Thinking of building an active tube preamp (or buffered preamp) and would like to add a bypass switch to have the option to operate it as a passive pre as most of my sources have enough output to drive the power amp. The signal would be split after the input selector and attenuator. My question is… can I leave the preamp powered on and simply switch back and forth between the active and passive preamp? I understand tube power amps need a load or else bad things happen. Thanks!
You don't need a load for a normal preamp. The requirement for a load on power amps is to prevent damage caused by high voltages occurring in the unloaded output transformer when or if the amp is driven hard. Assuming your preamp isn't transformer coupled at the output then that situation can not occur.
Make sure that you pay attention to tying any capacitor coupled outputs (and inputs) to ground via a high value resistor to help prevent any thumps as you operate any bypass switches. Any points that switch must be a at zero volts DC to prevent noise.
Make sure that you pay attention to tying any capacitor coupled outputs (and inputs) to ground via a high value resistor to help prevent any thumps as you operate any bypass switches. Any points that switch must be a at zero volts DC to prevent noise.
Most tube preamps have a high value volume control (100k - 500k) that would not be suitable
for a passive line stage. Such values would cause early HF rolloff in conjunction with the
load capacitances present. If yours has a 10k - 50k pot, that would be ok.
for a passive line stage. Such values would cause early HF rolloff in conjunction with the
load capacitances present. If yours has a 10k - 50k pot, that would be ok.
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thanks for your replies!
it wouldn’t be transformer coupled so i guess that addresses my concern! noted about the caps and will implement when i finalize the design.
i’m still working on the design but i was planning on a 25k or 50k stepped attenuator. so should be good to go, ye?
Use the lowest value that is compatible with your sources, and with your tube phono stage (if any).
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Are turn-on and turn-off thumps an issue for a tube preamp anyway, especially if using SS rectifiers in the B+ ???
Never thought about it really. I would guess not from a cold start if if solid state reccys are used.
The making sure all points to be switched are at zero still applies though, otherwise you'll get a step change in voltage and a loud thump.
The making sure all points to be switched are at zero still applies though, otherwise you'll get a step change in voltage and a loud thump.
Preamps, yes; no-load operation will not harm them. Tube power amps are another story--running them unloaded could cause damage to their OPT's. They need to be properly loaded when operated.
There are transformer output preamps. If you follow those designs, you will need to have the output loaded.
Otherwise not a problem.
Otherwise not a problem.