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-   -   Bravo V2 tube headphone amp - high pitched noise? (http://www.diyaudio.com/forums/tubes-valves/216406-bravo-v2-tube-headphone-amp-high-pitched-noise.html)

Magister Mundus 18th July 2012 06:55 AM

Bravo V2 tube headphone amp - high pitched noise?
 
Hi.

I'm a total newbie in the world of tubes, tube amplifiers etc.

I bought a Bravo V2 tube amp which uses a chinese 12au7 tube... (Valve Class A Tube Headphone Amplifier pre Bravo V2 | eBay)

(I'm not sure if I can post the link, sorry if I can't, I just want you guys to know what I'm talking about no advertising intended).

Well, it's the second day I'm using it and... I was about to turn it on, so I did the following:

1- Put the power supply connection (24V if I'm not mistaken)
2- Put the "In" signal (from computer soundcard)
3- Turned it on
4- Waited like a minute or something
5- Put the headphone on the "Out" of the amp
6- Raised volume from 0 to "something", not much

Now here is the deal, every single time I turned the amp on before, I put the headphone BEFORE turning it on...

So, when I started to hear the music, there was a screaming HIGH PITCHED sound coming through my headphones... This high-pitched sound was never there, not even with volume close to maximum, now it appeared in like 20% volume...

So you guys think maybe my tube is "dead"?
Next time should I be careful and turn the thing on AFTER I insert my headphones?
Also, can I use it with speakers?

Thanks and my apologies for asking too much!

Edit: The high-pitched noise (sounds like a 18khz tone or something, to describe it vaguely) increases as I increase the volume. I believe this info might be important somehow!

soundguruman 18th July 2012 04:07 PM

The amp must connected to a load (headphones) before turning it on.
The amp must have an input, recommend before turning on.

Allowing the amp to oscillate, without a load, is very bad. It can damage the tubes or other components.
Tube amps must be loaded before operating...

If the oscillation is still there, it could be a bad tube, or a broken solder connection on the circuit board.
The input / output jacks are susceptible to cracked solder connections- gently wiggle the jacks to test. Also, your cords that connect the amp could be bad, wiggle the cords around to test.

kevinkr 18th July 2012 04:27 PM

I believe this design uses a discrete fet or bipolar output stage, there is no way that a 12AU7 is going to drive a low impedance headphone directly with anything like the claimed performance. Since there do not appear to be any output transformers there should be no issue at all with plugging in the headphones after applying the power.

Based on the fact that the whine is affected by the volume control it seems more likely that it is coming from the external source. (Not impossible that the thing is also oscillating.)

Do the following checks:
With volume control all the way down and the unit off disconnect it from the source, then turn it on and check to see if the whine is still present.

Alternately or in addition if you have an iPod or other MP3 player plug that into the headphone amplifier and perform the same check.

Magister Mundus 18th July 2012 07:31 PM

Quote:

Originally Posted by kevinkr (Post 3096707)
I believe this design uses a discrete fet or bipolar output stage, there is no way that a 12AU7 is going to drive a low impedance headphone directly with anything like the claimed performance. Since there do not appear to be any output transformers there should be no issue at all with plugging in the headphones after applying the power.

Based on the fact that the whine is affected by the volume control it seems more likely that it is coming from the external source. (Not impossible that the thing is also oscillating.)

Do the following checks:
With volume control all the way down and the unit off disconnect it from the source, then turn it on and check to see if the whine is still present.

Alternately or in addition if you have an iPod or other MP3 player plug that into the headphone amplifier and perform the same check.

1-"With volume control all the way down and the unit off disconnect it from the source, then turn it on and check to see if the whine is still present. "

Nice! No whining, just a big BUZZ when listening through the headphones... Exactly like those when your speakers are on but the cable is disconnected from the source and you pick the cable and touch it on the tip.

2-"Alternately or in addition if you have an iPod or other MP3 player plug that into the headphone amplifier and perform the same check."

I got no MP3 Player! Lol... I got my notebook, HP DV4-1430US, which I don't use much, and connected its output to the input of of the amp (with it OFF, of course, headphone was already connected)...

Turned it on USING THE SAME SOURCE CABLE (with the music not playing) waited like 20 sec, hit play, put the headphones in my ears, and raised the volume...

NO WHINING AND NO BUZZ =D...I left it playing for like 15 minutes... Omg what did just happened ? I mean, I listened through my desktop soundcard (ASUS Xonar DX) after the amp started whining yesterday and it was absolutely fine, it wasnt damaged...

Any ideas?

Stefkorn 20th July 2012 12:55 AM

I believe those amps have no input caps, your output signal from your sound card is
( very likely ) not DC de-coupled. Put - as a test - two caps, sayn220 nF 100V between line out and line in and check again whether this will solve the problem.

Magister Mundus 20th July 2012 01:04 AM

@Stefkorn
I think it's not gonna be necessary, see below...

@everybody

I tried to reply the other day, but somehow couldn't...
I did the tests kevinkr suggested, everything went fine, and then I tried to use my Xonar DX soundcard again... And it's back to normal =D

Everything is good so far, no whining whatsoever... I bought two tubes in case this one decides to fail, one is a RCA and the other one a miniwatt... For some sound customization also...

I think the administrators may close the topic now, thanks for those that helped =D.


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