A good audio amplifier based around 2N3055(s)?

[Vincent]

Here's the thing:

This week I found a nice big power supply using four 2N3055s. The circuit boards didn't have much interesting stuff, but I did grab the transistors. Then while searching information about them I saw they apparently used to be among the top-notch transistors for audio amplifiers in the 60s!

A few days later I was examining the frame of an old Toshiba projection TV to see if the boards were worth salvaging, then I noticed the 15W speakers. They're not exactly big in terms of physical size, but I'm fairly sure they could provide more powerful bass for my Yamaha NP-31 electronic piano, which has a pair of 6W speakers. They sound just fine, but the bass sounds a little weak. Hence this project LOL.

Since connecting anything on my piano's audio output automatically disables internal speakers, I'm going to need an amplifier circuit for another pair of speakers covering full-range. As for the drivers themselves, I already have a pair of 80s Diatones that sound particularly good with higher frequencies.

I assume the amplifier for the full-range drivers can be built using parts that are fairly easy to find. For the bass driver(s) however, I'm pretty much stuck to the four 2N3055s I got, if I don't buy anything that is.

So, to summarize this, I'm not sure which amplifier circuit(s) I should choose among all the schematics I can find on the net. My project can take the form of multiple circuits (say one circuit per driver, thus 4 circuits), this way I can give both the bass and full-range their own on/off and volume control if necessary. It's almost a full-range amplifier with bass-boost, come to think of it LOL. That may make it suitable for other uses, eventually.

As of the power supply, I can easily find an old stereo system and salvage its power supply transformer, from which I can design my own linear supply. Not the most efficient, yes, but it won't be much hassle xD

[calexanian]

2N3055's were used in audio because they were stable and cheap. Thats about there their virtues end. By the late 70's they had been replaced by higher beta transistors in nearly every audio application. The do work well as audio amplifiers, provided the circuit can provide them with enough drive current and the feedback loop is good. On the Elliott Sound Products site there are several articles about basic Quasi Complementary amplifiers. Quasi Complementary means the final output transistors are on the same configuration. In the case of a 2N3055 NPN type. Look up the El Cheapo if memory serves me correctly to see a typical low cost implementation of that sort of circuit. Not the best performance, but not thermally critical and good for the wattage range you are talking about. +/- 15 or 20 volt type of thing. There are also a number of transformer driven circuits out there that have various performance characteristics. Most of them were used in Musical instrument amplifiers (Rhodes pianos, later Leslie speaker systems, later standel amplifiers, etc etc) where overall low distortion was not required.  With the low cost of TO220 NPN and PNP driver transistors in the late 70's and 80's these circuits practically disappeared in all but the cheapest crap from Taiwan. Start with that Elliot Sound products website and look up the original AP note from Motorola on the full complementary amplifier and you will get pointed in the right direction. The Crown DC300 was originally a quasi complementary circuit, and those worked pretty good.

[theatrus]

For example, here is one:

http://sound.westhost.com/project12a.htm

[Conrad Hoffman]

3055s are nothing to write home about, but if you do the rest of the amp well, it might surprise you. Have a read at Doug Self's site- http://www.douglas-self.com/ampins/dipa/dipa.htm The amp at the bottom of the page is very good and can be morphed into various other designs.

[calexanian]

Sorry about the google link, but its the fastest one that came up. Here is Motorola AN-485. This is the place most people start for design of practical solid state amplifiers. Most AB class amplifiers are based on this design.


http://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=2&cad=rja&uact=8&ved=0CCUQFjAB&url=http%3A%2F%2Fwww.manuallib.com%2Fdownload%2F2647525%2FC6AFF17160972FE127DFCAB70FC3411B%2FMOTOROLA%2520AN485%2520DATA%2520SHEET.pdf&ei=qjG2U6W0GczwoASLtYLQCQ&usg=AFQjCNFrbripHUJUCkdll-iZ3RFfcNF4AQ&sig2=9uPNPmjpHngXdDnezPWlBQ&bvm=bv.70138588,d.cGU

[Riotpack]

The legendary NAD 3020 used 3055s and its complimentary pair.

You will have to use emitted resistors as the old 3055s were an epitaxial type.

[Richard Crowley]

Sorry about the google link, but its the fastest one that came up. Here is Motorola AN-485. This is the place most people start for design of practical solid state amplifiers. Most AB class amplifiers are based on this design.

That app note is 29 years old!  Can you even get those devices anymore?  Do you really think there aren't more modern, better-performing, easier to build, more stable, designs avaiable in 2014?! 

Why do people even make audio power amps with bipolar transistors anymore when there are modern power FETS?

[Richard Head]

Why do people even make audio power amps with bipolar transistors anymore when there are modern power FETS?

That is the million dollar question.

[dannyf]

Different people have different definitions of "good".

To me, you should be looking for an amplifier with wide open loop linearity and low open loop gain.

On that, I would recommend JLH1969 - open loop frequency typically to 25Khz and incredibly fast, being a CFB. Up to 10w and class A only.

Along the same lines, Hiraga is also very good.

For class B amps, 3020 was mentioned earlier, with quasi complementary OPS. Aksa, closer to a more typical audio amplifier, can be built with just npn output devices as well.

Unless you are truly gifted, I wouldn't touch the vertical types.

If you have to go with MOSFETs, try the lateral types from Toshiba or power Jfets (very difficult to find now).

Unless you are truly gifted, I wouldn't touch the vertical types.

[GeorgeOfTheJungle]

The legendary NAD 3020 used 3055s and its complimentary pair.

You will have to use emitted resistors as the old 3055s were an epitaxial type.

I have fond memories of this beautiful BRAUN  http://www.hifiengine.com/manual_library/braun/csv-1000.shtml , it uses a couple of paired 3055s per channel that break everytime you accidentally pull out a speaker plug.

[GK]

To me, you should be looking for an amplifier with wide open loop linearity and low open loop gain.

On that, I would recommend JLH1969 - open loop frequency typically to 25Khz and incredibly fast, being a CFB. Up to 10w and class A only.

Pseudo-technical drivel. What is "wide open loop linearity"? 

The ignorance reveled by your few sentences is profound. Amplifier "speed" is not related to open loop bandwidth. Low feedback and high open loop bandwidth does not make a technically better performing amplifier - it makes a worse one.

[GK]

Why do people even make audio power amps with bipolar transistors anymore when there are modern power FETS?

That is the million dollar question.

Umm.... there are only 1 or 2 boutique semiconductor manufacturers actually making complementary MOSFETs specifically for "linear" (as opposed to Class D)  audio power amplification these days, and for the most part they are prohibitively expensive (eg Lateral devices from Profusion/Exicon). 

The most practical power devices currently available are BJT's from On-semi.
 

[Psi]

You should make a 50W true class A amp out of them using a big ass heatsink
Run them at 150+C, most have a max operating temp of 200C

Just for kicks.

[T3sl4co1l]

Here's the thing:

This week I found a nice big power supply using four 2N3055s. The circuit boards didn't have much interesting stuff, but I did grab the transistors. Then while searching information about them I saw they apparently used to be among the top-notch transistors for audio amplifiers in the 60s!

Er, they were certainly...common.  But Walmart is common, too.  And I don't think anyone would accuse them of being "top-notch".  Get my drift?

Personally, I would've left the power supply as is (or fixed it), since that's about all 2N3055s are good for.

That app note is 29 years old!  Can you even get those devices anymore?

The oddball ones in the datasheet?  Probably not, but also plenty of close enough or better substitutes.  Transistors aren't picky things, not the way they're generally used.  As for 2N3055s, yes, they are still around, though not likely the same fab as the original parts.  fT spec is minimum, so you could get anything from a near-germanium-level slacker to a 50MHz+ screamer.  Needless to say, the latter will oscillate in a whole lot of circuits!

Tim

[dannyf]

With just four 2n3055, you have a chance to dissipate 50w in a class a amp but no chance of outputting 50w in a class a amp.

[Andreas]

I would build 2 devices of the legendary "Elektor Edwin 40W" amp with 4 2N3055.
Its a class A/B device.

With best regards

Andreas

[f5r5e5d]

likely the projection TV had perfectly fine linear audio power chip amps designed by a team of pros, built on a few decades more recent semi process

[dannyf]

Dynaco ST120

Another CFB amp way ahead of its time.

[nctnico]

Sorry about the google link, but its the fastest one that came up. Here is Motorola AN-485. This is the place most people start for design of practical solid state amplifiers. Most AB class amplifiers are based on this design.

That app note is 29 years old!  Can you even get those devices anymore?  Do you really think there aren't more modern, better-performing, easier to build, more stable, designs avaiable in 2014?! 

Why do people even make audio power amps with bipolar transistors anymore when there are modern power FETS?

Not to mention complete chips and class-D amplifiers.

[c4757p]

Why do people even make audio power amps with bipolar transistors anymore when there are modern power FETS?

Because they work and they're easy to understand.

Not to mention complete chips and class-D amplifiers.

For fun.

[dannyf]

More importantly, because those people know what they are doing.

(vertical) MOSFETs are incredibly difficult to design in an audio amp, and sometimes practically impossible - try to build a CFB OPS with vertical mosfets, for example.

Aside from older Toshiba mosfets and power jfets (no longer available), you don't see many (any?) quality high power mosfet amps - I think if you were to ask Pass, he would tell you to not build Citation 12M today.

So far, (vertical) mosfets success in (linear) power amps are mostly in Class A designs by Pass, or JLH1969M, or Zeus. Those are 10w or less power amps.

Advocates of MOSFETs in (linear) power amps are either fooled by (audiophile) marketing or not understanding the challenges.

[f5r5e5d]

... or have read Cordell's book, http://www.cordellaudio.com/ - looks like he succeeded in building a vertical Mosfet amp 30 years ago http://www.cordellaudio.com/papers/mosfet_with_error_correction.shtml

[nctnico]

I have seen this circuit used a lot as a basis for PA amplifiers:
http://www.electroschematics.com/4/mosfet-audio-amplifier-irf9530-irf530/

[retrolefty]

My favorite site for all things audio is Elliott Sound Products. Tons of schematics, theory, and practical ideas. He has blank PCBs for many of his more popular projects.

http://sound.westhost.com/projects-0.htm#pwr

[AlfBaz]

Silicon Chip magazine published in Jan-Feb 2003 an updated version of the venerable ETI480 (Electronics Toda International magazine) amplifier that was originally published in Dec-1976. Even then it was an updated issue of an earlier amplifier.

The ETI480 was over 25 years old even back in 2003. I stand to be corrected, but I seem to recall it was a very popular kit and probably had been the longest running Australian kit ever. SC described it as a dog in the article, the front page of which is visible here http://archive.siliconchip.com.au/cms/A_30285/article.html and the schematic here http://archive.siliconchip.com.au/cms/gallery/article.html?a=30285&i=4 .

LOL that was the very first kit I ever built.

Can't remember the 4 channel mixer/preamp kit that was around at the time but I have to tell you I hooked it up to a couple of 12" speakers, plugged my guitar in and if memory serves me well, my friends and I concluded it's crisp punchy sound was on par with the venerable Fender twin reverb!