Dear TT Forum,

I just finished my PX18 Kit (by the way many thanks to @Stephan61 for the help on power supply connections: the kit worked from the first switch on!).
I find the sound really great, however I am quite surprised by the background noise of the High Treble channel.
I made no modification to the kit (e.g. Master volume or different cap values).
Symptoms:
- Normal channel makes almost no noise, no humming, even at 10.
- High Treble generates a disturbing 'breathing' (maybe 'atmen' would be the German term? In French we call it "un souffle"), and this noise is proportional to the volume of the Channel. At HT volume at max (10) and all knobs to max (10), I would describe it as the sound when ones fries a steak in a hot pan with butter! Attached some sound samples.
- The noise is highly dependent on three pots, by order of influence
   1/ Presence
   2/ Mids
   3/ Treble
If all pots are set at twelve o' clock (EQ and HighTreble volume at 5), turning the Presence to zero reduces the noise A LOT. Turning both Presence, Treble and Mids to zero make the noise clearly low, but still to hear at Volume 10 - and the sound is dull.

I have read that Marshall are often louder than others (I returned a DSL20 because with 0 volume it was already super loud), however I wonder if there is a way to reduce it to an acceptable level - or, even more fatal, if the symptoms evidence maybe something wrong in my build.

Troubleshootings I made up to now:
- COmparing influence of inputs with and without guitar: no difference. Same with patch or no patch (Normal channel remains nice and quiet)
- Comparing the noise with the same cab (incl. cable) on another amp: I did that with Emma, that turns out to be so quiet that I have to turn everything to 10 to start hearing a similar noise, but at a clearly lowere level
- Checking Bias again: I did have ca. 60 mV on R30. Modifying this value did not have a big influence on the noise, unless I turn the variable resistor all way down to a super low value (e.g. 10 mV, but probably not right?).
(By the way: R30 is supposed to be 1 R, but I measured 1.3 R. Is the correct target bias supposed to be a current of 60 mA or a tension of 60 mV - in the first case, I should even aim at 78 mV which has an even higher noise level).

I have attached recordings of the noise level at ca. 1m of the cab, with all pots (incl. volume) at 12 o'clock and turning either Presence, Middle or Treble from 0 to 10. Presence knob has a huge impact.

Any advice or comment more than welcome - I would also welcome "hey, you are trying to make silent an amp that can't be silent !!", I would then stop turning the wheel like a guinea pig :-)
Thanks!
Regards
David

Hi David,
First of all good to hear that your work was successful!
Regarding the noise, the correct term in english will be 'hiss'. (In german 'rauschen'). Thats only btw.
Its normal for that amp that the treble channel produces more hiss than the normal channel, especially in case of treble or presence pot is wide open. Thats because it is correctly called the "high treble " channel. 
The root cause for the hiss is multiple, every part of the amp and also every connection (quality of the solder joints!) contributes some portion to the all over hiss.
You may reduce the hiss by patching both channels like most players did in the past times, for blending the harsh treble sound of the HT channel with the darker sounding normal channel. It needs only a short guitar cable to connect the channels. This works very well with the 4 input Marshall amps and same with the PX18.
If you want to go deeper and modify the circuit, the you can play with the cap over the HT volume pot. It will reduce the high treble and also some hiss.

Regards Stephan

Thank you very much Stephan !

I looked also into some Marshall forums and it seems that the SV20 (very close to the PX18 in term of power and circuitry) is also very noisy on the treble channel, so I must live with that!
As a quick win, I tried today to swap the cable toward the treble volume pot with shielded wires (that's how it is recommended for the JC18), but it did not provide any notable reduction of hiss. I intended to try the same for the HT input jack but from what you write, that would also not be sufficient, so I'll just try to live with that Marshall feature :-)
Actually you are right, blending with the Normal channel (with majority of the blend on Normal Vs Treble) makes it more bearable.
Question on the cap value on treble volume:  the stock one is 4.7 nF. What is the next order of magnitude to reduce it a bit while not losing too much the character of the sound? Would 100 or 200 pF already be too low?
Thanks!
Kind regards
David

Hi David,
220 pF or similar is quite low and it will sound more like the bright switch on at other amps and not any more like the HT channel of a Marshall.
Try approx. 1 nF as a starting point and you will hear if it would be ok for or you like more or less treble.
As you already experienced, shielded wires will not help in terms of hiss reduction.
Regards Stephan

Hi, if there is no hiss then the amp is broken.
There are some simple "noisegates" made with a view diodes which help tu reduce hiss. Just ask your favorite search engine.

KInd regards, Dirk

Thank you both Stephan ans Dirk!
With your recommendations I am sure I will be able to get the best of this great amp.
I made another comparison with a Vix AC10, and with much volume the hiss is there and the amp is already loud with no volume, whereas my PX18 is totally silent if no treble volume is in, do +1 for the PX18 :-)
Best regards and many thanks for helping ´newcomers' build their dream amps!
David