The worst resistor ever?

I wound some 2KΩ copper wire resistors for DHT Rob a few years back and he used them to bias a 10.   He reached back out to me to do some 1300Ω to bias an AD1 @ 35ma.  

These got the slightly nicer package pictured above @ 25mm X 25mm. One of the biggest problems with the copper resistors is the value is not stable with temperature.  My solution to this for this case is to let them make some heat to stabilize them when in operation.  These units have a cold DCR of 1140Ω when cold and 1302Ω after an hour at 35ma.   Since these are intended for cathode bias it must be noted that the tube will run a bit hot initially before the temperature comes up and things stabilize.

The units are wound to be non-inductive and that results in a fair bit of parallel capacitance.  Since there will be a bypass cap in parallel with the resistor anyways, that is of little consequence.   In a crazy world one could use them unbypassed to compensate for loss of HF info elsewhere in the amplification chain like Kondo did with the Ongaku.

The other use for this idea is the loading of MC cartridges.  Since the proper way to load a cart is at the primary of the SUT, the values are typically much lower.  Knowing that it is also at a point where linearity of value across the audio band is a concern.  Here are some units that I wind onto an RCA plug for ease of termination and use.

These have much better bandwidth and are flat through the audio band.  Unlike the 1300Ω units which go capacitive at high frequencies, these go slightly inductive. 

The big question is why go to these insane lengths when there are plenty of off the shelf options are available?  Well if you look at it on the most basic level a resistor is simply a really crappy conductor. Given the extremes we go to for the quality of our signal wiring it isn't beyond reason to me that a longer length of a superior conductor with all of the baggage that it comes with may be better than an alloy specifically done to impede conductance.  Or you could just listen and damn if they don't sound good.

dave

Tannoy Redux

A decade or so ago, I played with some nickel cored versions of the Tannoy crossover parts for Arthur Loesch.  Recently, Chris from Jupiter Condenser had me do a complete set using his 0.99999 pure cotton served copper wire.

The cotton serving takes up a fair bit of the winding space compared to typical magnet wire insulation so some creative license was needed to keep the parameters identical to the original.  Luckily, the higher perm of the 80% nickel cores made this easy!

In the original design the ~0.5mhy inductor was air cored and had a rather high DCR and was in series with a ~4Ω resistor and I needed to juggle the DCR and and resistor values to net the same end result.

Since the theme here was 80% nickel, it only seemed appropriate to use that to my advantage for the above choke and sneak a few lams into the typically air cored inductor above to keep the family character.

FFM..... festivus filament modules

It's beginning to look a lot like Festivus with all the airing of grievances and dancing around the Festivus Pole of late.   This must mean is time to give something away.  The newest thing in our bag of tricks is a simple LC filament module that can provide up to 2A or 3.5A of DC current.

Originally designed to power coleman regs, these modules give you a nice easy way to mount and wire a source of low ripple DC from an AC input.

All of the sordid details of what they are and what they can do can be found HERE

Air your Grievance in a comment below (with your name) for a chance at a free pair shipped to you as a Festivus Miracle.  As usual... in a few days Random.Org will pick a winner.

Enjoy!

Sut Selection (Why I don't do a one size fits all unit)

I just recently had a customer ask for a single SUT to match three different cartridges with a range of internal impedances from 1.5 to 15 ohms.  While that range is seemingly small one only needs to look at the frequency response plots to see why I like to match a impedance to a transformer.

These are the plots for 4 different cartridges (2r, 6r 9r and 15r) all feeding the input of one of our phono stages.  This particular SUT was designed for a 10 ohm cartridge and once that is known I was able to wind it so that the cartridge stayed well behaved to beyond 100Khz and only had a slight attenuation at 10hz.  I would be comfortable uning this cartridge in the 6-15 ohm range but if I were to drive this transformer with a low Z cartridge like a Lyra Olympos SL, suddenly a 10dB peak would appear above the audio band.  In this case, since this peak is happening at close to 200Khz one could argue it is sonically benign but armed with the knowledge that a 2 ohm cartridge will be the source I simply adjust things to minimize the out of band resonances like in the plots below.

I know there are those that will argue that adding a snubber network to the first transformer driven by a 2r source can eliminate the ringing and give a very beautiful frequency plot but somehow doing that also steals a fair bit of the music along the way.   For more of my thoughts on loading SUT's take a peek at this.

Puzzle Part II

I hinted about this concept last July and John Chapman at Bent audio hit it out of the park.

I had some fun with the first one and thought the ability to A-B between two different settings would be cool so John stepped it up a notch.

 

It is a stereo unit with only 2 RCA Jacks.

 

This is just another prototype and for those of you who haven't figured it out, it is a remote controlled load resistor.  My initial use for it was to adjust the load a MC cartridge sees and it is a great educational tool!

Stay tuned.... we hope to have a finished unit and John will have boards available for OEM's to add this functionality to their devices.

dave