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Old 28th Jun 2020, 10:41 pm   #41
paulsherwin
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Default Re: Ceramic cartridge preamps revisited

Yes, even I assumed that stuff
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Old 28th Jun 2020, 11:00 pm   #42
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Default Re: Ceramic cartridge preamps revisited

Quote:
Originally Posted by mhennessy View Post
as we're not in "absolute beginners" territory I've assumed that anyone contemplated building this would realise that.
Fair enough, but it's an open forum on the internet and things hereabouts are being stashed for posterity, so I'm inclined to make as few assumptions as possible about who may read it in the future.

Hello posterity! - I think it was Donald Swann said it.

David
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Old 1st Jul 2020, 10:53 pm   #43
mhennessy
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Default Re: Ceramic cartridge preamps revisited

I had a brief play earlier. Still quite impressed with the performance for such a simple circuit - though it's far from "blameless", it's really not bad. A couple of "free" tweaks:

Earlier I reduced the 100k resistor to 82k put the mid-point closer to the err, mid-point. I wasn't paying much attention to that earlier because it's just a formality, but having looked just now, 62k is better. Feel free to adjust on test...

As I said, I wasn't sure why 33k was the best value for the collector load of the first transistor - I remember being surprised at the time, but it definitely was the case. Checking today, it turns out 4k7 is best. That feels a lot better. I can't think what might have changed between then and now, but that was several days ago, and I don't trust my memory! No matter - I said it wasn't ready for release.

I tried testing at higher output amplitudes (all THD figures were at 1V at the output), and as you'd expect the distortion rises with level (though it remains "easily good enough for the application"). Adding a bootstrap cap between the output and the mid-point of the 20k collector load for the second transistor (a pair of 10ks in series for just this reason) mostly improves things, but at smaller amplitudes it makes the distortion slightly worse. For the cost of the extra cap and resistor, it's probably not worth it, but I'd like to explore that further...

I also tried a 3mA current source at the output instead of the 3k3 resistor. I suppose I should call it a current sink, but hopefully we all know what I mean. There are many ways to build one of these, but I usually go with the "ring of 2" version - 2 transistors and two resistors. This made a positive but not massive difference to the distortion, with improvements were most obvious at higher output levels. Given the "KISS" theme so far, it's hard to justify, especially if the pre-amp is being asked to drive high impedance loads. But it's definitely an option.

I'm tempted to try a current source as a load for the second transistor to see if it's more beneficial there. If there was no limit to the number of transistors in use, then it would be an easy decision (plus replace the first transistor with a long-tail pair), but there's something really quite satisfying about extracting the best you can from just 3 transistors. I don't shy away from using lots of transistors for my own designs, so I might present a silicon-heavy version for the fun of it, but I'll focus on the core 3-transistor design in the short term.

I also tried measuring at 10kHz as well as 1kHz. As you'd expect, the distortion is higher, but still perfectly acceptable.

I hope to return to this in a few days time, but things are really hectic at the moment (this is a nice diversion!), which is why I didn't provide any measurement data this time. But either way, it's a neat little thing as it stands, but perhaps there might be one or two further "wins" left to uncover...
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Old 1st Jul 2020, 11:12 pm   #44
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Default Re: Ceramic cartridge preamps revisited

I haven't got around to building it yet, though I did dig out a couple of BC184 transistors in preparation. Maybe tomorrow...

Please don't put too much time into this Mark, I appreciate your insight and advice but this is hardly an important project.
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Old 2nd Jul 2020, 12:39 pm   #45
unitelex
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Default Re: Ceramic cartridge preamps revisited

What are you intending to power this pre-amp from Paul?
Not sure you would have a ready 12V supply.
If it is driving an EL84 then maybe you have the heater voltage that you can rectify and smooth to get ~9V - rectifier drop.
Or you could possibly harvest some power from the EL84 cathode voltage?

If the output valve is UL84 then maybe you have a higher voltage available from that heater.

Best Regards
Chris
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Old 2nd Jul 2020, 1:47 pm   #46
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Default Re: Ceramic cartridge preamps revisited

There are lots of potential sources of power. The easiest is to use the HT line via a resistor, which should be fine if the preamp only pulls a mA or two and potentially allows high voltage operation. There is DC available from the output valve cathode resistor, or the heater supply could be used, via a voltage doubler if necessary.

I am just playing around with this, and not building a practical installation at this stage. Others are of course welcome to use any of the ideas in this thread for their own projects.
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