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Old 22nd April 2015, 08:37 PM   #1
Sempati is offline Sempati  Netherlands
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Default RIAA network adjustment

Hi all,

Recently built a tube phono preamp.
It sounds quite nice, but could be less noisy, so I will try CCS instead of anode resistor.
But how (if at all) will this affect my RIAA component values?

(I have been using the well known RIAA calculator to find the original values)

Harm
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Old 22nd April 2015, 10:02 PM   #2
DF96 is offline DF96  England
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The 219k series resistor will need to be a slightly smaller value.
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Old 23rd April 2015, 04:53 AM   #3
gofar99 is offline gofar99  United States
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Hi, A related question....how did you determine that the resistor needed to be smaller. I can see that if the CCS effectively increases the output impedance of the circuit but I was not aware that the CCS would do this. In all the designs I have seen using passive RIAA compensation the resistor plus the effective output impedance of the preceding stage are added together to get the right response.
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Bruce
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Old 23rd April 2015, 07:36 AM   #4
Sempati is offline Sempati  Netherlands
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This is exactly my question Bruce. If 219K series resistor needs to be smaller, then this will inluence the 33K2 and capacitor values as well. Or am I wrong?

Harm
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Old 23rd April 2015, 08:33 AM   #5
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Sempati View Post
Recently built a tube phono preamp.
It sounds quite nice, but could be less noisy
What kind of noise? A CCS is very unlikely to provide much improvement and may even increase noise. What size is the input grid stopper? The unbypassecd cathode resistors are also not helping.
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Old 23rd April 2015, 09:53 AM   #6
DF96 is offline DF96  England
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In the original circuit the impedance at the anode of the first stage is anode impedance in parallel with the anode resistor. In the modified circuit it is just anode impedance, so will be greater. Hence the build-out resistor needs to be smaller in order to present the same impedance to the rest of the RIAA network. Given this, the rest of the network needs no change. Note that 'anode impedance' is modified by the unbypassed cathode resistors, so will be greater than the datasheet value.

I agree with Merlin. Why do you think a CCS will improve "noise"?
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Old 23rd April 2015, 10:46 AM   #7
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Did you try with a 12AX7/ECC83? The 5751 has less gain, I am not sure if this is a factor. In any case I would tie the cathodes together, bypass the cathode resistor, and lower the anode resistor to set the gain.
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Old 23rd April 2015, 12:18 PM   #8
Sempati is offline Sempati  Netherlands
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What I learned from reading several forums on the internet is that the use of active loads (CCS’s) in vacuum tube amplifiers has several advantages over resistive loads. These including full mu of the tube, lower distortion, and greatly improved power supply noise rejection.
I've never experimented with this philosophy, but would like to give it a try.

The tubes that I'm using are 5751 in the first, and 12AX7 in the second stage (2nd stage not on the attached schematic)

To increase gain is not my primary goal, (less gain would even be better), but
to reduce hum is what I'm looking for
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Old 23rd April 2015, 12:32 PM   #9
Merlinb is offline Merlinb  United Kingdom
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Quote:
Originally Posted by Sempati View Post
To increase gain is not my primary goal, (less gain would even be better), but
to reduce hum is what I'm looking for
Ah, you mean hum rather than (random) noise. Are you using DC heaters?

To answer you original question, the original source resistance of the input stage is about 45k ohms. With a CCS it will be about 78k ohms, so you would need to reduce the 219k resistor by 78-45=33k ohms (i.e. you need 186k)

Last edited by Merlinb; 23rd April 2015 at 12:39 PM.
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Old 23rd April 2015, 12:45 PM   #10
Sempati is offline Sempati  Netherlands
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Yes, I meant hum rather than noise and yes, I 'm using DC heaters, based on LM317 voltage regulator. Each channel independently supplied and lifted up to appr. 80V to keep voltages within tube limitations.

Initially I used 12AX7 in the first stage too. Less hum, but too much gain. With 5751 sufficient gain, but too much hum. Hence the CCS option.
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