Saturday, 24 September 2022

QRPHB Bench Tour

 Above —Unedited video. Poor audio. But all I have time for at the moment. Cheers !!

Above —  Our 2 lab supervisors

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The resource I lack the most is time.

Sunday, 4 September 2022

QRP HomeBuilder — Season 24 —

For Season 24, after a hiatus from RF, I plan to share some radio projects beginning in late October.

Above — I've cleaned up my bench and began ordering some RF parts so they'll be here for Fall experiments. This summer, countless people have emailed me and requested some RF content, so I'll oblige. After a few years away from RF experiments, I feel reinvigorated. I've kept up my reading, but not RF bench experiments --- plus I'll have to brush up on my lab procedures.

In addition, I just got a modern camera to add some video content to my blog posts. Therefore, I've got to learn V logger stuff and choose plus learn how to operate some video-editing software. That will take time. Time is the 1 resource I lack the most and I'm jealous of those retired experimenters who have time to work on stuff at their leisure. My experiments occur late at night with a push-through-to-the-end philosophy.

Until then, here is a gardening topic. I'm an avid gardener and for me it's served as my mental health re-constructor over the past ~ 3 years.

Single Dahlias

I Iove Dahlias and so do the pollinators around our place.  We have more solitary bees than honey bees and I’ve taken measures to provide them a variety of spaces to nest in and around our property.
I mostly grow single row Dahlias for these pollinators and actually the so-called "single row" collection can be divided into Anemone, Collarette, Orchid, Orchette, Single, Micro-Peony and Species (non-hybrid) Dahlias. Unlike the fancy modern hybrids that people grow for weddings and for beauty, "single rows" have just 1 basic row of petals and their private bits are exposed so pollinators can easily access their pollen and nectar.

2 examples of species plants include Dahlia merckii and Dahlia apiculate. I grow the latter.

I wanted to share my adventures with an Anemone hybrid that I’ve worked with for 2 years. My basic plan is to get a variety of seeds, germinate them in late February under lights, grow them indoors until the frost is gone and then plonk them in our gardens. If the plant performs well, then continue that line by saving the tuber, and perhaps growing-on some cuttings for next season.

I’ve noticed that all my "single row" Dahlias struggle in the scorching sun of late June to middle August. I planted some in a shady garden and the flowers plus foliage stays much nicer during the record setting, super-hot summer sun we got in both 2021-2022.  In that shady garden, flowers get about 6-7 hours of sunlight then dappled shade to full shade.

Above — The anemone style Dahlia that blew us away this season.

Above — The seedlings on Feb 28, 2022. The largest seedling is the flower I'm describing. I had to re-pot it twice before planting in in our full sun front garden on May 17th. It started blooming in the house while sitting in a south facing window.

Above — We endured a cool, wet Spring and most plants grew slowly.  I took this photo on July 10 and by this time, despite the Spring weather, this plant was a "flower machine". Note how green the foliage looks.

Above — Photo taken today, Sept 4, 2022. In late July into middle August, the leaves got scorched by the blazing hot sun during our heat wave. Many of the flower buds "cooked". They did not flower, rather they were burnt & turned black. I cut off these dead buds. This was not due to lack of water and a good mulch at the base of the plant — our hot weather has been brutal for the past 2 years. For awhile, there were very few flowers on this plant.

Above — The plant continues to recover and once again transformed into a flower machine. In addition, these flowers last longer than other "single row" dahlias in our gardens. This 1's a keeper and serves as a bee magnet. Grown from 1 seed this year, it out performs our other tuber-grown Dahlias in terms of bud production. Last year, we had the first frost kill of our Dahlias on November 3rd.

Above — Take care of yourself. Connect with nature. It will help clear your mind so you can do some serious electronics.  Best to you!

Saturday, 6 August 2022

Mini-Circuits Labs TC1-1-13M+ balun transformer - quick evaluation

I quickly evaluated a MCL or Mini-Circuits Labs TC1-1-13M+ transformer as a balun.  Although we live in a single-ended dominant RF world (single-ended LNAs, IF amps, bandpass filters, low-pass filters, VFOs, VCOs, etc.) there are times when you want differential output. That’s 2 outputs inverted, or 180 degrees apart with hopefully equal amplitude in each arm. 

It’s easy to make a homebrew balun, but I wanted to see how the MCL part compares to my home-built stuff which might be 4 or 5 twisted turns of #28 AWG on a FB-43-2402 binocular core, or something like that.

The MCL part looks tiny and its small footprint [ 3.81mm X 1.65 mm ] would well suit my hybrid SMT/through-hole build style using Ugly Construction with a few carved copper islands for the SMT parts and AC input/outputs, or DC inputs. I prefer my homebrew projects to be as small as possible. 

MCL specs this part at 50 Ω port impedance from 4.5 to 3000 MHz. An amazing wide band response. I measured the inductance of each arm at 21.6 µH. This is about the same as 4 turns through a FB-43-2402 balun I had lying around in my ready-made transformer tray. Some of these transformers are really sketchy and have been soldered in out of quick bench experiments in some cases for over decades.

My current interest is for lower HF, and in particular, ~7 MHz. 

URL for the TC1-1-13M+ data sheet:

Above — The schematic of my testing jig.

Above — The MCL output into both 50 Ω terminated channels of my DSO with a 7.039 MHz signal applied. The phase difference is perfect as shown and lies in spec at all frequencies I tested -- really amazing.  The amplitude difference isn't as good at HF but was within spec at lower VHF and all the way to ~3 GHz.

Above — My homebrew balun [4t #28 wire, twisted in a FB43-2402 ferrite] into the DSO. Amplitude difference looks better than the MCL at 7.039 MHz.  The phase difference looks unimpressive. I will pay more attention to physical symmetry in my layout when I make actual new baluns for my Fall project.

Above — Tracking generator plus spectrum analyzer sweep. I terminated 1 output port with a 50 Ω RF terminator and swept the other output port. This screen shot is just an HF sweep starting at the lower limit of the MCL part @ 4.5 MHz. It's pretty flat and insertion loss is on spec on this and on wider sweeps. I'm impressed with this low insertion loss across a huge bandwidth from my measures.

Above — The sweep of my balun with identical settings.The #43 mix ferrite material rears its ugly head.  ~ 3 dB insertion loss that gets worse as you move up into VHF and above. I've only got #61 and #43 material ferrites for making transmission line transformers. C'est la vie!

It's fun and humbling to compare your homebrew parts against premium industry components. 


Sunday, 3 July 2022

Reading Material 2022

In 2022 I l stopped most of my subscriptions to magazines and journals. I've got 2 left:

Above — I've belonged to Radio Amateurs of/du Canada for over 3 decades. The quality of our RAC journal 'The Canadian Amateur' has grown — and the build + technical articles rank as first class.


Above — Having subscribed to Nuts and Volts for many years, I still enjoy reading each new issue online. It helps keep me up-to-date with the hobbyist forefront. My interests aren't confined to amateur radio; they include a spectrum of electronics-related topics and general sciences.

Above — I still scan professional EE journal databases and read abstracts on topics I favour. Some articles are open-source and free. Occasionally I purchase articles of great interest to me.

My main hobby in Spring|Summer:


Monday, 6 June 2022

Obsolete parts for an obsolete hobbyist?

The story of life as a hobbyist builder --- you get this email from a vendor from whom you buy parts:

Monday, 7 February 2022

Jazz Guitar Amp Experimenters Series — Definitely Not Your Father's Tone Stack

Hey gang !  

Let's further examine some guitar amp tone control circuitry. I'll show you results from a few of my Winter 2021-2022 experiments. Our context = clean, jazz guitar-focused amplifiers.

After publishing Gibson GA-50 Inspired Guitar Preamplifier Tribulations
I improved the basic tone circuit to reduce resistor shot noise and show the schematic below:

Above — The evolved schematic of the adjustable low and high frequency shelf guitar preamplifier. Calculating the 3 dB high + low frequency turnover frequencies gets done by the standard formula Frequency = 1 / (2 pi *  R * C ) with each potentiometer set to its minimum and maximum frequency for the low and high shelf.

Although enjoyable, after experimenting with different capacitor values in the 2 shelving circuits, I abandoned this basic circuit. Why?  Too much knob fiddling; plus I found I only liked a single 3 dB cutoff frequency for both the high and low shelf. Why bother with all this circuitry when a fixed high and low shelf frequency will do?  I also wanted to focus on middle frequency circuits.

I learned that I prefer shelving tone equalizers over peaking or resonant types for both the low and high frequency. For mid range control, I seem to prefer peaking type albeit only if the resonant circuit Q is less than ~1.5

Above — Two basic fixed-frequency shelving tone controls. These often are combined a single op-amp stage plus/minus isolation resistors. Those 2 circuits follow the familiar Baxandall topology and prove easy to design, build and use. 

Above — A bass, middle, treble tone circuit taken from the out of production Carvin Sx-2000 preamp. The trio of equalizers use a single, unity-gain inverting op-amp stage and all 3 are peaking types. Please view the series capacitor(s) coming off each tone control potentiometer's wiper. 

Thus each active tone circuit uses 2 signal capacitors that convert it from shelving to peaking. The design formulae for peaking tone equalizers gets considerably more complex than shelving designs, however 1 capacitor establishes a 3 dB point below and the other above a centre frequency which give the bell shaped response of a band-pass filter.

Above — Another circuit that provides a peaking equalizer response; the Wien bridge design. This particular design offers a different time constant for each half of the Wien band-pass filter. You'll see this often in guitar amps since it allows the potential of a slightly higher Q (and a sharper response). I really enjoyed the 753 Hz 3 dB frequency version as a low middle control on my bench test guitar amp. All 3 scaled designs use standard value caps and the 3 dB frequency may also be manipulated by tweaking the 22K resistor value.

Guitar amps often feature a single middle frequency tone control; or perhaps bling out and offer 2 middle frequency tone controls such as low and high midrange. Which middle frequencies should I choose vexes many amplifier designers. This is likely the reason we may go with adjustable midrange frequencies, however as aforementioned, I want to move away from that. I built a guitar preamp with 2 separate Wien tone circuits and painfully tried many different time constants to see what worked best for me.

Above — My current experimental guitar tone circuitry. I absolutely love this circuit and it's now my benchmark to compare new designs against. I drive this circuit with a single op-amp voltage amplifier affair identical to U1a and U1b shown in the first schematic on this blog entry.

To avoid potentiometer interdependence, each tone stage gets its own op-amp. You'll occasionally see this in high-end console mixers. Since most of us are just making 1 home brew guitar amp and not a production run where a higher parts count costs your company money, perhaps we can afford to bling out and put in as many op-amps stages as we choose?

To keep design simple, I chose identical time constants for the low-pass and high-pass circuits of each Wien band-pass stage. Thus, the standard Frequency = 1 / (2 pi *  R * C ) formula is in play.

1 design consideration = what order do I put the bass middle treble circuits in?  I did some experiments and with active tone circuits ,you may simple choose what sounds best to your ears. Some amps, however, run the middle, then low + high frequency circuits in order and this worked OK in my experiments.

Lower Middle 883 Hz    Peaking

My middle frequencies were chosen for standard value capacitors. To my ears, choosing a low middle of  800-900 Hz offers the optimum single frequency to tailor the lower midrange. Using a Fender Telecaster + a Gibson ES-175 as my test guitars, I preferred the low middle frequency slightly scooped on the neck + bridge pick up combination; or with the treble pickup alone -- and slightly boosted when playing the front/ neck pickup alone.
I tested this board through my Popcorn PA with 3 different 8 Ω speakers: A 10 inch speaker in open-back mounting, a 12 inch speaker in a open-backed cabinet + another 12 inch speaker in a closed-back, ported cabinet.

Higher Middle 1540 Hz    Peaking

I seem to prefer a higher middle frequency between 1200 and 1600 Hertz. Most often, I tended to boost this frequency in my listening tests, but occasionally left it flat. 1540 Hz is still low enough in the spectrum to adds some punch to your sound while avoiding the nasal sounding (when boosted) 1 KHz frequency. Boosting around 1500 Hz added some grit to my neck position humbucker pickup guitars, although too much boost sounded a little tinny, but not especially, since the Q only lies around 1.5 at maximal boost.

Low and High Tone Controls    Shelving

Simple shelving circuits boost or cut the low and high frequencies. The low-pass filter uses the familiar topology of the low-pass variable frequency shelving stage shown in the first schematic of this blog post. Bass response rolls on & off smoothly and capacitor values of 0.39, 0.47 and 0.56 µF were tested. I prefer the really low 72 Hz turnover frequency at this point in time. You may also tweak the 3K9 resistor value slightly, or put in a temporary trimmer resistor to find your dream low frequency 3 dB point.

Chosen for a 12 KHz 3 dB turnover, the high-pass circuit follows the standard Baxandall design & added considerable shimmer to my guitar tone when boosted. Some builders may prefer a 10 KHz cutoff. Build and test stuff! You're the King on your bench.

On most of my experiments, I ran factory original Texas Instruments 5532 op-amps. On the low and high tone circuits, the 1.0 K end-stop resistors may be changed to limit the boost or cut as you prefer. They don't have to be symmetrical. 

The 20K pots could easily be 10K potentiometers to lower resistor shot noise, however, at extreme settings of 10K control pots distortion may arise & you may have to increase your end-stop resistor values. As it goes, Baxandall circuits offer low input Z when boosting hard and heavy loading via the negative feedback path when cutting hard. Evidently, the 5532 performs better than many other popular op-amps in these extreme situations.

Further, conventional wisdom purports we use FET input op-amps since DC flows through our tone control pots.The TL072, or OPAx134 series come to mind. Bipolar input op-amps may drop noise and boost distortion performance if you don't mind adding a few DC blocking capacitors.

Other Experiments

Above — An active equalizer with all stages centred at 500 Hz with 4 different Q factors to allow listening tests. I also performed this maneuver at 190 Hz and 1.6 KHz. I placed 2 or more capacitors in parallel to get as close to each non-standard value design capacitance as possible.

I felt amazed how differently the same frequency band sounded when changing its Q. Obviously, moving between a Q of 0.85 and 1.0 wasn't staggering, however, I heard a clear difference. I liked a Q of 1 and 1.7 best. However, between these 2 values, I preferred a Q of 1 better as a boost and liked a Q of 1.7 better as a cut. I never imagined such a observation. LTSPICE did not inform me about my preferences either — gotta do listening tests.  When Q = 5.1, my boosted or cut tone sounded unnatural @ 500 Hertz.

Above — A simple middle range Baxandall design. I've shown capacitor values for 3 frequencies, but I also scaled the cap values to test at 350, 400, 573 and 4000 Hertz. I preferred the Wien bridge middle range circuits over Baxandalls for midrange when both are centred on the same frequency. Likely, peaking sounds better than shelving to my ears for middle tones. Admittedly, by adding a capacitor to any  Baxandall frequency band you may convert it to a resonant filter. The math poses a little more difficult than a Wien circuit, but its definitely an option for you to consider.

I think this might prove useful in a bass-middle-treble tone circuit sharing a single op-amp stage. You could make the low and high shelving, and then add a series cap to convert the middle control to a peaking type.

Above — The Carvin contour control for scooping the low mids (or not). No boost. Somehow this circuit, a modified Wien bridge, fascinates me. I ran it in a practice amp for about 1 year and simmered a love-hate relationship with it. I scaled it to reduce shot noise with lower value resistors and a 20K control pot. Very creative circuit.

Above —A Laney scooper. Another fixed frequency, Wien inspired, player defeatable, low middle frequency scooper. The Wien bridge circuit has spawned much cool circuitry and certainly Max Wien sits in the pantheon of electronic designers. Mid scooping circuits feature heavily in guitar distortion circuits and often serve as "mud cutters". 

 Click here for my Guitar-related index

Saturday, 1 January 2022

Guitar Amp Experimenters Bench PA — 12 Watt Popcorn PA


I sought a low power bench audio PA to use for guitar preamplifier development. This brick measures ~ 18 x 11 x 8 cm and delivers 12 clean Watts power into an  8 Ω load. On the front panels 3 RCA jacks provide regulated +/- 17 VDC plus a signal ground connection. Additionally, the signal input jack lies on the front panel. The rear panel sports a speaker jack, fuse and the AC mains input.

Above — A PA is built around a power supply transformer. This tall, heavy & old Hammond transformer has sat in my lab for decades: Although the label shows 25v CT, the AC voltages on each side measured 13.8 VAC under no load. Rectified and filtered this transformer provided +/- 18.89 VDC under a heavy load with no sag until pushed to the extreme. Thus it will function as a 10 -12 W guitar amp power supply — good for my purposes.

Above — The +/- 17 volt rail regulator/filter circuits. My rectifier included a full wave bridge and the main reservoir capacitors = 4700 µF.  I re-purposed The main Hammond chassis from some old project and this entire project falls under the low cost category since most of the parts were ancient specimens from my collection.

Above — Popcorn PA schematic. This is a slightly tweaked version of this Polytone-inspired PA. I recommend this PA over my original version since it offers way less distortion.

The small signal transistors = 2N4401/2N4403 - a pretty decent general purpose BJT I use from DC to HF. The input pair feature some emitter degeneration via a 100 Ω trimmer potentiometer. If you lack a trimmer pot, place a 49.9 to 68 Ω resistor on each BJT emitter instead.  

I ran out of BD140s plus sought a VAS with an fT of > 150  MHz, a low Cbe and a hfe as high as possible. VAS transistor choice seems to shrink every decade. Gone are the luscious  high voltage, high current, high beta offerings from companies such as Toshiba or Sanyo. For example, the KSA1381/KSC3503 or 2SC2911/2SA1209. Yes, these are still available from online auction sellers, but they seem very expensive and the whole bootleg part worry looms heavy. The BD139-140 seem the only low cost, readily available choice for a low budget PA like mine. The BD139/140 pair specs also widely varies - and some are just garbage. I now get mine from Digi-Key and test a couple to confirm they are OK.

In the end, I placed two 2N4403 BJTs in parallel with 10 Ω current sharing resistors for the VAS. This worked OK. Guitar amps are generally not hifi amplifiers giving ultra low distortion. Some might classify as hifi, but that is the exception. I avoided current sources, driving the VAS with a emitter follower and other distortion lowering techniques. 

The virtue of this PA = easy to build, easy to debug and sounds very good for the parts count. If you scratch build a complicated HiFi PA, you suffer a high probability of failure. Especially when current limiting circuitry and 2 current sources that work together with negative feedback go into your circuit. Often enough, your first sign something is wrong comes in the form of smoke; and by the time you figure out why, you may have destroyed some parts.

Above — FFT of the Popcorn PA. All harmonics are down at least 70 dBc. I tweaked the input pair 100 Ω trimmer to crush the second harmonic. Some of this spurious output comes from IMD in the TIP142/147 pair. In a PA, distortion can arise from many points along the signal path. I'm quite happy how this particular PA turned out.

Across the base inputs of the Darlington final pairs, I measured 2.064 VDC. Three rectifier diodes seemed to eliminate any detectable crossover distortion ( a dominant source of distortion in many PAs).

If you listen to a  guitar amp that lacks enough forward PA base bias and suffers crossover distortion, you'll hear a faint, fuzzy sideband sound along with your main guitar sound. You really hear this when playing single, sustained note phrases. This popcorn PA sounds clean & punchy with no hum except with single coil pickups.

Above — I built this version of my PA first. The BD-139/140 emitters get 120 Ω resistors so they don't need a heat sink and hopefully won't burn up if I made a mistake. I measured DC voltages, calculated current by voltage drops across selected resistors — and also tested it with a signal generator and dummy load. Only 2 diodes drops are needed to properly bias the BD139/140 pair.

With the amp working well, I then pulled the BD139/140 off the main board and wired up the chassis mounted TIP142/147 after adding 1 more diode to set the correct PA idling base bias. Finally, I added the current feedback loop. I chose the 7K5 Ω resistor during listening tests.

Above — FFT of the circuit shown in the schematic above (with the BD139/140). The distortion is about as low as I can measure with my DSO. All that is needed = a set of complimentary finals and a good PA might arise. A decent LoFi popcorn PA for guitar amps.

Above — The "brick". The Popcorn PA lies on 2 boards with carved islands for the positive, negative and 0 volt rails - and to anchor the rectifier diodes. After taking this photo, I screwed on the cover plate to seal it for safety against the pile of wires and other mess on my bench. This amp is not connected to AC mains unless I am using it and does not have an on/off switch since all my audio test equipment goes on a dedicated switched AC mains power bar.

Above — Rear view of the Popcorn PA. I'll start work on my next preamplifier circuit tomorrow and audio test it using this PA. I really like the small bench footprint this brick offers. I place the preamp circuit just in front of the brick and have small cables built for the 17 volt rails and input.

Click here for my guitar related index.