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Old 4th June 2009, 02:23 PM   #1
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Default RCA 1972 Basic amplifier MODS

Looking recently for information to construct a simple set of amplifiers to complete my HTPC amp , I attempted to discuss the issue on the DX thread.
Since I was forced to modify the layout because of my application (stealth amp retrofit) , I could not use the DX board. I also have a surplus of 2sa992's , mje/ and other on- semi devices.

The amp in question was originally an RCA application note (1972)...

http://www.hilberink.nl/amps/amps2.htm
Click the image to open in full size.

As most of us know , this is the "original" .. skillfully upgraded and modified by "uncle charlie" , aksa , and too many others to count.
Also very interesting is that on the other pages (link above), many of our DIY SS amps found their beginnings in these RCA designs.

This thread is to address ALL modifications that are possible to this simple topology (even minor ones). In other threads it was suggested
that the use of new high Ft devices would render both the sound and stability of this amp unusable AND that if a constructor does modifications,
strays from the path,or asks questions, they will learn nothing !!

I know this to be false , as many clones of the RCA exist of world class performance that DO use modern parts. (I already have 2 that are 1 year old )
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Old 4th June 2009, 02:29 PM   #2
Bigun is online now Bigun  Canada
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Many amplifiers of this topology discussed on this forum did not keep the protection circuit from this RCA design. That always seemed like an oversight to me. People say they don't sound good, but is that true ? Do we keep the protection circuit for this thread ?
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Old 4th June 2009, 02:47 PM   #3
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Quote:
By bigun - Do we keep the protection circuit for this thread ?
That is a good question... A properly configured VI limiter can be good for those that are careless. At low rails with large speakers , the rail fuses would blow, but at 40-0-40vdc (my version) ,it would most likely damage my small sony surround speakers.

Maybe 2 versions ??

It would be interesting to find the characteristics of the original RCA semi's to see EXACTLY how the "baseline design" performed.
This would allow us to compare the new to the old (FFT , loop gain.etc) , to see what the best mods are to retain the original "flavor". We might find that the old devices DID have limitations (most likely) that our new ones can easily outperform (also most likely).
OS
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Old 4th June 2009, 02:49 PM   #4
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Default Nice schematic, i have tried it into the seventies


sound is not good...has more harmonics than a tube amplifiers...measure almost the same but sounds worse than a tube amplifier.

Good that you gonna ressurrect this one..if keep the topologie and recalculate all that one, without placing CCS, mirrors, complementary VAS, tripple darlington, double simetrical differential and all that modern stuff, then you will have a warm and delicious sound with less than 0.03% of harmonic distortion... and beautifull fourier graphic without bars, only the fundamental tone

And this because of the topologie... but keeping the bootstrapp there and re-adjusting it to the entire amplifier.

Good luck, have fun and enjoy

I did it watching the Sony 33 schematic and could finally produce my amplifier..i have tried many possible options and combinations and could not make it better than it has resulted. (to my ears, this is personnal)

There's nothing to learn, it is try and error basis, just ohm law to calculate and to try subcircuits...hard work and swet only.

Maybe you gonna learn that keeping the topologie the way it was before was a very good idea.

regards,

Carlos
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Old 4th June 2009, 03:18 PM   #5
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Hi, carlos. I am glad to see you.

When you did build the RCA ,did you use the EXACT parts??

These...

RCA1A01 Si npn 5W 70V 4V 1A 200>C 120MHz 40MIN MOT TO39

RCA1A02 Si pnp 7W 50V 4V 1A 200>C 50MHz 30MIN MOT TO39

RCA1A05 Si pnp 5W 75V 4V 1A 200>C 60MHz 50MIN MOT TO39

RCA1A06 Si npn 5W 75V 4V 1A 200>C 120MHz 50MIN MOT TO39

RCA1C05 Si npn 40W 140V 120V 5 4A 200>C 4MHz 40MIN RCA TO66

RCA1C06 Si pnp 40W 140V 120V 5 4A 200>C 10MHz 40MIN RCA TO66

Quote:
By DX - could not make it better than it has resulted. (to my ears, this is personnal)
You yourself said that the DX could not match the aska in either soundstage or high frequency clarity/detail. (lost these qualities)

Quote:
by DX - CCS, mirrors, complementary VAS, tripple darlington, double simetrical differential and all that modern stuff
NO intention of adding any "fancy" things here. Just basic reverse engineering. The only thing might be a CCS. the DX's zener "acts" as a crude CCS , rejecting "garbage" from the positive rail. The RCA's diode/R - C also decouples the LTP from the "garbage"(rail).
Also , when you built the RCA , you used the 3 diodes instead of a proper Vbe , which made for bad sound (Xover distortion).
OS
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Old 4th June 2009, 03:34 PM   #6
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If I get a chance, I will doodle the topology in LTSpice since people like to play with these things there.
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Old 4th June 2009, 03:55 PM   #7
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Quote:
By bigun - If I get a chance, I will doodle the topology in LTSpice since people like to play with these things there.
Just a few changes from the DX,TGM. The "trick" is the devices , since it is RCA's design for those devices.

BTW ,On your TGM, what is the LTP current. I did notice , depending on device , the bootstrap/RCA amp has a "window" (.9 to 1ma -2sa992) where the differential becomes balanced , offset goes low, and distortion really drops). This is not the case with the blameless or symasym topologies. Also , I noticed that this amp can keep its low output distortion up to about 2/3rd's peak to peak of the rail voltages , then a big increase in THD (another bootstrap effect)??
Have you seen this errata ?
OS
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Old 4th June 2009, 04:48 PM   #8
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Default No.... only some parts were original...i was very poor those early days


as a student i could not buy too much parts.

Some were original, others not...but do not worry with parts..they do not sound...topologie sounds.

Also i do not remember very well what parts i have used..was a very long time ago..but i remember i dislike the sound and had to tweak in something to make it work..the original schematic had not operated properly.....i cannot remember details...just the feeling about the amplifier.

That diode there is an audio killer... after assemble, remove it and place it once again and listen to high level sound... full power, and observe the hell harmonic distortion you gonna listen.

No!.. when trying amplifiers i use no diodes or VBE multiplier, i use resistance into the prototype and i use to take care of the heat as i will not have thermal control.

regards,

Carlos
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Old 4th June 2009, 04:51 PM   #9
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Well , here is the amp that is supposed to oscillate ..


Click the image to open in full size.

I have 2 of the the NPN LTP version (not as refined), not a problem for a year. Attached is a FULL (models included) .ASC for the above schema. Not a PPM amp , just 9 devices.

A few posts later , either me or BG will "doodle" the original. I also have a PCB layout in the works with integrated DC supply. At 5.5" X 3.5" , it will leave nothing else to build... just hook up the trafo - and enjoy..
Also should be good for multichannel/HT use.

OS
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Old 4th June 2009, 04:54 PM   #10
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Default Oh!...is this amplifier oscilating?.... was you that made it?


It looks nice,.

Is this your target?... to develop the RCA to be pretty that way?

regards,

Carlos
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