01-02-2019, 12:19 PM
Ah yes - I recognise that tweeter! I have not got around to measuring them, but it's on my list.
On the original design that they come from, a pair of them was wired in parallel, and that combination was fed via a single 1uF capacitor. That would have resulted in a fairly high crossover frequency - impossible to calculate without seeing a plot of impedance vs frequency. Running a single one via 1u8 will result in a lower frequency, of course. But the frequency you need is going to depend on what the large drive unit is doing. In the absence of measuring gear, then trial and error is perfectly OK.
Are you not tempted to add a second Sinclair amp and try an active design? If nothing else, it would be a fun experience!
Yes, you can change the caps to alter the turnover frequencies. Of course, the Rotel design is a hi-fi amp, and so the turnover frequencies will be closer to the ends of the spectrum than might be the case for a non-hi-fi design. Increasing C514 is - as you've found - the way to bring down the turnover frequency to make it more appropriate for the context.
Noting the reversed treble control in the test hook-up, do bear in mind that it's usual to use logarithmic pots in passive Baxandall designs, so if that's what you have there, you might want to try reversing it to double-check, else the action (and response when at centre) will be different when you build the final circuit. I haven't actually looked inside the amp to verify if that's what Rotel used.
Hope this helps,
Mark
On the original design that they come from, a pair of them was wired in parallel, and that combination was fed via a single 1uF capacitor. That would have resulted in a fairly high crossover frequency - impossible to calculate without seeing a plot of impedance vs frequency. Running a single one via 1u8 will result in a lower frequency, of course. But the frequency you need is going to depend on what the large drive unit is doing. In the absence of measuring gear, then trial and error is perfectly OK.
Are you not tempted to add a second Sinclair amp and try an active design? If nothing else, it would be a fun experience!
Yes, you can change the caps to alter the turnover frequencies. Of course, the Rotel design is a hi-fi amp, and so the turnover frequencies will be closer to the ends of the spectrum than might be the case for a non-hi-fi design. Increasing C514 is - as you've found - the way to bring down the turnover frequency to make it more appropriate for the context.
Noting the reversed treble control in the test hook-up, do bear in mind that it's usual to use logarithmic pots in passive Baxandall designs, so if that's what you have there, you might want to try reversing it to double-check, else the action (and response when at centre) will be different when you build the final circuit. I haven't actually looked inside the amp to verify if that's what Rotel used.
Hope this helps,
Mark
