Pro Audio Design Forum

"Programmable Integrator Has 6-Decade Range," Mike Chang, Amber Electro Design, EDN, January 22, 1987.

This circuit interests me for a number of reasons:

Amber was an audio test equipment manufacturer.
It's part of an oscillator and/or voltage-controlled filter.
It has applications in electronic music.
It used a Valley People TA-101 as the control element.

In the pdf Chang shows two sections being used as a voltage-controlled state variable filter.


"Programmable Integrator Has 6-Decade Range," Mike Chang, Amber Electro Design, EDN, January 22, 1987, Figure 1.

"Programmable Integrator Has 6-Decade Range," Mike Chang, Amber Electro Design, EDN, January 22, 1987.
Full pdf: https://proaudiodesignforum.com/images/ ... 2_1987.pdf

For more information on Valley People's TransAmp, TA-series and ECG-101 VCA see: https://www.proaudiodesignforum.com/for ... 114&p=2914

That looks a lot like a VCA...

JR

Yes it does.

The TA-101 apparently was the "VCA version" of the TA-series array having quad complimentary pairs.
The only data I have is on the TA-103 (8 PNP) but it also shows the TA-101 and TA-102 (8 NPN) versions in a pinout drawing.

It looks like the design could be modified to use a THAT2180-series VCA or two THAT340 dual complimentary pairs.

The last time I did a vaguely similar design was for the voltage controlled sine wave generator in the TS-1, but Back then VCAs were expensive and complex so I used an OTA (LM13600). I had difficulty getting reliable operation over 3+ decades. But my way was pretty inexpensive.

JR

I thought about the LM13600-based oscillator in the TS-1 when I stumbled on the Amber schematic.
If they were using the TA-101 as a state variable filter in the THD section as well as in the oscillator, the Amber box must have used at least four of the TA-101s.

John recently sent me better scans of the Loftec TS-1 and I've updated the schematic here:
viewtopic.php?f=12&t=404&p=4531

Thanks for sending in those TS-1 scans!

Not to veer off into TS-1 trivia but yes I could have made the TS-1 OSC better behaved with VCAs and a SVF but the distortion in the TS-1 is dominated by the need for fast settling time. Speaking of VCA's If I added a VCA in front of my TS-1 dB level meter I could have doubled the gain-bandwidth of the level meter. Stock it is 20kHz bandwidth at -50 dBu dropping to 10kHz at -60dBu etc. With a good VCA in front of my rectifier I could pretty much extend the 20kHz bandwidth down to the noise floor of the VCA (-90s dBu).

Of course now I would use different technology but you probably know that.

JR

John you could do an entire thread of all the circuit tricks you built into the TS-1.

When I worked for SSL I visited a lot of studios and virtually every maintenance shop had one.
It was ubiquitous. Like Yamaha NS-10s.
Everyone had one.

Some people modded them to make the frequency control a 10 turn which I'm sure would have driven up the manufacturing cost considerably.
The TS-1 does so much with so few parts.

mediatechnology wrote:John you could do an entire thread of all the circuit tricks you built into the TS-1.

When I worked for SSL I visited a lot of studios and virtually every maintenance shop had one.
It was ubiquitous. Like Yamaha NS-10s.
Everyone had one.

Some people modded them to make the frequency control a 10 turn which I'm sure would have driven up the manufacturing cost considerably.
The TS-1 does so much with so few parts.

I probably could have reduced the desire for 10-turn pots if rounded off the frequency counter to 10 Hz, but since people could read 1 Hz resolution they wanted to adjust the frequency pot that fine. A more practical solution is rough and fine frequency pots, I think a later version had that.

Glad that people liked it, I didn't make money from all those sales. In fact when I quit the company, I got paid off for my 25% ownership stake with TS-1s.. So I sold them as a dealer for several years, so I did make some profit from them as a reseller.

JR

mediatechnology wrote:It used a Valley People TA-101 as the control element.

TA-101 looks identical to the Blue Allison VCA we used at Calrec in da early 80's. I'm told Allison were either bought over or changed their name. Are they Valley People?

I did the investigation that led to Calrec's choice of Blue Allison over Black DBX for our VCAs.
______________________

In the pdf Chang shows two sections being used as a voltage-controlled state variable filter.

Baxandall did a 3 decade oscillator using semiconductor junctions as the variable freq elelment. They were used at KEF & Calrec for production testing for decades.

http://www.aes.org/e-lib/browse.cfm?elib=3941
A Low-Distortion Acoustic-Measurement Oscillator Using Semiconductor Junctions as Variable-Tuning Elements
John Vanderkooy had another look at it this Millenium http://www.aes.org/e-lib/browse.cfm?elib=14135
A Low-Distortion Fast-Settling Audio Oscillator: A Tribute to the late Peter J. Baxandall, Audio Analog Expert

There was also a State Variable, High Q tracking filter to go with this test gear.

I was the poor idiot who had to put this into small scale production. Though I have the highest regard for Great Guru B, his thinking was often from WW2 when he worked with Alan Blumlein on early radar. He often expected 'production' to have loadsa expensive virgins to tweak pots etc. (rather like JR's experience with a legacy 4 channel cassette deck )

We juuu..ust got a good 3 decade frequency range to obey a nice log law with selected transistors glued to a Aluminium bar. This was most obvious when trying to get the 'tracking filter' to track the oscillator.

I'd be interested today in how Chang's stuff follows an accurate log freq law but I'm implementing the 'same' thing with DSP to measure speakers & mikes in my Cooktown shed.

ricardo wrote:

mediatechnology wrote:It used a Valley People TA-101 as the control element.

TA-101 looks identical to the Blue Allison VCA we used at Calrec in da early 80's. I'm told Allison were either bought over or changed their name. Are they Valley People?

Yup same people... I think Paul's first wife when he was living out in CA was named Allison. Valley people, Valley Audio, all variations on the same company/designs after he moved to Nashville. Paul escaped audio business a few decades ago and started making photography strobe lights.

JR

I did the investigation that led to Calrec's choice of Blue Allison over Black DBX for our VCAs.
______________________

In the pdf Chang shows two sections being used as a voltage-controlled state variable filter.

Baxandall did a 3 decade oscillator using semiconductor junctions as the variable freq elelment. They were used at KEF & Calrec for production testing for decades.

http://www.aes.org/e-lib/browse.cfm?elib=3941
A Low-Distortion Acoustic-Measurement Oscillator Using Semiconductor Junctions as Variable-Tuning Elements
John Vanderkooy had another look at it this Millenium http://www.aes.org/e-lib/browse.cfm?elib=14135
A Low-Distortion Fast-Settling Audio Oscillator: A Tribute to the late Peter J. Baxandall, Audio Analog Expert

There was also a State Variable, High Q tracking filter to go with this test gear.

I was the poor idiot who had to put this into small scale production. Though I have the highest regard for Great Guru B, his thinking was often from WW2 when he worked with Alan Blumlein on early radar. He often expected 'production' to have loadsa expensive virgins to tweak pots etc. (rather like JR's experience with a legacy 4 channel cassette deck )

We juuu..ust got a good 3 decade frequency range to obey a nice log law with selected transistors glued to a Aluminium bar. This was most obvious when trying to get the 'tracking filter' to track the oscillator.

I'd be interested today in how Chang's stuff follows an accurate log freq law but I'm implementing the 'same' thing with DSP to measure speakers & mikes in my Cooktown shed.