered that I did not check the DC conditions on the triodes. I was so much focused on the output stage, that I overlooked that preamp stage.
I discovered that I had placed a 100k resistor in R5 position, probably copied from some other schematic. Va at the first triode (also the grid of the scecond) was 111 volts. Grid to cathode voltage was - 0.8 V.
I swapped a 470K resistor at R5 and all the distortion was gone. I could increase the volume of the output from the CD player (source) to maximum without audible distortion, the sound is loud and clear, though I wanted louder. Thank you Mr. Gribnau for the spot on comment.
Va became 83V (as far as I remember). Grid-cathode volyage is -0.75V.
The thing that was there and still is, but I did not previously mention, is that the input volume potentiometer (0.5M), was always left on maximum volume (max. resistance to ground). If I try to bring volume down a bit) I immediately get a buzzing sound in the speaker, sounds like it is picking EMI. Considering the state of chaos in wiring this contraption, may be it is too early to cure this. However I would like to explore it with you.
I would also like to mention that I had omitted the tone control part of the Dansette schematic, at least for the time being. I thought I would start with a minimalist design to get the least sources of trouble.