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Old 5th December 2006, 08:51 AM   #1
kegger is offline kegger  United States
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Default Output section on preamp questions/breakdown.

I see many different values for the output cap and the resistor to
ground after the cap, then I see a series resistor sometimes after
the cap to the output but I find no real good discussions on how
all these play out, with pro's and cons and what each part does.

I know the cap blocks DC and the size of it will determine your
bass rolloff when connected to an amp but what is the series
resistor doing? And what if you left out the resistor to ground
and things like that? How do you pick resistor to ground size?

I'm just wondering if someone could breakdown (or point to a link)
that describes all 3 of these pieces, there functions and how they
intereract with one other.

I know they are determined by the tubes used and what the device
will power but a simple breakdown of each element might be handy.

I don't know, just throwin this out there as it may be beneficial for
othes as well as myself. Thanks.

I tried searching and found no real insight into this.
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Old 5th December 2006, 10:47 AM   #2
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Default Re: Output section on preamp questions/breakdown.

Quote:
Originally posted by kegger
I see many different values for the output cap and the resistor to
ground after the cap,
this resistor is in series with the insulation resistance of the cap and prevents DC on open output terminals


Quote:
Originally posted by kegger
..then I see a series resistor sometimes after
the cap to the output...

this prevents the tube stage for short circuits on the output terminals (for AC).

Andreas
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Old 5th December 2006, 03:13 PM   #3
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If a preamp has feedback, it may not be stable with all possible loads - a series resistor can help, though it raises output impedance. The cap is chosen as you say for the lowest frequency and lowest expected load.

The shunt resistor just has to be small compared to capacitor leakage - since film caps usually measure 5,000 megs or more, a 1 meg resistor will do. A lower value will reduce the THUMP if your preamp is turned on last... best to just avoid this though...
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Old 5th December 2006, 05:53 PM   #4
kegger is offline kegger  United States
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Thanks guy's, all good stuff!
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Old 5th December 2006, 06:23 PM   #5
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But there's more...

The shunt resistor is in case somehow someone leaves the amp disconnected. It acts as the load for the amp.

A series resistor is in there in case there is a short circuit. It keeps the amp from seeing a short circuit.

It is all for the protection of the amp.

The value of the output cap determines the lowest frequency it will put out for a given load. So if the preamp specs say that it mates to a 100K load power amp, then the cap will be selected to insure the roll off at the low spec frequency, in most cases 20 Hz.

Depending on the size of the series resistor the same logic might apply. The cap will be selected to provide at least a flat response to 20 Hz no matter what the load of the power amp is, whether 100K or 600 ohms.

The shunt one might also play the role of draining the output cap so no DC will be stored at that cap. It also is used to determine the gain of the previous stage. In calculating the gain of a tube amp stage, one uses the plate resistance, the plate resistor, and the next stage load resistor in parallel to determine the gain of the stage. Now this is applicable only if the output is anode coupled. If it has a cathode follower in the output, then it doesn't apply.

Gabe
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Old 5th December 2006, 06:43 PM   #6
kegger is offline kegger  United States
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More great info for sure, keep it comin if anyone feels like contributing.


I'm running a 76 based pre at the moment in a "Ultra-Linear Cascode"
circuit as found in the tube cad journal with my outputs coming from
the anodes of the top tubes. I'm going to add a second output from
the cathode of the top tube as well so I have 2 different gain choices.

The amps I'm running now, 2 of them run 220k to ground on the input
and the 3rd runs a 100k to ground on the input.

But this info should be useful for anyone possably wondering about
the output section of there preamp and how it works.
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Old 6th December 2006, 12:54 AM   #7
kegger is offline kegger  United States
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Well installed a 3uf (poly) cap followed by a 470k resistor to ground.
Seems to work out nicely. (May bypass the poly with a nice oil cap)

What do others think of the series resistor, do you use one or not?
If so what values have you worked with? I see around 220 ohms to
be in a few applications where I do see the series resistor is used.
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