Add Gain and Load settings for Pearl 2 Phono?

Whats the best way to go about adding a selection of gain levels and load settings for the output in this design? It would be nice to not have to remove the case to make changes on the fly.

Gain output of the Pearl 2 is controlled by a resistor ( R 14 ) on the PC board (right?) The standard value for this Resistor is 1K and the gain is 55 db in standard configuration. Substituting a 300 Ohm resistor for ( R 14 ) bumps the gain up to 65 ohm, for those wishing to use a LOMC cartridge.

This had me wondering why phono stages don't have variable gain selections to allow LOMC, MM and High output MM.


What about adding a 3 position switch, or a 5 position rotary switch. and a few resistors.

what about using pots?

Any noise issues to consider in doing so?

Could this be done for the input Loading as well??

has anyone tried this, or is everyone sticking to the basic build? What do you think?
 

6L6

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2010-10-22 6:43 pm
Denver, Colorado
I can think of 3 instances in the last 6 months or so where it was done. One used a bank of DIP switches, another a couple of toggles, and one used a rotary for input loading, but I don't think he switched the gain...

Don't use a pot.

Have you read the entire Pearl 2 thread? It's worth it.

Stick to the basic build first. Adding a function like that is fairly easy to do as a modification later.
 
Based on what I've read Ive decided to go with 10pole rotary switches (from digikey) for gain and resistance settings. Pots will be poorly L/R balanced and noisy and too variable, proving difficult to match channels on the fly. Shouldn't be tough to build switches and wire it in instead of the respective fixed value resistor in the schematic.

Wire the switch as follows: One resistor to each tab (except v in) and then a common wire to all and out to the board.

---but capacitance selection might prove more challenging to implement..

DIP-switch from 50's or rotary switch to separate board and rows of series caps....?
can a 100pf and 50pf cap be wired in series to safely make 150pf?

Would like to add a few capacitance settings.. . rotary switch would be nice to operate, but dip switch might be easier to build. i Plan to have multiples of 50 and 100.
(50,100,200,300,400, 500..) ... Based on the limited number of pF cap values it looks like a dip switch would be the easiest idea... But I've never worked with one, and believe the caps must be in series to add... Is this correct?

For cas, radial C0G ceramics or pp film? Other ideas?
 
I have a suggestion which I will be trying in fact. I have MV102 (for MW radio)vericap diode and NSL-32SR2 optocoupler. So i plan to use vericap diode for tuning capacitance which will be controlled with some single pot variable DC voltage or stepped switch for pre configured voltages through fixed resistances. This will keep signal wires up to only PCB.

Same thing for resistance loading through voltage controlled switch and optocoupler. What you say?
 
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I have a suggestion which I will be trying in fact. I have MV102 (for MW radio)vericap diode and NSL-32SR2 optocoupler. So i plan to use vericap diode for tuning capacitance which will be controlled with some single pot variable DC voltage or stepped switch for pre configured voltages through fixed resistances. This will keep signal wires up to only PCB.

Same thing for resistance loading through voltage controlled switch and optocoupler. What you say?

Hi! I not so well aquanted with those components because im new to this hobby. I dont know if i perfectly understand your idea, but i think i have a faint idea. i believe if you want to try something you should! That sounds like a more sophisticated idea than mine. I feel that using a rotary switch to select between capacitor and resistor values would be most convenient and easier for me to understand and build at this point... Though my idea will take up a good amount of space in the case.

I breifly considered variable capacitors and reisistors for loading and gain but i decided against them for a couple of fundamental reasons. With continuously variable components or potentiometers it would take alot of time to dial the settings in and make sure both channels match exactly.. however you be able to get more exact match. im guessing you would need a very stable voltage. With basic components i feel im not changing the signal path or circuit voltage too much.

for me, Fixed value capacitors and resistors have close enough tolerances and can be matched so easily. also several can be installed and wired to dip switches or other types of switches to allow quick selections between them and make it easy to return to a previous setting if desired.

I like thinking of ideas to add because it makes me feel more creative and it challenges me to learn more than how to follow directions. Im glad to see you are also thinking of ideas too!


If i can leanr more about the riaa section i might contemplate adding alternative equalization schemes for older recordings... But thats a long way away for me.
 
Update:
So ive finished my pearl 2 and it sounds fantastic. Truly top notch. If youre considering it, stop and just get started on your own!

In my build ive intgrated 4 rotary switches mounted to the front pannel to allow custom tuning a few key components to accomidate any cartridge one could ever dream of using. Ive made my selections made on a few assumptions about the pearl and cartridges, but im not a circuit designer and just a music enthusiast who enjoyed adding a little flair. The benefit was that i can instantly compare one set of variables to another, and swap cartridges and cartridge types at a moments notice. Ive had few oportunites to do so at this time, but thus far ive discovered that i enjoy slightly different settings for my fidelity research fr1 mk3f mc cart than the manufacturer recommends. For example, instead of 10 ohm loading resistance, i find that 30ohms sounds better, and gets more output from this extremely low output cartridige. My 10 ohm setting had this low output cart input signal so low thati was having to turn my preamp up and use the highest gain setting of 65db on my pearl... This made the pearl demonstrate its max amount fo normal hiss which was audible, but quiet at normal listening levels when the record wasnt playing but resting on a still record. There was no audible hiss while playing as it was deep enough in the background to not hear. But, by uping the input resistance to 30 ohm or 50 ohm the input level was greater and the hiss was so low it could no longer be heard at playback levels.
This also permits me to use a lower gain setting and attenuate the slight background hiss further. The bass and mids are more authoritative, though not over blown, at these settings. The 10 ohm setting displayed perhaps more neutral character but required so much more gain to achieve the same listening level that amplification noise was starting to creep in .. So.. With my set up i can please my picky ear as the music calls for it.

There are 9 resistor settings from 10ohm to 1000ohm plus "blank" on one knob wired to R-user for mc cart load setting, and 10 settings for 14.7k to 110k wired to R-19 for mm load settings. The blank setting allows me to bypass the mc resitor load settings and use the mm settings. When using the mc settings the parallel reisitors are of such a magnitute of difference that the input values are virtually the same as indicated by r-user.

There are 10 selectable polystyrene capacitors for C22 from 43pf to 470pf.

there is also 10 selectable gain settings for variable gain tuning from about 45 to 65db.

Ive done this by using rotary switches and matched components. if you consider doing the same, It is important that these are make before break type to avoid noise when switching in the r-user resistor for mc settings. You van use break before make switches in the r19 and capacitor locations withput any switching noise. The gain and r-user slots require make before break type to prevent noise.

The results are pleasing and clean.


Ive noticed that 10ohm and similar lower resistance values in R-user decrease the input and ultimately the volume from the phono stage, requiring higher gain settings. Higher values in mc playback empart more weight and energy to the recording without changing the equalization. This is undoubtably due to less signal loss at input as im guessing low input resistors permit more signal being shunted to ground than say the default 47k. I dont understand what goes on with loading resistance though so id love to be educated!

Changing capacitance settings hasnt had any big change to the music that ive noticed yet, but ive been using a mc cart so this may be why. Mm may be more affected by input capacitance loading.

---- if anyone has experimented with the cx, r15x, or any of the riaa caps to move the subsonic roll off back, id love to hear about your results. I think i may have read that increasing c12 just a pinch can move the subsonic rollback down a bit. I havent felt the need to change this or anything about the riaa equalization as it is simply superb as it is.
 
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