Phono Resistor Value? - Audio Research SP14

[Noahpdavis]

Howdy!

I just received an SP14 and got it set up. The line stage sounds amazing. The phono stage sounds like everything above 10khz is rolled off. Speakers are under MANY blankets sonically

I tried swapping 2 different tubes in, and it didn't help. I'm wondering if the cartridge loading resistors got swapped at some point. I'm using a 2M Bronze with the following specs -

Internal impedance, DC resistance - 1,2 kOhm
Internal inductance - 630 mH
Recommended load resistance - 47 kOhm
Recommended load capacitance - 150-300 pF

I took a picture of the section of the board where you can replace/add resistors to change the cartridge loading. I think if I remove the ones seen in this picture, the stock/factory load is 47k.

Can someone confirm/deny this for me?

 

[arts]

Removing the resistors shown should return the unit to it's stock factory loading.

The stock input capacitance,which is stated to be 600pf when the 560pf caps (installed on the same turrets as the resistors) are present,seems to be a little high for most MM cartridges,especially when you factor in the added capacitance of the interconnects and tonearm wiring.

As the Bronze requires between 150pf-300pf for optimum frequency response,just the preamp alone at this point is exceeding this load requirement by a factor of 2.

When you remove the resistors,you should realize a fair increase in gain and a restoration of the highs,but I'm guessing it's gonna be awfully bright with those caps in place.


Edit: Nice cartridge,I'm using the 2M Black.My preamp is a little older than yours, Audio Research SP3

 

[Noahpdavis]

Awesome - thanks! It sounds like I should maybe replace those capacitors with a lower value as well? If so what value or should I remove them all together?

SP3 - niiiiiiice

 

[arts]

The stated input capacitance without those capacitors is 40pf. What you need to do is determine the capacitances of your interconnects and tonearm wiring,then add those to the 40pf value.You need to see where you land in relation to the 150-300pf requirement before you can determine what needs to be done.

 

[Noahpdavis]

Cut the resistors and all the highs and gain are back, and you’re right - it’s super hot and bright! The phono stage has more gain than the line level inputs. I compared with an external phono pre and it’s def too bright, Is there a simple way to test capacitance of my tone arm wiring and interconnects with a DMM?

 

[arts]

Ha ha,Audio Research was never shy with the phono gain.

Most DMM's aren't accurate enough,even if they have a capacitance test feature.What make/model arm or turntable & interconnects? Possibly you might see the brand or cable type printed on the wire itself.

Just for the sake of experiment,try cutting the wire at one end of each cap,but as close to the terminal as possible so you can re-attach it later if desired.
Depending on the interconnects and arm/table that you are using,they just might have enough capacitance on their own to suit the cartridge.

 

[Noahpdavis]

Okay, so I clipped the caps out and gave it a shot and *still* too bright! I'm using pro-ject phono cables - probably 3.5 ft in length and i have an SME 3009 tonearm. Hmmmm. Any idea of what to try next? All this help is VERY appreciated!

Also, I checked the resistance on the post and it was right around 47kohm. I've seen a few reviews that state that the 2M bronze can sound less peaky at 38-42kohm. What value resistor should I strap to the posts to give this a try?

 

[arts]

No problem,glad to help!

The SME shouldn't be a problem capacitance-wise,nor should the preamp,without those additional caps.

Are you certain the Project cables are actually intended for phono use? If you have the model # of the cable,try Google or contact Project and ask them what the capacitance is.A lot of ''general-use'' interconnects can have very high capacitance values.

The Bronze,like all other cartridges using ''exotic'' stylus shapes,really needs to be set up perfectly in order to sound correct. Are you set up perfectly?

 

[Noahpdavis]

Circling back after having this for almost a year. I initially strapped 220k ohm resistors to the posts dropping the load to 39k which helped. I had it that way for a while, but felt over time it was still a little too crispy. Also I upgraded to a Pro-Jet Classic SB and had the 2M bronze cart installed on it. When I'd A/B it to a HQ spotify stream, the vinyl still had more treble-y high end (in a bad way). I tried 100k's in dropping the load to 32k, then 47k/24k, then came back up to 100k/32k. I *think* this is the sweet spot for the 2m Bronze cart. Also for some reason I can't actually measure the load on the posts, where I'm almost positive I was able to do when I made this post. It shows 1k when I check the post pairs. Does anyone know another spot on the board to measure the cart load with a meter? Also curious what other settings people have found with the SP14, 2M Bronze combo?

 

[Noahpdavis]

Starting to think I've got a gain inbalance issue. Apparently the 2M Bronze pairs well with about phono pre with 36db of gain. The SP14 phono stage has 46db of gain. I'm guessing the crispiness I'm hearing is overload distortion. This morning I tested with a borrowed iphono2 set to 47k w/ 36 DB of gain and the highs were present, but not crispy. Gonna try setting the iphono to similar gain to the SP14 and see if it reproduces the crispiness I'm hearing. Will report back.

 

[FuzzyJ]

Starting to think I've got a gain inbalance issue. Apparently the 2M Bronze pairs well with about phono pre with 36db of gain. The SP14 phono stage has 46db of gain. I'm guessing the crispiness I'm hearing is overload distortion. This morning I tested with a borrowed iphono2 set to 47k w/ 36 DB of gain and the highs were present, but not crispy. Gonna try setting the iphono to similar gain to the SP14 and see if it reproduces the crispiness I'm hearing. Will report back.

Did you happen to solve this issue? I have 2M Bronze as well and am currently bidding on a SP14, so I'm hoping you have some insight that you could share. Much appreciated!

 

[Mr Boochie]

My limited experience with ARC, I've found the LS3 preamp voiced on the bright side. It is sitting in my closet, awaiting a rainy day --- for mods.