Capacitor Coupled Amp?

Printer2

Member
2010-04-02 6:34 pm
OK stop laughing. Oh wait, here's the diagram.

SSamp01_zps40945ee5.jpg


Just want to mess around with some transistors and like the idea of capacitor coupled stages, (partly so as to not blow all the transistors when I screw up) that is simple cheap and dirty. I would like to come up with a 5W Class AB amp using a TIP 31 / 32 (not decided on complementary or quasi copmlementary output) running about 50% bias current. Not set in stone though after reading some on the JLH Class A amp thread. and even the old 2N3055 looks acceptable.

Been a few years since taken transistors in school, as they say, a little bit of knowledge can be a dangerous thing especially when much of it was forgotten. I am pretty much expecting a 'that will never work', just wanted to give an example of where I want to go and maybe someone can direct me to something similar that has been made to work.
 
Capacitor coupling won't stop you blowing transistors, but it does give you some safety of your speaker. Also you may find only using a single supply easier than a symmetric supply. There's certainly nothing wrong with using output coupling if you want to learn.

There's plenty of cheap transistors for starters. The good old TO-3 2N3055 is still very usable. There are flatpack versions such as TIP3055 too. You can build a decent amp with cheap parts such as 2N5401/2N5551, MJE340/350, MJE243/253 or BD139/140, and the above mentioned 2955/3055's. They're so cheap, if you blow them up it's not much hardship!
 
Those are what I am looking for, Mr Tandbergen. Capacitor coupled output to protect my $300 each speakers. These seem to be a bit more modern than the Leak Delta 70 or the Dynakit ST120. Please may you could provide a link or something to drawings where I can actually read the resistor sizes or something? These images don't increase much with the plus control.
Mr. Printer, if you want the ultimate in interstage protection, look at the TC-10 tone cabinet from Hammond. It has transformer interstage coupling. Right out of the GE transistor manual ver 6.
 

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Printer2

Member
2010-04-02 6:34 pm
Those are what I am looking for, Mr Tandbergen. Capacitor coupled output to protect my $300 each speakers. These seem to be a bit more modern than the Leak Delta 70 or the Dynakit ST120. Please may you could provide a link or something to drawings where I can actually read the resistor sizes or something? These images don't increase much with the plus control.
Mr. Printer, if you want the ultimate in interstage protection, look at the TC-10 tone cabinet from Hammond. It has transformer interstage coupling. Right out of the GE transistor manual ver 6.

Capacitors are cheap, transformers...

Actually I just have to figure out how to do the output stage. The stages before it are simple textbook examples.
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Thanks for the quick replies. Actually I was more thinking more along the lines of all the stages being capacitor coupled to each other (dc isolation between the transistors) rather than just the output being capacitor coupled.

You sacrifice a lot using that approach. Any reason for wanting to do that ? or just for the fun of it :D

Its a great way to learn though and there is nothing stopping you making a very decent 4 or 5 transistor amp. Throw a couple of PNP devices in there and its a runner :D
 

Mooly

Administrator
Paid Member
2007-09-15 8:14 am
Those amps are based on the classic "Lin" amplifier. You'll see they use germanium devices which means that, although the configuration is OK they won't work correctly with silicon unless the design is modified.

Can sound very good though :)
 

Printer2

Member
2010-04-02 6:34 pm
You sacrifice a lot using that approach. Any reason for wanting to do that ? or just for the fun of it :D

Its a great way to learn though and there is nothing stopping you making a very decent 4 or 5 transistor amp. Throw a couple of PNP devices in there and its a runner :D

Well I do have ulterior motives. Not really looking to do a hifi amp but something to mimic tube amps of days gone by with some of their quirks due to capacitive coupling. Much better not mentioning it though as I don't want to get banished to a rarely traveled subforum, a larger knowledge base here.
 
1) your original example was an *every_stage_capacitively_coupled* which *may*be built but is not practical.

2) if you are worried about your $300 speakers, there are tons of speaker protection circuits around.

The most drastic and fastest/safest is the triac crowbar used in big PA power amps.
It often suicides protecting the speakers and even destroys even more the power amp , but that's cheaper and less distressing than killing big $$$ Pro speakers.

Or the regular Relay protection one, although it takes a few milliseconds to act.

3) or you can use a Capacitor out amplifier.
Two examples :

Acoustic 125
http://music-electronics-forum.com/attachments/7130d1259735926-acoustic-pa-170080.pdf


Yamaha G100
http://music-electronics-forum.com/attachments/11058d1285307419-g100-service-manual.pdf
 
Those are what I am looking for, Mr Tandbergen. Capacitor coupled output to protect my $300 each speakers. These seem to be a bit more modern than the Leak Delta 70 or the Dynakit ST120. Please may you could provide a link or something to drawings where I can actually read the resistor sizes or something? These images don't increase much with the plus control.
Mr. Printer, if you want the ultimate in interstage protection, look at the TC-10 tone cabinet from Hammond. It has transformer interstage coupling. Right out of the GE transistor manual ver 6.

I could send You the complete schematics on the two amplifiers I showed You.
They are 25W/8 Ohms, and 35W/8 Ohms.

But there is ways to protect Your speakers from DC. I could send You schematics for that too. All I should need is Your Emailadress.
 
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Or the regular Relay protection one, although it takes a few milliseconds to act.

I used a FET based 'solid state relay' on my TGM5 amplifier (search for the thread) which is able to react almost instantly in comparison with a relay. But remember that it takes finite time to integrate the signal in order to determine if it's really a dc problem and not just low frequency 'music'.
 

Printer2

Member
2010-04-02 6:34 pm
I can understand wanting something that sounds a bit different... not sure that capacitive coupling is the secret though.

I do not know if it will do it either but I think it would be worth looking into.


Seen them and really like the idea of them. A lot like the L'il Tiger amp I used in the late 70's. The L'il Tiger's part of the reason I am looking into this. I used it in an application that seen it spend much of its time moving into and out of clipping and yet it did not sound harsh like some amplifiers did in the same type of service. I came to the conclusion that the simple circuit with minimal NFB caused it to behave better under these conditions. We did blow the outputs from time to time but I have no doubt the SOA of the transistors were exceed.


if you are worried about your $300 speakers, there are tons of speaker protection circuits around.

Not worried about the speakers so much but I would rather the amp perform within its limits and keep on chugging no matter what you throw at it.
This amp is quite simple and you will get 5% THD at 15 Watt.

Still want to try the capacitor coupling between all the stages first.