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Old 22nd March 2005, 03:29 AM   #1
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Default RIAA help

The network below was designed to be placed after the plate of an EF86. I built the design as a stand-alone on a breadboard (no power), fed it with Pink Noise and compared the input & output using the spectrum analyser in Spectra-Foo
(http://www.mhlabs.com/metric_halo/products/foo/ - Mac only I'm afraid). I'm a Beta tester for the analyser so I sort of have a clue using it (they hope).

I cannot see any alteration to the original signal except a slight gain loss, I know I am looking at the output correctly as I do see a slight 60Hz bump when the input is disconnected.

Is my circuit incorrect? I presumed it would work as a stand-alone passive filter network. Does it need to "see" the EF86 plate and associated signal ground in order to function correctly? Any input from the diy brain trust would be very much appreciated.

Thanks a mill' anyway.

P : not_sticking_my_tongue_out_short_for_Philip
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Old 22nd March 2005, 03:45 AM   #2
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2 questions

1. what is the source impedance?

2. what is the load impedance?

If your using a low impedance function generator the source impedance is most likely 50 ohms. If your using the output of a sound card it could be as low as 8 ohms. For the network shown there needs to be a resistor in series with the input for the circuit to function.

This filter looks like it is designed to drive a high impedance load, something greater than 100k ohms. I don't know what the input impedance of you sound card is but I doubt it is that high.

I hope this helps

bz
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Old 22nd March 2005, 04:07 AM   #3
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You're a genius! Brewski on me!

I put a 150K on the input (before the cap) and voila, I have a slope, 24dB difference between 60Hz & 20KHz. 150K was the first one out of the box, how do I actually calculate what it should be?

According to the Ruskies at Svetlana the EF86 has a plate output impedence of 65K and I will be (eventually) driving the input of a Rozenblit Grounded Grid Pre with input resistors of 10K feeding the grids of 12AU7s with an impedence of ??

Thanks again...

P
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Old 22nd March 2005, 02:02 PM   #4
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The value of the input should probably be the tubes plate resistance in parallel with whatever the plate load resistor is in the schematic.

For output impedance, you are generally looking at the volume pot of the linestage. I'm not familiar with the preamp you'll be using, but they usually are 50k, 100k, or 250k.
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Old 22nd March 2005, 02:55 PM   #5
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FYI, here's what it looks like with a 154K input resistor (blue trace) & a 220K (purple trace).

Philip
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