Input Level Devil please help

tcaster · 27th January 2011, 02:07 PM

I'm using a tube preamp into a Coleman 3-1 line switcher then into a new, cathode biased, homemade by veteran builder, 4 el34 power amp (and/or 2 other amps).Each amp has it's own speakers.
When I switch out the line (or or even unplug the RCA input cables at the amp) to the el34 amp, it hisses like white noise , not a ground noise, more of an input level issue I think.
As long as line level inputs are plugged in and/or active it is very quiet but I can't use my system switching set up the way I'd like to without having to turn the amp off.
I've also tried it with a stepped passive attenuator in line and the el34 amp doesn't like that either, it's noisy like an old radio on all but a couple stages of volume.
The other amps (one being a rebuilt Magnavox 175) are totally quiet when switched out or when using the passive volume control.
Can anyone help me trouble shoot this or recommend what steps I can take, either with outboard gear or internal amp mods, to remedy this input level devil? Much Thanks!

mastodon · 27th January 2011, 03:05 PM

Posting the schematic of the EL34 amp is probably going to help. What does the input circuit of that amp look like?

DF96 · 27th January 2011, 04:00 PM

Yes, a circuit will help. Two possible causes: a high value input resistor is generating noise but this is attenuated when a low impedance input is connected; the first stage is oscillating and needs a grid stopper.

tcaster · 27th January 2011, 05:25 PM

Thanks for the replys.I'm not a tech and I don't have a schematic but this thing is pretty simple ptp wired etc.Is a grid stopper something a novice could install?
If it's an impedance issue do you think using a line buffer between the Coleman and the inputs would send the correct impedance value to the amp?

27th January 2011, 02:07 PM	#1
tcaster diyAudio Member Join Date: Nov 2009	Input Level Devil please help I'm using a tube preamp into a Coleman 3-1 line switcher then into a new, cathode biased, homemade by veteran builder, 4 el34 power amp (and/or 2 other amps).Each amp has it's own speakers. When I switch out the line (or or even unplug the RCA input cables at the amp) to the el34 amp, it hisses like white noise , not a ground noise, more of an input level issue I think. As long as line level inputs are plugged in and/or active it is very quiet but I can't use my system switching set up the way I'd like to without having to turn the amp off. I've also tried it with a stepped passive attenuator in line and the el34 amp doesn't like that either, it's noisy like an old radio on all but a couple stages of volume. The other amps (one being a rebuilt Magnavox 175) are totally quiet when switched out or when using the passive volume control. Can anyone help me trouble shoot this or recommend what steps I can take, either with outboard gear or internal amp mods, to remedy this input level devil? Much Thanks!

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27th January 2011, 03:05 PM	#2
mastodon diyAudio Member Join Date: Jan 2011	Posting the schematic of the EL34 amp is probably going to help. What does the input circuit of that amp look like?

27th January 2011, 04:00 PM	#3
DF96 diyAudio Member Join Date: May 2007	Yes, a circuit will help. Two possible causes: a high value input resistor is generating noise but this is attenuated when a low impedance input is connected; the first stage is oscillating and needs a grid stopper.

27th January 2011, 05:25 PM	#4
tcaster diyAudio Member Join Date: Nov 2009	Thanks for the replys.I'm not a tech and I don't have a schematic but this thing is pretty simple ptp wired etc.Is a grid stopper something a novice could install? If it's an impedance issue do you think using a line buffer between the Coleman and the inputs would send the correct impedance value to the amp?

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