3 Crown ComTech 210 amplifiers and a Question?

[solderdude]

You need to measure what the output of the amp does. Not the voltage at the input of the inverting input.
The feedback voltage comes from L300.

 

[DualTriode]

You need to measure what the output of the amp does. Not the voltage at the input of the inverting input.
The feedback voltage comes from L300.

@solderdude,

Please tell me more.

If I measure at L300 as you say, what am I measuring and why am I measuring it?

The amplifiers are assumed to be identical. When I measure at the Error Amp input of shorted input amplifier I am measuring exactly the same feedback voltage as in the driven amplifier.

That is what I am interested in.

Later after measuring things with the 500Watt 8Ohm load resistor I will connect a sealed enclosure sub-woofer and watch and measure the feedback voltage and phase as I sweep the frequency through resonance.

While this thing is open on the bench I will measure what you are speaking about as well. Tell me more about what results you are expecting to see.

Thanks DT

 

[solderdude]

Please tell me more.

If I measure at L300 as you say, what am I measuring and why am I measuring it?

You are evaluating the output resistance of the amplifier and how it handles back EMF.
You can also feed each channel with 2 non related frequencies.

The amplifiers are assumed to be identical. When I measure at the Error Amp input of shorted input amplifier I am measuring exactly the same feedback voltage as in the driven amplifier.

At the - input of the error amp you should see no voltage as it tries to keep the input voltage 0.
When you measure with a scope at that point or other equipment you will see noise and perhaps other things that aren't there in reality.
The - input is a current input so there will be nothing to measure there.

What you want to see is the output of the amplifier as that is the - input differential to the + input (which is 0V) amplified by the amps closed loop gain and easy to measure as you don't influence and potentially destabilize the feedback loop by inserting capacitance to ground at a sensitive point.

Later after measuring things with the 500Watt 8Ohm load resistor I will connect a sealed enclosure sub-woofer and watch and measure the feedback voltage and phase as I sweep the frequency through resonance.

While this thing is open on the bench I will measure what you are speaking about as well. Tell me more about what results you are expecting to see.

Thanks DT

Depending on how good the feedback loop and output stage are you might see issues with output stages having trouble with inductive or capacitive loads having to supply (damp) currents that are not in phase with output voltages.
Any signal you will see (on a scope or recorded with a sound card) that are measured before L300 is where the feedback loop/amp fails to deliver current.
The more signal you see there the more the feedback loop is compromised because of back EMF.

The more you move your test probe to the output post (after L300 or heaven forbid at the binding posts) the observed signal (when in phase and with a linear amplitude) will actually be Ohmic losses of the L300 and internal wiring of the + and - output wiring.
It is essential to observe before L300 and put the ground test lead as close as possible to the 'ground' of the amp module.
Not to the chassis or - binding post.

 

[DualTriode]

You are evaluating the output resistance of the amplifier and how it handles back EMF.
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At the - input of the error amp you should see no voltage as it tries to keep the input voltage 0.
When you measure with a scope at that point or other equipment you will see noise and perhaps other things that aren't there in reality.
The - input is a current input so there will be nothing to measure there.

...clip



...clip
The more signal you see there the more the feedback loop is compromised because of back EMF.

...clip

Hello,

The purpose of the experiment is to evaluate feedback and how it handles EMF from the connected sealed box woofer.

There will be voltage at the non-inverting input of the Error Amp. When there is 0 volts coming from the Voltage Gain Stage (inputs shorted) the only remaining voltage input at the Error Amp non-inverting input will be by way of feedback loop. Take a look at page 7 and Figure 1 in the Com Tech 200 Model CT200B Service Manual. Each stage in the amplifier front end operates by voltage input.

“The more signal you see there the more the feedback loop is compromised because of back EMF.”

Interesting choice of words, “compromised because of back EMF”. That is the design, back EMF becomes feedback, or input at the Error Amp non-inverting input.
https://www.crownaudio.com/en/site_elements/ct200b-service-manual

Error Amp (copied from page 7 of the service manual)

The inverted output from the VGS is fed to the noninverting input of the Error Amp op-amp stage through an AC coupling capacitor and input resistor. Amplifier output is fed back via the negative feedback (NFb) loop resistor. The ratio of feedback resistor to input resistor fixes gain from the Error Amp input to the output of the amplifier at 26 dB. Diodes prevent overdriving the Error Amp. Because the Error Amp amplifies the difference between input and output signals, any difference in the two waveforms will produce a near open loop gain condition which in turn results in high peak output voltage. The output of the Error Amp, called the Error Signal (ES) drives the Voltage Translators.

Thanks DT

 

[solderdude]

I understand what you are trying to do.
However as explained before the - input is a current input and there will be no voltage to measure there.
The little voltage that will be there (due to errors) is very, very small and when looking at a scope you will see noise.
That noise will be from the scope itself rather than what is there.
You will only see the input voltage there when you have a signal on that + input but it is not (is grounded)
The difference between + and - input is basically what you seem to want to know.
This will be very small as the open-loop gain of the amp will determine this.
You cannot measure this with a scope it will show things that aren't there.

So in order to see what the feedback loop does all you need to do is measure the voltage before L300.
When the feedback loop does NOT work properly you will see a voltage there (the output resistance of the amp).
When you know the gain you can calculate the current at the - input.

 

[DualTriode]

Hello All,

I took the bottom cover off this Crown amplifier. Reedy for access is the back of the low level PCB.

Turns out that this Com Tech 210 amplifier does not match the part numbering for the CT 200 amplifier on the data sheets provided on the Crown web site. I found that the same quad ON MC33079 Op-Amp is common to all or most of the family of amplifiers.

Looking at the data sheet, pin number 10 is the input to op-amp #3 on the IC, both the output from Gain Stage Amp and the feedback loop land on pin number 10. I am going to probe around with the DMM and confirm things then solder on a couple of test leads.

See the attached photo of the back of the amplifier low level PCB.

Thanks DT

 

[DualTriode]

Hello All,

I hooked it all up to the APx analyzer.

Using a Crown Com-Tech 210 amplifier both outputs are attached to the same 500Watt 8Ohms load resistor. The input to channel 1 is shorted. The APx555 output number 1 is the input to channel 2 of the amplifier. The APx55 input number 1 is connected to the inverting input of the error amplifier of channel number 1 of the amplifier. The APx55 input number 2 is connected across the common test load.

The test input signal to amp 2 is 2.35 volts at 1000Hz.

Any guesses about the recorded voltage?

With the input test signal at 2.35 volts 1000Hz for only one channel attached to the 8R load the output is 6.32VAC or ~ 5watts.

With the above test conditions the recorded voltage at both the channel number 1 error amplifier input and across the test load were within a few micro volts of 0.

I did wonder what the effect of the same feedback going to both amplifiers was going to be. The effect was equal and opposite. The output at the load was 0volts and the voltage at the error amplifier of amp 1 was also 0volts.

Thanks DT

 

[restorer-john]

How did you go with this DT?

 

[DualTriode]

Hello,

This is a little longer than the short story.

I just quit / Retired from my last part time job that was trying to become more than full time. Put a fork in me I am done.

Now we are in the middle of packing up and moving to the north coast of California everything is in boxes. So we get a letter from the Nor Cal County Health Department Director telling us to stay away. #!&k her.


Back to the topic, I do have some new capacitors to try out.

I did do some tweaking of the input level selector switches on the back side of one of a CROWN 210 amplifiers . Step #5 was the highest SINAD.

At 5 watts into 8 ohms ch1 was SINAD 92dB ch2 was SINAD 94dB. Not bad for a cheap old school amp.

See post 73 at link below.

https://www.audiosciencereview.com/...wer-amplifier-review.14287/page-4#post-442961

When we get settled I will look more at the feedback measurements.

Thanks DT