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dirkwright 16th October 2012 06:18 PM

SV Oscillator PCB group buy
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Who would like an ultra-low distortion audio oscillator PCB? - Page 1

He's trying to get enough interest in a group buy for the generator (oscillator) PCB. It is a multi-range oscillator. Frequencies are selectable in 4 ranges in 1-2-5 sequence (10-20-50Hz, 100-200-500Hz, etc) at what appears to be less than 1ppm THD. The design is based on Bob Cordell's THD analyzer he published in 1981.

"This is a complete, self contained T.H.D. analyser for measuring the distortion of audio circuits to levels below 0.001%. The design goal is have an analyzer with a residual noise and distortion "measurement floor" for the most part at or below 0.0001% (-120dB) Internally, in the signal source section, the unit consists of an ultra low distortion audio oscillator and a variable gain differential-output line driver with a selectable output impedance of either 50 ohms, 150 ohms or 600 ohms . The measurement section consists of a differential-input signal attenuator, preamplifier and buffer, an auto-tuning notch filter, a distortion residual amplifier with automatic level setting, measurement bandwidth filters and a true R.M.S. detector and analogue meter movement for measuring/indicating the amplitude of the distortion residual as a percentage of the level of the fundamental signal applied to the input. The measurement bandwidth is ten times the fundamental frequency
This analyser project is in fact a "baby step" to a much more complicated analyser project I have been dabbling with for some time, which is essentially a homebrew work-alike to the Audio Precision System One, with my own operating software for the computer control. That project just kept getting stalled for one, simply because I lacked the measurement capability with which to adequately and straight forwardly evaluate the distortion performance of the various modules and circuits under development of which the low-distortion audio frequency sections of that project are to be comprised. The development of a suitable alternative to Audio Precision's patented hybrid "MDAC" low-distortion multiplying digital to analogue converter, which is used as the resistive element (for high resolution, digitally-controlled frequency tuning) in the System One's state-variable filters is a case in point.

While the project described here will eventually be made relatively superfluous to my needs in the distant future if my above mentioned plans for the much more complicated and capable unit pan out, I still didn't want to invest a great deal of time an energy into hobbling together something of limited capability and usefulness. For the instrument detailed here I've have taken design cues from various sources including the (very comprehensive) A.P. System One technical manual and the T.H.D. analyser design by Cordell as published in Audio magazine, starting July 1981. Like the latter, the design presented here operates at fixed, switch-selectable spot frequencies covering the audio frequency band (and beyond), rather than a continuously variable operating frequency, however with some necessary rationalizations.

The design by Cordell, while relatively simple and straight forward electrically, was complicated a great deal, implementation wise, principally by the rotary-switch switching, with multiple gangs and wafers, of the passive components responsible for setting the loop gain and operating frequency of the state variable filters as used in the oscillator and notch filter sections. This isn't a constructional and parasitic-mitigating layout design issue I felt a desire to contend with, and especially so when a design goal was to make use of, in this design, of the superior distortion and noise performances provided by current generation op-amps from National's "LME" range, which have over five times the gain bandwidth product of the old and venerable NE-5534.

All of the switching in and out of passive components in this design is achieved intermediately via small-signal relays in the signal handling circuit paths that are distortion sensitive, and via analogue multiplexers or JFET switches in almost all of the rest. These small signal relays and analogue switches, along with the decoding circuitry required to drive them, add considerably to the electrical complexity of the design, as well as to the P.C.B. real estate required and overall cost. It is for this reason the frequency coverage of the instrument, from 10 Hz to 50 kHz, is covered by only 12 spot frequencies, in four decade ranges, in 1, 2, 5 steps. The Cordell design, in comparison, provided 11 spot frequencies in each decade range, but the signal relay complement required to switch so many steps just becomes prohibitive. However in light of the improved overall performance of this contemporary design, the limitation of only 12 spot frequencies isn't really much of practical hindrance to the instruments usefulness."

I have no financial interest in this group buy. I'm just trying to find more interested parties. The board is at the back left in the photo.

markus22ch 19th October 2012 06:55 AM

I'm in for one or two boards!

greetings, Markus

Nikitas 19th October 2012 10:48 AM

I'm in!

dirkwright 19th October 2012 03:50 PM

That's great. Thank you. I think they are going to be about $17 each. He is in OZ.

Ryssen 19th October 2012 04:19 PM

Im in for 1 board.
Is there a BOM for it?

dirkwright 19th October 2012 04:26 PM

He says he's going to make a complete BOM this weekend. He's a busy guy.

What I like about this board is that there aren't any controls or connectors soldered directly to it. So, we can use whatever connectors and controls we want to, and arrange them as we please. The board is about 4" by 9". He has more information on his website:

He's basically taken Bob Cordell's THD analyzer, with inspiration from an Audio Precision System One manual, and came up with something pretty good.

dirkwright 21st October 2012 11:28 PM

OK, here's the latest from Glen:

"OK, things seem to be gaining traction......... I will go ahead and get the boards (20) made. I'll sell them for $25 USD each plus postage via standard air mail. That way my 6.3L car engine won't get me into the red after multiple trips to the post office
I'll order them on the 12 day turn around option. I think the best way to handle payment is via Pay Pal. There is no rush as I won't have the boards for likely 2 weeks, but it would be convenient if those who want one could start e-mailing me their requests and delivery addresses. I'll send out money requests via Pay Pal once I have the boards. The BOM will have to wait until tomorrow. My e-mail is glenk (at) internode (dot) on (dot) net "

So, please email him if you want one.

dirkwright 22nd October 2012 01:57 PM

The BOM is available on the other thread:
Who would like an ultra-low distortion audio oscillator PCB? - Page 2

I can't attach an Excel file on this message board so I can't provide the BOM here.

He's ordered the boards and all you have to do is contact him via email to reserve one.

dirkwright 23rd October 2012 02:13 PM

Out of the initial order for 20 boards, 10 are gone already.

tagheuer 23rd October 2012 02:37 PM

Can I have one board reserved please ?


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