Only capacitance and noise can account for distortion that's frequency dependent. Actually measuring 1N4148's shows the leakage at 15V to only be about 6nA:
1N4148 Reverse Bias Leakage Current – 2N3904Blog
The capacitance has been measured too:
1N4148 Diode Reverse Biased Junction Capacitance – 2N3904Blog
A variation of 0.4pF between 2V and 12V bias seems very small indeed to me. Is it enough to explain the distortion?
Could the actual phenomenon be shot noise or light pickup in the glass encapsulated 1N4148's?
1N4148 Reverse Bias Leakage Current – 2N3904Blog
The capacitance has been measured too:
1N4148 Diode Reverse Biased Junction Capacitance – 2N3904Blog
A variation of 0.4pF between 2V and 12V bias seems very small indeed to me. Is it enough to explain the distortion?
Could the actual phenomenon be shot noise or light pickup in the glass encapsulated 1N4148's?
Those measurements were made with DC bias voltages so they may not be relevant for discussing frequency-dependent behaviour.
Looking at the datasheets of 1N4148 and FDH300 the only clear difference is the leakage current. Note also that in the FDH300 datasheet the max leakage current of 1nA is given at 125Vr so it is probably well below 1nA at 15Vr.
FDH300 is also glass encapsulated but that really should not matter since the PCB is enclosed in a box.
I've ordered the board, and have been able to locate most of the parts. But I'm totally stuck on the rotary selector switch (71BDF30-01-2-AJN). Nobody has it in stock, anywhere.
The solder-lug version (71BD30-01-2-AJN) is readily available. At this point I'm considering getting that and putting it in a project box with a short umbilical, until/if the PCB mount version starts to reappear.
Any other suggestions?
(BTW, if anyone needs pointers on how to find the display on eBay, PM me. It's not immediately obvious. On the upside, they're cheaper than Pete's spreadsheet says.)
The solder-lug version (71BD30-01-2-AJN) is readily available. At this point I'm considering getting that and putting it in a project box with a short umbilical, until/if the PCB mount version starts to reappear.
Any other suggestions?
(BTW, if anyone needs pointers on how to find the display on eBay, PM me. It's not immediately obvious. On the upside, they're cheaper than Pete's spreadsheet says.)
This is an Arrow company, and so is trustworthy.
AD536AJQ Analog Devices | Analog ICs / Power Management (PMIC) | RMS/DC Converters - chip1stop.com
AD536AJQ Analog Devices | Analog ICs / Power Management (PMIC) | RMS/DC Converters - chip1stop.com
It's just a subminiature 2P6W rotary switch... I would panel mount any other one and use small flywires to the board for the moment whilst waiting for the 4-month lead time that most folk seem to be quoting.
It's a good quality PCB so will easily take removing the flywires with a bit of wick and inseting the correct switch when you get it (if ever!).
They want $40 in shipping. So like $65 total. The part is now discontinued, so it'll never come back in stock. There appear to be some versions in a tophat package available, so that's an option. I'm also considering just going without it - I have a whole pile of voltmeters I can use. Can I assume everything else will still work without the chip?
Got everything all built, and tested it out late last night - looks like it works! I'm just using the crappy onboard sound card in an HP ultra-slim desktop, and even so the noise floor is down around -105dB, and using an external function generator I'm only seeing its own distortion (third harmonic around -95dB).
Had a question while I was poking around in it, though. I noticed that the number on the voltmeter cuts in half just by attaching the common lead of my Fluke 115 (or my scope probe) to circuit ground - not even connecting the other lead. And if I use my scope (with X10 probe), the signal is incredibly noisy anywhere I try to pick it up. I've tried it with all combinations of front panel switches and attenuation switch settings. I've also tried floating my scope off of earth ground, which didn't help.
It's just so weird to me, because hooked up to the sound card it's super clean. It works fine, so I'm not going to drive myself crazy trying to figure it out, but I'm definitely curious why this might be.
Had a question while I was poking around in it, though. I noticed that the number on the voltmeter cuts in half just by attaching the common lead of my Fluke 115 (or my scope probe) to circuit ground - not even connecting the other lead. And if I use my scope (with X10 probe), the signal is incredibly noisy anywhere I try to pick it up. I've tried it with all combinations of front panel switches and attenuation switch settings. I've also tried floating my scope off of earth ground, which didn't help.
It's just so weird to me, because hooked up to the sound card it's super clean. It works fine, so I'm not going to drive myself crazy trying to figure it out, but I'm definitely curious why this might be.
I am working on sourcing all of the parts for the interface. I've ordered everything except the Analog Devices chip. I found one at Mouser for $57 plus shipping - is that the best price around? I tried to link above to the Arrow company - and it's not available there.
For the selector switch I ordered the solder lug version of the Grayhill switch that was mentioned above. I plan to wire it to the board with individual flywires. The display was found on eBay - as mentioned in this thread. All in all, a little more than I planned to spend at first - but I figure this is a great solution to migrate to a modern, PC based testing setup.
For the selector switch I ordered the solder lug version of the Grayhill switch that was mentioned above. I plan to wire it to the board with individual flywires. The display was found on eBay - as mentioned in this thread. All in all, a little more than I planned to spend at first - but I figure this is a great solution to migrate to a modern, PC based testing setup.
https://www.ebay.co.uk/itm/324355742034 probably doesn't help you (Sounds as if you are stateside, but may help someone in UK. He seems genuine
I got mine from digi-key in 2020 for $25...
I got mine from digi-key in 2020 for $25...
I liked the look of this project, so I've made my own mini/lite version of the circuit, without the RMS detector, and using a jumper headers in place of the rotary switch. Just before sending the PCB design to be made, I quickly put together a daughter board for selecting the range using reed relays and a display.
Mostly populated
In use, with daughter board
I had an ancient external USB sound card (only 16-bit/48ks.s-1) but it was still better than the on-board sound in my desktop PC. Results with a near-fullscale sine compared to the unconnected soundcard are:
Not brilliant, but as a first use it has enabled me to start looking at the response of an active crossover:
So thank you for the original design!
Mostly populated
In use, with daughter board
I had an ancient external USB sound card (only 16-bit/48ks.s-1) but it was still better than the on-board sound in my desktop PC. Results with a near-fullscale sine compared to the unconnected soundcard are:
Not brilliant, but as a first use it has enabled me to start looking at the response of an active crossover:
So thank you for the original design!
Time for a re-think with the AD8436 True RMS Converter replacing the AD536. (FWIW, I have used the LTC1968 and one development board is housed within an HP403 for noise measurement.) AD8436ARQZ is a $12 chip with much higher accuracy, built in JFET buffer and greater dynamic range compared to the AD536 family and LTC1966, 67, 68.
Folks still building the original design?
Here are my finds about the components hard to come by…
Digital Panel: MDMV0000 can be replaced by Mouser: 580-20LCD-0-5B-C
Bit smaller in size almost same functionality mit DP select. Fits original case!
Rotary switch Gray…, can be replaced by Mouser: 633-MRA206A
Terminals different but same function DTDP 2-6 (2x6 pos) with adjustable stopper pin. Connect with short wires to PCB. Comes with a knob.
Case BUD Mouser: 563-PC-11403
Original BUD case now at Mouser.
Polyfuse Mouser: 530-0ZRG0110FF1E
1.1A, 30V. 16V type available at Aliexpress.
AD536AJQ: several sellers at Aliexpress. Probable fakes among them! I usually pay with Paypal. If I get a fake I report to Ali and Paypal and get my money back! I will work on an adapter to fit a AD8436ARQZ in the original 14pin socket, so we can use the original PCB.
Here are my finds about the components hard to come by…
Digital Panel: MDMV0000 can be replaced by Mouser: 580-20LCD-0-5B-C
Bit smaller in size almost same functionality mit DP select. Fits original case!
Rotary switch Gray…, can be replaced by Mouser: 633-MRA206A
Terminals different but same function DTDP 2-6 (2x6 pos) with adjustable stopper pin. Connect with short wires to PCB. Comes with a knob.
Case BUD Mouser: 563-PC-11403
Original BUD case now at Mouser.
Polyfuse Mouser: 530-0ZRG0110FF1E
1.1A, 30V. 16V type available at Aliexpress.
AD536AJQ: several sellers at Aliexpress. Probable fakes among them! I usually pay with Paypal. If I get a fake I report to Ali and Paypal and get my money back! I will work on an adapter to fit a AD8436ARQZ in the original 14pin socket, so we can use the original PCB.
Last edited:
Last edited:
