Measuring Potentiometer Resistance Taper

I had some fun measuring the resistance of a linear 10Kohm potentiometer from my junkbox and designated B10K.

I then measured an ALPS 10Kohm pot that is intended for audio applications. P/N RK0971221Z05. The taper for this pot is designated as type 3B by ALPS.

Refer to the following link: Potentiometer Taper Measurements

Homebrew Pot Taper Meter with 10 degree graduations. Hooked to a DMM

David Clark Headphones amplifier design 2

The simple common emitter circuit was redesigned with an increased target collector current of 10mA. This allowed enough current to supply the 150 ohm load and ensure that the transistor was operating as a constant current source.   

Varying emitter degeneration and/or collector current(by increasing the rail voltage)  demonstrated the effect on amplifier voltage gain and dynamic range. As can be seen from the measurements, collector current increase results in amplifier dynamic range improvements (seen as reduced gain compression in the measurements). Also increasing emitter degeneration improves dynamic range but at the expense of amplifier voltage gain.

So you cannot 'have your cake and eat it'. Better dynamic range comes at the expense of increased battery consumption or reduced gain.  With a 7V supply I found the gain limit to be less than 6.5. Increasing the gain to the 10 - 13 range required a collector current of 18mA. Clearly either option was problematic for this application. 

After increasing the emitter degeneration resistor to 15 ohms I then tried the amplifier connected to my K2 and using a Vcc of 7.5 volts.  Listening to our club evening 80m net with high QRN I was very interested to note slight distortion in the headphones. The received CW note sounded undistorted to my ear at normal listening levels. However the background noise sounded slightly distorted. The measured 3dB points of the amplifier were 200Hz - 1800Hz. When the K2 audio gain was turned down to a low listening level no distortion was detected. So in spite of measuring an undistorted (as far as I could tell) 1kHz output sinewave approaching 600mV (p-p)  on my bench. In practice the noisy band sounded distorted. Could this be due to noise spikes from the QRN exceeding the dynamic range of the amplifier? Using a single signal input to simulate a real radio channel is simplistic. 

Next it would be interesting to try a 2 transistor amplifier using an emitter follower on the output. This would allow an increase in the load resistance of the common emitter input stage which would allow improved dynamic range and gain at a lower collector current. However at least 10mA would be needed to drive the emitter follower. Is this a zero sum game?

20mV input, 128mV output, Vcc=7V Gain (V) = 6.4 'undistorted'

100mV input, 500mV output, Vcc=7V. Gain (V) = 5 'distorted'

250mV input, 1.3V output, Vcc=7V. Gain(V) = 5.2 'distorted'

250mV input, 1.55 V output, Vcc = 12V. Gain(V) = 6.2 'undistorted'

David Clark Headphones amplifier design. 1

I was lucky enough to receive a pair of David Clark Headphones from my kids as a birthday gift. The model I have is the classic headset and mic designed for use in General Aviation cockpits and known as Type H10.13.4. This is the best selling headset in aviation. They fit tightly over the head and have a 33dB (@1000Hz) external attenuation.

For amateur radio use I found them to be a bit quiet and require me to turn up the AF Gain on my K2 Elecraft rig to near maximum volume before I could hear them.

I figured it would be a great project to try to build an audio booster amplifier from scratch.

The design in this blog post did not meet the objective.

The headset has an impedance of 150ohms. I determined through listening to a 1KHz tone that a peak-peak voltage of 1V maximum was required to drive the headset to a loud volume. However output distortion occurs when greater than 250mV. This distortion could be easily heard in the headphones. The reason has not been determined. Next I will try raising the emitter voltage to greater than 1V.

As a part of the design process a 150Ohm resistor was used. The results were the same when the headphones were actually connected to the output.

Design Input requirements:

1. Rail voltage = between 7V and 12V to allow use of a 9V internal battery and an external 12V supply.
2. Total current drain = <3mA.
3. Frequency range = 200Hz - 3000Hz.
4. Maximum input signal amplitude = 100mVp-p.
5. Output signal amplitude = 1V.
6. Load impedance = 150 Ohms.

The common emitter design is based on achieving a gain of 10. 

Calculated and installed component values are shown on the included schematic. 

Calculated and measured parameters are shown on the included schematic.

The 3dB filter roll-off points were measured as 200Hz and 1800Hz (approximately).

250mV Output for 22mV Input. Gain =11.4. No Distortion

800mV Output for 100mV Input. Voltage gain = 8. Distorted.

Prototype 1

'Fun in the sun' with a true homebrew QRP field station. ZS6AZP

Thank you to my friend Dave ZS6AZP and the Centurion Radio Amateur Club ZS6CEN for allowing me to post a blog covering Dave's recent QRPexpedition. As usual Dave projects a 'no compromise' approach and a field operation in the true spirit of Amateur Radio (IMHO).

Check out the details at this link. Fun in the sun with ZS6AZP

I especially like the pool pole antenna support :)

Thanks Dave!

72 de Dick N4HAY

Improving PCB enclosure building

I was recently inspired by the fantastic website of K7QO to try to do something to improve my PCB enclosure building, both from an efficiency perspective as well as from a neatness point of view.

I built a fixture in accordance with Chuck's directions on building enclosures and it turned out very well. Last night I was able to test solder together two sides using a piece of 24AWG insulated wire to ensure that the angle was 90 degrees once the solder dried (see pics below). I used a 30W Weller iron.  I found that the technique worked better by having the vertical piece of PCB mating directly with the bottom of the fixture and in front of the 24AWG wire.

Enclosure Fixture as per K7QO. The walls are a bit high. A lot of time was spent ensuring that the corner consisted of all 90 degree angles.

Much as I would love to own a shear I found that I could accurately cut PCB's to exact size using my table saw. More on that later when I get to build a prototype enclosure as the next step.

Thanks to Chuck for sharing his extensive knowledge with us.

A piece of insulated 24AWG wire is used to compensate for the solder shrinkage

Clamps are  used to hold the pieces in place

The 'shrinkage compensating angle' can be clearly seen

Completed prototype effort. In fact the thinner black wire resulted in a better right angle finish 

CMoy pocket headphone amplifier measurements

Reference The CMoy headphone amplifier details

Gain and DC offset measurements were made using two different rail voltages as follows:

Input signal: A sinewave at 1kHz with variable amplitude. Source impedance = 50ohms.
Load: 270ohms on each channel to (simplistically) simulate the Beyerdynamic DT880 audiophile headphones owned by my son.
Both channels showed the same results.
LED current = 1.43mA
With rail-to-rail voltage = 7.68V, DC current = 9.1mA. (Same w/wo the 270ohm loads connected). Thus DC power = 70mW.
Vgnd-Vplus = 3.77V.  Vgnd-Vneg = -3.91V. Thus DC Offset = (7.68/2)-3.77=70mV.
At the point of clipping on positive voltage swings Vp-p(out) = 4.6V. Vp-p(in) = 430mV.
Thus Vgain = 10.69. Headroom on the Vplus rail = |(4.6/2)-3.77|=1.47V.
P=(Vp-p)^2/8R = 4.6^2/(8*270)=9.7mW.
Thus efficiency = 9.7/70 = 0.14.

With rail-to-rail voltage = 15.31V. DC current = 11mA. (Same w/wo the 270ohm loads connected). Thus DC power = 168mW.
Vgnd-Vplus = 7.59V.  Vgnd-Vneg = -7.72V. Thus DC Offset = (15.31/2)-7.59=65mV.
At the point of clipping on positive voltage swings Vp-p(out) = 10.9V. Vp-p(in) = 1V.
Thus Vgain = 10.9. Headroom on the Vplus rail = |(10.9/2)-7.59|=2.14V.
P=(Vp-p)^2/8R = 10.9^2/(8*270)=55mW.
Thus efficiency = 55/168 = 0.33

CMoy pocket Headphone amplifier. Input coupling capacitor tests

My son recently became the proud owner of a pair of audiophile headphones. A pair of Beyerdynamic DT880 premiums. See the specs http://zs6rsh.blogspot.com/2015/07/headset-technical-specification.html

 Motivated by my friend Chris KD4PBJ, I built a prototype of the CMoy pocket headphone amplifier.  As specified, I used an OPA 2132PA Burr-Brown Op Amp. The rest of the components came out of my junkbox.

In reading the section on Input Capacitors for Headphone Amps, the author recommends the use of Polypropylene Film Capacitors as the best choice followed by Polyester Capacitors and to stay well away from Ceramic Capacitors. A very interesting read. I understand that a major issue in audiophile amps with AC coupling is Phase Distortion caused by the low frequency roll-off due to the RC high pass filter formed by the coupling capacitor and the amplifier input impedance. This causes a smearing of the Bass notes due to Phase delay from about 10Hz through 100Hz for a 0.1uF input coupling capacitor looking into an input impedance of 100Kohms.

Curious about this, I set up a test fixture to measure the phase distortion differences between what looked like a film poly capacitor in my junkbox (and which I deployed in my prototype) and a ceramic capacitor. I also ran an ideal model using LT spice. 

The high pass RC filter measured, consisted of the 0.1uF test capacitor and a 100Kohm resistor. As input I used a function generator set to approx 20mVp-p sinewave and two 10X probes connected to an oscilloscope to measure the phase delay between the input and output of the circuit. Lissajous figures were used to measure the phase angle to approximately a plus/minus 2 degree accuracy.

Attached find a graph of the plotted result. I noted that the measured -3dB filter knee for the poly capacitor was 17Hz plus/minus 2 Hz and the ceramic was 21Hz plus/minus 2Hz.

As can be seen, I was unable to measure a big difference in the phase delay of these capacitors using my 1970's era home lab equipment. The measured results compare favorably with the ideal capacitor phase delay calculated results.

In delving into the fascinating world of the audiophile at http://www.head-fi.org/ I came to realize that what constitutes a 'good sonic' is something highly complex and thus, highly subjective. So what is it that makes a polypropylene film capacitor 'sound' better than a ceramic capacitor? It is not revealed in my simplistic measurements. Perhaps driving the amp with a step signal (squarewave) would reveal much more about the differences in these capacitors?

The amp works very well with the DT880's to my ear although clipping of Bass notes was noticed when driven with 9volts. No clipping when using an 18volt source. Leading to an idea to build a buffer type virtual ground for the next version.

Thanks to the excellent website http://www.tangentsoft.net/, I have experienced a glimpse into the  complex world of the audiophile.

Now can I use one of these amplifiers to replace an LM386 in my regen receiver? :)

CMoy Prototype 1. The input and outputs should be isolated from the chassis!

The junkbox Wesco input capacitors can be seen. Next version needs 18volts to drive the DT880's. The Op Amp is a Burr-Brown OPA2132PA. Amazing sound! Zero hiss.

RC Network High Pass Filter test fixture. Input = 200mVp-p sinewave from a Function Generator (50ohm source impedance). Phase angles were measured using Lissajous figures. 10X scope probes used.

Adventure Radio Society Spartan Sprint August 2015

Great fun was had in the Spartan Sprint Contest last night Monday evening.  Spartan Sprint Rules.

I operated my old trusty ATS3 rig into a BLTuner feeding an inverted Vee 80m doublet up about 55ft at the apex. Power output at the beginning of the contest was 2Watts on 40m and 20m. The battery was a 750mAH Li Po battery as the power with a set of mini paddles and earbuds. The total weight for this setup was 13.4oz (0.838lbs) of which 7oz was attributed to the ATU! There is definitely room for improvement and weight loss with my station.

QTH is near Raleigh NC operating from the kitchen table since it was extremely hot outside.

Band conditions were good and I was able to work all stations copied. 20m was excellent for the first hour and then switched to 40m. QRN was high on 40m but the signals were strong. Towards the last 30 minutes I switched back to 20m but no signals were copied. In particular I enjoyed the contacts with N4SX (1 Watt on 40m in GA) and KA5QQA (1 Watt on 20m in  TX).

Thanks to all for another great Spartan Sprint evening.

RESULTS

Total weight = 13.40oz = 0.838lbs
20m QSOs = 15
40m QSO's = 11
Total QSO's = 26

QSO's per Lb = 31.03

Flight of the Bumblebees 2015. N4HAY. Jordan Lake NC

I really enjoyed the FOBB this year. Unlike last year the bands were in good shape and the wx was beautifully mild. I set up in roughly the same spot as last year only this time I deployed a far superior antenna system. My operation did not involve a major trek since I merely walked from the car park to a nearby picnic table. Oh well perhaps next year I will commit to more of an adventure hi.

The FOBB was held on Sunday afternoon from 1pm - 5pm Eastern time zone. Check out the following link for The Adventure Radio Society Flight of the Bumblebees details of the contest rules.

I set up at the Ebeneezer State Park at Jordan Lake North Carolina. This is about a 20minute drive from my QTH. I arrived on site at noon. First I had to build a 40m dipole. I was not well prepared for this contest! I then deployed my 20m and 40m dipoles. This turned out to be more of a challenge than I had judged. Thus I was only QRV an hour into the contest. However once my antennas were deployed they were very effective. My 40m dipole was beaming N-S and was up about 55 ft agl. My 20m dipole was beaming W-E and was up about 45ft. I used my trusty slingshot to launch the antennas to this height.

Rig was a K2 running on the internal battery. I ran a frequency and S&P about 50/50. Battery indicated voltage on the K2 was down to 11volts by the end of the contest.

Band conditions were good and I was able to work almost all stations that I heard. I had the feeling that I could have done better with a W-E facing 40m dipole. Lesson learnt here was to stick to a simple inverted-vee doublet center-fed with ladder line next time. Ideally deployed in such a way that I could change the orientation easily. I am sure I could have done better if I had been better prepared.

I also had problems with my pen that would not write properly! This brings to mind the reason why early astronauts used pencils on their early missions hi!

I was unable to copy my fellow Knights from the Knightlites QRP club Sirs Derek W4FI, JP AB4PP and Paul AA4XX indicating that the 40m band was long.

I look forward to next year's adventure with much anticipation. Many thanks to all stations who worked N4HAY. 72 de Dick.

Log of N4HAY. FOBB 2015. Jordan Lake

N4HAY. FOBB 2015. Suffering no pain!

Headset Technical Specification comparison

I did a little technical comparison, based on online technical specifications, between the shown headsets. I found it interesting. Perhaps not surprising. On a qualitative level I note a 'very big' difference in sensitivity between the David Clarks and my Sennheisers. However I assume that I am driving them with an amplifier (a current audio  amplifier in my Elecraft K2 ham radio) and  may not necessarily be optimized for either.

I also do notice a difference in both low and high range frequencies between my Sennheiser and Earpods on listening tests. But still the Earpods are very very good and have to be some of the best value for money on the planet?

I would say that all 3 headphones would benefit from a Headphone Amplifier. 

Also note that the SPL measurement is made at one frequency only. Generally seems to be 1KHz. So the sensitivity may seriously degrade at the ends of the claimed bandwidth.

It is interesting to note the technical differences between the sets as optimized for different applications.

Given the complexity of comparing headphones due to many parameters way beyond this list I would think that the 'crowd sourcing' method of opinions may be the best way to make a determination as to which headset is best for you.

Headset technical comparison

Field Day 2015. WQ4RP. The Knightlites

As in all previous years for me, Field Day 2015 with The Knightlites was a total pleasure! A great adventure and an opportunity to learn and appreciate the challenges presented from operating in the great outdoors in less than ideal circumstances. Certainly that was the case this year!

Four intrepid Knights Sirs Derek W4FI, Joe WA4GIR, JP AB4PP and Dick N4HAY operated Field Day as WQ4RP, Class 3A Battery, NC from the Holly Point State Park at Falls Lake North Carolina.

The Great Setup. Pics by Joe WA4GIR

Sir JP and Sir Derek before the action began. Pic by W4CHX Karl

Violent thunderstorms on Friday evening, and Saturday, destroyed a tent and relegated some of the Knights to sleeping in their cars. But the intrepid few remained at the Key...Sending and Receiving in spite of it all! Great adventure, camaraderie and plain good fun was had for sure.

MRE's issued by Sir JP just in case.  Pic by W4CHX Karl

A shelter destroyed by a violent thunderstorm. Pic by JP AB4PP

20m operating position.

20m operation position. Pic by Joe WA4GIR

Sir Derek WF4I pounding the key on 20m on Sunday morning

40M operating position. Pics Joe WA4GIR

Sir JP at the key. 80m, 15m, 10m operating position. Pic by Joe WA4GIR

Mason (or Consul) Jar LED lamp. A South African product

The Knight's Trebuchet built by Sir Gary N3GO.
This weapon gave us at least a 3dB advantage!

Some finer details of the event.

Background

Results

Operating Conditions

Lessons Learned

Inventory Checklist

DUP Sheet

Here find Band analytical information: 

10 meters

15 meters

20 meters

40 meters

80 meters

All Bands

WQ4RP The Knightlites. Field Day 2015 QTH.  Pic by W4CHX Karl

Homebrew Attenuator Measurements contd

It was interesting to measure the insertion loss and maximum attenuator accuracy over a wider frequency range.

The BIG attenuator has a maximum attenuation of 68dB.
The Copper attenuator has a maximum attenuation of 71dB.


As for the previous measurements I used the HP 8657B as the 'standard' against which to compare.

I also used a 0dBm input level for the 71dB (copper attenuator) and 68dB (Big attenuator) measurements in order to move the measurements comfortably up from the lower end of the Power Meter measuring capability. The Power Meter in fact showed a consistent slope ranging from 20.5mV/dB for 1.6MHz to 19.69 mV/dB at 450 MHz. Quite cool for a homebrew power meter!

Reference the following links:

68dB slope calculation

Insertion Loss Measurements

Maximum Attenuation Measurements

In conclusion both attenuators show significant insertion loss and maximum attenuator errors for the 148MHz, 224MHz and 450MHz measurements.

'BIG' Attenuator

'Copper' Attenuator

Homebrew Attenuator Measurements contd/

Today I characterized another attenuator recently acquired and known as the 'BIG' attenuator on this blog. The performance of this attenuator over the HF frequency band is not as good as the 'copper' attenuator (results shown in the previous blog). The worst case measurement was on one of the 20dB pads at +1.06dB error. Although this unit is solidly constructed, I assume that the fact that it does not have shielded compartments between each attenuator must degrade performance over the frequency range. Perhaps also the switches used are not as good as slide switches for this application.

The shown measurements in fact are optimistic since they do not include insertion loss. I performed separate insertion loss measurements which are shown in the next blog.

As a part of these measurements I also characterized the slope of the Power Meter.

In conclusion, the attenuator certainly is useful for many functions at HF. Also the use of 1/4 Watt resistors will allow use in higher power applications.

The following links

Measured Attenuation

Pi network resistor values and calculations

Slope Calculations

Homebrew Attenuator Measurements

I recently acquired a homebrew attenuator (called the copper attenuator in this blog). The construction is modeled in line with the QST article of September 1982 titled "A Step Attenuator You Can Build" by Bob Shriner WA0UZO and Paul Pagel N1FB although the named attenuator pad values were different.

On opening up the unit I was mystified by the values used for four of the PADS. The parallel and series values appeared to have been transposed in error. I reworked these values using standard nearest value resistors.

The switches are in excellent condition with no signs of wear. As can be seen from the linked spreadsheets below, I tested the attenuator over the HF frequency range only. A maximum of 0.1dB insertion loss was observed at 28MHz.

The worst case error observed was +0.4dB for the 16dB PAD.

With all PADS switched in to yield a maximum attenuation of 71dB the maximum error was 0.8dB at 10MHz.

Click on the link below for details of the measurements made. To return to this page please use the back button.
Attenuator Measurements

Click on the link below for details of the resistor Pi Network Values. To return to this page please use the back button.
Pi Network Resistor Values, dB(calculated), Return Loss (calculated)