3 Crown ComTech 210 amplifiers and a Question?

[DualTriode]

Hello All,

Still Under House Arrest here.

I bought, count them, 3 Crown ComTech 210 amplifiers with the idea of using them for bi-amp / Tri-amp application. I tested all three of them on the APx555 with the SINAD for the 6 channels in the range of 90 and above except for one.

This performance is pretty decent, near the top tier of the amplifiers tested here on AudioScience. See the FFT’s and SINAD’s posted below.

There is a request for ideas?

What is up with the one channel that is 6 dB low?

I am thinking that there may be old dried up electrolytic capacitors that should be replaced.

What do you all think?

Thanks DT



ComTech Amp 2

Ch1 0.005597 %

Ch2 0.002745 %

Ch1 85.061 dB

Ch2 91.146 dB

Crown210 #2 0.240 v in



ComTech Amp 3





Ch1 0.002711 %

Ch2 0.003375 %

Ch1 91.498 dB

Ch2 89.461 dB

Crown210 #3 0.240 v in

 

[sejarzo]

Those are 70 volt amps designed for commercial sound distribution, aren't they?

 

[DualTriode]

Those are 70 volt amps designed for commercial sound distribution, aren't they?

Yes and no.

There is a switch on the back to select between 70 volts or 4/8 Ohms output.

My tests were completed with 8Ohm resistors on the outputs.

Thanks

 

[sejarzo]

Thanks, I saw one advertised around here really cheap a few months ago, and that ad showed it was a 70 volt amp. Likely the seller was not really technical, now that I recall more of that posting!

 

[gene_stl]

Switchable between 4/8 ohm and 70 volt. I think they will work great tri amped.

 

[RayDunzl]

What is up with the one channel that is 6 dB low?

There are several control settings...

Are they all set properly for your application?

 

[DualTriode]

There are several control settings...

Are they all set properly for your application?

View attachment 58865

Yes and double checked.

Outputs are all 5Watts into 8Ohms.

It is one channel with greater distortion and noise. It is the SINAD that is 6dB low compared to the others.

Thanks

 

[DualTriode]

Let me bump this and ask the question a different way.

What effect will changing the old electrolytic power supply capacitors have on amplifier noise and distortion?

Will SINAD go up?

Thanks DT

 

[RayDunzl]

Trace the signal starting at the input connector and see where it gets noisy.

 

[RayDunzl]

What effect will changing the old electrolytic power supply capacitors have on amplifier noise and distortion?

Does it have separate caps for each channel?

 

[DualTriode]

Hello All,

I spent some time with the manual.

Each channel is powered by its own transformer winging.

Each channel has its own ripple filter.

Both channels share a common low-voltage supply.

I am taking the cover off and possibly taking some photos.

Thanks DT

 

[DualTriode]

Hello All,

I found a star screw driver and took the cover off.

Yes there are two power supply capacitors. Each channel output has its own full bridge rectifier and capacitor.

For grins I went to the bid site and bought a pair of “NOS” capacitors to try out and I also went to Mouser and bought a pair of slightly higher uf value capacitors. It will be a few days.

@solderdude, I also traced out the feedback loop, from the red out terminal to the input of the error amplifier.

When I put this thing back together we can see if the one channel or both noise and distortion improves.

Also I will do @solderdude ‘s inspired experiment: Connect both channels output to the same resistor load. Short one channel’s input and drive the other channels input with the generator and watch the feedback voltage, on the scope, at the point of feedback to the error amplifier of the channel of the shorted input.

Thanks DT

 

[RayDunzl]

Yes there are two power supply capacitors. Each channel output has its own full bridge rectifier and capacitor.

Maybe I isunderstand you, but two capacitors is not usually "one for each channel".

Two capacitors = one Positive and one Negative storage.

Are there 4 large caps?

I don't discount they may be doing something "different", but...

I have one of these:

Two big caps, one stores positive, one stores negative, shared among the two channels.

---

I suppose you could have two capacitors in one package, looking for that now...

Found some, Ok, I stand down.

Example: https://www.hificollective.co.uk/components/fandt-lfaz-dual-section-electrolytic-capacitors.html

 

[DualTriode]

Hello,

Only one capacitor per channel; the + terminal of the capacitor goes to +VCC voltage and the - terminal of the capacitor goes to -VCC voltage.

When I search Crown for the ComTech 210 schematic this what I get. Take a look at the top left of page 2 of 3.

https://www.crownaudio.com/en/site_elements/ct200b-schematic



Thanks DT

 

[gene_stl]

If it's a class D or similar it may only have single ended power supplies for each channel. The big caps don't look to be labeled as two section.

It's not class D. those big caps drive both channels. it's a plain vanilla class AB am fully complementary. linear supply. (ie regular split supply output stage)

If the waveform looks good and you are going to tri amp you probably won't hear that 6 db difference in sinad.
See if you can get the service manual. Sometimes Crown published the "acceptance procedures" for their commercial amps.
You may just need to adjust the bias but you really should have the procedure unless you are a tech ace.
Crown does publish fer free a service manual on this page

Page 12 has the acceptance check out procedure. If you do adjust the bias, watch the temp. On this unit the cooling fans were optional.

 

[DualTriode]

Hello All,

I have downloaded and spent days in the manuals.

The amplifier is classic Class AB plus some safety stuff.

Both outputs have their own separate power transformer winding a separate bridge rectifier and a separate single capacitor.

Commonly a power supply will have a capacitor between the positive rail and ground and a second capacitor between ground and the negative rail. In this case there is only one capacitor across the rails without a ground in between. Take a look at the schematic.

https://www.crownaudio.com/en/site_elements/ct200b-schematic

Yes, even as it is you may not be able to hear the SINAD difference between the channels.

Since we are locked down in the house, I need a toy, I will continue to play with it.

Thanks DT

@RayDunzl No reason to stand down

 

[oldsysop]

@restorer-john might give you some ideas, if decide to get out of self-imposed voluntary quarantine.

 

[DualTriode]

@restorer-john might give you some ideas, if decide to get out of self-imposed voluntary quarantine.

Self imposed? No !

I did not lock the doors at my employment. There are 22,000,000 unemployed as a result of the virus panic.

 

[solderdude]

Yes there are two power supply capacitors. Each channel output has its own full bridge rectifier and capacitor.

They seem to be using an 'active ground' which is tied to the chassis ground. Unusual config.

@solderdude, I also traced out the feedback loop, from the red out terminal to the input of the error amplifier.

When I put this thing back together we can see if the one channel or both noise and distortion improves.

Also I will do @solderdude ‘s inspired experiment: Connect both channels output to the same resistor load. Short one channel’s input and drive the other channels input with the generator and watch the feedback voltage, on the scope, at the point of feedback to the error amplifier of the channel of the shorted input.

Do note that the actual output (where you connect the + speaker terminal) is not exactly where you should measure what the amp feedback circuit does. You should measure on the other side of L300 (one side is connected to the + out terminal) to ensure you are not including the resistance of L300. That resistance is probably very low. Maybe just a few windings of thick copper wire.

 

[DualTriode]

@solderdude and All,

On the Crown.com page the CT400 schematic download is much clearer and easier to read and for what we are looking at is the same as the CT200.

On the schematic follow the feedback loop to the left hand side of the page to the High-Side Error Amp. Between R101 and R103 that is where the Gain Stage output and feedback add together, that is where I am going to measure the feedback voltage. Recall that the measured channel has the input shorted. The Gain Stage output is zero. The only voltage to be measured at the Error Amp input is the feedback voltage. Also recall that the input of the other channel is driven by the generator and that both amplifiers share a single load resistor.

What we are measuring is the output of the driven channel making its way from the common load resistor back through the feedback loop, inductor and resistor included, of the measured channel at the Error Amp input.

Thanks DT