Fisher SA-300 - How to eliminate Pot and alter-input

Hey Fisher folks.....

Working on an SA-300.....and have determined that I'd like to remove the 'level' control along with the 'filtered' input. In looking at the Schematic, I see some oddities that I'm unfamiliar with.....like the fact that it appears the the 'input' tube.....a 12AX7....is actually running both sections in parallel? Input appears to be going to the grids on pin 7 AND on pin 2. Anyway...I'd like to do away with the 'filtered' input....and the 'level' pot. To this end, I'm wondering if I can just leave the 470K resistor I see in line between both grids to ground ....and simply run a .047 cap between the one RCA unfiltered input and the tubes? There is a 68mmf cap in there I'd strip out, along with the level pot, another .047 cap, and all the parts in the 'filtered' section. My level pots are both bad....and I see no great use for them anyway.......as that's what the volume control is for on the pre-amp (or so I tend to believe.....). And, I see no use at all for the 'filtered' input......so I'd like to just scrap it all. Any help confirming my plan or other advice would be greatly appreciated. Many thanks...... Tom D.

To mimic the original design while eliminating the input filtering components and level control, leave C14, C15, C35, and C36 in place (Sams schematic), and where these two caps join in each channel, connect the unfiltered input jack, and a 270K resistor to ground. Then the level controls and filtered input components can be removed.

Power amp level controls are not for controlling volume per say, but for matching the sensitivity of a given power amp to a given preamp. If a power amp can be driven to full power output with (for example) .75 vac, while the preamp has a typical line stage gain of 20 db (10X), then that means it will only take a source signal of .075 vac to drive such a combination to full power output at the preamp's line level input jacks. Since most source signals are much greater that this, the result will be a very touchy preamp volume control and excessive low level noise as well, as the combination simply has more gain to it than needed. By including level controls on power amplifiers with input level requirements of less than (typically) 1.0 vac, the sensitivity of the power amp can be appropriately reduced to match the gain of the preamp, so that a much more normal .25 vac line level input sensitivity is achieved at the preamp inputs. This will then produce normal volume control action in the preamp, and reduce any preamplifier noise that would otherwise be over amplified by the power amplifier, were no power amplifier level control in place in this scenario. Such controls then are not for directly controlling the volume, but controlling the flexibility that power amplifiers with greater than normal input sensitivities provide.

I hope this helps!

Dave

Dave,

Perfect! Essentially that was my plan exactly....with the exception of the 270K to ground. I'd neglected to think about the fact that even when turned to full 'on' position, that pot is still acting like a load resistor in that input set-up. So, my bad, and I appreciate your help. This 300 is turning into quite a project! I hope for great things from it in the end. I do understand the original intent of the level control. Since Ipods and MP3 players didn't exist back when this thing was being produced, I know the level controls in amps that have them were for the 'matching' purposes you describe. It's just been my experience that....like this amp.... having a pot up front there is a possible breech point in the signal path......and I'd prefer not to have it. Further, I've not really encountered the type of scenario you describe where the pre-amp VC is 'too touchy' for general use. Guess I've just been lucky with that. Hopefully this removal of the pots and 'filtered' input won't 'hurt' the amp, as it's my intent to just get it as streamlined as possible....and working as best possible! Thanks again for your help Dave! Tom D.

Tom,
It just occurred to me that maybe you should have left the potentiometer in place on one input jack, and have the other input jack bypassed to eliminate the potentiometer and filter.
Then you could have the best of both worlds-a clear signal path and a filtered path with level control. You never know what the future may hold.

Rich

Raz,

Not a bad plan. I'm still a couple weeks away from getting back into the amp....though I'm already missing the fun 'soldering' away........ But, producing an income has it's merits....particularly after the shock I got yesterday when my Tax preparation expert called and gave me some news that about gave me cardiac arrest! Seems to me it's time for another 'tea party' of sorts! I gotta wonder just how much longer us working stiffs are going to keep footin' the bill for people think 'work' is a 4 letter word.....????? But, I guess it's those folks that put the administration in place to keep them in free cell phones, pre-paid credit cards for anything from new barbeques to the steaks to put on 'em.....and of course.....free housing!! Guess I'll get back to work so I keep those folks in their Escalades.

I'll take a Suburban if you're still handing them out Tom. And let DC foot the gas bill.