[SOLVED] MF X-CAN V8P significant noise floor

[madsherlock]

Hi,

I've been reading some posts on this forum and decided to buy a used MF X-CAN V8P (non-modded). It came with the X-PSU V8.

I turned it on, plugged in my 56 Ohm headphones, and I could hear noise. Not hum, just something resembling white noise, or "hiss" you would say.

It's what is called the noise floor, I believe.

It is not affected by the volume potentiometer.

I would like to lower this noise to an inaudible level as the first upgrade to my new used headphone amp.

Should I start with getting a better power supply? (I believe one is sold in these circles)

Or better tubes? (I have used stock tubes here)

Or something else? Debugging suggestions appreciated.

I've had a hard time finding which upgrade would lower this noise, as I don't understand all the terminology here. "Cleaning up" the audio, is that what I need here?

Based on my experience with other amps, this kind of noise would come from something switching at a high frequency and might be fixable with bigger/better capacitors on the DC supply voltage, and/or an inductor in series with the supply.

Additional info: Knocking slightly on the amp makes a low audible noise in the output. That noise increases in volume with the volume potentiometer.

Not a big deal, as it won't be knocked often, just wondering if this is an indicator that the tubes have gone bad, and that this might also explain the constant background noise.

I am using the LINE IN input, not the internal DAC.

Also, the amp sounds pretty good.

Best regards,

Mikael

[PinkFloyd]

Hi Mikael,

That sounds very much like the tubes (Valves) to me. When you tap the amp the noise is microphonics from the tubes. The first thing I would do is replace the tubes.

All the best,

Mike.

[madsherlock]

Hi,
Thanks, I will try that then. Plenty of tube suggestions for any budget on this forum, so I should be able to find something

Best regards,
Mikael

[PinkFloyd]

Jun 16, 2019 14:00:53 GMT madsherlock said:

Hi,
Thanks, I will try that then. Plenty of tube suggestions for any budget on this forum, so I should be able to find something

Best regards,
Mikael

I've opened a carton of 100 Voskhod 6H23N (October 1992 batch) and they are superb! I can sort you out a precision matched pair, PM me for details. rockgrotto.proboards.com/thread/7184/valves-6n23p-6h23n-tubes-stock?page=3

All the best,

Mike.

[madsherlock]

Update for anyone interested:

I bought the tubes from Mike, and they sound great out of the box. The microphonics from tapping the amplifier have been heavily reduced.

Installation of the tubes was easy. Just remove the front cover, unplug three plugs and take off the grounding terminal. Then slide the PCB and back cover out of the amp.
Stock tubes were removed by gently pulling them out of their sockets, and "new" tubes were installed by gently inserting them all the way into the sockets.

The "noise floor" / hiss problem is still there though, at the same volume.

After some experimentation, I've found that the volume of this hissing increases slightly when the volume knob is around 1-2 o' clock, and then decreases again when turned up higher.
This has led me to believe that the problem is either on the input side of the amp, or with the PS.
I don't think it's the tubes, since these have been switched to a pair from Mike's stock, and the noise is still audible on both channels.
Shorting the inputs of the amp does nothing.

So, I suspect it is either a bad potentiometer (on both channels though...), EMI, or the power supply.

I will try each of the following, and see if they affect the noise intensity:
* Turning off all other electronics, including lighting, in the vicinity.
* Different room and wall socket.
* Increasing the distance to the PSU. Right now the amp is stacked on top of the PSU. I don't see how the PSU would emit high frequency noise like what I'm hearing, but I don't know...

I'll also see if I have some headphones with different impedance, and test how much that has to say in the matter.

If none of the above work, I will try removing the case from the amp, and see if the increased EMI pickup "sounds the same" but louder.
Then I would suspect EMI, and look for bad grounding in the amp.

Last resort: Swapping electronics in the amp, starting with the potentiometer and/or semiconductors.


Any ideas what might be causing the noise?

[madsherlock]

Am I correct that the volume potentiomer is between the tubes and the output transistors?

If so, I suspect the noise is coming from the transistors.
Do lower-noise alternatives for the STD03n and STD03p exist?
Or might a simple replacement of these help?

Best regards,
Mikael

[madsherlock]

Scratch that. The volume potentiometer is between input and tubes.
Buying a headphone attenuator to see if it can lower the background hiss enough for me. I get quite loud volume at 10 o'clock, and this thing goes to 5, so I have plenty of room to attenuate.
Just hope the extra resistors won't affect SQ much

Also wondering if the gain or the output impedance of the amplifier can be changed. Lower gain or higher output could reduce the background hiss.

Anyone have schematics for the V8/V8p amp?

[madsherlock]

Finally reduced the background hiss to an inaudible level.

I used the iFi Ear Buddy passive attenuator.
Putting that between my headphones and the amplifier solved my problem.
I belive that product is very similar to the attenuation adapter described on diyaudioheaven: diyaudioheaven.wordpress.com/headphones/headphone-attenuation-adapter/

The only difference is the attenuation of the iFi is 16 dB, not 20 dB.

Regarding sound quality: I can't hear any difference. Used a 16-bit FLAC of "Mercedes Sosa - Kyrie" to test that.
Instead of listening with the volume at "9:30" it is now on "11". Those 16 dB of attenuation don't do any harm then

Thanks Mike for advice, and for the Voskhod space tubes; They sound great, and they are much less microphonic than the used stock tubes.


To summarize what I believe I've learned:

* Background hiss is inherent to amplifiers, and is usually inaudible.
* Background hiss like this has roughly the same volume regardless of volume potentiometer position.
* Background hiss can be heard if a) the output impedance of the amplifier + the impedance of the headphones is low enough to allow it, and b) the ears are sensitive enough.
* Background hiss can be effectively reduced by voltage division withough affecting sound quality (much); See link above. However, this lowers the maximum volume of the amplifier.
* Background hiss can be effectively reduced by lowering the gain of the component which is amplifying the "hiss" signal inherent to the amplifier. Whether that be the tubes, or the transistors, or both, I don't know. However, swapping these components for lower gain "equivalents" may affect sound quality.
* I believe adapters can be found which would allow for the installation of 12AU7 (ECC82) tubes in the XCAN V8P amplifier. These require more current, and have lower gain than the stock ECC88 tubes. However, I don't know if other adjustments would need to be made to the amplifier for optimal bias etc. Also, the adapter adds height to the tubes, and space is limited in this amp.
* If you have high impedance headphones (higher than 56 Ohms apparently...), there is less risk of hearing background hiss.
* Mike has some excellent sounding tubes for sale.

I hope this may be of help to others hearing background hiss from their amplifiers

Best regards,
Mikael