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Old 14th April 2007, 02:20 AM   #1
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Default A thought on Cathode feedback

Hello everybody!

The Michigan Technological University Audio Engineering Soceity is having an equipment listening session this sunday, and I have agreed to bring my EL34 SET amp along. I want the amp sounding its absolute best, so I am tweaking it.

The first thing I did was take measurements to make sure everything was on spec (it is ).

After measuring everything, I took off the Cathode Feedback, The amp's Midrange came to life, however the bass died a painfull death. So this leads me to thinking. What if I use One capacitor to bypass all of the high frequencies to gound, and another Capacitor to allow all the low frequencies to to become part of a cathode feeback loop. I even came up with a schematic in eagle to show everyone what I am thinking about, now, as you see this, you must understand that this schematic only shows the bias circuitry, not the entire output stage

Will this work? Does it look like a good idea? I know I will have to be carfull picking capacitor values, and The CK will have to be far smaller than CF (see Schematic).

my other questions about this are dependent on the fact that it will work, How do I find the impedances to base the cap values off of?

Any help would be sooo apreciated
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Old 14th April 2007, 02:41 AM   #2
SY is offline SY  United States
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Welcome to the world of tone controls!

If you can measure frequency response, this would be a good and educational time to do so. Ck may have far less effect than you think until you make it big enough to defeat its own purpose.
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Old 14th April 2007, 02:19 PM   #3
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okay, I have no fancy equipment or anything, however Another tube-head and I got together with some sweeps, and tones and saw which frequencies appeard to be dropping out. It appears that anything below ~80hz is just soft, quiet, and fluffy.
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Old 14th April 2007, 07:14 PM   #4
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Will anything bad happen if I just start playing around with this? because I think that is what I am going to do, just play around...
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Old 14th April 2007, 09:24 PM   #5
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Quote:
Originally posted by SY
Welcome to the world of tone controls!

Click the image to open in full size.

Also, think of frequency dependent load of the amp by capacitors across the speaker...

Quote:
Originally posted by alexmoose
Will anything bad happen if I just start playing around with this? because I think that is what I am going to do, just play around...
Nothing is bad in getting an experience. But as soon as you've asked we are answering.



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Old 14th April 2007, 10:22 PM   #6
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Quote:
Originally posted by alexmoose
Will anything bad happen if I just start playing around with this? because I think that is what I am going to do, just play around...

Consider a little resistance (50-75 ohms) between Cf and Ck to start. You won't lose much degeneration and the amp will see a much friendlier load.
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Old 14th April 2007, 10:42 PM   #7
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does anyone have any idea where to start picking CK anf CF values?
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Old 14th April 2007, 11:46 PM   #8
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Quote:
Originally posted by alexmoose
does anyone have any idea where to start picking CK anf CF values?
I would suggest to start from desired curve of dependence of gain on frequency.
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Old 15th April 2007, 03:09 AM   #9
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Okay, this is what I am thinking now However I need to figure out the impedance of this system so I can pick values.
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Old 15th April 2007, 03:41 AM   #10
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keep in mind the frequencies with more feedback will have less gain. So you will have to add gain to the low freq's somewhere upstream.
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