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Old 9th October 2013, 08:04 AM   #1
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Default GLA (good little amp) "survivor"

Here is the latest creation (re-creation ) from the "badger hole" .

- A really stable triple OPS - I've swapped outputs , hooked up without a zoble ...
all the abuse.

- This amp is 32 years old and I'm still abusing it daily (just needed a re-cap).

-My LT simulation worked first time , needed to tweak the unconventional compensation scheme - it never exhibited any instability even without the "tweak" .

- Since I actually own the real live amp (and listen to it a lot) , I was able to
compare LT's output to the real world operating points - 99% accurate.

Below are the "goods"
The schematic and the .ASC (spice file)

PS - the PCB is next , I REALLY like this little amp. (try to guess who designed it ? )
OS
Attached Images
File Type: jpg GLAsurvivor.jpg (127.5 KB, 1721 views)
Attached Files
File Type: asc GLA-v1.0.asc (13.4 KB, 94 views)
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Old 9th October 2013, 01:52 PM   #2
Bigun is offline Bigun  Canada
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I see you are back in the saddle on this one

Looks like a good solid performer from the schematic. All those LTP's - could include a balanced input option ?
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Old 9th October 2013, 02:39 PM   #3
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Could the board occupy just 1/3 of the UMS so we can make a 4-6 channel GLA in a store 4u case? That would improve my system's SAF.
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Old 9th October 2013, 03:01 PM   #4
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Quote:
Originally Posted by Bigun View Post
I see you are back in the saddle on this one

Looks like a good solid performer from the schematic. All those LTP's - could include a balanced input option ?
I'm not , but sansui sure was in 81' .

Here's some specs :
- .02% THD20 on the simulator, .03% rated in the OEM manual.
.002% full power @ 1K

- -107db noise. My real amp is noiseless (ear to tweeter-full volume).

I'm surprised they put such a good amp in a "mid-fi".
With my digital source(SB live -PC) , this really sounds wonderful on the Dan Richie audiophile 2-ways in the workshop.
(below link and attachment) X-LS Classic kit

The circuit - I've found that by loading the VAS (R25 -47K) you can control
the H2 / H3 balance in the Fourier results. 22k gives H2 dominant at the
expense of total THD. 33K seems to be the sweet spot , at least for these models.

The real amp uses 2sa992 /2sc1845 almost exclusively , 50VDC rails.
The longevity of this circuit is amazing .... found it in the college of Saint Rose dumpster
( many beer- soaked "enhancements" of "higher education" ).
It worked at lower volumes even with dry electrolytics -amazing.

The compensation is "weird" . I know C17 (lead comp.) and it's effect/purpose.
I'm curious about R7 /C5. I don't know what this network is called , but it greatly contributes to this amps stability.
The amp also exhibits the best clipping behavior , I would guess most of
32 years of clipping .

OS
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Old 9th October 2013, 03:14 PM   #5
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Quote:
Originally Posted by BobEllis View Post
Could the board occupy just 1/3 of the UMS so we can make a 4-6 channel GLA in a store 4u case? That would improve my system's SAF.
I would just go for a couple RSA's. (really sarcastic acronyms)

At least I gave a circuit , unlike the "revolution class a/b thread" ... with
78 replies and over 3000 views.

Too many EF2 OPS's around here.
If I scale this up to a mje340/350 - njw0302/0281 - mjl21193/94 output stage,
perhaps builders will be a little less apprehensive of triples.

OS
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Old 9th October 2013, 04:27 PM   #6
Bigun is offline Bigun  Canada
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The Pioneer amplifier I moded into my TGM6 amplifier (there's a thread with schematics somewhere around here) which also has this 'R25', same value in fact. The load on the VAS was a key part of the compensation (like your R7 C5) as it also had no Cdom. The lead comp does seem a little large at 22pF, but not by much.

I love the speakers - red, great colour. I hope these weren't punished with 32 years of clipping !
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Old 9th October 2013, 04:35 PM   #7
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Ahhhh .... while others are asleep and fussing over the "revolution" , I
continue to dissect this cool circuit.

Attachment 1 shows the clipping behavior. At least it is symmetrical , but
shows a small amount of "sticking" . The edges are rounded , not as bad as
"blameless" clipping.

Attachment 2 is driving a capacitive load. I had to really coax this amp to
give me a damped ringing response. Bridged the OP inductor , lowered the
load resistor. VERY stable ... I've not simulated too many like this.

All this is reflected in the real amp , 32 years with original semi's is a pretty good track record .....
This must become a project !

OS
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File Type: jpg gla-clip.jpg (36.4 KB, 1331 views)
File Type: jpg ringing.jpg (23.1 KB, 1251 views)
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Old 9th October 2013, 04:47 PM   #8
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Quote:
Originally Posted by Bigun View Post
The Pioneer amplifier I moded into my TGM6 amplifier (there's a thread with schematics somewhere around here) which also has this 'R25', same value in fact. The load on the VAS was a key part of the compensation (like your R7 C5) as it also had no Cdom. The lead comp does seem a little large at 22pF, but not by much.

I love the speakers - red, great colour. I hope these weren't punished with 32 years of clipping !
That was the "stock photo" of the AV123's , I have piano gloss black ones "made in Cali,Columbia"
(checked the boxes for contraband - found these
outside a local tavern - perfect condition).

I'm nice to my AV123's - I use them as a reference to hear the imaging and
subtleties of my amps , for clipping I use my "bombed out" realistic 3-ways
(10"/5"/3") - only $20 , even with new woofers.

I haven't done much amp stuff lately , I have been increasing my loudspeaker
"collection".
OS
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Old 9th October 2013, 06:39 PM   #9
wg_ski is offline wg_ski  United States
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Quote:
Originally Posted by ostripper View Post
Too many EF2 OPS's around here.
If I scale this up to a mje340/350 - njw0302/0281 - mjl21193/94 output stage,
perhaps builders will be a little less apprehensive of triples.

OS
Way too many. Especially when they parallel a dozen or so outputs and think they can drive an ohm. And then wonder why the VAS runs out of gas at normal current levels. I hate to run VAS's at 50mA because they burn up the PCB and drift all over the place. Much better to use EF3 with a couple miserable mA in the predriver.

A lot of builders have trouble with triples (tribbles?) because they insist on doing weird things with them. A straight complementary EF3 is pretty darn stable with either high or low fT parts in the driver/predriver.
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Old 9th October 2013, 07:43 PM   #10
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Originally Posted by wg_ski View Post
Way too many. Especially when they parallel a dozen or so outputs and think they can drive an ohm. And then wonder why the VAS runs out of gas at normal current levels. I hate to run VAS's at 50mA because they burn up the PCB and drift all over the place. Much better to use EF3 with a couple miserable mA in the predriver.

A lot of builders have trouble with triples (tribbles?) because they insist on doing weird things with them. A straight complementary EF3 is pretty darn stable with either high or low fT parts in the driver/predriver.
I came to these same conclusions after reviewing some of the more popular
threads that use those EF2's with 5 or more OP's and claim 2R loads.

In the attempt to not "run out of gas " VAS wise , some are using drivers
to do the job. Besides the much higher Cob of these devices (mje15032/33 or
2sa1837/2sc4793 -not too bad) , you also have a limited choice of a 1A SOA drivers to run 5 or more pairs of "hungry" outputs.

Moon audio and other overpriced high-end amps use outputs as drivers. You can't (or shouldn't)
do this with the standard EF2.
I have the opinion that 3 pair of outputs is the maximum for the EF2.

OS
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