Audio Research D76-A and bass response

[tomp]

David Berning's amps git rid of this problem by doing away with the output transformers.

Tom

[m1garandusa]

OK,
I've just lost confidence in my understanding of all the help that has been provided. This is what I intend to do to measure THD:

Input to the amp is a low-THD signal generator with a grounded plug. This will put the amp chassis and 4-Ohm tap at ground potential (even though the amp is just powered by a 2-prong plug)

My THD analyzer is also a grounded device, but it will draw power from an isolation transformer via a cheater plug, so my test leads are floating, right?

As per the manual, I will connect a dummy 16-Ohm load between the 0 tap and the 16-Ohm tap. Still safe as the dummy load is floating and not touching anything other that those 2 terminals

Here is where I have lost confidence: As the distortion meter is floating, I can measure THD between the 0-Ohm and 16-Ohm taps.... or do I need to measure between the 4-Ohm tap and the 16-Ohm tap?

Is there smoke, burned transformers and a chassis at lethal voltages in my immediate future?

I already set the DC bias using a custom lead and a battery powered meter.

Thanks for being so understanding!

V/R
James

[dberning]

You can make any measurements you wish using the 4-ohm tap as the common for your signal generator/distortion analyzer set, and connect the hot leads of the analyzer to either the 16-ohm tap or the 0-ohm tap. The readings should be very similar whichever of those taps you use.

David

[dberning]

Remember you can measure power output with a floating ac meter between the 0 and 16 ohm taps with your 16-ohm load resistor. You can separately measure clipping between the 4-ohm and 16-ohm tap with an o-scope as the o-scope needs to be connected to the 4-ohm tap ground reference. Then simply remember that the actual voltage you see on the 16-ohm tap should be doubled.

David

[m1garandusa]

Ok, thanks. I will not be using an AC voltmeter nor a scope (though I did by a true differential probe set, if it was needed), just the THD meter. Since it is floating, would anything bad happen if I ran it between 0 and 16 Ohm taps? I'm tracking on the amp schematics, but I really don't know what is happening inside the THD meter.

I've been poking around and building tube stuff for 16 years, but this amp has made me realize that there are aspects of electronic fundamentals that I accepted as convention without really understanding the underlying truths.

Again, thanks to everyone for helping and not treating me like the idiot I am!

V/R
James

p.s. I had watched a bunch of youtube videos on minimizing THD with an O-Scope, but even the old-timers had some difficulty judging wave symmetry with just their eyes. That's why I decided to just use a THD meter.

p.p.s I have less hope than before that new tubes will fix the perception of weak bass. I told you all before that the power tubes were all old, but it turns out that just the VR series pass 6550 was old (GE 6550). The remaining 4 6550's are new production Tung-Sol 6550's, and the sticker on the base that I thought was an ARC sticker was actually a "tubedepot.com" sticker. None the less, they could still be 6+ years old, but I haven't bought a tester yet, so we'll see how well these new Telefunken (derived from JJ 65550, and cryo treated) tubes perform.

[tomp]

If you want to bring the amp over I have the application LAUD that I use to measure distortion in electronic equipment and speakers.Here is what the 2nd and 3rd order harmonics look like on the VFET amp I built using that system.

Tom

Attachments

Figure 8 web.pdf

(177.61 KiB) Downloaded 570 times

[m1garandusa]

Tom,
Thanks for the offer, and I will take you up on it, but just not with this amp, which I intend to sell (unless this re-tubing and AC-biasing makes an earth-shattering transformation for the better)
V/R
James