Thursday, 5 January 2017

Polytone Amplifier Page

This is a reprise of the Polytone Amplifier Page from my old QRP HomeBuilder web site (2008).

Above — The Mega-Brute has an eight inch speaker and is über-portable. This amp tucks into almost any corner. On the rare occasion when I play for a function, I may use a 12 inch extension speaker, depending on the situation.
I've owned 4 different Polytone amplifiers and like their tone and compactness. I must disclaim this blog post by stating that what I have written is just 1 opinion. Discussing guitar amplifiers is akin to stepping into a quagmire. Guitarists often become quite emotional and speak passionately, or possibly even overly-critical when discussing gear. When you consider that most players don't even perform in front of an audience, or record for mass distribution, the "gear wars" diatribe can get really quite silly. If your tone sounds good to you, then perhaps your amp is suitable (at least for this month).

For jazz guitar, there are countless amplifier choices and the list of of good jazz guitar amplifiers has really grown in the last few years. The trend seems to be towards more hi-fi sound (less distortion-more headroom and more power), smaller/lighter designs at somewhat increased cost.

Consider talking to Michael Biller at Sound Island Music if you wish to talk to someone with considerable knowledge and practical experience regarding modern jazz guitar amplification.  Michael stocks many products -- and his passion about helping you obtain your perfect jazz or double bass guitar tone really shines through when you talk to him.

Polytone Mega-Brute Amplifier

Canadian guitarist, Glenn Murch once told me that Polytone guitar amps basically have one sound and you either like it or not. I agree with him. This sound is not
ultra-high in fidelity ( compared to more contemporary designs ), has a dark voicing, a distinct midrange honk and starts to distort at high volume settings. This is exactly why I like Polytone amps in some situations. To each, their own.

The Mega Brute combo amp has an 8 inch speaker. This is probably not the best amp to use if  you play in a big band, but it works okay with a trio if you are happy with the sound it provides and the drummer uses brushes and/or soft hands with sticks.

I have owned Mini-Brutes with 15 inch and 12 inch speakers as well as the Mega-Brute combo amp and head. The various Brute-series amps are worth a trial if you're in the market for a mid-price jazz guitar amp.

Official Polytone Page  Click

Old Murch Music Polytone Page  Click

Old Murch Music Polytone Schematics Page  Click

Above — Rear view. The tolex work is quite excellent. I love closed back speaker
amps for that bass thump, although, in my opinion, ported designs are preferable. The porting seems to be via the via the low input instrument, pre-amp out and FX loop jacks!

Above — Top view showing the various control pots and switches. The sonic circuit was a great addition to the Brute series. The previous Brute design had an overdrive circuit which failed to merit wide acclaim. I personally do not use the sonic circuit and rely upon the main amp circuit. I like the slight break up of the power amp when driven hard, although it is by no means a Marshall-style crunch tone. Baxandall equalizer plus decent spring reverb.

Above — My Mega-Brute atop a 1 by 12 Marshall cabinet. This proved a pleasant
combination for R & B plus jazz-fusion work.

Above — The other Mega-Brute.  An amp head which became known as the
Mega-Brain. This product was discontinued. The speaker is a Raezer's Edge Stealth 12 speaker cabinet. I sold this head in 2004 and now regret it.

Above —  Inside the Raezer's Edge Stealth 12 speaker cabinet. Foam and
fiberglass insulation absorb standing waves + reflections, lower the box Q and hopefully smooths out the bass response.

Above — Another view of the Raezer's Edge Stealth 12 speaker cabinet.

Above — I like Eminence speakers; including the Eminence Patriot Swamp Thang mounted in another cabinet for my stereo amp rig.

 Above — My old Mini-Brute with a 15 inch speaker on its side ready to carry. Bass for days!

Sunday, 1 January 2017

Fender Telecaster Jazz Box

Leo Fender invented the solid body electric guitar. Eventually, his invention became the legendary Fender Telecaster®. This basic guitar: withstood the test of time; exudes versatility; sounds great; and most importantly -- proves fun to play.

I tried out a number of Fender Telecasters at local music stores and 1 stood out. A blue American Special Telecaster®. This particular guitar sounded nice acoustically and the neck felt great under my fingers. I play jazz fusion, or clean-only, jazz chord style blues on mostly the front pick up.

While rolling the guitar tone knob back about 50% provided a decent jazz tone, the 60 hertz hum plus the front pick's thin sound left me wanting to place humbucking pickups in this axe.

I 've replaced a lot of pickups over the decades and the aftermarket choices today seems incredible. Still, too, I tend to go with what I know. I bought a pair of Seymour Duncan hot rails for tele pups and mounted them. Click for a link to those pickups on the Seymour Duncan web site.

Above — The stock Telecaster® pickup just sitting in its cavity. The protective cellophane still lies in-situ.

This guitar came stock with Texas Special™ Single-Coil Tele pickups. They sound good and would likely work well for the classic rock, blues and country player. When I removed the pick guard, I felt surprised to see the guitar body was already routed for a full size humbucker pickup, although the routing seemed more shallow than standard.

Above — The new Duncan humbucker sitting in the neck cavity.

Above — Alternate view of Duncan humbucker just sitting in the neck cavity. I removed the bridge pickup. You can see the black ground wire I added to ground the bridge since the Duncan pickup lacks the classic Telecaster brass grounding backing plate used with stock Tele single coil pickups.

Above — The front pickup screwed into the guitar wood. I lacked the hardware to mount it in the pickguard and so just screwed it to the guitar. In my 22 fret telecaster, in order to put the pick guard back on, I had to remove the neck. I also had to sand my pickguard's front pickup slot a little to better fit the pickup width.  You can see, that an early attempt to place the pickup in the pickguard shaved off a little of the tape that covers the pickup front.

Above — To unsolder and solder new pickups, the tele's control plate gets removed and set on a towel to avoid scratching the guitar.


Above 3 photographs — Control panel, switch, and the deep control cavity. Wiring guitars seems so simple compared to the microwave circuitry I've toiled with lately.
I also changed the stock Fender tone circuit to my preferred tone circuit: that used by Gibson® guitars in the 1950s.

Above — The bridge pickup mounted in the vintage style Fender bridge.
Above — Both pickups installed and the bridge screwed in and grounded with my new ground wire. I then temporarily set the pickguard and neck on top plus fitted the E string shown to determine the ideal front pickup height. The tele rails pickups feature a strong ceramic magnet so I opted to keep the front pickup low to avoid decreasing string vibration. Once the neck pickup height seemed correct, I re-attached the neck and, screwed down the pickguard.

I played this guitar for about 2 weeks, and while I loved the sound of the bridge pickup, I only liked the neck pickup for my particular playing style. So, then, I ordered my all-time favorite neck pickup: The Duncan 59. Click for link.

I ordered a new aftermarket Tele pickguard cut for a pickguard mounted humbucker and waited for parts to arrive.

Further, I got some Bourns 500K panel potentiometers specified and distributed by Seymour Duncan and their dealers.

Above — (Bourns) Seymour Duncan pots that turn like butter. For me, the most important determinants in the guitar/amp system = the guitar's volume and tone pots.

Above — A rear view of the Duncan 59 neck humbucker. It gives that old PAF sound with some tweaks: slightly scooped mids + peaked highs on an ALNICO ( aluminum - nickel - cobalt ) 5 magnet. Sweetness indeed.  I measured the pickups DC resistance at 7.56K - right on spec.
Your DMM comes in handy for guitar wiring -- and can help you clearly establish correct grounding, plus help you detect short circuits.

Above — The 59 pickup mounted in a new aftermarket pick guard.

Above — Because the factory carved front pickup cavity loomed shallow, I got in there with a spade bit and created room for the Duncan 59 pickup mounting plate and screws.


Above — Testing to ensure that my spade drill bit holes were deep enough to allow the pickup hardware to clear and not bottom out. After a couple of gentle drillings, It fit perfectly.

Above — Time to bolt on the neck and secure the new pickguard.

Above —  I soldered the control panel in reverse fashion. I prefer the switch at the back and pots up front. This gives  easier access to the volume pot since I employ a  lot of volume swells and mostly just play on the front pick up so the switch is much less important. I oriented the switch so it points to the chosen pickup per normal.

I ran (black) shielded RG-174 wire to connect the switch output to the 500K volume pot. The tone capacitor value = 0.018 µF per my personal preference.

Above — You can see the reversed Tele control plate well in the photo. I'll enjoy boosted access to the volume pot.

Treble Bleed Network

As you roll down the volume pot, the sound grows muffled due to low-pass filtration of the pick up(s) output signal. Some builders/players place a "treble bleed" circuit across the volume pot's high impedance lug and the center wiper to restore high frequency at lower gain settings.  If you search this topic you'll find a large number of websites and videos showing simple C, plus parallel, or series R C networks to help solve the loss of treble that occurs as the volume pot's resistance gets increased.

Above — To audibly test various treble bypass circuits on a Tele, 1 way is to solder in a couple of wires with the control plate gently screwed down. Then you can connect your treble bypass network to find your favorite. Here,  I've got a 250K pot and a parallel 1 nF capacitor attached to my wires. It's important to test your "bleeder network" with your normal patch cord, pedal board and amp(s) since the entire signal chain exerts reactance that may skew your results.

Once you find your magic C or R C network, then remove the wires and solder these part(s) to the volume pot and you're done.

Above — My final guitar schematic after many listening tests. For treble bypass, I chose a 270 pF NP0 ceramic bypass capacitor. Some networks ( especially some parallel R C combinations ) can wreck the taper of your volume pot. As a player who swells the volume knob frequently, I prefer a simple bypass capacitor as shown since this modification often skews the volume potentiometer taper the least and makes me happy. 

Above — Guitar strung and tuned.  I'm not into hyperbole: it sounds great and noise free. The pots turn easily and I've got a new toy to enjoy.

QRP —  Postdata         Addition of a Hipshot Bridge on January 13, 2017

In January 2017, I placed an aftermarket bridge from Hipshot

Above — Hipshot's compensated stainless Telecaster replacement bridge overcomes the 2 strings per saddle intonation gitch in the vintage bridge.  Further, it lacks the Leo Fender bridge lip that may boost your ability to finger pick and palm mute. I love not having that metal lip to contend with.

Above — Installation proves easy. Remove your strings, then unscrew the bridge pickup and old bridge. Here, I've placed the rubber pickup spacers over the 2 pickup mounting bolts.

Above — Everything installed, I hooked up a strobe tuner. Now each string pair lies perfectly intonated thanks to the compensated bridge. I saw a compensated saddle bridge on 1 stock, higher end, American-built Fender Telecaster in a local shop this Winter.

I tested the action at a few settings before properly setting the guitar up for my normal flat-wound strings.

Above — After intonating the saddles + setting up the action came the best part --- the playing test. Lovely bridge and intonation. Final guitar photo.

Above — To mellow out my Tele for jazz-fusion and clean jazz-blues, I installed my favorite flat wound strings. In stock, I've got this and the .011 to 50 set; both with a wound G string.


Sunday, 27 November 2016

ADC -- Jupiter Modular Receiver


Click for the Master Index to this project

Radio astronomy requires a receiver output data stream to make sense of what you receive. Computer programs may convert the data into graphs, FFTs, spectrograms, or pool it with other data in hopes of digging the signal out of the noise. At this point, I'm happy with a simple X-Y graph and hopefully my methods will evolve over time.

After much reading + thinking and some hardware failures, I settled on a commercial product to stream data into my computer via a USB cable and then use homebrew + commercial programs to massage my data.

Above — A 4 analog channel, 15-bit ADC from Electronic Energy Control, in Mildford Center, Ohio, USA. The product is called the ADC-4U15. Click for the EECI Website
EECI provides amazing customer service plus innovative solutions for data streaming. 

The on-board USB-to-Serial Bridge Controller = a Prolific PL-2303 HX.  Click for Prolific's web site. The PL-2303 contains an on-chip clock, so I put the board in a RF shielded container fashioned with 2-sided copper clad board. Unlike some ADCs sold today, the PL-2303's drivers were MS Windows certified; they installed and worked on my first try.

Above — I carved out a hole for the USB, Type B connector with a hammer and chisel before soldering it to the rest of the enclosure. My USB cable has ferrite beads on both ends. On the RCA input port, I placed a 100n bypass capacitor.

I'm currently only using Channel 1 of the ADC, so the other 3 inputs are shorted to ground.

Data Streaming and Code  

To run the ADC--4U15,  you start a supplied application that establishes communication to the ADC and an output number between 0 and 32767 ( 15 - bits) appears for each of the 4 data channels.

Above — The ADC-4U15 controller application. Once I plug the ADC to its USB port and start this application, I normally hide it -- and it runs in the background.

Above — setup dialog box for the  ADC-4U15 controller application. Once you set the com port, sample rate and maximum voltage of your data channel(s), it's set and forget. This application provides a data logger output, but you don't have to use it. EECI provides some source code to get you going in both C# and VB. The ADC-4U15 application is a combination user interface and USB driver -- you read the 4 digital channels from a Windows memory label

In Microsoft Visual Studio ( free version) I followed Bert from EECIs instructions and created an application that read the Windows memory map and it compiled without errors!

Then it's up to you to add code to convert the 2 byte pairs of each channel into a 16 bit integer -- and then further, to convert it to some usable unit to graph. Of course, you'll need to write code to write to file -- or graph within your application. I chose the former for now.

I kept my units as DC volts since this allowed me to test and calibrate my receiver system to ensure my code was working. At some point when I get my noise figure system going, I'll plot the Y axis in degrees Kelvin.

Since my maximum input reference voltage = 2.05v, to convert a 15 bit full scale integer into 2.05V full scale float, the code goes:  inputVolts[i] = 2.05f * ((float)inputInt[i]) / 0x8000;

Then, code was added to write the data to a CSV file.

Here's my meat and potatoes source code file:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using System.IO;
using System.IO.MemoryMappedFiles;

namespace WindowsFormsApplication1
    public partial class Form1 : Form
        Byte[] inputByte = new byte[8];     //create byte array
        Int16[] inputInt = new Int16[4];
        float[] inputVolts = new float[4];
        Label[] MyLabelArray = new Label[4];
        StreamWriter csvWriter;
        DateTime startTime;

        public Form1()

        private void Form1_Load(object sender, EventArgs e)

            string csvFilename = string.Format(".\\{0}.csv", DateTime.Now.ToString("yyyy-MM-dd_HH-mm-ss"));
            string csvPath = Path.GetFullPath( Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments),csvFilename));
            csvWriter = new StreamWriter(csvPath, false, Encoding.ASCII);
            startTime = DateTime.UtcNow;

                "UTC Internet format"
                + ", " + "UTC Excel format"
                + ", " + "Local Text format"
                + ", " + "Local Excel format"
                + ", " + "Elapsed seconds"
                + ", " + "Data");

            // update the readings every 200ms
            timer1.Interval = 200;
            timer1.Enabled = true;

        private void Form1_FormClosing(object sender, FormClosingEventArgs e)

        private void timer1_Tick(object sender, EventArgs e)
            string csvLine = ReadInputMemoryMap();
            DateTime currentTime = DateTime.UtcNow;
            TimeSpan elapsedTime = (currentTime - startTime);
            TimeSpan excelTime = currentTime - DateTime.Parse("1900-01-01 00:00:00").ToUniversalTime();

                currentTime.ToString("yyyy'-'MM'-'dd'T'HH':'mm':'ss'.'fff'Z'") // UTC Internet format
                + ", " + (excelTime.TotalDays - TimeZone.CurrentTimeZone.GetUtcOffset(currentTime.ToLocalTime()).TotalDays + 2).ToString("0.000000") // UTC Excel format
                + ", " + currentTime.ToLocalTime().ToString("yyyy'-'MM'-'dd HH':'mm':'ss'.'fff") // Local Text format
                + ", " + (excelTime.TotalDays + 2).ToString("0.000000") // Local Excel format
                + ", " + elapsedTime.TotalSeconds.ToString("0.000") // Elapsed seconds
                + ", " + csvLine); // data value

            for (int i = 0; i < 4; i++)
                MyLabelArray[i].Text = inputVolts[i].ToString("0.0####");

            labelTimeStamp.Text = elapsedTime.ToString(@"d\.hh\:mm\:ss");

        private string ReadInputMemoryMap()
                MemoryMappedFile file = MemoryMappedFile.OpenExisting("EECI_ADC4U15_OUT");
                MemoryMappedViewAccessor reader = file.CreateViewAccessor();
                for (int i = 0; i < 8; i++)
                    inputByte[i] = reader.ReadByte(i);

                for(int i=0; i<4; i++)
                    // convert 2 byte pairs into 16 bit integer
                    inputInt[i] = (Int16)((((UInt16)inputByte[i]) << 8) | ((UInt16)inputByte[i + 4]));

                    // convert 15 bit full scale integer into 2.05V full scale float
                    inputVolts[i] = 2.05f * ((float)inputInt[i]) / 0x8000;
            catch (Exception ex)
                labelChannel1.Text = ex.Message;

            return string.Format("{0:0.0####}", inputVolts[0]);

        private void CreateLabellArray()
            MyLabelArray[0] = labelChannel1;
            MyLabelArray[1] = labelChannel2;
            MyLabelArray[2] = labelChannel3;
            MyLabelArray[3] = labelChannel4;

The application looks like this currently.

Above — Channel 1 is expressed as a DC voltage. All other channels are shorted to ground and read 0. Once the app starts up, elapsed time gets displayed. The only purpose of this app is to read the data from memory, massage the data bytes into something usable, and write that data to a CSV file in a format that meets my requirements. The file name = the current timestamp.csv. This allows me to keep track of the CSV files by a unique namesake.

Above —The 6 column CSV file allows me to graph in UTC or local time. Microsoft Excel seems to suffer some time stamp issues, so the source code was adjusted to provide an accurate output for my location.

Above — a test plot of my Jupiter receiver during the early evening when my local noise is louder than after midnight. I've calibrated my receiver's DC output voltage against known amplitude input signals from -121 to - 47 dBm and wrote some of them on the graph in red.

My graphing process will likely change over time. It's nice to finally have data to work with.

Sunday, 13 November 2016

Log Amplifer -- Jupiter Modular Receiver

SECTION 7  — Log Amplifier

Click for the Master Index to this project

After many experiments and 4 versions, I settled on my simplest design.

The AD8307 splendidly responds to input signals from DC to ~ 500 MHz, so it can work well for a DC or ZIF receiver. However, any pulsing noise on its positive rail, or at the input may get converted to a DC output and wreck your log response and accuracy. Not to mention -- any detected RF will likewise wreck your day.

Above all, I learned it's essential to put it in a metal box and filter the DC rail and input heavily. My analog meter registers a receiver input signal from - 121 dBm ( S1 ) to  - 43 dBm ( S9 + 30 dB )  Note: measures made with a low noise + distortion continuous wave available power connected to the 50 Ω port of the filter / preamplifer with the receiver tuned to generate a 1 KHz AF tone at the receiver AF output.

Above — Final schematic of my Jovian logarithmic amplifier module.

DC Filtration 

I'll first discuss the DC filtration. In past log amps featuring the AD8307, I ran battery power to eliminate any noise on the DC rail. In other versions, I added active ripple filters and for this project compared the results to just using big capacitors to filter the AF ripple. I measured no difference and opted for the 470 + 1000 + 470 µF shown. Further, careful RF filtration delivers reasonable attenuation of RF signals from HF to UHF.

Above — Assessing my wide band RF filtration with a tracking generator + spectrum analyzer. My final network plied a 10 Ω resistor instead of the first 22 Ω resistor as shown above. The reasonable Qul of the1 µH Coilcraft choke plus the 1n caps gives the added attenuation past 40 MHz.  When combining RF capacitors in parallel with low Q Audio Frequency capacitors, no high impedance peaks occur, so it's OK to do this.

Recall that when we combine 2 RF caps in parallel , the inductance of 1 capacitor resonates with the capacitance of the other to form a parallel resonance — leading to a high impedance — that wrecks RF bypass at a specific frequency, although this peak will also exhibit skirts like any band-pass filter. Hence, I've placed either a resistor, or a choke in between the  shunt 1n or 100n RF capacitors.

Input Signal Filtration

My inspiration came from Gary, NA6O ( ex-WB9JPS ) who penned An Accurate S-Meter for Direct Conversion Receivers for QST, February 2008. I recommend his article as a gold-standard reference on this topic. With Gary's permission, I copied his fast attack, slow delay post AD8307 detector circuitry, and the 5 volt supply rail-rail op-amp scheme. Further, Gary nailed the problems ailing the AD8307 at AF with particular insight.

I ended up making 4 circuits and settled on the 1 presented above. This, an experimenters circuit, and likely requires your own nuance. Based on my AD8307 projects, much discussion and emails over the years, accurate log amps may prove thorny and difficult for some builders.

Gary found common-mode noise afflicted his design when receiving small amplitude signals.
He opted for a front-end differential amplifier to reduce this noise while providing adjustable gain to optimize the  AD8307 input amplitude to any direct conversion receiver with known gain to get the best possible dynamic range. I found his conclusion about needing variable gain true plus invaluable. Gain adjustment can be done in the receiver, or the log module; or both.

Further, employing a low noise, rail-to-rail op-amp proved vital.

Let's quickly review common mode noise in differential op-amps since sometimes when I discuss this, the other builder's eyes glaze over.  Differential amps done right reject common mode noise. In differential op-amps, the inverting [-] and non-inverting inputs [+] are driven 180 degrees out of phase. Since unwanted noise or hum appears in-phase on each input ( common mode ), this noise/hum gets rejected and does not appear at the op-amp output. Of course, you need to 1% match the input circuit capacitors and resistors to yield exemplary common mode suppression in an op-amp filter.

In my lab, with a differential amp/filter I obtained good results, but still, my AD8307 lacked sensitivity to measure signals when I applied < -110 dBm to the front end of my single-signal receiver.  For my particular receiver, I found that brute-force R-C filters worked better.

Simple R-C filters get a bad rap; likely because for a first order filter the roll off is only 6 dB per octave and we get so much better with our popular active filter designs.

Let's trace the input of my circuit.

A 680 pF cap connected to the front panel jack filters VHF signals -- especially around the FM broadcast band. From there a 68 Ω resistor plus a 3.3 µF provides a 3 dB down low-pass response @ ~ 709 KHz.  A secondary filter gets formed with the 68 Ω resistor plus shunt 0.1 µF capacitor to filter any HF or LF signals getting into the input.

The 0.33 µF series cap provides some hum immunity into the op-amp unity-gain buffer. An identical 68 Ω R ( labelled RF in the schematic ) plus a 3.3 µF polyester low-pass filters the buffer's output. If your receiver output looks pretty noisy, you may increase RF to as high as 200 Ω to drop that filter's 3 dB cutoff point. Brute force indeed. Set the buffer input and output R-C filter values to what you require.

The two 709 KHz filters attenuate receiver AF wide band noise above 1 KHz that reduced the sensitivity of my log module to measure very low-level signals. With appropriate adjustment of  my receiver output level pot and the meter's zero and calibration knobs, my analog meter reads 0 or nearly zero when the  antenna ( or signal generator ) gets removed and replaced with a 50 Ω resistor terminator. Essentially, the log module nulls the receiver + AD8307 noise with no antenna input signal.

With the analog meter calibrated with a signal generator, and the zero and calibration pots adjusted correctly, I can measure down to -121 dBm or S1. With a voltmeter, I measured signals down around -130 dBm before getting clobbered by the receiver noise floor. Since my
receiver monitors low-level Gaussian noise for radio astronomy, I wanted the meter sensitivity focused towards the low end.

Calibrating my analog S - meter proved easy. I set a calibrated signal generator to 20.190 MHz and tuned the receiver for a 1 KHz AF output signal.  I injected - 73 dBm or S9 and adjusted the meter cal pot to go about 60% of my particular panel meter. Then I decreased power by 6 dBm to find S8 and so on.  When satisfied, I had the range correct. I marked my meter with a black marker for S1- S9 -- and then S9 + 10 dB ( -63 dBm) , S9 + 20 dB and then S9 plus 30 dB. I tested the meter by inserting in-line 6 and/or 10 dB attenuators and so forth. It check out and my meter marks account for any non linearity in the AD8307 circuit.

This gives you a calibrated measurement receiver. I love the S-meter -- and Gary's fast-attack, slow delay detector further boosts the experience. With it, no AD8307 output DC low-pass circuitry is required ( see the datasheet ).

The other output goes to a panel jack for a connection to the an ADC for data acquisition. The 2K7 shunted pot will help calibrate the noise power output on a graph.

Above — First "real world" test with analog meter before I painted its face white and then after, marked the S units with a felt marker in a calibration set up. When I close the -6 dB switch on my preamplifier - filter module, the meter drops by 1 S-unit.  I'm happy!

I've attached 2 of my discarded circuits for experimental fodder.

Click for Module 8, the ADC
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