Hi all. I built an amplifier based on 2N3055. i wanted to use 2N3055 transistors as i have a swag of them in TO3 form. i used this CCT. below with some mi nor changes. I am also running it at +75 volts single supply rail. my only concern and query is the output capacitor.
Is the output capacitor going to be a problem and will the capacitor have a significant effect to the amplifier's performance? I wanted to use both NPN output transistors so this is why I used this CCT with a single supply rail. And its simplicity! The output cap was increased to 4700uf aswell and some minor resistor value changes were made. All other transistors are BD139/BD140 except the front small signal gain stage which is a BC559c. The amplifier works quite well with a continuous output of 50 watts RMS into 8 ohms and over 65 watts into 4ohms. 9 did not push the 4 ohm test to the limit. Gain is set at 27db. For such a simple CCT. I managed to get a distortion reading of about 0.07% at 50 watts RMS and down to 0.05% at all other power levels tested at 100, 1k and 10kHz frequencies
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?
Please let me know
Regards
Billy D...
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Is the output capacitor going to be a problem and will the capacitor have a significant effect to the amplifier's performance? I wanted to use both NPN output transistors so this is why I used this CCT with a single supply rail. And its simplicity! The output cap was increased to 4700uf aswell and some minor resistor value changes were made. All other transistors are BD139/BD140 except the front small signal gain stage which is a BC559c. The amplifier works quite well with a continuous output of 50 watts RMS into 8 ohms and over 65 watts into 4ohms. 9 did not push the 4 ohm test to the limit. Gain is set at 27db. For such a simple CCT. I managed to get a distortion reading of about 0.07% at 50 watts RMS and down to 0.05% at all other power levels tested at 100, 1k and 10kHz frequencies
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?
Please let me know
Regards
Billy D...
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The output capacitor should be around 2k2uF and will last as long as the quality of it. Quad have used them for years.
I agree with the output cap being too high but I was catering for 4 ohm loads. Which I may not use the amp such low loads. If I can find them I will lower the output cap value. Otherwise I will leave it at that.
Given enough time, temperature, and current all electrolytics eventually wear out. The current through the output cap is on the same order as that through the power supply cap. If you're still using the amp in 10 or 20 years, you may need to replace the caps. Not any different than any other amplifier. Even listening at "loud" levels, a 50 watt amp will be putting out around 5 watts average, unless you like listening to heavy clipping. That's not THAT much stress.
Thats O.K. Its the first amplifier i have built with output capacitor coupling. I must admit its a very stable amplifier and the capacitor does give you some protection for your speakers. Its just that many say that it is inferior to direct coupled amplifiers as far as distortion and quality of the sound is concerned. The power supply is a choke filter supply so hum and noise is eliminated despite such simple design. I am impressed of its performance.
I will post a pic of the finished amplifier.
Regards
Billy D...
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I will post a pic of the finished amplifier.
Regards
Billy D...
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Many of the classic great sounding amplifiers used capacitor coupling, DC coupling was a more 'modern' innovation - and while it 'may' be capable of better specs. that doesn't mean better sound 😀
The capacitor gives MASSIVE protection for the speaker, it was almost totally unknown for such amps to kill speakers - I've never seen one do it in all my years as a service engineer.
No need for a choke PSU, and I've no idea where you'd even find a suitably sized choke either? - as it was never needed, they were never made.
One of the classic amps, the Quad 303, used a regulated PSU - but that was very much the exception to the rule.
I find the complaints about the "sound" of the output capacitor of single suply amps, to be mostly marketing hype to sell direct coupled "speaker burner" amps. Which were IMHO cheaper to build.
I've compared the sound at 1.5 W of my capacitor coupled djoffe bias modified dynakit ST120, to my direct through Peavey CS800s. No difference I can hear on Peavey SP2-XT speakers, on piano source records or CDs. I can hear HF vibrato with either for example, using 20' of 18 ga zip cord instead of 10' of 10 ga flexible SO cord, so I think I have a sense of what a piano "should" sound like. After all, I do have a Steinway console located between the speakers, for calibration.
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.
BTW, IMHO real homotaxial 2N3055 wimped out at 60 Vceo. Special 5 digit selected versions from the RCA stream had low enough Iceo to operate at a regulated 72 v in the dynakit ST120. Leak ST70 used the 5 digit RCA transistors, too. 75 v is pretty up there for real 1970-s parts. Newer transistors silk screened 2n3055 have higher Ft is one way to tell. The old ones need a lot of prompting with parallel runs etc to oscillate.
I've never had the output capacitor fail, although I replaced them at about the 30 year boundary just based upon good electrolytic practice.
I've compared the sound at 1.5 W of my capacitor coupled djoffe bias modified dynakit ST120, to my direct through Peavey CS800s. No difference I can hear on Peavey SP2-XT speakers, on piano source records or CDs. I can hear HF vibrato with either for example, using 20' of 18 ga zip cord instead of 10' of 10 ga flexible SO cord, so I think I have a sense of what a piano "should" sound like. After all, I do have a Steinway console located between the speakers, for calibration.
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.
BTW, IMHO real homotaxial 2N3055 wimped out at 60 Vceo. Special 5 digit selected versions from the RCA stream had low enough Iceo to operate at a regulated 72 v in the dynakit ST120. Leak ST70 used the 5 digit RCA transistors, too. 75 v is pretty up there for real 1970-s parts. Newer transistors silk screened 2n3055 have higher Ft is one way to tell. The old ones need a lot of prompting with parallel runs etc to oscillate.
I've never had the output capacitor fail, although I replaced them at about the 30 year boundary just based upon good electrolytic practice.
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Hi,
Search on the use of a "baxandall diode" for biasing non complementary
output devices, its use is a no brainer, e.g. see Naim stuff and D.Self.
rgds, sreten.
Search on the use of a "baxandall diode" for biasing non complementary
output devices, its use is a no brainer, e.g. see Naim stuff and D.Self.
rgds, sreten.
The Naim is a split supply, amp, thoroughly documented and fairly useless. For a long discussion of bauxendall diodes, scott diodes, and other topography on a single supply quasi-comp output amp, look up a long thread about the Panasonic SU380, a single supply amp that was simulated extensively by a member with various diode configurations. None of which diodes affect the idle bias circuit IMHO. Ygg-it was the member contributing, my schematic diagram names show.
I built a similar amplifier.
I found I needed a capacitor on the lower driver CB to stop oscillation on the output.
However my 2n3055 was a new one and the newer ones tend to have higher gain.
I found I needed a capacitor on the lower driver CB to stop oscillation on the output.
However my 2n3055 was a new one and the newer ones tend to have higher gain.
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.
Here is a better circuit.
Its a Maplin 225WRMS amplifier using 2N3055 and MJ2955.
If you use this with modern transistors the VAS capacitor needs increasing.
Its a Maplin 225WRMS amplifier using 2N3055 and MJ2955.
If you use this with modern transistors the VAS capacitor needs increasing.
An externally hosted image should be here but it was not working when we last tested it.
Maplin used +/-45 volts on their 2N3055 amp.
I used one in a disco for a good few years.
^I think the OP only has older NPN TO-3 outputs and an Xfmr with one secondary winding.
has not to fear using output DC blocking but he should look for slightly better schematic.
quasi topology was invented for lack of good PNPs OPT. 2955s aren't all that good compared to new.
has not to fear using output DC blocking but he should look for slightly better schematic.
quasi topology was invented for lack of good PNPs OPT. 2955s aren't all that good compared to new.
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Post 14 is a split supply "speaker burner". A better design perhaps if you have an infinite supply of trash speakers. Typical diyaudio madness for the tin eared that listen to car radio speakers from Salvation Army resale store. No protection circuits are shown in post 14, however troublesome they are in the long term (as relay contacts are unless they have very special contacts). Yeah, NFET circuits are great protection in theory: I have yet to succesfully connect the 25 amp load and the 10 microamp drive current in a long term reliable way to the SAME NFET.
My speakers were $300 each USED . The amp was $50 plus $30 in parts and repair and redesign work. Better driver board experiments are about $12 each in parts. I don't propose to build a diy speaker without an extensive test lab. I have heard some that sounded awful, with as many as four drivers.
The Panasonic SU380 discussion was the best I have seen of a simple single supply amp. Another useful discussion is the Apex AX6 thread.
You make one stupid mistake on a split supply amp with series speaker capacitor, you get to try some cheap new silicon and metal film parts tomorrow with the SAME speakers. You have a solder joint pop loose on the Maplin in post 14, you've bought a new $135 woofer, possibly a $100 horn driver instead if the problem was oscillation. My solder joint episode melted loose at the end of a four hour rehearsal - the heat sinks just weren't up to it. I still have and experiment with that rather premium sounding, undamaged PV1210 speaker.
My speakers were $300 each USED . The amp was $50 plus $30 in parts and repair and redesign work. Better driver board experiments are about $12 each in parts. I don't propose to build a diy speaker without an extensive test lab. I have heard some that sounded awful, with as many as four drivers.
The Panasonic SU380 discussion was the best I have seen of a simple single supply amp. Another useful discussion is the Apex AX6 thread.
You make one stupid mistake on a split supply amp with series speaker capacitor, you get to try some cheap new silicon and metal film parts tomorrow with the SAME speakers. You have a solder joint pop loose on the Maplin in post 14, you've bought a new $135 woofer, possibly a $100 horn driver instead if the problem was oscillation. My solder joint episode melted loose at the end of a four hour rehearsal - the heat sinks just weren't up to it. I still have and experiment with that rather premium sounding, undamaged PV1210 speaker.
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This is the finished amplifier. It runs quite well and I have incorporated a speaker delay cct. to eliminate the little thump at power up. I had no oscillation problems and the output is very stable at all loads. The choke in the power supply I have a few of them in my parts bin. They are in the order of 20mH. The ripple on the supply line (and hum on the output) is eliminated so I left it in there with four 4700uf caps. Two before and two after the choke filter.
This is not my cct. I found it on the web. I do not take credit for it.
Regards
Billy D.
This is not my cct. I found it on the web. I do not take credit for it.
Regards
Billy D.
Attachments
Very nice looking. Certainly adequate heat sinks for 60 w/ch if the thermal grease is properly installed and the angle to heat sink surface is flat enough.
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