Negative Feedback

[wy3d]

I have a Knight T-150A transmitter and the design uses a 470K feedback resistor from the 6DE7 modulator tube back to the cathode of the second half of the 12AX7 mic amp.  I have nticed in most designs there is a capacitor in series with the resistor used for negative feedback.  What is the purpose of the capacitor other than to block the DC? The resistor alone seems to be working OK.
Frank  WY3D

[w3jn]

To shape the audio response.  Example:  If you have a low value capacitance, say 500 pF in series with the FB resistor, the negative FB will be weighted towards the high frequency end due to the capacitive reactance.  Therefore, the gain of the amp at high frequencies will be less.

[K2FW]

To shape the audio response.  Example:  If you have a low value capacitance, say 500 pF in series with the FB resistor, the negative FB will be weighted towards the high frequency end due to the capacitive reactance.  Therefore, the gain of the amp at high frequencies will be less.

So, in other words, a smaller cap such as a 300pf in the feedback loop will in effect give the modulator a bit more low end response?

[w3jn]

No.  It will cut the high freq response.  Remember the more NFB the lower the gain, and when you have MORE HF NFB you have less gain at HF.  THe lower freq gain isn't affected (although with a straight R/C circuit it's gonna fall off at a linear rate)

73 John

[K2FW]

No.  It will cut the high freq response.  Remember the more NFB the lower the gain, and when you have MORE HF NFB you have less gain at HF.  THe lower freq gain isn't affected (although with a straight R/C circuit it's gonna fall off at a linear rate)

73 John

Ok John, in afterthought I guess I should have worded my response a little better. I really meant to say, that in using a smaller cap in the feed-back loop,would in effect appear to give the modulator more low end by limiting the high end response. Thanks so much John for your input into this discussion.  It's very interesting to say the least!

[w3jn]

Perhaps... it's not gonna boost low ends any.  As you know, a straight resistor (negligible reactance at audio) will broaden the frequency response as well as decrease harmonic distortion.  If you shunt the FB loop to ground with a cap, it will tend to effectively boost HF (essentially cut LF) as the HF aren't in the FB loop anymore, and your THD will increase.  

Of course if you choose your FB loop components improperly and get a pole somewhere in the bandwidth of the amp you'll end up with oscillation!

[wavebourn]

I hope it helps:

http://wavebourn.com/forum/viewtopic.php?t=29