Solid-state audio amplifier complexity vs valve/tube

[schmitt trigger]

The solid state designs have another advantage over tubes: they have complementary devices, NPN/PNP, Nchan/Pchan.
These allow direct coupling (with bipolar supplies).

Having said this, I love vacuum tubes, but only for nostalgia.

 

[dr pepper]

Thanks for the correction on ultralinear Nige.
My bad.

 

[Nigel Goodwin]

That's true.
I was thinking the diodes were facing the other direction.

So basically the diodes are biased by R17 to balance the offset base-emitter voltages of Q4 and Q6 so the pair act as a PNP emitter follower with low offset, complementary to the NPN emitter follower (Darlington) pair of Q3 and Q5.
These four transistor are driven by the bootstrap output of Q2.

It may look like a weird circuit but it works well.

I'd be interested to see some suitable tests on it

 

[crutschow]

I'd be interested to see some suitable tests on it

That would be interesting but all I have is an oscilloscope and a pulse generator.

 

[ronsimpson]

Solid-state ..... complexity ...... vs valve/tube

When I started electronics, I worked two hours to pay for a tube. The old men I learned from did not understand transistors.
I just checked and I am paying $0.02 for a surface mount version of a 2n2222a transistor.
One of the ICs I designed has 6000 transistors and we sold it for around $1.00usd. At some point I asked TI what it would cost to add 100 more transistors. They told me "nothing". When the price of parts approaches zero then complexity has no limit. My next part hit 100,000 transistors.

 

[Elerion]

f you want to built a good quality simple old amp, google for the Leak Stereo 70

Thank you! I will.

You can create a hybrid so that you don't need an output transformer.
Use transistors to buffer the output.

Yes. I just built a simple stereo preamp to use with headphones. I used a simple triode per channel (common cathode, grid biased, no feedback). The output from the tube goes to a source-follower NMOS, with a current source. I just used a variable resistor to adjust bias.

The sound is great. I can't hear any kind of distortion, even using good headphones. And it works with low voltage.

I know that I could have achived the same (or better) result using a bipolar (or FET) instead of the triode. Most of the power goes to heat the filaments. But it is more an aesthetic matter.
At 12 V, the preamp draws 240 mA (the heaters take 150 mA, so a solid-state version could have been around 100 mA).

 

[Elerion]

I just checked and I am paying $0.02 for a surface mount version of a 2n2222a transistor.
One of the ICs I designed has 6000 transistors and we sold it for around $1.00usd.

That's another story.
I was talking about simple audio amplifiers. Tube amplifiers sound really good to me. So, I started reflecting on why so much complexity was needed in newer designs (I said solid-state, not because of being solid-state, but because everything new is solid-state).

It seems that the reason is just: because it can be done, and it's cheap (and compact).
Thanks everyone for your thoughts about this.

 

[ronsimpson]

Tube amplifiers sound really good to me.

I think so to but:
There are certain things you can do (distortion) that sound good. "warm" Some people like tube sound better.

There are two thoughts about how to make good amplifiers.
1) Make each stage low gain. 5 to 10 Then just live with that gain and distortion.
2) Make each stage high gain and use feedback. High gain comes with bad distortion in part because we don't care & no one remembers how to make amps with out distortion. Then use feedback to make the output follow the input.

When a amp is over driven into distortion, Tube amps (and FET amps) have a tendency to round top distort. Amps with heavy feedback flat top. Worse yet, transistor amps, when driven into their supplies, get sticky. The output will get stuck at the supply and it takes time for the output to pull back off the supply. (I should send pictures)

My opinion is that tube amps, when driven too hard, have a distortion that is less objectionable.
I think amplifiers should not color music. It should reproduce exactly what it sees.

 

[audioguru]

A tube amplifier sounds "warm" because its transformer cuts high distortion (and program) frequencies. A vacuum tube produces even harmonics distortion that sounds "musical".
So of course the music is colored, it has highs cut and it has even harmonics added.
The distortion increases daily as the tube burns out until you cannot stand it anymore then replace the tube, over and over.
Some bands replace all vacuum tubes before each gig.

 

[Elerion]

I found the Leak 70 Stereo amplifier quite complex as a first project of this kind.
What do you think about these simple ones?
The specifications seem great for much simpler designs.

I'm looking towards learning, not excelent performace.

 

[audioguru]

You should look at modern amplifier designs, not amplifiers that used germanium transistors 56 years ago that are not available today.
Many low power amplifiers like those ones use amplifier ICs today. The LM3886 is 14 years old, is inexpensive and has excellent performance:

 

[Nigel Goodwin]

I found the Leak 70 Stereo amplifier quite complex as a first project of this kind.
What do you think about these simple ones?
The specifications seem great for much simpler designs.

I'm looking towards learning, not excelent performace.

As already mentioned, those are antiques using transistors that haven't been available for many decades.

The first one looks very similar in complexity to the Leak Stereo 70, you should be comparing the power amplifier - not the entire pre-amplifier/amplifier assembly.

 

[crutschow]

What do you think about these simple ones?
The specifications seem great for much simpler designs.

If you insist on a discrete design I would go with the 15W unit on the right as it uses mostly silicon NPN transistors, and I like the differential input stage for stable DC biasing of the amp.

 

[audioguru]

The cheap simple 15W amplifier:
1) The DC current of the driver transistor is in the speaker causing the cone to be offset. Adding one resistor and one capacitor will fix it.
2) A frequency compensation capacitor is missing possibly causing instability.
3) A zobel network is missing at the output further causing instability due to the inductance of a speaker.

 

[crutschow]

The DC current of the driver transistor is in the speaker causing the cone to be offset. Adding one resistor and one capacitor will fix it.

This is the 15W amp I was referring to.
It has an output capacitor to block any DC to the speaker.

 

[Elerion]

The first one looks very similar in complexity to the Leak Stereo 70, you should be comparing the power amplifier - not the entire pre-amplifier/amplifier assembly.

You mean the red portion of the image, right?

Could I just leave out the rest?

You should look at modern amplifier designs, not amplifiers that used germanium transistors 56 years ago that are not available today.

I was thinking on selecting new suitable transistors using SPICE simulation and some tweking.
I was aiming at understanding a simple design, so I can take decisions, not facing a complex design where I'm lost on many portions of it.

 

[Ian Rogers]

Interestingly! Marshall still prefer hybrid systems today!! So the valve output stage must be reasonable. Surely?

 

[Nigel Goodwin]

You mean the red portion of the image, right?

Of course, as you're only making a power amplifier. You might like to include the volume control as well though.

The Stereo 70 was a great amp, loads of power and sounds great - I've seen many of them used for PA or guitar use as well (including mine at times).

 

[Nigel Goodwin]

Interestingly! Marshall still prefer hybrid systems today!! So the valve output stage must be reasonable. Surely?

Depends what you mean by 'reasonable'

Marshall (along with most other guitar amp manufacturers) use valves to give the specific low quality sound they are looking for, poor frequency response and masses of distortion.

 

[audioguru]

This is the 15W amp I was referring to.
It has an output capacitor to block any DC to the speaker.

No. It uses the speaker as a resistor for "bootstrapping" then the current of driver transistor TR3 goes into the speaker which offsets its cone. Proper bootstrapping adds a resistor and capacitor.