HISS and HUM, acceptable level

[sebinHK]

Hello everyone.

When building tube amps I always try to have the HISS and HUM level as low as possible.
But different people have different expectation about what is an acceptable level of those noise produced by the amplifier.

If we take an amp without anything connected at the input and connect a speaker at the output.
Volume and tone controls can be increase until you are hearing noise, AC voltage can be measured at the output jack or at the speaker.

What is for you the maximum voltage you can accept in your amplifier, before you have the feeling that the HUM and/or the HISS are too loud ?

[Haze13]

Interesting thing that you ask... I'll check that out. Although I have only 2 builds, but one of them is a clone, and another is my take on SE marshall. 2204 and JCM800 78 (2 gain stage) pre with a loftin/white for a pentode (dc connected power amp) with a SMPS for B+ from 12v.
The thing is that a common belief that SE have the highest noise level. Well mine is dead quite. Even though it's a SMPS.
Do you talk about clones or just builds of some sort?

[sebinHK]

Any amp, you may have built it or bought it.

The question is not hardware-related, but hear-related.

For you dead quiet is : X.XX Volt
For you acceptable is : X.X Volt
For you too much is : X Volt

[Haze13]

Power Amp - 1 half of 12au7 DC coupled to a 6973. 2.8-3 watts at max with no visual clipping.
Pre amp - GTO Soldano clone - 4 gain stages - Everything at max (Volume, Gain, Tone)
Load - 7.1 ohm resistor.
~50mV amplitude of noise:
http://i1276.photobucket.com/albums/y47 ... dlwqt9.jpg

No Gain from a preamp:
10mV-15mV of noise
http://i1276.photobucket.com/albums/y47 ... hdqllz.jpg

[Haze13]

Sorry for a late response, I've had an exam.
I'll check my marshall tomorrow, it's at my work.

[sebinHK]

Thanks for the reply.

Your amp is a good case for this topic, it have 10 mv with the gain at 0 and 50 mv with the gain at max : it is a 4 gain stages preamp.

Please replace the load resistor by a speaker, to be able for you to listen the noise.

By turning the gain from 0 to max the noise will increase and you may feel at a certain value that the noise is too high, at this point check how many mv you have at the output, or maybe you won't feel it and you will consider the 50 mv good enough.

It is important to note the speaker sensitivity, as a 96db speaker will give less of noise compare to a 100db speaker.

[Haze13]

I've been looking for the less efficient speakers, but some how every 12" speakers (Jensen, Eminence, Celestion) rated at 100dB at 1 watt into 8ohm or 16ohm... Exeption is the Blackbird by Jensen - 98dB.
50mV of noise is a good thing for guitar amp considering 4 gain stages. I'll check and will write what I found.
Not so long ago I've been working on a vintage fender silverface and it was far worse than my amp... Or yours, cuz we have same results
There is just a huge amount of gain is going on, and there is no way that this will be lower. You can filter b+ with voltage regulators, more caps, chokes before and after the OT, metal film resistor with a good TempCo but to beat this result is very hard... The only way is to lower the gain. Tubes are far noisier than a Jfets (they are most quite semiconductors) that's the reason why there are not so many low noise Phono tube preamps. There has to be lots of gain, just like in the High-Gain guitar peamps, just in the guitar preamps all the companies are not interested in thing like Noise rejection, because high gain stage and low noise is not real.
If you want to explore that area, take a look at Morgan Jones Books about valve amps. You will find your self building differential pairs, push-pull gain-stages and some thing like aikido followers as drivers to cancel the noise + current/voltage sources. To much glass and SS devises... In the end some one whom you will show it, will tell you that a Plexi or Silverface Twin is his dream amp. I've seen it myself. They just don't want a quality! They need old, stock, barely working stuff!
But some how I will go on trying an exploring new/old circuits that never been used in guitar world!
Are you with me?

[sebinHK]

Warehouse have the G12A and the G12Q at 96 db.
Celestion have the G12EVH at 96db (mounted in the EVH cab) and the rocket 50 at 95db (mounted in the Fender Mustang cab)

It is quite easy to find a rocket 50 loaded cab at a decent price.

[Haze13]

Checked the setup with a speaker. Yes, I can clearly hear the noise. Input grounded, every thing on max.
Just like I said, there is nothing to do about it. There is no way to get rid of that noise, using regular gain stages. Push-pull though types or using CCS can help.