IGC Without Volume control

[budwiser]

I want to complete my gainclone without a built in volume control since I will be using a preamp with it. What changes do I need to make to do this.



Pete

[planet10]

ASFAIK, you would just replace the pot with a fixed resistor.

dave

[sek]

(Just to jump back in again)

Yes, what the pot would do is to set the desired (selectable) input sensitivity of your clone. This pot would have a certain setting whereever you like it (nominal sensitivity).

If you don't like the recommended single resistor for the sake of ensuring a certain (desired) sensitivity at a certain (desired) input impedance, you could simply "fake" that setting of your pot by soldering two resistors in place.

Just give them the exact values that a sufficient pot would have had. Or calculate a matching voltage divider, starting with your desired input impedance for the shunt resistor.

Recommendation from scratch: series 0Ohm, shunt 22kOhm. Build in a mounting space and pins for a series resistor, then either mount one or bridge the pins with a piece of wire, whatever suits the maximum output of your preamp to prevent the gainclone from clipping or current limiting!

Hope this helps,
Sebastian.

[Peter Daniel]

Actually you don't even need a shunt resistor. 10K series resistor sets the input impedance and that's all. You can omitt the pot without any additional changes/substitutions.

[Joe Rasmussen]

Originally posted by Peter Daniel
Actually you don't even need a shunt resistor. 10K series resistor sets the input impedance and that's all. You can omit the pot without any additional changes/substitutions.

Agreed Peter, except for safety reasons. The danger lies in this, if the interconnect is accidentally removed the IGC is now unity gain and will be UN-stable. Have you heard what that does?

If a resistor to ground was put there and it's value plus the value of 10K (to use your example), then devide these into the value of the main feedback resistor and provided that value is equal or greater than 10 (gain = 20dB) then IGC will be stable.

Let's say f/b resistor is 220K, then 10K (defining input impedance) plus a 12K to ground will keep it safe.

220/(10+12) = 10 (20dB)

But that means 5K454 input Z. A bit too low for my liking. Use higher values, 470K, 22K setting the same gain, then another 22K to ground. Input Z now 11K.

470/(22+22) = 10.68 (20.6dB)

What I am saying is this, using IGC as power amp without pot, you will need to double all values to keep input Z up. It works very nicely as I have done it.

Incidentally, the additional use of an LPF, which I recall you tried, cap shunts to ground at HF and keeps things stable. Likely then, there may be no need for ground resistor. The 3875 doesn't like much feedback above 200KHz, the phase goes severely beyond 90 degrees (positive f/b), so the closed loop gain needs to be kept up and f/b low. At 200KHz a 1n2 cap is 660 Ohm. So that's another way to keep things on even keel and steady in case somebody pulls out the RCA plug.

So, instead of using a grounding resistor, use a small cap perhaps as low as 220pF (3K617 @ 200KHz). Now your 10K stays near 10K input Z at audio frequencies. A neat trick.

Joe R.