Thread: PLZ pre-preamp
View Single Post
Old 20th February 2019, 04:57 PM
bikerhifinut bikerhifinut is offline
Senior Member
Join Date: Jan 2006
Location: Penrith, Cumbria
Posts: 1,093
Default Re: PLZ pre-preamp

I'm with Bob, Gain of 27db that means my ortofon MC would give a nominal 6mV give or take at the step ups output. Which is just about bang on for the average magnetic cartridge input, (somewhere between 2 and 5MV for most modern kit).
Or to put it another way, if it was for stepping up a VERY low output source into a MC input lets make a couple of assumptions and do the maths.
Assume 500uV ie 0.5mV for a fairly low sensitivity MC stage, divide by 27dB gain (22.4x) and you get a sensitivity of around 22 microVolts. Now thats so close to the natural noise produced by even the quietest metal film resistors for instance that I'd bet all you'd get is a very hissy noisy result.

If you can get into the guts of it ok and you think you need to alter the input loading then it should be possible to trace the wiring from the input socket to the loading resistor which will likely be the first resistor across the input. For an MC its usually somewhere between 50 to a couple of hundred ohms, many designs seem to fix it at about 150 ohms if they dont have adjustable loading.
You will have I hope the info for loading your low output moving magnet as I guess it may not be the industry standard 47k. The only low output magnetic I know of and have had practical experience of is the Grado moving iron job and that simply worked into a standard MC input, I think maybe it was 100 ohms? thats a function of the lower number of turns on the generator coil.
Incidentally, moving coil cartridges are, as a rule, very tolerant of loading resistance variations and their loading figures can often be regarded as a minimum figure. Certainly I can't hear any difference between 100, 150 or 300 ohms when I had a fiddle with my rega Fono MC's adjustable loading. The thing to watch with MC stages is that they could well have a higher loading capacitance than for MM.

I've attached an interesting little circuit that I fancy having a go at, it'll cost washers to build on a bit of veroboard and you can run it nicely off a couple of PP3 9V batteries in series.
Here's a link, they do a PCB kit too.

Best regards

Attached Images
File Type: jpg Boozhound-Labs-JFET-MC-Pre-Preamp-Schematic.jpg (26.9 KB, 11 views)
Reply With Quote