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PostPosted: 23 May 2016, 00:38 
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John Lenk's book, "Handbook of Simplified Solid-State Circuit Design", has designs for various all-transistor amplifiers. I'm looking at the one on page 75 which uses four transistors for a few watts of output. It would be an excellent amplifier for tube radio circuits which must feed a circuit with high input impedance. If I need something higher, I could place a JFET source follower on the input.

For a transistor compliment, I'm thinking 2N3904/06 and BD139/140. I can't get 1980s transistors and I'm not willing to try. My experience is that a good amplifier will perform well with any compliment of transistors, as long as their ratings are enough to handle the power in the amplifier.

I'm looking for feedback on my decision. It has been a while since I've dabbled in audio and driving a pair of transistors with an op amp feels cheap and dirty, as if I could get better performance with another circuit.

Ed

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PostPosted: 23 May 2016, 09:59 
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Not having the circuit to look at it is difficult to make a recommendation. What are the original transistor types?


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PostPosted: 23 May 2016, 11:25 
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Attached is the schematic from the book.

The four transistors are available from Ebay. I am not paying 8 to 10 each for them when I have modern transistors on hand. I intend to build this amp and see how it does. Then I may build one of higher performance, as the book has bigger amp designs.

Ed


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PostPosted: 23 May 2016, 15:48 
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That's the hobbiest standard "Need an amp! Breadboard now!" type of thing. Stable and sounds decent :D
(only in that case, it's 3904/3906 time, LOL!)

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PostPosted: 23 May 2016, 16:05 
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One of the things that I find strange is that people here tend to shy away from AB designs using transistors. For some reason, tube and ICs are okay for AB but everyone seems to prefer class A when it comes to discrete transistors.

I'm not trying to rustle anyone's jimmies here. There are some people who believe only tubed can sound good and some people are perfectly happy with TDA2050 amps. I've messed around with many types and I don't have a clear preference. I have used BD139 as an RF amplifier for 7 MHZ with good results.

My wondering is, short of using transistors that are incapable of handling an amp's output power, what could be wrong with substituting old transistors for new? After all, we're trying to take a waveform, change it only in terms of strength, and put it into a transducer.

Ed

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PostPosted: 23 May 2016, 17:21 
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BowToEd wrote:
One of the things that I find strange is that people here tend to shy away from AB designs using transistors. For some reason, tube and ICs are okay for AB but everyone seems to prefer class A when it comes to discrete transistors.


Not just here, but anywhere.... and it's pure ideological audiophoolery. Some of the best SS designs I've heard are AB.

Yamaha..... 'nuff said ;)

Cheers!

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PostPosted: 23 May 2016, 18:48 
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Here is something else I've noticed - most tube amplifiers that are supposed to be class AB actually don't run in AB unless they are running at full power. That's because you have to bias tubes rather hot to get the best sound out of them, so moderate input levels won't be enough to take one or the other tube out of conduction. Some people prattle on about how such amplifiers sound hollow or metallic because they can "hear the switching noises".

I think some of it is "I spent so much on my audio system, it HAS to be better", or "I built my own audio system based on some articles I read, so it's got to be better than a mass-produced amp". There's some truth to both of those but I don't think they're necessarily true in all cases.

I built a transistor amplifier back in 2012. I was proud of myself. I have since learned that my design needed a little work to increase stability among other things. I'd like to try it again and work my way up to a more powerful design. I've got some TIP35/36s that should work nicely for a 50w design.

Someday.

Ed

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PostPosted: 23 May 2016, 19:19 
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BowToEd wrote:
I built a transistor amplifier back in 2012. I was proud of myself. I have since learned that my design needed a little work to increase stability among other things. I'd like to try it again and work my way up to a more powerful design.
If interested in solid state amplifier design, I highly recommend Bob Cordell's book "Designing Audio Power Amplifiers". He takes a fairly straight forward topology (differential current input stage, gain stage, output stage) and then walks through all the major design topics showing how to alter the design to account for various factors. The result is a comprehensive review of virtually all the major topics encountered in solid state power amplifier design.

Although these days I am pretty much fully dedicated to tube design, this book is one of the solid state design references that I always keep on my book shelf. You an also check out his website at http://www.cordellaudio.com. Just remember that Mr. Cordell works for a living, so don't be surprised if he tries to sell you something. ;)

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PostPosted: 25 May 2016, 01:57 
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I've spent a spit-ton of time on Rod Elliott's website, reading his different articles and gaining a lot of information. My favorite overall layout is an op-amp driving a pair or a bank of transistors. I find that op-amps tend to work better than discrete transistors for certain things. I also do not claim to be an expert.

I've got an amplifier set up on the breadboard. I'm going to test it out in the coming days and see how it works. I may decide to go with a bigger, more capable design. The tests will occur at 12 volts but I could make a stronger power supply if I like what the rock is cooking.

In the realm of tubes, I am working on a huge bench supply using a 220v, 300w transformer. I have heavily considered an audio amp integral to the power supply. Since I'll be using a 1w speaker, I'll likely make an amp using a 12AU7 for 750 mW. I'm not going to need a lot of power to make receivers.

Ed

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PostPosted: 26 May 2016, 03:25 
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I breadboarded this circuit and now think I want to pursue a better design.

The circuit was laid out on an IC breadboard and powered by a 12v gel cell battery. The midpoint voltage between the two output transistors was a little low but nothing I couldn't design out. Other than semiconductor substitutions, the rest of the circuit was pretty much stock.

I listened to some PMJ featuring such talents as Haley Reinhart and the magnificent Robyn Adele Anderson. I noticed some fuzz that was slightly annoying to me and it could hardly pass for quality audio.

I'm not going to give up on this design yet. It's designed for a supply of 18 to 22 volts and I was using 12.9 via a battery. If I use a stronger power source, I should be able to realize at least one watt of power.

All in all, the thing makes noise in a speaker. I think I'll end up making a different design before building something permanent, though.

Ed

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