Building A Reactive Load for Amplifier Testing

[John Atkinson]

Values please?

I described the changes I made, including a circuit diagram, in the second page of the article at https://www.stereophile.com/reference/60/index.html.

John Atkinson
Technical Editor, Stereophile

 

[pma]

This is what I use to test with complex load

L1 is on the ferrite core and is nonlinear in a similar way as a real life woofer.

 

[MC_RME]

Still do, and use it for every amplifier review. See, for example, figs.1 & 2 at https://www.stereophile.com/content/vac-sigma-170i-iq-integrated-amplifier-measurements . The only change I made to Ken's circuit was to add a Zobel network---a resistor and capacitor in series---across the input terminals so that the top-octave impedance more closely resembled real-world loudspeakers.

John Atkinson
Technical Editor, Stereophile

I don't see from this text that I should conclude that the circuit presented online as 'Ken's' already include your modification with an added resistor and capacitor in series at the input. Nor does the circuit diagram show such a combination. So please tell me what parts that are in that circuit diagram. Or tell me the values of the added R and C. Thanks.

Edit: finally found the diagram by taking of my sunglasses...

 

[Head_Unit]

Values please?

- Do unto others as you would have them do to you
- Buy low, sell high
- Dream the impossible dream

 

[Head_Unit]

Still do, and use it for every amplifier review.

So as the king daddy of measuring amps, what are your thoughts about testing say low frequencies with (partially) capacitive and inductive loads? And do you feel what you measure pretty much shows what the listeners hear, or is there still mystery there?

 

[Head_Unit]

So how about this? (my complex dummy load is easy to search here on the forum)

View attachment 155466

Interesting! Overlay versus a resistive load!

 

[amirm]

FYI I built that load and measured a few class D amps (without the additional RC). It seemed to be more work than worth so I stopped.

 

[Ralph_Cramden]

I don't see from this text that I should conclude that the circuit presented online as 'Ken's' already include your modification with an added resistor and capacitor in series at the input. Nor does the circuit diagram show such a combination. So please tell me what parts that are in that circuit diagram. Or tell me the values of the added R and C. Thanks.

Ken Kantor’s circuit

JA’s circuit

 

[MC_RME]

Thanks. Finally got it: the link from JA does not go to page 2, but page 1... And the first resistor of the 'tweeter' part has also been changed in value.

 

[John Atkinson]

the link from JA does not go to page 2, but page 1...

You get to the second page by scrolling down to the bottom of the first page and clicking on the page 2 button above the comments.

John Atkinson
Technical Editor, Stereophile

 

[Bob from Florida]

Looks like the John Atkinson circuit should give a decent load variation for the amp to drive. Can the amp be tested with pink noise and a 20-20KHZ sweep? Seems like that would show how it handles individual frequencies in the sweep versus all frequencies at once.

 

[pma]

Can the amp be tested with pink noise and a 20-20KHZ sweep?

Yes it can. You will get nothing useful from the pink noise test. 20Hz-20kHz (or higher) sweep is routinely used for THD/THD+N vs. frequency test. So nothing new.

 

[Bob from Florida]

Yes it can. You will get nothing useful from the pink noise test. 20Hz-20kHz (or higher) sweep is routinely used for THD/THD+N vs. frequency test. So nothing new.

If I can explain further. The sweep will give us a measurement of how the amp responds to an ever increasing single frequency over time. The pink noise measurement will test how the amp responds to all frequencies at the same time. This could be like playing back low complexity music - acoustic guitar for example, versus complex music - full orchestra.