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 Post subject: Bass control circuit mod
PostPosted: Jan Tue 28, 2014 1:42 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 12015
Location: Warner Robins, GA
I have a Magnavox CR-198C console and I would like to alter the bass control circuit as I don't really like it as is.

Image

Any idea at what frequency the bass starts to be boosted and how to mod the circuit?

My thinking is this. The treble control is only cut and the bass control is cut and boost. It would be easier if I just disconnected both controls, but I'd rather not do that.

Part # 26 which has see table by it I upped to a .01 uF capacitor which is what it was in the CR-198 instead of the .005uF value the CR-198C is. Part 31 which says see table is directly across the bass control. Part 23 is a .0018uF cap. I am thinking those two values somehow control the frequency at which the bass is boosted.

The bass cut is simply a reduction in the amount of feedback going to the 6SN7 grid and also the lowering of the input impedance of the 6SN7 stage which because of the value of part 26 increases the LF roloff.

If I calculated right at the full cut position the LF roloff starts at 247 Hz with the .005 uF cap for part 26 given the impedance is only 129K when the bass control is full CCW. With the bass control full CW the LF roloff starts at 114 Hz given the impedance has increased to 279K.

With the .01 uF cap the LF roloff starts at 123.4 Hz with the bass control full CCW. The LF roloff with the bass control full CW starts at 57 Hz.

I haven't checked for sure so I am only guessing that at full CCW the feedback line is grounded by the bass control.

EDIT: I decided to use a .1uF cap for part 26 and the bass is improved more. I am thinking about rewiring the bass control so that it is across part 26 and I'll reduce its value. That way the bass control will be cut only. I'll try it temporarily first to see if I like it.

EDIT2: Had to put it back as it was. The only way to make the circuit work as I wanted is to use a 1 meg control across the capacitor.

I would like the amp to reproduce down to at least 35 Hz, but still have the ability to boost and cut the bass or just cut the bass if boost is not easy to do. So far increasing the value of part 26 just makes the bass control less effective, although it does help the bass some.

EDIT3: I replaced the .0018uF cap with a .001uF cap which reduced the amount of bass boost.

Did a frequency test and the bass boost starts around 80 Hz and I still have usable bass output at 40 Hz although the level is reduced somewhat.

Unless y'all can think of a better solution I am tempted to leave it as is.


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