First Watt M2 Noise

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IronmanIV

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Joined 2021
#1
I am getting some noise from my First Watt M2. It is noticeable on higher frequency compression drivers on a 800 hz. cutoff horn. So, the driver is VERY efficient and I don't think this noise would even be noticeable on typical efficiency speakers. But, high efficiency is what I use. I tried plugging the M2 into a power conditioner and no change. I don't think it's power supply hum, it's too high frequency. I also allowed the amp to warmup for several hours.

I have read about using a resistor and L-pad to bring amps up out of a noise floor. I do not know how to design this. Anyone tried anything like this with the M2? Any tweaks?

I love the amp and I fully embrace the zero feedback Class A design, so this is not a criticism in any fashion.

Cheers,

Jamie
 
#6
Thanks Mark Tillotsan (it won't let me quote you or reply explicitly), that's very helpful. My guess was it's a bit characteristic of the amplifier since it is a zero feedback design. I think I would just pad up the output resistance to get things above the noise floor. The drivers are too efficient anyway.
 
#7
IronmanIV said:
It was built by Pass Labs. I guess if I was hearing the same thing from other amplifiers, I wouldn't be here asking about the M2?
Click to expand...

Since this is diyAudio, a diy forum where many people build M2s and ask questions about their diy amplifiers, I think it was a fair question. It's good to have as much information as possible.

The transformer that provides voltage gain in the M2 is the main contributor to noise. You could try positioning the amplifier away from emi producers.
 
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#8
How do you know that it is the auto-former that is causing the noise? And, if the noise floor is intrinsically part of the amp design as identified by Mark's explanation of the noise specification in the manual as compared to other First Watt amps, how would that be cured by moving the amp?
 
#9
Many has built a small mu-metal box for the Edcor transformers (including myself). I also used screened toriods etc. and placed the toroid as far away from the Edcor as possible.

It is possible to reduce the output noise to below 100 uV.
 
#11
I think you guys are confused. From that link you provided, Nelson says that it is the autoformer, not the transformer, that is shielded with mu metal and that the 500 uv noise is achieved AFTER the “adjusting the power transformer and shielding the autoformers.”

He also says “put this amp on a 100db pair of speakers, and you will hear it.”

What Nelson is trying to tell us, is that the circuit is inherently noisy, compared to other amps, or when used with very efficient speakers. This is also the interpretation of the noise values for the Pass built version of this amp as provided by Mark.

I think your advice to modify a factory built amp as designed and built by Nelson to the best of his ability, to fix a problem he acknowledges is inherently intrinsic to the circuit and NOT mitigated by shielding, is bad advice.

Given it requires 100 db efficient drivers to be noticeable, per Nelson, and this is also what I’ve noticed, it is a much better solution to put an l-pad on the driver to attenuate the driver and lift the amp up out of the noise floor.

To really understand the operation of this amp, we need to consider how it interacts with the speaker drivers. Although it was intended to offer the benefits of a SET tube amp, it was not necessarily intended for use with super efficient compression drivers on horns. This convoluted discovery aside, I think it could still be good at this. Despite the noise floor issue, it has fairly low harmonic distortion and offers more power than typical SET amps, which may also have fairly high distortion, however pleasing to some. It might also have a damping factor more like tubes, but, I’m not sure about that.
 
#14
With factory build it will be difficult to do much. I am surprised that the factory build does have a mu-metal box on the Edocrs (voltage gain transformers).
Maybe have a look inside and if not implemented then this alone will do a lot to reduce noise.

I built it (called M2x as DIY) as mono blocks in large chassis. So I had space to move power transformer far away and implement extra PSU filtering (CRCLC).
The circuit itself can be made silent. There are other amps using same kind of "step up trafo". E.g. the VFET kit-amp. But here the PSU is in a separate box and this amp is very silent.
 
#17
While this is a DIY forum and there's a lot of enthusiam for the possible modifications, I don't get the sense the OP is looking to modify his factory built amp.

Since the OP is looking into an L-pad solution, it'll be interesting to see how much attenuation he ends up with before he's satisfied and if this attenuation is too much for him and the rest of his gain chain.

@IronmanIV : In case you haven't seen this, a lot of specs for the FW amps (including quote noise figures) can be found here: https://firstwatt.com/prod.html
 
#19
MEPER said:
Ok, the Edcor's are shielded in the factory M2 (so probably not much more you can do without much work):

View attachment 1231528

The Edcor's are the small boxes just below the RCA connectors.
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Thanks for the pic, I can at least crack the case and double check the Mu shields are in place.

Would you expect the auto-former noise or PSU induced noise to be like HF pink noise, or HF static and crackle? I probably should have been more explicit about the noise. I am high passing above 1200 hz to the M2 using an active crossover and the M2 output is going to a HF compression driver with horn. So, the noise is in the higher frequencies, more like pink noise or white noise.

I am speculating wildly because I don't have any direct observation of anything like this, but my guess would be that this noise is thermal noise from the JFET's and MOSFETs. I find it hard to believe that Nelson couldn't design a PSU and input voltage auto-former to reject this HF hash. But, in designing a circuit for no negative feedback on a simple circuit using transistors attempting to replicate SET sound, I wouldn't be surprised if thermal noise was present. There is not feedback or other operating point manipulation of these transistors, just a ramp into pure Class A.
 
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