2n3055 amplifier

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#22
I was driving back home and was wondering how good of a PA could be made with 'old' transistors like the-3055? Maybe all oldies for the transistors. I open DIYAUDIO and here we are!

On that output coupling cap -- just be sure there is no ac voltage drop across it or it will produce harmonic distortion... like all electrolytics. In that case, larger values work better.


Maybe a film bypass will help a little if the Ls and esr of the output cap is high (probably).



THx-RNMarsh
 
#23
indianajo said:
Admittedly the ST120 as sold had a room temperature low bias current crossover distortion problem, which your design seems to sidestep with the BC107 setting the bias current. Pity I can't buy those transistors without entering the e-bay market, where half of everything electronic is counterfeit.
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No need for a BC107, any small NPN silicon transistor would do, in fact pretty well nothing in that circuit is special in any way.

Personally though, I would also have used a BC107 - it's my standard small NPN transistor 😀 Unless I was mounting it on the heat sink with the outputs, where I'd use a plastic transistor instead, as the BC107 is metal.
 
#25
Kool Lites I sourced the case from old test gear from my work which were used in the 60's and 70's. Practically all my amps use this case as its so versatile, small but not too big, and aluminium fascias for easy drilling. I have made this amp, a class A JLH amp, two valve amps and a NOS DAC with these cases. Sorry they were not sold in shops. They were custom made for the test gear that were housed in them. So they are refurbished cases and will have extra holes in them but all underneath.

Regards
Billy D...
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#26
infinia said:
The OP image shows 50V loaded, 60V unloaded, should be OK depending on make and how vintage his devices are Vceo > 2Vcc. not many have 1st gen. devices . I ran real RCAs at 80V before.
I wonder about the bias stability and low feedback not using diff amp front end E.g. long-tailed pair. may as well do it a lil better.
I have built and heard very good quasi hi-Fi amps some with single supply and they sound really good with the output Capacitor. In fact I recommend single supply for the added piece of mind for newB's not with that schematic above tho.
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I have read that amplifiers with a single transistor input stage tend not to suffer from phase problems as with long tail pair designs, and amps with this input stage tend to be inherently stable. I read this in an article that Rod Elliott wrote in regards to an "El Cheapo amplifier project from the 60's that he was discussing.
Regards
Billy D...
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#27
Nice!!

I recently tried to replenish my stock of bc107/109/441 and the like and was disappointed ��

Had to settle for bc237/557 etc

Nothing is quite like those metal cans...had to settle for tip3055/2955 too (Nice Gbw, not so nice package)

Nice little amp ��
 
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#28
billbo said:
Just asking if someone can tell me about the output cap and will it fail me at some point especially at high output levels. Is the output capacitor a potential problem?
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If the output capacitor has too little capacitance (< 4,700 uF) it will interact with some loudspeakers and cause an unexpected audible increase in bass response.

The old Acoustic Research AR-3 and many amps of the time including the Heath AR15 were susceptible to this situation. If you like a little extra bass boost, it may not be something that you perceive as a problem. ;-)
 
#29
Hi Billbo
yes that's true about keeping track of the phase margins because you have also lost the many great benefits of negative feedback, which im sure you can find an article on ESP as well. I havent done a bias sensitivity analysis here, but I do know the 1/2 Vcc operating point as Vcc changes isn't simply the ratio of resistors anymore.
 
#30
arnyk said:
If the output capacitor has too little capacitance (< 4,700 uF) it will interact with some loudspeakers and cause an unexpected audible increase in bass response.

The old Acoustic Research AR-3 and many amps of the time including the Heath AR15 were susceptible to this situation. If you like a little extra bass boost, it may not be something that you perceive as a problem. ;-)
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What?

2k2uF is the chosen value of many manufacturers, Quad being one of them. I am sure if there had been a problem, Quad would not be the industry standard of the latter year.
 
#31
Most speakers don't have bass down to 20 Hz. You would notice the difference between 2200 and 4700 uf if you did - but not with the typical 40-60 Hz roll off. To "convert" typical speakers to a quasi-3rd order alignment with a cap usually needs 400-1000 uF. 4700uF is too big to "interact" with most speakers. For a dedicated subwoofer, maybe. You'd want to double all the values for 4 ohms, and double again for 2 ohms but that's just common sense. If you were doing really low impedances you'd probably consider a different amplifier topology.
 
#32
I have the same problem on an amp that i have build, it is the " My old faithful power amp design " project03.

What i've changed:

I have used 2sa940 and 2sc2073 for drivers and the lower npn 2sc2073
output transistors 2sa1695 / 2sc4468. it works well, but it oscilates at lower freq,
at 20hz for example at low low volume it sounds ok but when i slightly turn it a bit up it has a clicking sound on it + some higher frequency clicking or something i cant well explain, my speakers are good, transformer is 200w 2x 26.5 V ac.
it has something to do with the drivers ? I think i have to " tweak " the two 220p caps .
A little help please 😀
Thank you! , Bruno.
 
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#33
I think you have put too much gain in the amplifier. Do you need that much gain? 51k with 1k instead of 22k with 1k. If you need that much gain lower the 100uf cap back down to 22uf and see how stable it is. If this doesn't workwind back the gain. Lower the 51k down to 22k and see. Then wind it back up slowly.

Regards.
Billy D...
P.S. My one is still working well!
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#37
Surprisingly it sounds great with heaps of power. I have increased the voltage to about 80 volts and re biased it. The terminal block thing is awesome. Also the screw connectors on the main board is beneficial for repair. No desoldering off the vero board so no damage. I managed to score a handful of MJ802 transistors and have implaced them instead of the 2n3055's. I must admit it has more power output especially at 4 ohms with these transistors (look the specs up). But yeah the amp is a keeper and the output cap is not an issue.
Regards
Billy D...
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#38
billbo said:
...I have incorporated a speaker delay cct. to eliminate the little thump at power up.
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I would have skipped that, for Indianajo's reasons. An output capacitor of 1mF for 8 Ohms nominal but rather 5.6 Ohms far-below-lowest-resonance load gives a power limit of 28 Hz. Sure the capacitor distorts there, but show me a loudspeaker, which is clean there.

In the picture of the open case i see power transistors mounted without thermal grease squishing out of the gaps, not good.
 
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