By way of an update - I've tested the humming further by disconnecting the sources and turning up the volume on the preamp to full. The humming was through one channel only and appears to be an AC hum due to the wires running from the transformer to the power switch - these run directly alongside the signal path of the left channel. I've pulled the AC wires to the side slightly and noticed a decrease in the humming. However it is definitely still there at higher volume levels. Given there is very little space in my chassis, what is a good way of shielding the left hand side of the pcb from the ac wires?
Twist the flow together with the return of the two power lines running to the switch.
Twist the flow together with the return of the two signal lines running nearby.
And, move the switch to the back panel.
Only have remote operated low voltage DC triggering to turn ON/OFF running to the front panel.
And, check your grounding and earthing thoroughly, eg. where is your main audio ground?
Sorting the DC coupled input to grid PIN.
Move the DC blocking cap to the input of the pre-amp. That's where it should have been in the first place. In series with R11 and right next to R11 is perfect.
This gives you the operating flexibility to connect any style of preceding equipment.
I note that there is no RF attenuation at the input to the preamp.
Add that in. Since R11 is very high the capacitor to be added in parallel to R13 must be low value. Try 47pF. listen to hear if this cuts off too much of the HF response. You may need to adjust (swap) this 47pF cap from 22pF all the way upto 150pF
You might also find that you can reduce the value of R11 now that the DC block has been fitted. Any comments from the tube/valve Members?
Twist the flow together with the return of the two signal lines running nearby.
And, move the switch to the back panel.
Only have remote operated low voltage DC triggering to turn ON/OFF running to the front panel.
And, check your grounding and earthing thoroughly, eg. where is your main audio ground?
Sorting the DC coupled input to grid PIN.
Move the DC blocking cap to the input of the pre-amp. That's where it should have been in the first place. In series with R11 and right next to R11 is perfect.
This gives you the operating flexibility to connect any style of preceding equipment.
I note that there is no RF attenuation at the input to the preamp.
Add that in. Since R11 is very high the capacitor to be added in parallel to R13 must be low value. Try 47pF. listen to hear if this cuts off too much of the HF response. You may need to adjust (swap) this 47pF cap from 22pF all the way upto 150pF
You might also find that you can reduce the value of R11 now that the DC block has been fitted. Any comments from the tube/valve Members?
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R11 is part of a pseudo-balanced input attenuator. You might need to change R12 too. The whole amp is strange, so changing it could lead to unforeseen problems.
I've turned the switch upside down which allowed me to run the AC cables along the edge of the chassis, then twisted the cables as per the instructions above, this has improved the hum markedly! Unfortunately the holes are pre-drilled so cannot move the switch to the rear...so that's about as good as it is going to get I think. The capacitors I installed seem to have one end of their leads connected to the casing (also fabricated from metal) so they pick up hum and cause buzz when I touch them!
Interesting point re the 'pseudo-balanced input attenuator' - the balanced input is definitely QUIETER than the unbalanced input...what the?!?
Interesting point re the 'pseudo-balanced input attenuator' - the balanced input is definitely QUIETER than the unbalanced input...what the?!?
Are you referring to a low pass filter?
At the output of the AD1865 dac w/ 1k i/v resistor, I soldered some capacitors from signal to ground (tip to sleeve on the rca connector) using these calcs:
1/(2 * 3.14 * 1000 * 0.0000000068) = 23.4 KHz
So the value is 6.8nF (0.0068uF).
Will I also need one inside the preamp?
Out of interest - at the output of the dac, should it go:
dac chip -> i/v resistor -> dc blocking cap -> low pass filter cap
My current set up:
dac chip -> i/v resistor -> low pass filter cap -> preamp stepped attenuator -> dc blocking cap
I don't know what your DAC puts out, so I'll leave that to someone else.
Add RF filtering to the input of your equipment.
Add RF filtering to the input of your equipment.
Andrew - I've added the dc block to the input of the amp as suggested. Why would this mean I can reduce the value of R11/R12? (going from 10k to 4.7k here - according to the designer - would increase the gain by 100%)
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