ruffrecords
Well-known member
I am looking at binary relay control of the gain of one of my tube pre-amps. The gain needs to be varied from 10dB to 40dB in 5dB steps. The required gain setting resistance for each gain is as follows:
10dB 37K
15dB 12K3
20dB 5K6
25dB 2K85
30dB 1K5
35dB 820
40dB 430
A trivial way to achieve this is to use a 3 to 8 decoder and connects its outputs to 7 relays each one of which connects in one of the above values.
But I am sure it can be done with fewer by operating one or more relays and hence paralleling the resistors. In theory the minimum is 3 but I cannot find a set of 3 resistors which will achieve the above values. P.art of the reason is I need to achieve a 37K/430 to 1 span (that is 86 times) but the span of 3 bit is only 8 times. 5 bits would get me 64 times which could be close enough but I would like to do better. Another reason is there is already a resistor in parallel with these (a 47K) that cannot be removed as it is part of the dc feedback. This has much less of an effect at the higher gains but at the lower ones it means the values required are a lot higher.
For this reason it looks like it might be possible to achieve the range 25db to 40dB with two bits with reasonable accuracy and then maybe switch in some series resistors for the last three using another 2 bits.
I am sure this problem must have been addressed in the past but Google has not been much help. Any leads?
Cheers
Ian
10dB 37K
15dB 12K3
20dB 5K6
25dB 2K85
30dB 1K5
35dB 820
40dB 430
A trivial way to achieve this is to use a 3 to 8 decoder and connects its outputs to 7 relays each one of which connects in one of the above values.
But I am sure it can be done with fewer by operating one or more relays and hence paralleling the resistors. In theory the minimum is 3 but I cannot find a set of 3 resistors which will achieve the above values. P.art of the reason is I need to achieve a 37K/430 to 1 span (that is 86 times) but the span of 3 bit is only 8 times. 5 bits would get me 64 times which could be close enough but I would like to do better. Another reason is there is already a resistor in parallel with these (a 47K) that cannot be removed as it is part of the dc feedback. This has much less of an effect at the higher gains but at the lower ones it means the values required are a lot higher.
For this reason it looks like it might be possible to achieve the range 25db to 40dB with two bits with reasonable accuracy and then maybe switch in some series resistors for the last three using another 2 bits.
I am sure this problem must have been addressed in the past but Google has not been much help. Any leads?
Cheers
Ian