OK, so you can get polys at 400V.
My concern is bypassing R108, IC1 is in a delicate balance here where it could easily be destroyed by stray 300V.
My concern is bypassing R108, IC1 is in a delicate balance here where it could easily be destroyed by stray 300V.
Well the new ElectroHarmonix valves made no appreciable difference to the "hiss", whether they will improve the sound once I get the Impasse working correctly I don't know.
Next step is to bypass R108 but add a protection diode iaw the datasheet.
Do valve amps naturally exhibit a poor PSRR ?
I'm being driven more and more down the route that this is a PSU issue.
Next step is to bypass R108 but add a protection diode iaw the datasheet.
Do valve amps naturally exhibit a poor PSRR ?
I'm being driven more and more down the route that this is a PSU issue.
Last edited:
You really need to get in there with a scope so you can quantify what's happening and where instead of randomly flailing around. And look at your system gain structure- last I remembered, you had this running into another preamp?
B1 must have been before. Its single ended. How'bout some closeups Katie? Have you anything to measure with? Soundcard? That could tell us how the grids look like at least maybe.
I haven't had time to put the scope onto it yet.
The chain is CD - B1 - Impasse - F5.
The B1 is acting simply as the volume control and input selector.
The chain is CD - B1 - Impasse - F5.
The B1 is acting simply as the volume control and input selector.
Does it hiss with shorted input? Easy test so to know if the hiss is fed to or locally created in the particular ImPasse.
Well, I've finally got around to making some blanking plugs for the input.
With the blanking plugs fitted it is indeed quiet.
I guess I've got a problem with my inputs, however, it's not noise as I can plug the same leads into another amp and there is no "hiss".
With the blanking plugs fitted it is indeed quiet.
I guess I've got a problem with my inputs, however, it's not noise as I can plug the same leads into another amp and there is no "hiss".
It can be some weird interaction you can damp. You need a scope. The Impasse with transformer input and R1 C1 Zobel has a defined bandwidth when a B1 is 1MHz+ capable. Maybe raising R3 grid stopper in Impasse, but I don't know ask Stuart. Still, it will be stabbing in the dark.
What I think our esteemed friend is trying to say is get the B1 out of the picture. Get a pot for the ImPasse, wire it in properly and see how this circuit is actually supposed to work.
Other people, myself included, have been hinting (unsuccessfully) to encourage you to remove the B1 from your signal path. So I will come out and say it plainly, get the B1 out of the signal path, it is likely your problem, and if it isn't, it's still in the way of proper analysis of the ImPasse.
Agreed, but it does add noise. And the ImPasse has so much gain that the B1 noise is being amplified quite handily.
ImPasse adds another 26dB of gain (it was designed to drive unity gain power amps). The F5 has 15dB of gain. So that's 41dB of amplification of the input noise. -79dB noise floor ref full out is not particularly quiet- the hissy quality is symptomatic.
As I said a while ago I was just using the B1 as an input selector and volume control, but I will do as you suggest.
I've got a 50K Audio Log Taper pot, will that be high enough impedance or should I get a 100K ?
