554mw from TO92 too much ?? - diyAudio
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Old 12th December 2012, 06:14 PM   #1
Vostro is offline Vostro  South Africa
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Default 554mw from TO92 too much ??

I'm looking for a different amplifier topology to use,
I've been using Lin (Blameless) type till now.

So I've been scouting circuit diagrams ,

I came apon this Cambridge Audio Azure 840W,
It uses a Lin structure, but something caught my attention.

They use MPSA42 (625mW max @ 25 deg C) for VAS transistor running at 8mA+ and 66V on circuit says 554mW.

How reliable is this practice ?

Regards
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Old 12th December 2012, 06:49 PM   #2
Mooly is offline Mooly  United Kingdom
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Wouldn't like to say without seeing it all in context. -66 volts means theres a +66 volt rail above it and if its a normal configuration then there could be 100 volts or more swing on that transistor under full drive.

Impossible to say really...
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Old 12th December 2012, 07:14 PM   #3
Vostro is offline Vostro  South Africa
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Circuit is kinda standard, just has switch to inculde/exclude Class 'XD' circuit.
EF output with MJE340/350 Driving 4 2SC3264/2SA1295
Will I be pushing my luck If I use a TO92 for a push pull VAS with +/-50Vdc rails (7-8mA) VAS current
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Last edited by Vostro; 12th December 2012 at 07:17 PM.
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Old 12th December 2012, 07:29 PM   #4
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They used to make heat sinks for TO92, looked like an aluminum conduit clamp with a back folded over. I've got some. Would make this little less marginal but a TO220 package with a bit of metal attached is obviously better (but more expensive).
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Old 12th December 2012, 08:19 PM   #5
Bensen is offline Bensen  Belgium
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In my last build, I am also using to92 VAS devices. I started with 200 mW dissipation, but decided to reduce to about 100mW, because the devices felt uncomfortable hot.

A big difference with your amp is that I am using fet's as drivers (2sk216/2sj79), these will not load the VAS devices when the amp drives a very hard load!

Greetz

Last edited by Bensen; 12th December 2012 at 08:24 PM.
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Old 13th December 2012, 06:25 AM   #6
Mooly is offline Mooly  United Kingdom
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Quote:
Originally Posted by Vostro View Post
Circuit is kinda standard, just has switch to inculde/exclude Class 'XD' circuit.
EF output with MJE340/350 Driving 4 2SC3264/2SA1295
Will I be pushing my luck If I use a TO92 for a push pull VAS with +/-50Vdc rails (7-8mA) VAS current
It looks very much like the blameless class b circuit.

This looks under rated on face value. As you say , the data sheet shows an absolute max dissipation of 625mw/25C ? In simple terms and very approximately 100watts into 8 ohms means a 40 volt peak swing PLUS the rail voltage across each device as the output goes pos and neg.

Are you sure the circuit is really what it says it is ? i.e. a Cambridge production 840 amp.
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Old 13th December 2012, 10:45 AM   #7
Vostro is offline Vostro  South Africa
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Then service manual I used, looks detailed and authentic.
Rated power 200W 8R,

Basically 'Blameless' + uses DC servo for offset, IPS and VAS run from (+63V,-66V)
and outputs run from (+63V,-63V)
theres VI ptotection, and a Class XD section,
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Old 13th December 2012, 11:03 AM   #8
Mooly is offline Mooly  United Kingdom
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Something doesn't ring true then.

200 watts and 8 ohm. Thats 56 volts peak swing. So those MPSA devices will see that swing plus the constant supply voltage. 8ma at 122 volts is around 1 watt. I agree the 8ma looking at the circuit, in fact I would have put it nearer 9 if just asked without reference to the text on the circuit.

Why not pm the designer and ask ? He's on the forum.
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