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Reparing Class A symetrically driven Headphone Amp kit
Reparing Class A symetrically driven Headphone Amp kit
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Old 12th July 2020, 05:53 PM   #21
hpro is offline hpro  Switzerland
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Join Date: Apr 2012
Location: Bern, Switzerland
Quote by lawerence
Unfortunately the left channel dropped out after listening for half hour or so but then came back again so there is still an intermittent fault somewhere.
end Quote

this could be to bad soldering joints in the line, or one of the Volume controls is loosing conection. When you have switched on the amp, and the sound is on both speakers use the handle of a screwdriver and hit gently on the case of the headphone amp .. not on the board of course but on the case itself. if you have lose component you will find them that way..
If you have an Scope then check the output where the signals is going to the Poweramp if the signal is still there. you can compare with the side which still is working.
should not be hard to find that one..check your connection cables for short in the PLUGS., and also female Plugs inside both, the headphone amp and the Poweramp. Last but not least check the HEADPHONE Sockets when the signal goes away stick in the heaphone and remove it again, might help..if you do this and the sound comes back then there is only one thing to go.. the sockets.. also check that 6.8 Ohm resistor which is in Serial to Power amp Input.. it must be somewhere there.


Also check the source of sound.. sometimes it's the source.. change input Cables left and Right.. if the error is moving to the other channel then you have found it..
All this takes a few minutes.. much less than ripping all that amp in pieces and searching for something which does not yet happen. Also lose Speaker Cables and so on.. Make sure that to and from the AMP all is thight and works
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Last edited by hpro; 12th July 2020 at 05:58 PM.
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Old 12th July 2020, 06:23 PM   #22
hpro is offline hpro  Switzerland
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Join Date: Apr 2012
Location: Bern, Switzerland
Quote:
Originally Posted by Lawrence.M View Post
It seems that Q1(BC184) and Q2(BC214) are obsolete components.Farnell is suggesting the MPSA92 as a replacement for the BC214.would this be a good alternative? And what about the BC184.What would be a good replacement for that?

If these still work, then do not change them, other numbers other sound..

Download the datasheet of both..
Most important is HFE GAIN
VCE
VCB
Ptot
and also FT. FT is frequencey response for theat specific Component usually Transistor IC or Jfet or Mosfet.

For transistor calculate max FT divided by HFE GAIN
So if you have a transistor which has a FT of 1 MHZ and a gain of 100 then MAX FT in the Circuit is 10KHz

That's why Other numbers other sound..

MPSA92 is a bad match.. Ft50mhz Gain about 25 - max 40

I would go with BC212 or 213 ia the better match 280 respectivley 300mhz
BC 214 has Gain 200 and FT 300 so calculate which one is the better..

Then also VCE VCB of MPSA is not relevant to your circuit.. is to high

if this would be a output then it would be welcomed if the transistor makes VCE 200volts up.. but is in a headphone amp. the ones selected are just about good.
If you want to change, then go for the alsmost the best inpui device BC559 or BC560 complimentary is BC549 or BC550 I thought..
Both better than MPSA92 /93 But you decide..

There is a sentence we use here in Europe, I think you guys call it Murphis law..

Oh you are in good old England.. it says.
Never Change A Running System.. do not screw on something which works.
Cheers..
Regards Chris
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Last edited by hpro; 12th July 2020 at 06:25 PM.
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Old 14th July 2020, 08:03 PM   #23
Lawrence.M is offline Lawrence.M  United Kingdom
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Join Date: Aug 2005
Many thanks for your suggestions Chris.I went straight for the headphone sockets and that would seem to be the problem.After pushing the headphone Jack in and out with a bit of white vinegar as a cleaner the sockets worked as they should and so far I haven't had any drop out from either channel.
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Old 17th July 2020, 02:02 PM   #24
Lawrence.M is offline Lawrence.M  United Kingdom
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Join Date: Aug 2005
Default Mains Hum

After playing music for about 20 minutes I suddenly got mains hum coming through both channels while using the high output/headphone amp stage.This isn't happening on my normal level output so I'm thinking it's either the headphone stage or the power supply feeding it.
Just checked the output voltages from the regulated supply and I'm getting about +14.8V(should be 15v) and -14.8V(should be -15V) at the appropriate places.So that seems OK.I also tried to measure possible ripple voltage with my digital multi-meter set to AC and I seemed to be getting readings of 1.4V,1.7V and .3mV at the +15V DC and -15V DC outputs but would these be reliable measurements from a multi- meter? Unfortunately I don't have a scope.
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