How Distortion Free are the Distortion Measurers?

[1audio]

I would check to see if the 2n3906 would work or a 2n4250. Neither should cost more than $.25. If you were close by I'd give you some.

Keep in mind the device is looking at random noise. Distortion really doesn't matter much.

[klewis]

Originally Posted by 1audio

I would check to see if the 2n3906 would work or a 2n4250. Neither should cost more than $.25. If you were close by I'd give you some.

Keep in mind the device is looking at random noise. Distortion really doesn't matter much.

Thanks,
The 2n3906 look like a great replacement. Any thoughts on replacement for the 2n1304/2n1305?

Ken

[richiem]

Hi Ken -- I'd use a PN2484 for Q1109 and 2N3904/3906 for the other two. Might need a little more base spreading R for the Si devices.

[klewis]

Originally Posted by richiem

Hi Ken -- I'd use a PN2484 for Q1109 and 2N3904/3906 for the other two. Might need a little more base spreading R for the Si devices.

Hi Dick,
Fairchild says that the 2n5089 is the same as the PN2484. What do you think? Regarding base spreading R, I presume that you are refering to R1122 47 ohm. How much more?

Ken

[richiem]

Yeah, the 5089 is OK. Its complement is the 5087. Lots of Beta, low noise. The base spreading R just reduces or eliminates crossover distortion in the output. I would guess, not knowing the load that's being driven, that maybe having 3 or 4 mA of standing current in the output stage would be OK. Using a 100 ohm pot should give plenty of range. You could just tweak for lowest THD....

[klewis]

Went to the surplus store today and bought a small collection of 1/4 watt Alen Bradley 10% resistor, still in the orignal packaging. Measured them when I got home, some were at the limit or over spec. All were greater than the posted value. Do carbon resistors change with age?

[1audio]

That's one of many reasons carbon composition resistors are history. They also change with humidity, temperature, voltage and probably the phase of the moon. They also have higher current noise and distortion. The one virtue is momentary power handling.

[zigzagflux]

Bake them in the oven and most will return to a closer nominal value. Only place I will ever use them is snubber circuits, though specified performance of metal oxide is turning my attention.

[richiem]

I think carbon film resistors get a bad rap concerning noise and other issues -- in most circuits they work perfectly well, so long as you know that they have larger temperature and other stability drifts, and use better resistors in critical paths. If gain or amplitude stability is important, use metal film or wirewound (but watch that L/R factor); if frequency stability is important, etc.

[klewis]

Originally Posted by 1audio

I would do the following- first put it back together and feed a signal into the noise cal test point at the transistor jig. Look at what comes out with a good transistor in the jig. A 2n3904 running at 100 uA should get down to 1-4 nV /rtHz or so at 1 KHz or less if the base resistance is set to zero. If you add a signal via the noise cal it will be processed and should be visible at the output BNC at around 10X the level it went in at. The input Z is 600 Ohms so measure at the noise cal terminal to get the gain right..


The manual I have is for serial number 203 and really old. The internals of John Curl's are the same and his is 20 years newer than the manual. The unit has a gate voltage control for setting a fixed gate bias. I pretty much never use that and use the drain/collector current to set the biases. For matching transistors you can connect a more precise voltmeter (DVM) to the gate (base) terminal (D4) of the base supply board and readout the voltage for a target drain current which would be the operating point. Its a very clean way to match JFETs.

edits-
My manual is definitely older than the one you are looking at. It does not have the switch or relay and the connections are not brought out of the card. Your unit predates the relay and switch. I'm not sure that prevents it from measuring JFets but if you are up for the changes have at it. The older drawing will help identify what changed (attached)

Demian, Thanks for the help, luck would have it a Quantech noise generator came up on ebay! Got that, calibrated the noise, yeah. I've made replacement boards for the amp 1. The schematic has a bit less gain than the old schematic for amp 1 37db vs 32db. Could you post the Compressor schematic? It's the page after amp 1 in the manual. I want to compare the compressor schematics from my newer manual to the older manual.

I should add that the newer manual shows the implementation of the jfet switch, basically boosting the amp 1 gain by 10db. I want to make sure that I won't overload the compressor stage.

Appreciate all the help.

Ken