Thread: PLZ pre-preamp
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Old 21st February 2019, 02:52 PM
bikerhifinut bikerhifinut is offline
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Default Re: PLZ pre-preamp

Originally Posted by Black Stuart View Post
Error, yes it should be plugged into an MM stage.

Everything I've read on the 7500S is that a 1K load opens up the top end. I'd like to try 500ohm first and then 750ohm.

Zev audio has all the numbers on the 7500S.

The first resistor is 27ohm.

I will within a few days take some pics of the circuit.

No one who uses the PLZ said anything about it being noisy.
The comment about noise refers to if it was being used to raise very, very low level signals to MC levels, that was all Stuart. I wouldnt expect it to be noisy if good low noise semiconductors and High quality metal film resistors were used. The battery operation certainly will eliminate any mains borne hash and hum issues, It's not such a bad idea when amplifying the tiny signals from MC and these ultra low output MMs.
Ok it may be worth metering the inputs to see what its shunted with, but I'd be wary if there's a sensitive FET or something directly coupled to it. It's easy enough to trace the input circuitry if you can see the PCB tracks. The load resistor is obvious by being connected across the input from hot to earth. An idea if you are keen to try experimenting with load resistors would be to leave the load resistor out of circuit on the board, (you could stick a 1meg in there if you are worried about the input not having any reference to ground, it'll have negligible effect on your loading shunt.
Then, you could make loading plugs up. Simply have the resistor of choice wired across the pins, I am sure somewhere out there there are RCA plugs that are terminated in an RCA socket so you can wire a load resistor or attenuator potential divider something like thos Rothwell things. They must get their plug bodies from somwhere. I'll have a sniff round the internet. otherwise its a fiddle on to make something out of a plug and socket, but it would be feasible I think if you keep things short and use some decent shrink tubing. If theres room to get a couple of extra sockets in beside the existing inputs you can wire an RCA in parallel with the input and make little loading plugs up to go in. That would be a neat way too. theres at least one commercial design that uses that technique.
27 ohms seems very low for a loading resistor, I'd expect more like 100 or 150.
That said, ortofon, for example, recommend loading at "Greater than" 20 ohms and I have never noticed any fundamental difference between the various loading values on my MC stage.
I could of course have cloth ears.

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