Low-distortion Audio-range Oscillator

[davada]

Originally Posted by coluke

Could't agree more
The original 239A employed an HA-2625, which is unfortunately classified either as obsolete or retired by Intersil - it was a bipolar uncompensated opamp featuring high speed (100 MHz GBW, > 60 dB OLG @ 100 kHz, SR 35 V/usec), high input impedance (300 Mohm), low noise current density (0.16 pA/sqrt(Hz)): these are not-so-common specs even in modern opamps, unless you are going to choose expensive beasts like the OPA637 - all in all, OPA2134 is a good starting point...

L.

I have 10 of these in the plastic DIP package if anyone is interested.

[coluke]

Originally Posted by davada

I have 10 of these in the plastic DIP package if anyone is interested.

HA-2625? Lucky man! I would be interested in a couple of them

This is the 239A clone working at 20 Vpp out into 10 kohm - measured THD is almost only the THD of the EMU 0204 itself. A plot of the FET operating Vds (~88 mVpp) is attached too.

Ciao,

L.

[richiem]

My 339A/239A clone uses an LT1468 for the main amp -- works quite well, considering it has a 10:1 cap ratio instead of HP's 100:1 ratio. I call this version the IG-339B -- it's built into a Heath IG-18 chassis and uses the Heath tuning resistor switches. The Heath mylar caps however have been changed to polypropylenes.

The fundamental is 1kHz at 6.3VRMS, level-adjusted by a 10k pot to 1VRMS and fed into my Active Twin-T filter to reduce the fundamental by 60dB without attenuating harmonics at all. The ADC is an E-MU 0204.

[dirkwright]

Well, I'm certainly willing to get a PCB together and order it. I use Pad2Pad. Surface mount or through hole devices?

[dirkwright]

Originally Posted by dirkwright

Well, I'm certainly willing to get a PCB together and order it. I use Pad2Pad. Surface mount or through hole devices?

Reed relays ok for switching? 5V? I'm having some trouble figuring out the switching arrangement for the frequency setting resistors and capacitors. Anyone have a plan?

[davada]

Originally Posted by dirkwright

Reed relays ok for switching? 5V? I'm having some trouble figuring out the switching arrangement for the frequency setting resistors and capacitors. Anyone have a plan?

Everything you need is on Dick's web page.

http://www.moorepage.net/RC.html

[davada]

Originally Posted by coluke

HA-2625? Lucky man! I would be interested in a couple of them

This is the 239A clone working at 20 Vpp out into 10 kohm - measured THD is almost only the THD of the EMU 0204 itself. A plot of the FET operating Vds (~88 mVpp) is attached too.

Ciao,

L.

Just PM with an address.

[richiem]

IG-18 #4, the IG-339B

[Samuel Groner]

Any experts on SG505's here on this truly exiting thread?

Both my colleage and I recently purchased one of these and we already tried a few things to improve the residual harmonics but to no avail. I don't exactly find the SG505 easy to understand in all details (AGC section, notably) but I don't claim to be an expert. Both units are still fully within spec (as verified on the AP2722), so it's really about tuning the "last bit out of them".

So, anybody have a hint to known SG505 tricks (except for getting the hum down which is managable)? Not that I would expect the Tek guys didn't try tweak the hell out of it at their time, but after so many years there might be a chance that someone found ways to improve it with moderate effort.

Bruce Hofer did (as usual) an excellent job here. There are a lot of subtleties going on (and I'm sur I'm missing one or the other), and it is unlikely that we'll find an easy fix which will significantly improve performance. Just to highlight a few details:

* It looks like residual ripple of the peak level detector is cancelled through C1502 and C1603;
* The NE5534s implement various tricks--feed forward compensation (C1514, C1622), two-pole compensation (C1405, C1406, R1405), forced class A operation (R1621) and even a simple composite opamp (Q1620, see US-patent 4296381);
* The for higher frequency ranges unused integrator caps in the second integrator (C1312, C1320, C1322) are precharged to a specific voltage through R1421/R1422. I presume this speeds up settling after a range switch, because the capacitors are already in the correct phase relation to those in the first integrator (which a precharged to 0 V);
* The power supply of the peak detector is heavily low-pass filtered (R1611, C1600). This localises the current loop of the distorted currents, and avoids coupling to the main signal path.

One thing you could try is to make the drain feedback of Q1501 adjustable. Unsolder the ends of R1510/R1513 which connect to the gate of Q1501; take a (quality multi-turn) 1k trim pot; connect its wiper to the gate of Q1501 and the resistance element ends to R1510 and R1513 respectively. By adjusting the trimmer you might be able to reduce the 2nd harmonic for certain frequencies. But make sure it doesn't get worse for others!

Something else might be the addition of a 68r resistor between pin 5 and 6 of U1401 (as done for U1520). This forces the output stage of the NE5534 into class A operation, with possibly some distortion improvement.

[dirkwright]

Originally Posted by davada

Everything you need is on Dick's web page.

About RC oscillators

OK, then I just do this for me and this isn't going to become something for everyone. We're back to punting ideas around, like always.

His page is great for theory, thanks.