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Old 8th March 2009, 01:43 PM   #31
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The gentlemen I know are pensioner british citizens now living here, I see them sometimes as my girlfriend provides some medical care for them, they are in their late 70s, we didnt get into any details about amstrong amps, I only mentioned if they had ever heard of the 700 series amp by armstrong, to which the reply was immediately confirmed that it was a 200 watter that sounded darn good, The only other comment was that they sounded slighly warmer than the typical british sound, I dont know what that means though.

Andrew its a piece of cake just redesigning that front ltp, I dont like it at all, it spells disaster to me, but I like that vas, it works very well, I recently build prototype holman amp, slightly different but used cascoded fet input ltp, very nice amp, I m trying to find someone here that can measure accurate THD20. The sims shows 115db 20 khz performance at half power H2 dominant , if sim is out by 10 15 percent it is still very good. Ostripper is in love with his holman type amp as well, for some reason that topology gives awesome bass.
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Old 8th March 2009, 03:57 PM   #32
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Simmed, as I suspected this amp easily returns -120db 20 khz distortion figures, the original should be stable with just the rc across the ltp, no other compensation needed, mine is with a cascoded jfet input. Nice its fast too.
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Old 8th March 2009, 08:23 PM   #33
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Quote:
[i]The only other comment was that they sounded slighly warmer than the typical british sound, I dont know what that means though.[/B]


could be the heat dissipated by the 15v regulators????

actually the Apt-1 had a lot of innovative features in it. it had a darlington diff amp (LTP), a current mirrored cascode VAS, a baker clamp to prevent rail hang-up during clipping, and a triple darlington (triple EF) output stage, a very effective and reliable DC servo, and a protection circuit of which i have yet to see it's equal. they were truly the most "idiot proof" amps i've seen. and, yes, they did have awesome bass.
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Old 8th March 2009, 11:26 PM   #34
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unclejed613, you right about the holman amp, its pretty simple but very well designed. The proto I built I changed the cascode to a randy sloan type, referenced to the output, Im not sure what the right english terminology is for this, more defined mids and treble without losing that stumping bass. Also I used a cascoded jfet input instead of the bjts, the holman is an old but very good design.

This armstrong uses the same topology but it has added ltp frontend and no cascode. Its openloop gain is higher but what I like about it is the wider bandwith by far, and its faster. Reminds me of the otala amps and what that designer tried to achieve, Im am owner of electrocompaniet monoblocks and they sound awesome. The current mirror I included in my simmed circuit, gives better balance of the push pull currents but not stricly needed, does nothing for distortion figures. Well if I strap a triple darlington on this the distortion figures would be scary , this is in simulation offcourse. I dont care much about clipping or protection circuits, I find them detrimental to the sound, If I want a 200 watt amp, Ill design it so it can handle 300w but Ill change the gain in accordance with preamp so it wont go far beyond 200. I see this is what armstrong have said about protection circuits too, I happen to agree.
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Old 8th March 2009, 11:59 PM   #35
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actually the Baker clamp's only effect on the sound was to make light clipping undetectable sonically. and it probably saved a lot of users money, because the amp didn't do the "tweeter eater" thing when clipped. not that an amp should be run into clipping, but with music with a wide dynamic range (the intro to Boston's "Been So Long" comes to mind) it's almost inevitable.

the triple darlington used very fast predrivers and drivers, and the emitter loads did not return to the speaker rail. if you look at a schematic of the Apt output stage, you'll see what i mean. the beta on the output devices had to be below 45 or 50, to avoid common mode conduction, and this also improved the distortion figure a bit, since beta droop , while still there, was minimized as a matter of scale.
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Old 4th April 2011, 10:22 PM   #36
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Originally Posted by AndrewT View Post
Hi,
were the 700 series amps Armstrong's first foray into SS output stages without an output transformer?
Or did that tribute belong to the 600 series amplifiers?
The 600 series was also SS without output transformers

Quote:
Originally Posted by AndrewT View Post
The Armstrong was a fairly good value, cheap British amplifier that just got into the HiFi category, when HiFi used to mean striving for High Fidelity.

I have no idea how reliable the Armstrongs were nor how long it took them to develop transformerless output stages. This could be a risky project.
I've got a 600 series amp which sounds absolutely sublime (following a recap). If thats anything to go on then I would be very eager to attempt this build at some point. Only 200 or so units of the 700 series were ever sold, so it is a rarity.

Apparently the Armstrong 700 series was the companies break away from quality budget hifi to precision high quality (high price) gear, and due to this the company went bust. People weren't ready to pay that much for an Armstrong I guess when they could get a Quad or a Linn for the same price.

Having said that when I got in touch with one of the original designers (Mike Solomon) looking for schematics and pcb designs his initial response was "It's still working?!?!"
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Old 5th April 2011, 01:26 AM   #37
Tajzmaj is offline Tajzmaj  Slovenia
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Funny thing this Armstrong amp. Is it built for something like 100R speakers or is one of the first attempts to make self oscillating amplifier? I think, it will oscillate so much that it will overheat at idle. Or will burn when goes to clip. What would happen if you hook up 4R speakers? You need to water cool it. Please don't build it, at least not without any caps to calm down oscillations......
Have fun
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Old 5th April 2011, 02:31 AM   #38
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wow, no compensation caps, in fact no bandwidth limiting of any kind in this schematic. looks like a good topology, just poorly executed... with the brute force zeners and the obvious lack of compensation, are we sure this isn't an expensive space heater? somebody mentioned water cooling..... liquid nitrogen is more like it....(waddya mean the transistors won't work below -40 degrees?)
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Last edited by unclejed613; 5th April 2011 at 02:36 AM.
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Old 5th April 2011, 02:40 AM   #39
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Originally Posted by homemodder View Post
Andrew you are right, 90 volts is crazy
High rail voltage with 2 pairs of output transistors to get 200-300W to drive real world speakers, why not?

I wonder why I haven't seen here an effort to use a cascoded output stage?? I don't understand amp design so I wonder why not?!

I hope that sakis will come up with successfull result so I can follow building this amp. But may be not, I don't have that many bipolar pairs, I expect only 2 pairs of output transistors
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Old 5th April 2011, 04:58 AM   #40
Tajzmaj is offline Tajzmaj  Slovenia
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Why not? Because you can only dissipate some 50-60W per transistor if use mica insulators and have operating temperature of heat sink some 60 degrees celsius 8i don't know how much is this in farenheit). So there you get some 200W of heat dissipation and how effective is such amp? I suspect some 60%. So you can squeeze out just little over 200W. Things aren't so bad because w listen music not DC but such an amp will overheat sooner or later. Impendance of speaker isn't just simple 8E but can dramatically drop. Sometimes under 2E at poor constructed speakers. At +-90V juy will get some 60V output which gives you very high current peaks when Z drops. Sooner or later the outputs will fail. 100%.
Hmm liquid nitrogen is nice idea. Transistors will be superconductive and there will be no internal loses....it maybe will work. Lets try...he he
But if someone wants to see how looks output stage from that era then should find schematic of Phase Linear 700b. That amp have used +-100V. And runs hot as hell.
So, if someone think that is possible to build powerful and cheap amp(because there are only 4 transistors....), just because he find such scheme,than HE IS WRONG. Everything is possible on paper. Such amp will be very very unstable and unreliable. Especially if you install some modern transistors instead of venerable ,,heat dissipators,,
I know what happens when you just drop in new components in old circuits constructed with old materials. Try or believe.
Make a 1000cc car engine with 300hp.
Best regards
Taj
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