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Author Topic: Class A solid state  (Read 11220 times)

jfetter

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Class A solid state
« on: December 15, 2008, 05:56:23 PM »
hi im back....

doing a new design, i once biased a germanium doorknob power resistor 1/2ps and ran simple class a. it has a special sound for sure. ebay has lots of russian germanium transistors.
the physics may be more complex then simple spice circuit analysis. not sure but it sounded really good. problem is dumping all that heat. my test was a 10 watt unit, gets too hot.

has anyone tried ss class a guitar? ive studies the jlh and variants , think simple class a is best. mosfets better?

ps ive switched to eagle layout.

teemuk

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Re: Class A solid state
« Reply #1 on: December 16, 2008, 08:39:19 AM »
Never tested one, the poor efficiency is a somewhat turnoff for me.

But there's one circuit I definitely have to try out someday if I bumb into the proper components: Use a single-ended class-A output, output transformer coupled and powered from a moderately high rail voltage. This has decent efficiency (for class-A) to build about a 5 - 10W circuit. If the rail voltage is about 120V I think one of those 70V line transformers could suffice as the OT. Some cheap speakers that include the right kind of transformer that steps down from a 70V line voltage can be acquired rather cheaply. Any SS w. OT project likely isn't worthwhile unless suitable "iron" is readily available and with a modest price. In this case, one could acquire iron without having to search for it extensively or resort into ordering something that is custom winded. Biggest problem is likely finding the suitable high voltage power transistor for the output.

Got to try it out someday if I bumb into cheap 70V system speakers and right kind of transistors.

Enzo

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Re: Class A solid state
« Reply #2 on: December 17, 2008, 05:11:01 AM »
70v transformers are cheap, either pulled off a speaker or just purchased alone.  Parts Express $4.25 for a 10w after a quick look.

I don't really know, but wouldn;t the transformer have to have a gap in the iron like a SE OT in a tube amp?  The steady DC current through it might not make it happy.  The typical 70v transformer wouldn't be made that way.  Then again this is low power.

Is this project meant for germanium?

Jack1962

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Re: Class A solid state
« Reply #3 on: December 17, 2008, 06:58:16 AM »
I use class A amps , however as teemuk stated they suck much power for a realtivly low output power. However, they are the cleanest amps(what goes in comes out, just LOUDER).

jfetter

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Re: Class A solid state
« Reply #4 on: December 17, 2008, 09:35:12 AM »
yes the 70v output transformer is an option. I have a CBS champ and have looked at possibly using a IGBT or HV mosfet just for testing the concept. It has the se output transformer. The (genuine)class a sound is stunning.

I was thinking about using a automatic 'idle' bias ckt to reduce pwr dissipation when not running. Idle i think is about 30-40% of time on stage. 

Also thinking the input to final should be xfmr coupled.

By the way, on my old TDA2050 design, the air core first order output filter IS the way to go. Clipping sounds sweet. I used i think 330uH into 8 ohm. second order may be better don't know.

jfetter

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Re: Class A solid state
« Reply #5 on: January 04, 2009, 09:38:43 PM »
I'm thinking about using a RTD sensor on the load resistor for fast RMS detection.
Google and found this hifi example using the Russ T813 Ge.

http://www.hifi-forumi.com/forum/viewtopic.php?f=18&t=10229&st=0&sk=t&sd=a

I think I'll be able to do a dsPIC bias driver that will keep the waste heat down.





Boprikov

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Re: Class A solid state
« Reply #6 on: January 15, 2009, 05:32:52 AM »
http://www.vintagehofner.co.uk/selmer/schematic/ssmerc.html

This is an interesting vintage design. If i understand correctly, the primary of the output transformer must stand 400 mA. I have no idea what the impedance ratio should be. Is it possible to somehow estimate it? And what should the power supply voltage be? I think 10-20 Volts is not enough as efficiency is poor in class A.

Sorry my english... but i´m quite interested about what type of output transformer was used at those SS Selmers.

phatt

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Re: Class A solid state
« Reply #7 on: January 15, 2009, 09:31:52 AM »
http://www.vintagehofner.co.uk/selmer/schematic/ssmerc.html

This is an interesting vintage design. If i understand correctly, the primary of the output transformer must stand 400 mA. I have no idea what the impedance ratio should be. Is it possible to somehow estimate it? And what should the power supply voltage be? I think 10-20 Volts is not enough as efficiency is poor in class A.

Sorry my english... but i´m quite interested about what type of output transformer was used at those SS Selmers.

Hi Boprikov,
 I searched years back trying to find such a circuit and you found one,, Good find.
Here is one I built some years back similar in concept to Selmer Mercury.
I never went on to complete it because of the bass issue but insanely loud even though it was driving a small 100mm speaker.
I have no idea where the OT came from, just found it in some trash.
The output Tr has to be a fairly high voltage as the collector voltage can get quite high when using transformer coupling.
Q3 only needs a clip-on heat sink as it only runs lukewarm.

Jfetter, I do wonder about Exotic transistors and all that heat ,, my thoughts are go with transfomer coupling as it puts a lot less heat into the Amp,,heck line transformers are easy found.  Teemu is on the mark there,,
just my take on it. :)
Phil,  Nambour Australia

J M Fahey

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Re: Class A solid state
« Reply #8 on: January 15, 2009, 01:07:22 PM »
Interesting idea. I always wanted to build an "SS Champ" . Not efficient but the original tubed one is even worse!!!! I think that an IRFP250, which I use a lot in my 300W up amplifiers, will work nicely here . Of course I´ll have to design and build the O.T. What others describe (rightly) as "bass problems" , I think really means "very saturated core"; it definitely will need a gap.

jfetter

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Re: Class A solid state
« Reply #9 on: January 16, 2009, 08:10:24 AM »
that is an interesting design,
i was thing about zero watt idle by capacitive coupling the drain (or collector npn / emitter pnp) to PS and biasing using a "calibration resistor" which is then removed and hence a virtual 1/2 PS. within one half audio period the circuit is running true to the load. no pop or thump but no wast heat at all. A "cold" class a amp. I'm looking at the 2n297A and the ruskis either single or parallel. PS would be 30-40v. no feedback. not much dampening either.




http://www.vintagehofner.co.uk/selmer/schematic/ssmerc.html

This is an interesting vintage design. If i understand correctly, the primary of the output transformer must stand 400 mA. I have no idea what the impedance ratio should be. Is it possible to somehow estimate it? And what should the power supply voltage be? I think 10-20 Volts is not enough as efficiency is poor in class A.

Sorry my english... but i´m quite interested about what type of output transformer was used at those SS Selmers.

Boprikov

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Re: Class A solid state
« Reply #10 on: January 21, 2009, 03:56:40 AM »
Nice to hear that phatt has built a transformer-coupled class A amp rather successfully. I must some day try to build such a design just for curiosity. It seems that they are not notoriously inefficient, but sound quality may leave something to be desired...

jfetter

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Re: Class A solid state
« Reply #11 on: September 04, 2009, 11:02:51 AM »
http://www.vintagehofner.co.uk/selmer/schematic/ssmerc.html

This is an interesting vintage design. If i understand correctly, the primary of the output transformer must stand 400 mA. I have no idea what the impedance ratio should be. Is it possible to somehow estimate it? And what should the power supply voltage be? I think 10-20 Volts is not enough as efficiency is poor in class A.

Sorry my english... but i´m quite interested about what type of output transformer was used at those SS Selmers.

Is that a "70V" line transformer? You think about 400ma bias idle current is through primary? That is about 30w to dissipate at idle with 35v supply.
Hi Boprikov,
 I searched years back trying to find such a circuit and you found one,, Good find.
Here is one I built some years back similar in concept to Selmer Mercury.
I never went on to complete it because of the bass issue but insanely loud even though it was driving a small 100mm speaker.
I have no idea where the OT came from, just found it in some trash.
The output Tr has to be a fairly high voltage as the collector voltage can get quite high when using transformer coupling.
Q3 only needs a clip-on heat sink as it only runs lukewarm.

Jfetter, I do wonder about Exotic transistors and all that heat ,, my thoughts are go with transfomer coupling as it puts a lot less heat into the Amp,,heck line transformers are easy found.  Teemu is on the mark there,,
just my take on it. :)
Phil,  Nambour Australia

J M Fahey

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Re: Class A solid state
« Reply #12 on: September 04, 2009, 06:05:19 PM »
Just for kicks I fired up my old worn calculator. Assuming +B=24V, the 2N3055 would have to support Vce around 50V. So far, so good. Knowing the no signal bias of 400 mA and a saturation voltage of 4 V, the peak excursion would be 20V. That calls for a load impedance of: 20/.4=50 ohms. The transistor would have to dissipate 16 watts. Hot but bearable. Theoretically the disign would supply around 8W RMS, which in the real worls would be enough to supply the 5W RMS specified with some margin.
The sound would be good.
My *very first* SS amplifier was a class A 2W job, the output section of a car radio with a Germanium 2SB176. In those dawn-of-humanity days (around 1967/68), complete kits could be bought, specially the transformer sets (did I mention the *driver* transformer?), because that was state of the art car entertainment.
The almost 1 Amp steady consumption meant nothing in a car.
Car tube radios used 10 times that.

phatt

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Re: Class A solid state
« Reply #13 on: September 05, 2009, 08:39:40 AM »
I have been messing around with transformer coupling Again :loco

It would seem that the OT imp matching is not over critical in fact I'm getting better perfomance using
a *Mains Tr* ???
This particular unit is a 240volt pri /25 volt secondary. Simply connect the pri side to the output transistor and connect your speaker to the 25 volt secondary. :tu:

I think the main problem with my circuit (Schematic on previous post) is the signal can't swing negitive on Q2, causing excessive distortion but as it has to pass through a transformer anyway then any high freq hash is wiped off and it does sound quite smooth
even though it is heavily distorting.

If you replace the OT with a droping resistor and Cap couple the output you loose power and it sounds like a very  bad Amp. (which is what it is of course)
I guess it just shows that even a badly designed circuit can be sonically improved via the use of a Transformer.
I'll have another go at it sometime soon but one has to find the time.

*Enzo* asked way back about the OT GAP For SE Amps;

I don't know how far you can push the boundries but I DO Know that MANY of those old Valve radio gram units from the 50/60's used SE OTr's with no gaps.
From what I've read the DC current (which is what will kill it) would not be high enough to cause problems. Most of those old valve units only pumped out about 5 watts at best but over that you would certainly need to consider it.

The only reason I've been messing with this circuit is the possible use as a pedal.
Just useing a voltage divider off the output and send it off to a big amp.
Have fun, Phil.



J M Fahey

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Re: Class A solid state
« Reply #14 on: September 05, 2009, 01:03:03 PM »
Hi Phil. Those old transformers *did* have a built-in gap, even if apparently it was not there.
To begin with, the EI lamination was *not* interlaced, as in "regular" transformers, but had all "E"s facing the same side, and all "I"s together closing the magnetic circuit. This already provides a usable gap, simply because of die cutting defects and irregularities. To this some added a single sheet of paper (regular printer paper will do) which becomes invisible after varnish impregnation.
For *mechanical* mounting reasons, the first and last E/I laminations were interlaced, simply to act as clamps enclosing the others.
Thus, on visual inspection the transformer looks exactly as an ungapped one, it can be only seen by dismounting it.

 

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