Re-greening the Heathkit IG-18

[PB2]

Originally Posted by JimT

It is quite difficult to get a USB sound device to sample at the higher rates due to limits to USB and computer latency. That said I did get an EMU 0202, but I have to turn off all other software and my wireless ethernet to get low distortion readings at high sampling rates, eg 96 kHz or above. There is some information on the web about doing this. Sorry, but I have lost the links.

Whenever USB 3 comes out things will probably be fine.

Jim

Thanks Jim, I had a feeling that USB might be an issue but I don't mind disabling things to have a portable system. I'll look for some tips on the web if I ever get a better card.

[Enzo]

Thanks to Bill and PB for that response in posts 48,49.

[PB2]

I'll probably try a current source load for Q3 next. Was also thinking about the beta multiplier on the VAS and wanted to point out that Reg's load resistor of 220K is way too high, it needs to provide the turn off current for Q3 and 1 to 10K is more reasonable. A good example of a high performance OP amp is the Jensen 990 where 2K is used in that position:
http://www.johnhardyco.com/pdf/990-2007.pdf

Simulation of the JE-990 OP Amp By Deane Jensen

The VAS is biased at about 6 mA, and the outputs even higher. It is designed to easily drive 600 ohm loads which lines up well with this design since it has to drive potentially a 600 ohm load in parallel with the low Z feedback network.

[PB2]

Originally Posted by richiem

As to the feedback pot, the lamp runs at between 800 and 1200 ohms in normal operation, and when the output is fully across the lamp, the current is limited by the lamp, so I dont' think there's a possibility of damaging the pot. After that, I don't think it matters which way you wire it up... I don't see a performance difference being likely.

Yes, I measured the lamp here, with 10V RMS out, at about 900 ohms by putting a 100 ohm resistor in series with it and measuring the current then voltage across the bulb.
The pot measured OK, but I tried swapping pins 1 and 2 and the adjustment seems to work better. Perhaps it is just the wiper making and breaking contact.
I agree, I do not expect much if any difference in performance. There was the possibility of output loading on the amp, since with the pins swapped the lamp is always in series with the full 750 ohms of the pot for minimal loading.

[PB2]

I should probably get an HP 339; will have to look for a bargain.

[davada]

Originally Posted by PB2

I should probably get an HP 339; will have to look for a bargain.

Hi Pete,

If you're going to get a 339A get one with option 001. These ones are special.

David.

[PB2]

Thanks David, I'll keep that in mind.

[PB2]

I'm starting to see HF oscillations and was wondering if anything in the grounding helps to stop this.

What I have found partly by accident, is that the oscillations seem to be very sensitive to the switch settings and wiring. I tested this theory by adding a series RC of 1K and 10 pF from H to K on the PC board to give the negative feedback path a low inductance local shunt path. This seemed to help a lot but it is still not completely gone. I missed, at the time that I tried this, the fact that Cx shunts this path and it would be easy to put say 30 pF local to the PC board and make the X1000 500 pF cap 470 pF instead. The 30 pF introduces a very small error with the other larger caps.

The Cy return to point J on the PC board is essentially going to AC ground and since there is no DC component it could go directly to actual ground. Has anyone tried this or have any thoughts concerning the change? If point J is strongly decoupled to ground such as with a good quality 100 uF then it is not an issue but with the .68 or even 6.8 uF it just seems wrong - directly to ground would be better.

[PB2]

I forgot to mention that, of course, the theory is that there is some inductance in the wiring from the switches to the PC board and while it is probably not significant even at 100 kHz, the amp is flat out to 1-3 MHz in this unity gain configuration where it does make a difference. The cap range switch is wired to the Tens switch and it would be better to star wire all the switches to the PC board to minimize the inductance for each run. The caps have the greatest potential for problems since they can form another resonant circuit with the lead inductance so that switch should certainly go straight to the board.
There are two ways to get 20 kHz:
The Tens set to 20, Units 0, Decimal 0, range X1000
Tens set to 10, Units 10, Decimal 0, range X1000
These use the same caps and the swtich settings provide the same resistance, with different resistors and wiring, and the distortion is different between the two settings .0045% second HD in one case and .0065% with the Tens set to 20.
I expect that this is due to wiring inductance and might be cleaned up with star wiring for the switches - I'll give it a try. Push button switches with short traces to the amp would be best as was done in the HP.

I'm getting a low of .0024% THD at 9 kHz but it is quite touchy at this point.
I'm thinking that less than .01% THD from 20 to 20 kHz is a reasonable goal without changing transistors or any drastic mods, but I'll try for less than .005%.

[JimT]

Thanks for the updates. I would be happy with 0.005 for most of my testing and 0.002 would be great. Like a typical Canadian, everything else is on hold while I enjoy the summer weather, mainly hiking, swimming, fishing and the occasional BBQ and beer, but will return to the soldering iron during the fall. Getting back to this will be one of my first projects, so any progress you make will be welcome.

Thanks
Jim