How to pick a good audio transistor?

[understress]

I'm assuming this has been covered before in the past, but I could not find it with searches.

I've been reading on and off for a number of years and have followed a number of threads with great interest, there is some amazing work shown here. But one thing that I haven't seen (and probably just missed), is how to pick a good transistor for audio?

There are a number of parameters regarding transistors, and some of them I assume don't apply when you think about 'sounding good'. For example, Vce I would think would only be important to make sure the transistor can work in a given design.

I would assume just using the transistors that Nelson Pass uses would good choices (I'm a big fan of his since the Threshold days). Or copying one of the many designs found here would a good starting point.

But what if you wanted to pick some transistors? Where do you start? I would assume something claiming to be low noise would be a good start.

So, where does one start? What makes a given transistor a good selection for audio use?

Thanks in advance for any advice you can offer.

[ThorstenL]

Hi,

Originally Posted by understress

But one thing that I haven't seen (and probably just missed), is how to pick a good transistor for audio?

It is very simple, just use J-Fets and Lateral Fets. They are "good for audio".

The rest is selecting enough power handling, voltage handling, current handling and transconductance for the Job and if you design looped feedback circuits you also need get enough Ft.

Bipolars have so many issues, you really need to characterise the part yourself (as most datasheets no longer show H parameter variability with voltage/current) to be even able to make good guesses, their parasitic capacitances are all over the place with signal, their thermal behaviour throws further spanners into the works, so why deal with all this pain?

Originally Posted by understress

There are a number of parameters regarding transistors, and some of them I assume don't apply when you think about 'sounding good'. For example, Vce I would think would only be important to make sure the transistor can work in a given design.

For Bipolars look at Hfe, Hie, Hre and Hoe and more critically how they change with the Voltage across the device and the collector current. You also need to look at how Ft changes with current changes and parasitic capacitance changes with voltage change. And then you may weep and go and use something else for audio, like tubes, that is actually suited for the job.

For J&L Fets you only really need to look at Gm vs. current and voltage and possibly the parasitic capacitances. VFets are just plain bad news in terms of parasitic capacitances and their modulation with signal. After looking at Fets you may also weep and go and use something else more suited for audio use, like tubes...

Originally Posted by understress

So, where does one start? What makes a given transistor a good selection for audio use?

For Bipolars it is transistors that show minimal variation of H parameters with Signal (current/voltage) and have low parasitic capacitances and high and stable Ft with signal changes.

For J/L Fet's is a GM that is constant with signal changes and low capacitances.

Ciao T

[a.wayne]

Glowing bulb fantasy ...? ....

Show me an published IEEE or AES paper showing the superiority of tubes over solid state devices for audio.

-Peter Aczel

[ThorstenL]

Hi,

Originally Posted by a.wayne

Show me an published IEEE or AES paper showing the superiority of tubes over solid state devices...

Why should I?

A single look at the various datasheet suffices to illustrate the points. Please just peruse them, they are all over the internet.

Ciao T

[kevinkr]

I think a.wayne's post was probably at least somewhat tongue in cheek, noting who he discloses as the author of that remark.

[a.wayne]

[evette]

Hello Mr. Wayne Sir,,I always enjoy your posts..But sometimes you may be taking what other people are telling you a little to seriouly. Please enjoy the sprit of things someone is telling you. This person did not tell you anything bad In fact. So enjoy Regards Evette. Forgive my bad spelling Youi know I was brought up BAD.

[Easyamp]

I’m no pro but here is some info.

Well it honestly depends on the intended application and what stage/type of amplifier or equipment you're designing. I don't know of any Holy Grail transistors, they're always trade off's with spec’s price availability high tide maybe low tide.
If I’m designing a standard 3 stage negative feedback amp I use low noise small signal pnp's for the input comparator that have vce's above at least a single rail. I'm sure hfe, capacitance, and bandwidth are important to somebody but I've never noticed much difference between your standard low frequency small signals’s at this stage. I will however make sure that the transistors intended working current versus gain bandwidth (Ft) is somewhere on the data sheet curve.

I'll throw a couple of low voltage high gain small signal npn's at the bottom to make a current mirror which will help in overall gain/lineararity in the feedback loop. Voltage and current seen by these is minimal so again check your datasheet curves. A couple 2n3904's or the like are usually ok.

Next an upper PnP CCS is a good practice, again a vce above the upper rail is required and current is minimal 3~8ma so some more small signals will work fine.

VAS/VBE multiplier stage: you're going to need some transistors you can physically mount to something and in the case of VBE multiplier transistor it will need to be mounted to the main power transistor heat sink. So first off its case type, I usually go with medium power to-126/sot-32/82 types. More then likely you'll also use these as power transistor drivers, but not always, so I buy complementary PnP/NPN pairs here to save cost later. Your upper PnP completing the upper ccs will see both rail voltages across it so its vce needs to be above both rail voltages added together and maybe throw in a 20% error of caution. The same goes for the lower class A NPN driver. Currents here will be between 10~20ma for this hypothetical amp stage so check your datasheet curves. The vbe multiplier: just use whatever you got in the through hole NPN variety even though I have seen to-92's used here mounted to the main heat sink so?? The voltage here is low and current 10~20ma. The ac characteristic's are not important due to this being bypassed with a cap and is only used to set the forward vbe of the output stage and thermal tracking.

I like multi element Darlington’s for outputs because of how simple they are and they include a reverse bias diode. But many do not because of the lack of control and tailor ability you gain with individual drivers and power trannies. These are load/cost decided for me. Data sheets are quite important here. You need to project worst case operating scenarios for you output stage including: vce above rails with at least 30-50% for emf talk back, pp current for intended load must under the worst heat condition never be aloud to touch the secondary breakdown curve on the data sheet. Complementary Devices are best but that NFB can really fix a lot. Then make sure your drivers can handle the same vce as your outputs. PP current divided by the HFE of your output transistors must not exceed the secondary breakdown curve of the drivers and don’t forget to de-rate both curves for heat.

In a negative feedback loop amplifier you can really get away with a lot of short cuts in the ac characteristics of transistors but heat volts and amps will burn you every time. This is but one of thousands or tens of thousands of audio circuits out there with many having more specific needs.

[ThorstenL]

Hi,

Originally Posted by evette

Please enjoy the sprit of things someone is telling you. This person did not tell you anything bad In fact.

I must admit my comments of "just go and use a tube" are a bit tongue in cheek as well, even though as in a joke a jester may make, it also contains a great truth.

Transistors, Fets and tubes are what we have to amplify a signal. No device is "perfect". They all suffer from problems and hence there is no "universally good" solution.

We want the the signal to be music. And we want to listen to it. So if we had an absolute ideal amplifier and speaker we would all agree that that is is well suited to the Job.

With real devices and real amplifiers and speakers we have problems and all is a compromise and not everyone agrees what the best compromise is.

And so we get people championing each and ever technology.

If we bend our will to dealing with each devices problems and are willing to compound circuits out of many different devices to maximise the gain from their strength and to minimise their drawbacks, we may be able to get closer to at least some version of the ideal that is close.

Ciao T

[kevinkr]

I design with semi-conductors during the work day and just as studiously avoid them at night when I am designing my own toys. Hard to imagine a much more linear device than a D3A without feedback - so you know what camp I fall into.