Changing input capacitor value

derekva

This ain't no picnic...
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One thing I've noticed with pretty much all vintage tube amps is that they have an input capacitor on the RCA plug to block DC. However, in Fisher amplifiers, the value of the capacitor seems rather small when compared to that of other tube gear. For example: the Fisher 30A series uses a 0.022uF cap on the input (into a 500kohm input impedence), while a Dynaco ST35 uses 0.1uF into an identical 500kohm input impedence. Is there any potential harm that could result (not magic smoke, but sound quality-wise) in increasing the value of the input cap from 0.022uF to 0.1uF? From what I understand, the smaller value acts as a low-pass filter and actually decreases the bandwidth of the amplifier on the bottom end.

Thanks!

-D
 
Derek -- The reason that the cap is smaller on the 30A versus the ST35, is that the Dynaco amplifier has the ability to produce full power output right down to 20 Hz with its Z565 OPT, where as the Fisher's OPT is likely power bandwidth limited to about 30 - 35 Hz. The smaller input cap on the Fisher then helps limit the amplitude of lower frequencies presented to it, that might otherwise overload the bandwidth limitations of its OPT.

As long as the power you are requiring from your Fisher is within its capabilities, then the cap can be increased, or eliminated altogether.

Dave
 
If you increase the capacitor's value to where you exceed the bandwidth capabilities of the output transformer...or for that matter, ANYTHING in the circuit...at high levels you could wind up with a large negative feedback "error" signal going back to the input stage...it could be large enough to cause nonlinearity in the input stage, and perhaps even overload it. Either way, the amplifier won't be reproducing music, but making noise. I'd say Fisher probably knew what they were doing.
 
Derek -- The reason that the cap is smaller on the 30A versus the ST35, is that the Dynaco amplifier has the ability to produce full power output right down to 20 Hz with its Z565 OPT, where as the Fisher's OPT is likely power bandwidth limited to about 30 - 35 Hz. The smaller input cap on the Fisher then helps limit the amplitude of lower frequencies presented to it, that might otherwise overload the bandwidth limitations of its OPT.

As long as the power you are requiring from your Fisher is within its capabilities, then the cap can be increased, or eliminated altogether.

Dave

Thank you for explaining the rationale behind the decisions made by Fisher. Based upon this coupling capacitor calculator (http://www.v-cap.com/coupling-capacitor-calculator.php), it looks like with an input impedence of 500k and a 0.022uF cap, the roll-off begins pretty darn high. Going to a 0.1uF should put the optimal low-frequency response at 30Hz, so I may just give that a shot. The input cap is very simple to remove and replace, so if I don't like the results, I can always undo the mistake.

(it's hooked up to a 102dB 1W/m speaker, so I don't think I'll be taxing it much, unless I want to make my eardrums meet in the center)

Thanks, Dave!

-D
 
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