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Old 20th December 2014, 11:12 AM   #1
Benji90 is offline Benji90  Italy
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Default phono FVP5A

Hi everybody!
I would like to build the fvp5a phono stage as a stand alone phono preamp to be then coupled with a existing line preamp.
I have two questions about the schematics that I have found on vacuum state's website (Schematics).
First I don't understand what should the capacitor bypassing the resistor (6k8) on the first tube's grid be. It is signed as 25/25. What does it mean?
Second on the project it says that the gain control must have an exact and stable 50k impedance. Since I don't know which is the entry impedance on the line preamp and I would like to make it somehow "plug&play" I thought I could add a 50k resistor in parallel with the 1M resistor and then a capacitor in series with the output signal pathway (after the 50k resistor). Is it a good idea? I hope I have made myself clear.
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Old 20th December 2014, 12:07 PM   #2
scafas is offline scafas  Italy
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il condensatore e' da 25uf 25volt
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Old 20th December 2014, 01:15 PM   #3
Bigun is offline Bigun  Canada
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if the design needs to see a constant 50k load (?) then it means not just DC but also across the audio range. if you choose the circuit with a cathode follower at the output it will not be so sensitive to the load. The link you provided shows some schematics with cathode follower output.


This may also be of interest to you: https://darklanternforowen.wordpress...ardi-φ42/
http://mmiworks.net/sprc/SP14-Nardi.pdf
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Last edited by Bigun; 20th December 2014 at 01:23 PM.
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Old 20th December 2014, 04:11 PM   #4
Benji90 is offline Benji90  Italy
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Thank you for your help!
On the schematics there are both the phono circuit with an anode output and subsequently the line circuit with the cathode follower output. I would like to build just the first part since I already have a line preamp. The designer says that there needs to be a stable 50k load so that there are no changes in the riaa equalization. Because of this I thought that adding a 50k resistor and then a coupling cap may help. Thank you for the design you posted! It looks very nice but I would like to build a mc phono stage without the need for a step up transformer.
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Old 20th December 2014, 04:28 PM   #5
rayma is offline rayma  United States
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Quote:
Originally Posted by Benji90 View Post
I would like to build just the first part since I already have a line preamp. The designer says that there needs to be a stable 50k load
so that there are no changes in the riaa equalization. Because of this I thought that adding a 50k resistor and then a coupling cap may help.
The 50k load spec (for EACH of the balanced sides) is for the total loading, and includes your line amp. This is critical for accurate RIAA.
This loading must be AFTER the two coupling capacitors however, like the volume controls in the original circuit

Last edited by rayma; 20th December 2014 at 04:44 PM.
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Old 20th December 2014, 04:43 PM   #6
Benji90 is offline Benji90  Italy
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Ok thank you! So how could I solve this problem? For example in my line pre I have a stepped attenuator which can change a bit the load.
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Old 20th December 2014, 04:47 PM   #7
rayma is offline rayma  United States
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Quote:
Originally Posted by Benji90 View Post
Ok thank you! So how could I solve this problem? For example in my line pre I have a stepped attenuator which can change a bit the load.
Assuming your preamp is balanced, you could use different (50k) attenuators. Or, use a buffer circuit after the dual 50k loading. What's the circuit for your line amp?

You may be able to use a pair of directly coupled cathode followers from the phono circuit (with the 100k RIAA loading between the grids), and the coupling capacitors at their outputs.

Last edited by rayma; 20th December 2014 at 04:53 PM.
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Old 20th December 2014, 04:51 PM   #8
Benji90 is offline Benji90  Italy
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It is a clone of a Kondo M77 (Kondo - Audio Note M7 Tube Pre-amp Clone | DMS Audio)
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Old 20th December 2014, 04:55 PM   #9
rayma is offline rayma  United States
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Originally Posted by Benji90 View Post
Ok, that's a straightforward circuit. Is your version a balanced or single ended line amp? The phono stage is balanced.

Last edited by rayma; 20th December 2014 at 04:57 PM.
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Old 20th December 2014, 05:28 PM   #10
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I haven't read the replies carefully. The reference to the 50K being exact at any volume level is because it affects ( is part of !) the RIAA equalisation. You will notice that there is no 1meg grid to ground resistor. So a 50 K volume pot will look like 50K at any volume position. If you add the usual 1 meg at the grid, it will NOT be 50 K at all positions.
While you could use a switched 50K volume switch, the wiper must be MBB ( make before break ) type. Otherwise you will get a bang each time you switch the volume. This is IF you do not use a 1 Meg resistor at the grid. But as mentioned before you cannot use a grid to ground resistor.

Alternate method is to just use a cathode follower with a 50K input impedance and then follow up with a stage with gain. You will have a slight noise penalty here. But possibly not of much consequence.
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