How to eliminate the "white noise" for the 300B tube amp?

[Angelpat]

I was build a 300B mono block with 2 ECC32 at SRPP as the input and driver. Everything goes well but the only disappointing that I have is the white noise when the signal from the preamp have been "mute".

From the schematic that I got, there is 2 x ECC32, one at the input stage and one work as the driver for a SE 300B tube. Both the ECC32 have been connected at SRPP.

I was tried to remove the cathode bypass capacitor from the input tube and all the white noise have gone but it do lose lot of mid and low....

Also tried to adjust the gain of the input tube by changed the valve of the resistor(s) of the cathode(s) but did not show much improvement.

Any recommendation?

[Yvesm]

Is the noise still here with the input grounded (use a shorted plug for exemple) ?
If not, the noise comes from the preamp, not from the amp.

Yves.

[mastodon]

Is the noise still here with the input grounded (use a shorted plug for exemple) ?

Everything goes well but the only disappointing that I have is the white noise when the signal from the preamp have been "mute".

So I guess the noise originates from the amp itself, and not from the source/preamp, right?

Is it possible to trace the source of the noise by disconnecting either one of the .22/600V coupling caps?

[Yvesm]

Not sure what the preamp does when "muted".
Disconecting the preamp and shorting the amp input is just a "easy to do" sanity check before other investigations.

[Angelpat]

i was tried to disconnect the preamp and do not find any white noise (hiss) out from the power ampat the amp....

however, when i connected my preamp to another factory made power amplifier, there is no hiss at all too

not sure where is the problem came from?

[Yvesm]

Looks like if the amp has too much gain.
The 300B needs roughly 150V pp on its grid for full output while the two SRPP stages can provide a gain of near 600 times.
That means an input level of (150 / 600) = .25V pp or 90mV rms . . . ten times too sensitive !

Perhaps just turning down the input potentiometer . . . or removing the preamp

Yves.

[jrenkin]

Could he eliminate one input stage?

[Yvesm]

Probably not !
Gain should be 50 to 60 and a single ECC32 (similar to 6SN7) can't do that in SRPP configuration.

However, a 6LS7 (same pinout) should do !

[Angelpat]

i still wanted to see if this problem can be fixed without changed the tube.

have the following findings when i did a trail further.

1. shorted the RCA jack of the power amp : without noise at all

2. connected the power amp with the RCA cable but not connect to the preamp : "de .... de" noise came out from the tweeter.

3. connected the power amp to the preamp with both power ON : hiss noise from the tweeter (can be heard from 3 ft away).

4. connected the power amp to the preamp with the preamp power OFF : "de .... de....de..." noise the same as the situation of above point 2.

[quikie22]

With input grounded and no noise, it means the source of noise cannot be from the power amp itself. This is further substantiated by the fact that when the preamp is switched on and connected, you get the hissing sound, meaning the noise is coming from the preamp. As Yves said, probably the gain of the power amp is so high, it is amplifying the low-level noise from the preamp to audible levels. Your solid state amp may not be doing that because it may have a lower input impedance and therefore causing a loss of the higher frequencies due to Miller capacitance of input stage, hence attenuation of the noise.

The "de...de...de..." sound that occurs with RCA jacks connected or even with the preamp turned off is because the cables are probably picking up some stray noise in its "ungrounded" form (happens on my system too), with too high an input sensitivity. I just make sure my preamps/inputs are all turned on and running before I turn on my poweramp.