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Variable phono input loading
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Fatmangolf View Drop Down
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    Posted: 19 Aug 2010 at 9:58am
In the interest of experimentation I am going to develop a way to safely change the input capacitance of my Genera without switching it off. I have written elsewhere about modelling the electrical circuit, this thread is more practical.
 
I appreciate the following:
  • The sound will change over time after a "cold" capacitor is added
  • A cheap and nasty capacitor will sound bad
  • The impact of changing the LC tank and the damping resistor is only part of the HF response of the cartridge
  • The input capacitor also helps to stabilise the preamp (please see Graham's work on the Genera)
  • The Spice model suggests the resonant peak and roll-off changes slightly
  • And yes this is like nudging the treble control

But I am still curious. My thinking is to use snipped pieces of component lead or tinned wire through the board, solder across a smaller capacitor i.e. 50pF and 2 old cartridge connectors or sockets from a D-SUB connector. The extra capacitor would plug into this.

Before I get the tools out has anyone any practical tips from trying this (or similar mods) themselves?
 
Jon
 
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Graham Slee View Drop Down
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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 20 Aug 2010 at 2:32am
Suggestion...

Some stripboard (veroboard), a DIL switch and the selection of caps you wish to try - one end to DIL switch the other to ground - the DIL switch commons to the input.

Correction...

The input cap does not help stabilise the amp - it is stable without. It prevents the input slew rate being faster than the output slew rate to combat slew rate distortion.
That none should be able to park up and enjoy the view without a smartphone and the knowledge in how to use apps
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Fatmangolf View Drop Down
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Points: 9695 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 20 Aug 2010 at 7:58am
Great, that would be a very neat and reversible approach. Correction noted. Thank you.
 
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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Points: 9695 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 21 Aug 2010 at 12:07pm
The%20input%20cap%20switching%20from%20aboveWith an early start the Genera was back on for a quick test by 9:30am. All seems to be fine but I want it to have some running time after being switched off so I have left all 4 capacitors switched on in parallel.
 Input%20caps%20from%20sideEven with Graham's helpful suggestion you can see the extension boards aren't pretty. It is more robust than the approach I had considered where the leads would have probably wobbled out! Anyway these extensions will allow me to try different capacitor loading when the preamp has warmed up. Even now the differences can be heard on percussion sounds.
 
(I deliberately did not re-use the original 220pF capacitors on these extension boards so I can easily reinstate the original PCB/circuit)
 


Edited by Fatmangolf - 21 Aug 2010 at 10:51pm
Jon

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Points: 9695 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 21 Aug 2010 at 10:43pm
This is why I wanted to experiment with the actual sound of my Goldring 2500 and the Genera. After some issues with my Rega Exact (low self-inductance for MM), I tried the Golrding 1012 (550mH) and was tempted to buy a 2500 (720mH), which I would recommend trying with a Rega arm.
Electrical%20response%20for%20different%20input%20capacitors
The curve shows a Spice model of the circuit described elsewhere in the Forum, specifically the electrical parameters of the cartridge (dc resistance and self inductance) and their interaction with the input capacitance and resistance of the preamp.
 
You can see I have kept the input resistance at 47K and modelled the effect of different preamp capacitors. The cable capacitance is about 100pF, so 100pF above gives a combined load of 200pF. To my inexperienced eye the phase and other curves seemed quite similar so I have just used the response curves above.
 
You can see the more extreme effect of having all the DIL switches on with the 570pF curve's peak, that is just as dramatic in real life as in Spice! I made the modification to hear the quite subtle difference between the "stock" 220pF and the 100pF. I prefer 100pF with the Goldring 2500 but probably the other way round with the 1012, although I did add a damping resistor to tame its treble.
 
After all this I think the 50pF and 100pF sound best! Hopefully someone else at some time will find this helpful.
 
Jon


Edited by Fatmangolf - 21 Aug 2010 at 10:45pm
Jon

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Points: 16314 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2010 at 12:42am
A comment: I find the Evox PFR polypropylene we use for the 220p input cap (as well as in the EQ) differ considerably in sound to polystyrene. If you were to solder a couple of bits of wire to the original 220p cap and compare it against the 220p polystyrene and let us know your findings?
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Points: 9695 		Post Options Post Options   Thanks (0) Thanks(0)   Quote Fatmangolf Quote  Post ReplyReply Direct Link To This Post Posted: 22 Aug 2010 at 9:43pm
To be fair the batch of polystyrenes were used because 1) I could get them easily and 2) they would all be the same age and 3) the comparison would be between the pF values, not the compositions. I would definitely have used a 50pF Evox PFR instead if I had one!
 
I will do the 220pF listening comparison you suggested and send you a note. I will try a blind test as well, don't expect a surprise win for the p/s if the first impression was anything to go by! The new P/S 220pF did not compare well with the 2 month old Evox 220pF, I could hear why you chose the latter component.
 
Graham, where can I order Evox PFR 50pF or 100pF from please? Selecting between one of those and the 220pF that came with the kit would have been my ideal test of the LC tank for my cartridge.
 
Jon

Open mind and ears whilst owning GSP Genera, Accession M, Accession MC, Elevator EXP, Solo ULDE, Proprius amps, Cusat50 cables, Lautus digital cable, Spatia cables and links, and a Majestic DAC.
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