A Little Help From My Friends

[Gepetto]

I set up 2 meters and a signal generator to indirectly measure the impedance of the zobels at several frequencies. I measured the current through the input of each channel. The following settings were used:

Frequency: 20, 100, 1k, 10k, 20k.
Source Voltage 2.0 volts
Measured A Current (AC Current is a redundant statement. Sound it out)
Instruments: Loftech TS-1 oscillator, Fluke 87, Fluke 87V, Vici micro ohm meter,


Frequency. R Ch. L CH .

20, 24 uA 24 uA

100HZ, 119 uA 119 uA

1K, 1.17 mA 1.21 mA

10K, 11.8 mA 11.7 mA

20k, 24.1 mA 23.3 mA

Resistors measure 5.1 ohm Leftch cap .092 uf, rt channel .095uf.

Mark, you could do the same thing and compare the results. Note that the board does not need to be installed to do this. just inject the signal at the L or R channel. All this does is verify the zobels are functioning.

A note on the instruments. The Flukes have not been calibrated since date of purchase. I have no reason to believe they are out of cal. The Vici micro ohmmeter is $100 or so on Amazon. I really like this instrument and it seems to be accurate.

Spot on Don, the calculations call for 23.6ma at 20KHz with 2.0VACrms into the network

Makes you wonder why folks talk about burning up Zoebels?? And why they put that heavy duty Zoebel in the amps sold to Clair Brothers?

A brain teaser indeed. At 350W full power output at 20KHz it is less than 2W dissipation in the Zoebel. Of course, no one is putting out 350W at 20KHz CW anyway...

 

[wattsabundant]

The calculation Joe did determined the impedance of the cap at 20 Khz. The formula is
1 / (2 × π × f × C).

π= 3.14, f = 20Khz, c= 0.1 uf. That works out to .001256.

1/.001256 = 79.6 ohms.

To determine the impedance of the R/C network, add the cap impedance 79.6 ohms + 5 ohms from the resistor = a total of 84.6 ohms. Using Ohms law, E (2.0 volts) divided by 84.6 ohms = 23 milliamps (023 amps).

That's all fine and good. What I find really amazing is I did the calculation on the Sharp EL-506-S calculator I used in tech school more than 40 years ago. If it has a battery I don't see any way it has lasted 40 years. The calculator does not have a solar panel. The only possible explanation is F.M.

 

[Gepetto]

We all need that Don...

 

[J!m]

Nuclear calculator.

 

[mlucitt]

Mark, you could do the same thing and compare the results. Note that the board does not need to be installed to do this. just inject the signal at the L or R channel. All this does is verify the zobels are functioning.

Just to close out the original issue...
I could not inject more than .2V to the Right channel of the amplifier before it started motorboating and the DBT got bright and the DCP opened the relay due to the DC voltage spiking. When I removed the Zobel Network, the Right Channel behaved normally.

 

[laatsch55]

We had that problem years ago.....may have been in your walkabout period..

 

[mlucitt]

We had that problem years ago.....may have been in your walkabout period..

If so, it CAME BACK! Anyway, back to our regularly scheduled program.