hazel said:
iprovlek said:
Need help with gain reduction with this THAT 4301 compressor!
I bought from DIY-TUBES.com!
Can be 1mA meter,or another meter use here in this project!?
If meter can be use what I must connect to work for gain reduction!?
Thanks
Looking at schematics
http://diy-tubes.com/image/data/manuals/that4301-v1.5.2.pdf you can see first page bottom right corner, after Ratio wiper pot there's R19 15K optional resistor, just replace it with your 1mA meter and it should work
No, it wouldn't work. The load presented by the meter would be utterly wrong for the circuit. The voltage here is -6mV per dB of Gain Reduction. It is necessary to insert a buffer there; in fact, more precisely, it needs be a voltage-to-current converter.
One has to decide first what scale is targetted. Generally, people opt for about 30-40dB GR for full-scale deviation.
Let's say 30dB with a 1mA meter.
You need a converter with a characteristic of 1mA for 180mV (6x30). It can be as simple as a simple 180 ohm resistor.
This resistance must include the meter's internal resistance, which is often higher than that.
So there should be some gain, which the buffer would also provide.
It's a problem with more unknows than equations, so there is an infinity of solutions.
Let's see:
r is the meter's internal resistance in ohms
R is the series resistance in ohms
I is the full-scale current in A
GR is the maximum GR corresponding to full-scale deviation in dB
G is the buffer's gain (no dimension, just a number)
We get:
I=6. 10e-3.G.GR/ (R+r) or
R=[6. 10e-3.G.GR/I] - r
It's a problem with more unknows than equations, so there is an infinity of solutions.
For the buffer, you may use any old opamp (TL0, 741...).
You must choose a value for R+r that doesn't imply too high output voltage from the opamp. For a 1mA full-scale meter, that would be about 10k.
