Changing output capacitor from .1uf to 1uf

I asked this question as an aside in an older thread and it was briefly addressed, but wanted to better understand the function/impact of changing the capacitance of a capacitor in a given location of my FM-50 tuner.

Some time back I bought a "resto-pak" kit from Fisherdoctor. The kit was fine, but one thing I was curious about was his direction to change two capacitors from their original value of .1uf to 1uf to "improve bass response".

Both of these capacitors are immediately preceding the output of the "amp" and "recorder" outputs, which are identical circuits of each side of the output 12AX7. The only difference is that the "amp" output has a gain pot on the rear of the chassis, and the "recorder" output uses a "recording level" pot/knob on the front of the unit to allow its use as a simple volume control so the FM-50 can be directly connected to an amplifier.

I am using the FM-50 with a 30-A in this configuration with good success.

The two capacitors in question are C33 and C40 in the Fisher FM-50 Service Manual. In the Sam's Photofact the capacitors are C44 and C49. Their original value is .1uf 250V

Fisherdoc's kit replaces both of these with a 1uf capacitor noting in the instructions that: "Increasing the value of C-40 [and C33] as noted will allow the audio output of the tuner to be more efficiently coupled providing true bass response down to the lowest frequencies, even if the unit is used with a solid state preamplifier."


So, with all of that said, I am interested to understand how this change accomplishes this? I am not doubting his specification, but simply want to understand the theory that applies in this example.

I had a Jantzen 1uf capacitor in hand, so went ahead and used that for C33 versus the more generic poly film in the kit.

Thanks,
Al

Link to the service manual for the FM-50

BTW, for reference, here is a link to a previous post of mine with scans of the Fisher FM-50 Service Manual:

http://audiokarma.org/forums/showpost.php?p=6050651&postcount=7


......we just finished watching "It's a Wonderful Life", and the kids are now all asleep.


Have a Merry Christmas!

Al

The roll-off frequency will depend on the input impedance of your amp.
For example, if your amp input is 50k ohms, the roll-off with 0.1 uF would be at approximately 30 Hz. Increasing to 1 uF would lower it to 3 Hz.

These numbers are inverse to the input impedance of your amp, so if it's 100k ohms, the numbers would be approx 15 hz original and 1.5 Hz modified; and so forth.

did it make any difference ?

old_tv_nut said:

The roll-off frequency will depend on the input impedance of your amp.
For example, if your amp input is 50k ohms, the roll-off with 0.1 uF would be at approximately 30 Hz. Increasing to 1 uF would lower it to 3 Hz.

These numbers are inverse to the input impedance of your amp, so if it's 100k ohms, the numbers would be approx 15 hz original and 1.5 Hz modified; and so forth.

Click to expand...

Thanks, that's very helpful.

I get the math of the changing capacitance, but is there some formula for the overall coupling interaction described above? Interesting

So a follow up question would be, what is (or how can I determine or measure) what the input impedance is on a Fisher 30-A? If someone can tell me how to determine that from the specs of the schematic or via measuring, I'll be happier to do that and learn But if someone also knows that impedance off-hand for a 30-A, that would be a good "check" as well.


As far as "did it help"? .... I actually can't say. Both the FM-50 and 30-A required a complete recap (e.g. they both had near lethal voltages on their chassis from leaking filter caps, among other problems) so I rehabilitated so many components from two barely functional units to "like new" that I don't have a clear before and after case for this single capacitor swap.

In theory I could put the .1uf cap back in and see, but it sounds like this is pretty straightforward in the explanation given above.

Thanks again, much appreciated.

Al

little-al said:

So a follow up question would be, what is (or how can I determine or measure) what the input impedance is on a Fisher 30-A? If someone can tell me how to determine that from the specs of the schematic or via measuring, I'll be happier to do that and learn But if someone also knows that impedance off-hand for a 30-A, that would be a good "check" as well.

Al

Click to expand...

If you post a schematic, someone could walk you through the calculation. Measuring is not as simple as using an ohmmeter, since there may be input coupling capacitors.

The input circuit of a 30A consists of a .022uf input cap feeding a 500K level control. This produces a hi-pass filter with a corner frequency of ~ 14 Hz. For all intents and purposes then, the input impedance can basically be considered as the value of the level control (or 500K) over virtually all of the audio spectrum.

This cap could potentially be removed today, since no properly designed signal source would have DC appearing at the output anyway. On the other hand, leaving the cap in does help limit amplification of LF pulses from normal or quick interruption power cycling events.

Dave

Thanks, see link to schematic in my second post.

old_tv_nut said:

If you post a schematic, someone could walk you through the calculation. Measuring is not as simple as using an ohmmeter, since there may be input coupling capacitors.

Click to expand...

Thanks, see link to schematic of the FM-50 in my second post. :thmbsp:


Here is a link to the three versions of the 30-A. I have the 10000 series with the 2.5 Ohm output.

http://audiokarma.org/forums/showthread.php?t=476973


Thanks again!

dcgillespie said:

The input circuit of a 30A consists of a .022uf input cap feeding a 500K level control. This produces a hi-pass filter with a corner frequency of ~ 14 Hz. For all intents and purposes then, the input impedance can basically be considered as the value of the level control (or 500K) over virtually all of the audio spectrum.

This cap could potentially be removed today, since no properly designed signal source would have DC appearing at the output anyway. On the other hand, leaving the cap in does help limit amplification of LF pulses from normal or quick interruption power cycling events.

Dave

Click to expand...

Thanks Dave. I assume when you are referring to "this cap" you are meaning the .22uf (C8) capacitor in the 30-A...

If so, yes I've kept that in.

So with your interpretation of 500k input impedance for the 30-A, what is the net effect of changing the output capacitor on the FM-50 from .1uf to 1uf?

Thanks,
Al

dcgillespie said:

The input circuit of a 30A consists of a .022uf input cap feeding a 500K level control. This produces a hi-pass filter with a corner frequency of ~ 14 Hz. For all intents and purposes then, the input impedance can basically be considered as the value of the level control (or 500K) over virtually all of the audio spectrum.

This cap could potentially be removed today, since no properly designed signal source would have DC appearing at the output anyway. On the other hand, leaving the cap in does help limit amplification of LF pulses from normal or quick interruption power cycling events.

Dave

Click to expand...

This means the original 0.1 uF output cap was raising the corner frequency about 20%, not terribly significant. Changing it to 1 uF ensures its effect really isn't significant at all.

Little -- the only schematic I have of the Fisher 30-A amplifier is the Sams version. On that schematic, I am referring to C3 which runs from the input jack to the top of the level control. On this schematic, it has a value designated as .022 uF.

In my notes, I show the value of C3 and the level control to be identical in all versions of the 30-A.

Dave

dcgillespie said:

Little -- the only schematic I have of the Fisher 30-A amplifier is the Sams version. On that schematic, I am referring to C3 which runs from the input jack to the top of the level control. On this schematic, it has a value designated as .022 uF.

In my notes, I show the value of C3 and the level control to be identical in all versions of the 30-A.

Dave

Click to expand...

Thanks Dave, yes on the Sam's this is C3

Al