Parallel Resistive Loading

Application of an incremental parallel resistor is a very simple and low cost method of delivering variable phono stage resistive loading.

Here’s an overview of my approach to delivering this.

First up what’s the effect?

Although most MM/MI/HOMC’s are supposedly designed around what’s become the standard 47k/50k ohm MM stage resistive load,
this may not be the optimum setting for individuals personal taste or from a system matching perspective.

The application of an incremental parallel resistance will reduce the overall MM stage resistive loading.
This tends to dampen higher frequency response, but it’s not like a simple tone control that would affect a small range.
It provides a total tonal change, dampening the lower mids all the way up and typically used to tune perceived "bright" carts.
The effects vary depending upon cart design, it can also tighten up a slightly woolly or loose lower end, especially with HOMC's.

Some carts are affected more than others and only need a subtle reduction to make a significant effect.
HOMC’s tend to require bigger swings in resistance before the audible differences can be heard.

Personally, I’ve found very few MM/MI/HOMC’s that deliver their optimum performance bang on 47k and tend to tweak most of them.
For example here are a few preferred loadings (to my ears and on my system) of some popular cartridges;

Audio Technica AT 440mla (MM) – 32k
Shure M97xE with JICO SAS Stylus (MM) - 35k
Denon DL 160 (HOMC) – 1k
Dynavector 20XH (HOMC) – 1k
Grado Reference Sonata 1 (MI) – 38k

Examples

Here are some downloadable test clips at a various loadings for a selection of cartridges
Shure M97xE with JICO SAS Stylus (MM)
Denon DL160 (HOMC)
Dynavector 20XH (HOMC)
Grado Reference Sonata 1 (MI)

And here's a bit of a special that I'm particularly fond of;
Grado Z+ with MCZ Stylus (MI)

This one (as discussed here) needs dialling down quite low to get it to sing in my system, but is well worth the effort.

Before we get into "how", it needs to be stated that increasing the resistive load (to liven up the general character) requires phono stage surgery.
The load resistor within the phono stage has to be removed and replaced with a higher value item.
I have done it, but IMHO it really is only for those who are totally comfortable with electronics and use of a soldering iron.
I am not intending to go into that here.


OK so how’s it done?

Well as I say it’s relatively simple, you need to apply a parallel resistor to the MM input stage to reduce the overall resistance.

Here’s the maths;

1/Rtotal=1/Rphono+1/Rplug

Where;
Rtotal: The modified delivered phono stage resistance
Rphono: The quoted resistive load of the phono stage
Rplug: The incremental resistance wired in parallel

So if you had a 50k ohm phono stage and applied a 200k ohm resistor in parallel you’d get a total load of 40k ohms.


So how do we do it?

First thing you need is a pair of good quality RCA Y splitters (1 male to 2 female) such as the ones pictured here;


The male plugs into the one of the phonostage RCA connectors and the equivalent channel TT male connector goes into one of the females on the splitter.

You then need to add your parallel resistance.

The simple way is to buy some resistors and male RCA plugs and solder up like this;



If you don't have access to a soldering iron then you could use male RCA plugs with 2 pin connections such as these;


Search for "RCA Screw Connector" on the internet and you'll find them.

Snip your resistors to the required length and insert across the screw terminals.
I have not used these, but they should work fine.
I would however, strongly recommend using insulation tape to insulate the connection points and resistor wires.


Ultimately you can make up a little kit that looks like this;



All of these items are readily available through electrical retailers either in store or on the internet.


You then need to connect up as per the below pictorial which also includes a table (rounded to the nearest kOhm) showing the delivered loading reduction.


Once the Y Splitters are in place it’s then a very simple process to replace the resistor plugs to vary the loading and understand it’s effect.

Don’t forget! No plug at all means you’re right back where you started with the standard Phono Stage resistive load.

IT IS STRONGLY ADVISED THAT THE PHONO STAGE AND MAIN AMP ARE SWITCHED OFF WHEN FITTING THE KIT AND SWAPPING THE RESISTORS

You can get some pretty harsh spikes if you leave the amplification on, risking circuit and speaker damage.


And Finally

I personally have taken it one step further and built up this infinitely variable loading box;


It uses a pair of calibrated potentiometers in two ranges.

I use this to adjust resistance “on the fly” to quickly tune in the optimal loading.
But this has its dangers. I did induce some hum with this unit and had to spend quite a bit of time getting the earthing sorted.

I had originally built this to first verify optimal loading to then build up a dedicated plug to this value.
However, after slaying the hum demons I am now more than happy to leave it as a permanent fixture to the rig.

It is especially useful as I have three turntables in play and one (the Dual 1219) has 7 cart/sleds which are regularly swapped.
The loading box get's adjusted for each table and cart combo that's selected.

It is also occasionally (not often) used to tweak loading for the odd disc that may be a little "hot" or even dull.


There you go.

I certainly have not looked back since delving into this.
It really is a simple and low cost method of optimsing an individual cartridge's response in any given system.


Created by Mail To: davey_w@hotmail.co.uk: March 2011

Last Updated February 2018: Loading Box Usage Updated