tskguy said:
Ok let me rephrase my question... Earllier in this thread thre was a conversation between 3nity and sr1200, you guys talked about adding a 5k pot and a 5k resistor at R4???
I can only assume you ment R5 Correct?
Eric
Eric, instead of painting by numbers, try to understand the circuit behind.
This preamp is based on a simple non-inverting opamp gain stage with variable amount of gain. Add up the gain from the input transformer and output transformer, depending on type and wiring for your total gain.
Voltage gain of this opamp gain stage is 1+(Rfeedback/Rshunt) for DC conditions, ignoring the AC response from C6 and C7 in the frequency range of interest just for examples sake.
Rfeedback is the resistor between opamps output and its inverting input, from Jorges schematic R5 with value 10K.
Rshunt is the resistor between opamps inverting input and reference voltage 0V, from Jorges schematic R4 with value 100R in series with the 10K potentiometer with reverse logarythmic law for the variable part of gain, so this shunt resistor can be adjusted anywhere within 100R and 10.100R on paper. (ignoring the shunt arm cap C6 in between just for now).
When you fill in the numbers in the voltage gain formula above, you come up with a voltage gain of 1+(10000/10100)=1.99 with the pot dialed in full CCW (highest resistance 10K) and a voltage gain of 1+(10000/100)=101 for the other extreme setting with the pot dialed in full CW (lowest resistance zero ohms).
From the 1+(.. in the formula above, you'd also see, voltage gain will not go lower than 1, so the gain pot will never be a volume adjust to -infinity.
Make these above voltage gain readings a dB reading is LOG(voltage gain)*20, giving LOG(1.99)*20=+5.98dB and LOG(101)*20=+40.09dB, roundabout +6...+40dB.
If -for whatever reason- you want a perfect 10K feedback resistor, your (flawed) idea seems to be substituting the fixed 10K resistor R5 with a series string of a fixed 5K resistor and a 5K trimmer, connected as rheostat, so its effective value could be varied between 5K and 10K on paper.
You already know, there is no such thing as a perfect part, so FI resistors come with tolerances, maybe +/-1% for your fixed resistors and +/-20% for variable resistors. Counting in these in their tolerance extremes, this on paper 10K series string with trimmer dialed in full CW for its max.resistance setting might result into something in between (5000-50)+(5000-1000)=8950 ohm and (5000+50)+(5000+1000)=11050 ohm. If tolerances are on the negative side, you never could adjust it to the desired exact 10K value. To make it (still flawed) work on paper and keeping your trimmer, you'd calculate for the trimmer in its about center position (5K/2=2K5) to catch these tolerances, so the fixed resistor better would be 7K5 instead for this example of a series string.
The flaw with this series string is, trimmers or pots might fail one day or the wiper might lose contact for the blink of an eye while dialing and this open circuit resistance value now increases to infinity, resulting in an opamp without feedback, running open loop. Using a maybe 15K resistor for R5 and placing a 27K in series with your 5K rheostat in parallel to this 15K will leave enough variation to dial in your perfect 10K value and limit the feedback resistance to 15K, if the rheostat fails.
Only using a fixed 10K resistor for the feedback resistor R5 will spare you the prementioned parts cost and hassle.
.) For usual there is a reason for every parts value. Nothing wrong with changing it, but its not the worst idea to know what you're doing. You could learn so much by simply ignoring these BOMs and have a look at the parts datasheets, app notes or just applying ohms law. Besides obvious errors, you might notice some essential parameters not listed in these BOMs, such as FI the power rating of R1 and R2 from this builds BOM when you imagine a -not so uncommon- shorted mic cable dumping +48V phantom to gnd. You do the math, why a FI 0.25W resistor rating in this spot might not be the best idea. You might read about parts temperature limits when soldering to their leads, especially for caps, switches or semiconductors....
