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Graham Slee Power Amplifier

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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 28 Feb 2011 at 8:19pm
Graham, Have you seen the Dynaco ST-120A power amplifier? I has a linear 72 volt regulated supply with built in over-current shut down and each channel has short-circuit and over-current shut down. Mine is now 38 years old (rebuilt in 1996 with 1% resistors and 105 temp caps and all new semiconductors). It recently took a direct short (my fault testing new speakersDead) on the right channel and only the zener in the amp protection circuit shorted! The other channel still worked fine and the regulator was still at 70 volts with one channel shut down! I replaced the zener, scrutinized all other semiconductors in the right channel and repaired a burned PCB land on R17, then compared all voltages with my "as-built" measurements from 1996; ALL WORKED FINE. The design description is very accurate in stating this amplifier can handle shorts, I know it would have been better if that weak solder joint on R17(right) did not open up.
If you like I can send you any info needed on this unit, including specs, updated schematic, my personal upgrades.Wink
 
Notice the feedback circuit...Smile
I think you would like this design. It is very simple and yet has specs like the highest top end units.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 01 Mar 2011 at 8:01am
Yes, it is to a degree intriguing. The multiple negative feedback to the DC coupled pair input stage configuration was much loved for its quite complex but educational baptism into transistor biasing techniques. With so much gain however, there is a considerable "Miller" capacitance on Q2 which sets its HF limit - this is compensated by C13 to hopefully assure stability. Short circuit protection looks as if it all depends on the zener diode, but I still have to get my head round that - I've only studied it a few minutes (just waking up). The output stage bias is really clever - D2 and 3 pulling Q4 emitter into conduction - however, C6 that shunts D2 and 3 can upset the HF or should I say VHF? R28 offers additional local negative feedback to the phase splitter (Q4 again) but whether it was thought as that or became essential for stability is not known - PNP transistors back then were not known for their good HF performance. Q1 offers the qualities of an inverting input in a non-inverting design (clever) and provides excellent input linearity (it is essentially a summing node or virtual earth). Much as I hate plagiarism, this is very tempting indeed.
Not simple enough for Google-Bot to understand...
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 01 Mar 2011 at 6:49pm
Originally posted by Graham Slee Graham Slee wrote:

Q1 offers the qualities of an inverting input in a non-inverting design (clever) and provides excellent input linearity (it is essentially a summing node or virtual earth).


I must be going blind! Didn't see C5. Without C5 it becomes a summing node or virtual earth input, but with C5 there is no local negative feedback and the open-loop input linearity of the entire amp is down to the transistor base and its emitter resistance. As the junction between R4 and Q1 is a summing node due to the negative feedback paths from R9 R10 and C15, then it's the transistor base and its intrinsic (unseen) emitter resistance (25/Ic) which determine open-loop input linearity. R1 and R2 help in a small way but linearity is only assured closed-loop by the negative feedback via R9 and R10.

So what's the problem? With a sine wave it's not a problem, but with a complex signal the speaker load and component imperfections can cause phase shifts or delays and therefore the portion of the signal fed back can be at odds with the input signal, and when it's at odds the input linearity is screwed-up.

The answer is to add emitter resistance between Q1 and the junction of R4, R9, R10 and C15. However, that reduces Q1's gain which may not be a good thing for the overall amplifier stability.

It is exactly the same with the JLH 10 watt class A amp as can be seen by examining TR4 emitter. Great idea compromised by open-loop input linearity - 60mV p-p for inputs that are much more than 60mV!








Edited by Graham Slee - 01 Mar 2011 at 6:51pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 03 Mar 2011 at 2:34am
I think I ought to move or continue this post in the Amplification section, and will do so shortly.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote BAK Quote  Post ReplyReply Direct Link To This Post Posted: 03 Mar 2011 at 9:58am
Topic: Graham Slee Power Amplifier
Posted: Yesterday at 9:34pm By Graham Slee
I think I ought to move or continue this post in the Amplification section, and will do so shortly.
Graham, I would like to follow your thoughts on this. Wink I will look next on the Amplification section.
Please note my comments on piezo tweeters. In that thread I mentioned my test results for the Dynaco ST-120.
Distortion is rated at "less than 0.5% ae full output power and much less at lower power".
 Also, the driver transistors were upgraded to TIP-31C and TIP-32C.
 
I only thought you could gain some ideas from this design.Big%20smile
I don't think anyone is using this design any more unless they have a Dynaco ST-120.
What makes this amp unique is the output transistors don't draw the idle (quiescent) current, R16 and R17 do. This reduces heat building up in the output and thus stabilizes the amplifiers' heat dissipation and transfer curves.
All is described in the "Servixe Notes 1" and "TIP mod info" in the website mentioned earlier.
 
At full power the outputs only get warm, not hot to the touch at all. The power supply regulator does get warm idling, and hot at full power , but still not over 135 f. Of course, I have beefed up the heatsinks.
 
Bruce
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 03 Mar 2011 at 6:30pm
Bruce, I've just been simulating the Dynaco ST-120 in Easy-PC in an attempt to learn about its amplifier circuit. Some of the features are indeed unique and provide uncomplicated (to some...) and novel output protection. The resulting plots show that it's quite wide-band - considerably so, without the input filter. Not having the exact transistor models I substituted Q2, 5 and 6 for something closely similar. The phase margin is a bit slim and the gain margin could be better, but here I must add that this amp was designed in 1966, 45 years ago, long before the advent of simulation, so Ed Laurent (amp designer) puts me to shame Cry

I will have a play with the design if only to completely understand it. My immediate thought is that the secondary negative feedback which includes the output cap doesn't do the bottom end frequency response (and phase) any favour (puts a slight hump at 2Hz), but then again is probably needed during output stage protection.

It is stated that the output is class B but I would not want anyone to believe it has true class B crossover distortion, it cannot as the output transistors have some forward bias demonstrated by a quiescent voltage drop across R27.

I will write some more on the subject in a while.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote mrarroyo Quote  Post ReplyReply Direct Link To This Post Posted: 03 Mar 2011 at 11:39pm
This is very exciting!
Miguel
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