Yamaha M70 ZDR

[Yamaha5000]

fed into spectrum/audio/whatever analyzer through some sort of coupler for observation/measurement,

This coupler is external to the QA?

 

[Sparkplug]

This coupler is external to the QA?

Yes. Refer to your analyzer’s manual to find out the max input sig level, it may be marked on the front panel too.

 

[Yamaha5000]

42db

 

[Sparkplug]

42db

dB what? dBm? dBu? dB something else? dB by itself isn’t a unit of anything …

 

[Yamaha5000]

Maybe this will provide a better understanding of what is going on from all angles:

 

[Yamaha5000]

Performance Evaluation

I am not sure if this is the correct Thread to post this question, but I will give it a try. The manufacturers of Audio (Home Stereo) go to great lengths to supply performance specifications of their equipment. How can a small shop implement these testing procedures into daily practices. My...

forum.quantasylum.com

 

[Sparkplug]

v

That means a max input of ≈125Vrms, that’s pretty high, are you sure? I’m not home now so can’t read lengthy texts but a cursory look at their diagram shows they’re taking CA-2000 ‘rec out’, not the direct output. A power amp has no rec out so you need a coupler.

 

[Yamaha5000]

OK, so the true amp gain is 29 dB, which is what the spec says

Now, if the amp is good for 250W into 4 ohms, that is 31.6Vrms = 30 dBV. That is on the edge of not needing an attenuator. But let’s run it and see what happens without an external atten.

 

[Yamaha5000]

Last week I measured the RMS with a generator set at 1.6v input and 42v at the output, this was just before clipping. DMM for measuring RMS and a Scope to see the clipping point.

 

[Yamaha5000]

 

[Yamaha5000]

216w reading without any additional attenuation

 

[Yamaha5000]

 

[Yamaha5000]

Continuation of discussion

 

[Sparkplug]

Last week I measured the RMS with a generator set at 1.6v input and 42v at the output, this was just before clipping. DMM for measuring RMS and a Scope to see the clipping point.

I’d keep the input to the analyzer at least 10dB, preferably more, lower than the spec to get very linear response without having to worry about causing intermodulation with the analyzer.

 

[Yamaha5000]

What is your summary of thoughts on measuring the ZDR Test Points?

 

[Sparkplug]

What is your summary of thoughts on measuring the ZDR Test Points?

Personally I wouldn’t bother with it unless you’ve made drastic changes to the board (new non-OEM output transistors, etc.). As far as M-70 is concerned, the procedure is a bit unclear to me, does the ZDR adjustment voltages at test points go up and down as you adjust? Do you need to be monitoring the output on an analyzer visually? It doesn’t say.

Also the SM prescribes an output of 200W (≈53dBm) at the output but says 34.2dBm, which is only 2.6W, an error? Wouldn’t be the first time.


In general I don’t believe you’ll hear any difference if you’re making minor adjustments to the harmonics. If you’re adjusting the non-linear nature of the class B finals (turn-On/turn-Off time lags), then maybe there’ll be an audible difference … maybe, I haven’t performed one of these adjustments so couldn’t tell for sure.

To digress a bit, when I worked on 900MHz cellular mobile transmitter circuits, in one project, the customer spec for modulation accuracy (TDMA transmitter) was an FER of < 0.04% or some number like that. One day I didn’t have too much to do and decided to actually degrade it and see at what point I’d hear poor audio. I started to pump noise into the channel and observed the FER (frame error rate) climb up but the audio remained just fine. Finally at FERs of 30% to 40% I noticed audible audio degradation such as missing words, chirping, and other sound artifacts. So I really wouldn’t worry about the distortion settings, chances are you won’t hear anything different.

 

[Yamaha5000]

you’ve made drastic changes to the board (new non-OEM output transistors, etc.)

The output transistors were changed
All trim pots were changed

 

[Yamaha5000]

Current condition

 

[Yamaha5000]

Current condition

 

[Yamaha5000]

Have you had an opportunity to look at the QA Forum that I sent last night?