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Old 18th September 2018, 12:06 AM   #1
deafbykhorns is offline deafbykhorns  United States
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Default Frequency compensation-Audio Analyzer

Using an RE084 preamp tube which is about an 8K plate resistance, cap coupled, I have a relatively high output impedance. When connected with cables, it forms a nice low pass filter when the cables are several hundred pf. I get about a 2-4db drop at 20khz.
When measuring frequency response, it clearly shows whats going on with higher impedance. Using just an 8k resistor and the cables, i get similar results.
So, how do you analyze a preamp with high output impedance's properly?
Do you run it thru a precision opamp that will drive the cable impedance without changing the signal being tested? Or build something to compensate for the output impedance?


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Old 18th September 2018, 01:52 AM   #2
PRR is offline PRR  United States
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How much cable??

20KHz and 8K Ohms looks like 1,000pFd and maybe 30 feet 10 meters of ordinary cable. I don't know how you can see the meter that far away.

Classically you use SHORT cable to your Ballantine: AC-Voltmeter which has a high-Z input. It will read ACV directly, and it has an output for your plotter, 'scope, or other tools. Keeping a 1947 Ballantine is today awkward; as you say, a 19 cent TL072 and two 9V batteries will give >10Meg and <10pFd, if done carefully.
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Old 18th September 2018, 11:31 PM   #3
deafbykhorns is offline deafbykhorns  United States
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Need something more like a dongle to plug into the output of preamp.
Build it with a high Z input
Surprised there's not something already available to do this.
Then again, this immediately shows you how a preamp is effected by high capacitance cables.
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Old 19th September 2018, 08:05 AM   #4
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by deafbykhorns View Post
Using an RE084 preamp tube which is about an 8K plate resistance, cap coupled, I have a relatively high output impedance. When connected with cables, it forms a nice low pass filter when the cables are several hundred pf. I get about a 2-4db drop at 20khz.
When measuring frequency response, it clearly shows whats going on with higher impedance. Using just an 8k resistor and the cables, i get similar results.
So, how do you analyze a preamp with high output impedance's properly?
Do you run it thru a precision opamp that will drive the cable impedance without changing the signal being tested? Or build something to compensate for the output impedance?


Attachment 704167
I would think you would measure it with the same type and length of cable you use to connect it to the power amp. That will be what you get in practice.

Measuring without a cable may improve the picture but that's just fooling yourself, it has no meaning because you will not use the preamp like that.

If with the actual cable you still have too much roll-off you need to change the design.

Jan
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