Hiss in 12B4 amp

[JDeV]

I just finished this pre-amp and have a constant hiss (too loud to ignore).
It does not change with the volume control.
I tried various PSU configurations, but it stays the same.
(HV MOSFET regulator, CRCRC, CLCRC)
On the PSU I have a 20mV 100Hz ripple and on the outputs I see a 40kHz 20mV ripple. I have no idea where that is coming from. It is not on the input, I also do not see it on the base or emitter of Q1.
Anyway, I don't think this is the hiss I hear, way too high for my ears.

I tried another set of valves as well.
I lowered the heater voltage also to 12V.

How can I fix this hiss noise?

Thank you for any help.

 

[artosalo]

On the PSU I have a 20mV 100Hz ripple and on the outputs I see a 40kHz 20mV ripple.

Is there also any audible hiss ?
Try to replace the CCS with a resistor to see if the hiss comes from the CCS (or from tube).

 

[JDeV]

Yes, I should have mentioned it. I can see 40mV of noise on the output, but it is to noisy to see the frequency on my old CRT scope . I will try the resistor instead of CCS.

Also, I use AC heaters, but do not hear any humm.

 

[JDeV]

You are a genius, that was it. No CCS, no hiss, and with only the resistor it still sounds great.

Now why would the CCS create the noise?
Everybody says it is better then the plain resistor.

 

[JonSnell Electronic]

Just goes to prove the humble resistor doesn't produce noise like the transistor does.
Noisy transistor I reckon the BC is your source.

 

[DF96]

Could be parasitic oscillation in the CCS or the valve. Try increasing the grid stopper to a few k.

 

[audiowize]

What's your B+ voltage? Do you have any PN2907, 2N2907, or MPS4250 available to use as the top transistor?

 

[JDeV]

I have a 220v B+, tried a 2N2907, same noise. With only a resistor, and B+ 270v, dead quiet.

 

[audiowize]

What LED's? (They aren't all quiet)

 

[Hearinspace]

Good question audiowize. That load circuit is the first CCS I ever built. The HLMP 6000 series red from HP was the recommended part (for low noise) at the time.

If you have the parts handy you might try 100R in series with the base of each transistor and .1uF in across each of the LEDs.

Though it was quiet I soon opted for another circuit.

Somewhere in my playing with this circuit I also tried 47uF from the Vin to the base of each transistor at the junction of 100R and Base.

 

[audiowize]

For a linestage, adding base stoppers or caps across the LED's shouldn't be necessary if quiet LED's and transistors are used from the start.

 

[Hearinspace]

I just thought that if he has a few parts handy then what's the harm in trying?

 

[JDeV]

What LED's? (They aren't all quiet)

Cheap, red, chinese ones. I will find some others and try them.

 

[Bassivus]

I have built preamplifier on this scheme and it works perfect for years now.

 

[JDeV]

Did you use "quiet" leds?
Was there any hiss without the 100n caps?

 

[Bassivus]

Well I got CCS already built on small pcb from a friend. I just wired all up and it worked flawlessly from the first moment here is the pic

 

[tequilamike]

Isn't that .047uF output cap also adding high frequency gain? I'm seeing a lot of values greater than 4.7uF in comparable designs.

 

[JDeV]

I tried various values there, there bigger the values I used, the worse the sound was in general. I am now just using a 10k resistor on the anode and going to keep it like that.

I am happy with the sound at the moment.

 

[JDeV]

... but there are always room for improvement.

I now use another CCS, 2x DN2540 set to 17mA.
HISS is almost gone (slightly more then only the resistor, but MUCH less then the LED version) and the general sound is even better then before.

I also changed the heater supply to a 12V 3A SMPS. To get rid of the faint humm, I also implemented the "ground breaker" scheme, works like a charm.

 

[JDeV]

I currently use this linestage with some Mullard 4-20's and Fostex FE207e speakers, which is very good but too sensitive I think.
With what can I replace them, not DIY up to £500.

Link to my amps: Mullard 4-20's