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Old 5th November 2012, 12:25 PM   #51
jaycee is offline jaycee  United Kingdom
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VAS Transistors

The problem is getting most of those. The 2SA1209/2SC2911 used to be easy to get - but since the Onsemi take over, cant find anyone selling them except Profusion - not really suitable for hobbyists.

The 2SC3421, 2SC3423 and complements are now discontinued by Toshiba.
Likewise, the Sanyo devices have become unobtainium.
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Old 5th November 2012, 03:05 PM   #52
jaycee is offline jaycee  United Kingdom
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Diodes Inc/Zetex DZT5401/DZT5551 seem interesting. They are in a SOT-223 package and rated for 1W.

Also ZXTN5551G/ZXTN5551G similar...
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Old 5th November 2012, 05:13 PM   #53
Vostro is offline Vostro  South Africa
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I used 2SC3953 (from RS) for my other amp, but Vce(max) is 120V, to low for this amp.

I've gone through the list and cannot source those transistors.

What transistors are current commercial amplifiers using ?
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Old 5th November 2012, 05:24 PM   #54
CBS240 is offline CBS240  United States
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I think you need to go with a small signal device and a few mA bias for the VAS and add another follower stage, a small signal pre-driver making the output stage a triple. After all, your driving 3 pair of output transistors...

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Old 6th November 2012, 01:14 AM   #55
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You can obtain various suitable VAS transistors around the globe that were once used everywhere as video drivers in CRT Televisions, as were the KSC3503/A1381 Fairchild clones.
RS and E14 have both reduced their through hole lines in recent years so I don't use them any more for anything other than large electrolytics, on which they seem to be out of stock with every purchase anyway. Manufacturers have a habit of dropping obsolete parts so, if you are spending considerable money on freight, as many outside Europe and the US do, buy a stock that will last for some forseeable years. They are cheap from Fairchild, so why not get as much as you can afford, as well as any other through-hole semis that will only become more difficult to source in the future.

It sounds expensive, but a $200 free freight order on Mouser or Digi-key gets many times the value of product that the equivalent in 5 or 6 smaller lots from RS or E14 does. This is how I buy most of my semis and specialised components now and I give those other guys a wide berth.
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Last edited by Ian Finch; 6th November 2012 at 01:20 AM.
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Old 6th November 2012, 09:46 AM   #56
AndrewT is offline AndrewT  Scotland
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If you add a cascode to the VAS, then the cascode takes the big voltages and you can select a low voltage device for the VAS duty.
Low voltage devices have different compromises from high voltage devices and you should find it easier to get a suitable low Vce0 VAS device.
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Old 6th November 2012, 10:43 AM   #57
djk is offline djk
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Common base (cascode) may be the way to use the MJE340/350 and get good performance. It looks like they can do 60Mhz at currents around 10mA~20mA in common base mode.

The Musical Concepts replacement for the Hafler driver boards use the MJE340/350 in cascode around 2N5551/5401.

I wonder how a MJE340/350 cascode around a 125Mhz Darlington like the MPSA13/63 would perform? Mark Levinson did this with a 100Mhz Darlington and a 30Mhz cascode device.

The 340/350 and 13/63 are common industrial devices available in through-hole as well as surface-mount and should be widely available now, and in the future (because of their widespread use in industry).
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Last edited by djk; 6th November 2012 at 10:48 AM.
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Old 6th November 2012, 10:54 AM   #58
AndrewT is offline AndrewT  Scotland
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I think you picked the wrong pair.
The 139/140 is the lower Cob and higher fT. Due to low Vce0 cannot be used in amplifiers with >+-40Vdc, unless some protection against over voltage is implemented.

The 300Vce0 mje340/350 is compromised to achieve that very high voltage capability. It is not suitable for VAS duty in conventional LIN topology.
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Old 6th November 2012, 11:01 AM   #59
jaycee is offline jaycee  United Kingdom
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Ian, problem is CRT's are obsolete so the parts are being discontinued. Im quite surprised its taken this long - most CRT driver boards use an IC such as LM2402.

Commercial separates are often using LM3886 at the low end, or MJE340/350 based circuits. Mid range is now commonly becoming driver IC's like LM4702 or LME49810.

Suitable SMD parts exist but the problem is then power dissipation. So I guess cascodes are going to become the answer.
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Old 6th November 2012, 11:31 AM   #60
AndrewT is offline AndrewT  Scotland
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some smd have higher Pdmax.
eg.
To92 500mW
sot223 2000mW
for the same component.
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