I already have a good solution for the way too sensitive inputs of the Sugden. First I wanted to put an active attenuator between the preamp and the power amp boards inside the Sugden, but I discovered that with an input signal of 1.77 Vrms the pre-amplifier started to distort when the potentiometer was set to 2/3. My Meitner DAC even has 2.3 Vrms.
So attenuation of the signal must take place before the Sugden preamplifier.
I looked at the input circuit in the Sugden and determined that the input is a 475 ohm resistor in series with the 50 Kohm volume potentiometer. Only after the potentiometer does the Signal encounter a HF blocking capacitance.
That means that I can place a resistor attenuator at the input of the pre-amplifier board without any problems. I chose 10.2 Kohm in series and 475 ohm parallel. Seems to work fine. High frequencies such as 50KHz are also not attenuated. So with there resistance values I end up with a damping factor 22.5x. or -27dB.
now the potentiometer can be set to twelve o'clock while listening and the sound is still beautiful.
So attenuation of the signal must take place before the Sugden preamplifier.
I looked at the input circuit in the Sugden and determined that the input is a 475 ohm resistor in series with the 50 Kohm volume potentiometer. Only after the potentiometer does the Signal encounter a HF blocking capacitance.
That means that I can place a resistor attenuator at the input of the pre-amplifier board without any problems. I chose 10.2 Kohm in series and 475 ohm parallel. Seems to work fine. High frequencies such as 50KHz are also not attenuated. So with there resistance values I end up with a damping factor 22.5x. or -27dB.
now the potentiometer can be set to twelve o'clock while listening and the sound is still beautiful.
That is very high sensitivity - I rinderpest why they did that?
If you attenuate the input, just be careful to think about the thermal noise implications. Depending on the preamp schematic, I’d think reducing the preamp gain would be a better solution.
If you attenuate the input, just be careful to think about the thermal noise implications. Depending on the preamp schematic, I’d think reducing the preamp gain would be a better solution.
The input of the Sugden already has a 475 ohm resistor in series with a 50K pot. The thermal noise will barely increase due to the 10.2K resistor with 475 ohms to ground, which attenuates the noise from the 10.2K resistor by a factor of 0.0455. The noise from the DAC and cable are also attenuated by the same factor. With the DAC on and the streamer paused, I can turn the volume knob on the Sugden all the way up without hearing any noise. I don't have a schematic of the preamp, but it "only" amplifies 6x, so I still have to apply additional attenuation to get to 22.5x.
Ahh Ok - so you have a c. 20:1 attenuator. That’s cool. 👍
If you just put a series resistor with the pot, that would be noisy.
If you just put a series resistor with the pot, that would be noisy.