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How to measure the output impedance of a valve preamp?
How to measure the output impedance of a valve preamp?
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Old 29th May 2019, 06:05 AM   #51
jan.didden is offline jan.didden  Europe
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Originally Posted by i3alan View Post
I would like to make the output compatible with as many devices as possible. So the preamp could be connected to the front of guitar amp, to the fx return input of an amp, line level audio input or daisy chained with another guitar pedals. So I would guess the input impedance of the load would be >10K, the signal level in the range of -10dBv to +4dBu. Please see the schematics of my last two gain stages and the output. As I mentioned, I would like to attenuate the signal, say 100:1, and get a lower output impedance.
So, with the master volume pot being used as a part of the voltage divider, where would you place the rest of the voltage divider circuit - before or after the pot?
With this scheme, the Zout depends on the setting of the last volume control.

Remember, Zout is what you see when you look back into the output. With the control all the way down, you see 250k. With the control all the way up, you see 3.3k in parallel with 250k. With the control in mid position, you see 125k plus 125k in parallel with 3.3 is about 132k.

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Last edited by jan.didden; 29th May 2019 at 06:07 AM.
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Old 30th May 2019, 02:41 AM   #52
PRR is offline PRR  United States
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Originally Posted by jan.didden View Post
I think the switch is not corrected correctly. The label on the switch connection CH2_Vol should be CH1_Vol.
Also, the Zout is dependent on the pot setting...
I assume there is a CH2, not shown in the image I cribbed; and that what is shown is CH1. In any case, that's his problem (flip the switch until something happens).

I clearly noted "Zout 0 to 3K". Did not say "depends on knob setting", no. However for the intended uses, it hardly matters; anything under 10K is "low" in guitar-cord work. (Typical input impedances are far over 100K, over 500K for best sound straight from a guitar.)
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Old 30th May 2019, 03:04 AM   #53
Gnobuddy is offline Gnobuddy  Canada
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I think Jan didn't notice PRR changed the final pot from 250k to 10k...

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Old 30th May 2019, 04:24 AM   #54
JMFahey is offline JMFahey  Argentina
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This is what I suggested when I said "the pot will be part of the attenutor".
With R22=100k youŽll have 10:1 attenuation, good to drive a power amp
With R22=1M youŽll have 100:1 attenuation, which is Guitar/pedal level.

Output impedance will vary from 0 to 10k according to Master Volume setting, which is fine.

Pick desired R22 depending on your needs.
You may even make it switch selectable: "Guitar/Line level"
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Old 30th May 2019, 05:30 AM   #55
Gnobuddy is offline Gnobuddy  Canada
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I have one suggestion regarding the channel select switch. Currently it's wired in series with the signal. Feeding into a 100k or 1M impedance, even a few pF of stray capacitance across the switch will "leak" signal, so that "offness" will be poor - there may be audible cross-talk from the channel that's supposed to be switched off.

I suggest wiring for shunt (parallel) switching instead:

1) Add one more 100k resistor from the top of the Master Vol pot to the wiper of the channel 2 volume pot.

2) Wire the switch pole (moving contact) of the channel select SPDT switch to ground, one throw to wiper of CH1_VOL, the other throw to the wiper of CH2_VOl.

The output of one channel or the other (from the individual channel volume pot wipers) is now shorted directly to ground. The 100k resistors allow the other channel signal to get through to the master volume pot. Offness will be better this way.

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