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Old 15th October 2001, 03:08 PM   #21
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Default Beta droop

I think this unlikely a source of additional distortion. The propely designed transistor circuit does not rely on beta too much. Nobody else but Duglas Self said this.
Let me add, that if an amplifier uses multiple pairs of output transistors, it is even less significant. The output transistors in good circuits never run at their full rated current except for the short pulses.
I started with Toshiba 2SC3281/2SA1302, but I found them far less reliable than Motorola ones. Maximal die temperature of 200C means something! Indeed, when I switched from Toshiba to Motorola devices in the same amplifier circuit, I noticed no change in both value and spectrum of distortions for any frequency.
By the way, there are motorola's transistors designated MJ3281 and 1302! They have TO-3 cases and excellent SOA.
Indeed, I saw them only in databook.
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Old 15th October 2001, 07:40 PM   #22
mlloyd1 is offline mlloyd1  United States
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This is interesting discussion to me! Has anyone experienced any stability issues in their designs due to the much higher than typical (for a power bipolar transistor) bandwidth of the the xxC3281/xxA1302? I have read in Randy Sloan's book on amplifier design (and also somewhere else; I just can't remember the other source) that the much higher than typical Ft for causes some less than desirable anomalies if you're not careful.

Michael
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Old 16th October 2001, 01:14 PM   #23
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Default Stability

I think, the troubles begin when more than two-stage Darlingtons are used. If the output stage consists of a standard two-stage Darlington, an unwanted oscillation may occur if the front-end transistor has too much an input capacitance. I always use the small transistors like 2SB861/2SD1138(several pairs if needed) and small anti-oscillation resistors in their base circuits. Of course, the wiring layout is critical.
Indeed, now I see no advantage in using the output transistors with Ft>2MHz from the sound quality viewpoint.

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Old 16th October 2001, 02:24 PM   #24
djk is offline djk
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Did you ever look closely at a 3281? The gain bandwith product at 6A is only 3Mhz! A 21194 is 4Mhz at 6A! The SOA of the 21194 at 100V is double that of the 3281! As for beta droop, 400W at 8 ohms is 10A peak.With three pair that is only 3.3A per device.The beta is pretty good out to about 8A or so.Even if you are driving 600W at 4 ohms you are under 6A peak per device.
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Old 16th October 2001, 02:53 PM   #25
Geoff is offline Geoff  United Kingdom
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Agreed, the 2SC3281 bandwidth is as you say, but the datasheet for the MJE3281A indicates 7.5MHz at a Vce of 5V and 45MHz at a Vce of 10V, for an Ic of 6A.

The benefits of the MJL3281A and MJL1302A are not confined to the lack of gain droop at higher current levels. I have recently measured ten of each of these devices and found that the gain varies by less than 6% for the 3281A and 3% for the 1302A over a collector current range of 0.33mA to 100mA. After that, the datasheet graph is realistic (unlike some). Compare this to the wide variatons in gain for some other devices. For example, the measured gain for a TIP35C varies from 1055 to 230 over a collector current range of 100mA to 2.5A, a wide variation with actual gains well in excess of the datasheet graph for a typical device.
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Old 16th October 2001, 09:53 PM   #26
Jon T. is offline Jon T.  United States
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Default Toshiba transistors and SITs

The 2SC3281/2SA1302 transistors are actually now obsoleted by Toshiba. They have a whole new family of audio power output transistors:



2SA1943/2SC5200 - 150W

2SA1942/2SC5199 - 120W

2SA1941/2SC5198 - 100W

2SA1940/2SC5197 - 80W

2SA1939/2SC5196 - 60W



All have very linear gain, and virtually no beta droop. Their transistion frequency is only specified at 1A collector current, but it is quite high at 30MHz.



You can get the datasheets off their website, and I believe MCM stocks them.



You might also want to look at the Sanken transistors.



To answer Denis's question about static induction transistors, there are a couple of manufacturers supposedly still making them for military applications, but I've never found a source or even datasheets.



The Japanese audio magazine MJ has run a series of articles over the past year discussing amps using SITs from Tokin. Unfortunately, they have been discontinued. There is a DIY electronics store in Tokyo (Hino Audio) that bought up the last remaining stock and is selling them for about $600/pair.



It's too bad they are so expensive, since they look like an ideal device -- triode characteristics but transistor level currents and power handling. They would be nearly perfect for OTL like applications, but at those prices, I'd be afraid to experiment with them.



If anyone is interested, I can provide copies of the Tokin SIT datasheets and the MJ articles. Unfortunately, its all in Japanese, but the graphs and schematics are fairly self explanatory.



-Jon
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Old 17th October 2001, 04:19 AM   #27
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Default Motorola/ON Semi catalog

A check of the ON Semiconductor site shows there is an apparently more rugged version of the MJL1302/3281 in the
MJW series. There is no metal TO-3 device, which sucks for me because I can't substitute them into my Leach amp without
going to new heatsinks.

Their bipolar device catalog is available in .pdf format:

http://www.onsemi.com/pub/Collateral/DL111-D.PDF

It's something like 10 megabytes...
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Old 17th October 2001, 09:53 AM   #28
djk is offline djk
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"Agreed, the 2SC3281 bandwidth is as you say, but the datasheet for the MJE3281A indicates 7.5MHz at a Vce of 5V and 45MHz at a Vce of 10V, for an Ic of 6A. " The Motorola data book for the MJ3281A, a device built under a technology transfer between Motorola and Toshiba, has a fig.2 showing the current gain-bandwidth product.At 1A it is 30Mhz, it peaks at 50Mhz at 2A, beyond it crashes.With 5V CE at 6A it is 3Mhz. At an operating temperature that would be typical in a large amplifier the current gain is flat as a ruler to about four amps where it begins to fall.At 7A the gain is about half what it is below 4A.The SOA changes to secondary breakdown limited at about 70V.The gain of the MJ21194 from 4A to 7A changes less than 20% and is still good out to 10A.I have about 50pcs of the 1302/3281 on hand.They make a great driver transistor.I think the 21193/21194 makes a better output.I should mention that I am using six pair at +/- 95V.


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Old 17th October 2001, 10:08 AM   #29
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Default Motorola, Toshiba and SIT

I too can add that Motorola's datasheet are those I trust most. And the Motorola versions of various widespread bipolar transistors are much improved over their original versions. Let me take for example the famous 3055.
I always prefer to use the metal TO-3 output devices even for the replacements of the plastic ones! Indeed, such substitution is not always possible. In these cases it seems better to replace even the modern Toshiba transistors with Motorola MJL types.

Ft of 30 MHz is not essential for good sound. It is not difficult to get a full-power bandwidth of more than 100 kHz with any type of modern linear bipolar transistors. Otherwise, the nonlinear input capacitance is to be managed first. Neither the bandwidth nor the slew rate are now the limiting factors.

I think SITs are the dead branch of silicon technology. They have virtually no advantages over other silicon devices,and they have poor reliability. They draw current from the signal source just like bipolars, and they are vulnerable to second breakdown. From other hand, they cannot withstand a die temperature of 200C.
Finally, their output characteristics look very undesirable for audio applications, yes!
Even the vacuum triode cannot be considered the ideal audio amplification device by definition. The triode tube has internal negative feedback. It is this feedback that makes the triode a voltage amplifier. Of course, this is a simple way to build a voltage amplifier, because no external feedback is necessary. Indeed, this internal feedback is nonlinear, what is reflected by variation of triode gain factor mu over a whole range of plate currents and voltages. When a triode is used as an output device, this nonlinearity cannot be omitted. The most significant manifestation of triode's gain nonlinearity takes place when a triode amplifier, especially a single-ended one sees a reactive load like a loudspeaker. There is quite simple explanation why those SET amps like so much the sensitive loudspeakers, they operate in small-signal region far from the voltage and current clipping. Othewise, some more ideal amplifiers will make good dynamics at just 10W of 8 Ohm power rating even with 'speakers of medium sensitivity, say, 86 dB/1W.
The SITs have the same drawback of nonlinear internal feedback plus their thermal unstability. Need one say more?

From other hand, a bipolar transistor is, in fact, a voltage driven current source. (Again Douglas Self is right.) This device is described by such parameter as transconductance. What is essential, the small-signal transconductance of any bipolar transistor is solely defined by collector current and chip temperature, and is independent of particular transistor type! What makes the real difference between power transistors, is their SOA. The beta is a parameter of least importance. It is just necessary to have the beta high enough. There are simple and effective circuit topologies which make the transistor circuits virtually insensitive to beta variations. At least, most modern linear transistors have quite narrow spread of this parameter, and it is not the beta variation what is responcible for high-order nonlinearity.


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Old 25th October 2007, 03:57 AM   #30
Daveis is offline Daveis  United States
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Quote:
Originally posted by tvi

With so few commercial audio products using IGBTs it makes me wonder about their sound/relability.
Reliability? Dunno. Have two Threshold T-200 amps from the 90's with lots of IGBT's. Sound is quite good.

Also listened to Forte Model 4 which used IGBT's around the same time.

Both sounded nice enough.

Sorry for digging up an old thread...
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