I've built myself the green channel (clean) of the famed Randall RG preamp to along with my TDA2050 power amp. It more or less the same thing as the famous red channel but with a treble bleed capacitor on the gain pot and no clipping diodes. I did however make some adjustments based what components I had and the amp it was going into:
Alltogether it sounds pretty nice, a nice JFET overdrivey sound. The issue I have is that I suspect its more noisy than it should be, possibly due to the changes I made in regard to what reference voltages are.
With the input grounded, If I set the volume to 0 its dead silent, if I put my ear up to the speaker I can barely hear 50hz hum.
If I set the gain to 0 and thus effectively mute the first stage, theres a waterfall of noise coming from the second gain stage alone whenever volume is past 50% (where it needs to be for full clean volume if I set the gain control to around 25% for the cleanest of cleans with my hot pickups xP ).
If I put a mic in front of the speaker and watch the VU meter in my computer I can measure that each stage is adding about 12dB of white noise.
To be fair, this is mainly evident when dimed, and with signal applied from 2/3 master volume and up the power amp is already clipping but still, lower noise would be better tjomg.
Is this just a fact of life with preamps of such design? if I were after a quieter amp I should have used something you're actually adjusting the gain of the stage and not just attenuating the signal afterwards? Use clipping diodes instead of clipping the transistors themselves so you don't have to amplify the hell out of your signal to get clipping?
- moved the master volume to after the tone stack
- made the entire thing run off bipolar 12V power (so that slightly changed the layout of the biasing)
Alltogether it sounds pretty nice, a nice JFET overdrivey sound. The issue I have is that I suspect its more noisy than it should be, possibly due to the changes I made in regard to what reference voltages are.
With the input grounded, If I set the volume to 0 its dead silent, if I put my ear up to the speaker I can barely hear 50hz hum.
If I set the gain to 0 and thus effectively mute the first stage, theres a waterfall of noise coming from the second gain stage alone whenever volume is past 50% (where it needs to be for full clean volume if I set the gain control to around 25% for the cleanest of cleans with my hot pickups xP ).
If I put a mic in front of the speaker and watch the VU meter in my computer I can measure that each stage is adding about 12dB of white noise.
To be fair, this is mainly evident when dimed, and with signal applied from 2/3 master volume and up the power amp is already clipping but still, lower noise would be better tjomg.
Is this just a fact of life with preamps of such design? if I were after a quieter amp I should have used something you're actually adjusting the gain of the stage and not just attenuating the signal afterwards? Use clipping diodes instead of clipping the transistors themselves so you don't have to amplify the hell out of your signal to get clipping?