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Old 9th April 2012, 11:22 AM   #2361
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Originally Posted by guitar89 View Post
LC, I'm been searching through alot of website, the only good VAS transistor candidate are as below : 2SA1209/2SC2911, 2SA1360/2SC3423, 2SA1541/2SC3955
However, the source is from ebay, so i'm kinda risky of genuinity. Which one is better for VAS ?

About last time my destruction by spike/surge, only the Transistor are destroyed of positive portion. Negative is unharmed at all. 2 BC560C of Input and 2SA1707 for VAS, are either shorted or fused, one of them can see one piece chipped away from the transistor.

I want to ask upon your advise, that how to prevent such destruction. Does voltage regulation will safe them, by using resistor and zener only , or using 2N5401/2N5551 for cascode will do the job ? For input only, since VAS i'm going to change either one of them of above.
Try to catch 2SA1209/2SC2911, 2SA1360/2SC3423 or even 2SA1381/2SC3503. The problem is that these are mostly video CRT BJT-s, also suitable for VAS stages and their days are numbered till final expiration. I really don't know where to get them to be solid genuine, trial & error is probably the only method here.

There shouldn't be any spikes in the SSA circuit, since 15 V potentials are build-up slowly because of 1000 uF elcos charging characteristic (dV/dt). Bias currents in all SSA stages rising slowly just because of charging mechanism, so there's probably some other mistake present, to burn your BJT-s.

Last edited by Lazy Cat; 9th April 2012 at 11:31 AM.
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Old 9th April 2012, 11:38 AM   #2362
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Originally Posted by sonnya View Post
Look how busy lazy cat and boscoe (tssa) is!! They could not sleep as the ssa is spinning in there heads.
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Originally Posted by MiiB View Post
hehe....
You "lucky ba..." you are at least one step ahead , poor me

You're just and say nothing, probably having a lot of fun

Andrej

Last edited by Lazy Cat; 9th April 2012 at 11:41 AM.
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Old 9th April 2012, 11:49 AM   #2363
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Originally Posted by Lazy Cat View Post
Try to catch 2SA1209/2SC2911, 2SA1360/2SC3423 or even 2SA1381/2SC3503. The problem is that these are mostly video CRT BJT-s, also suitable for VAS stages and their days are numbered till final expiration. I really don't know where to get them to be solid genuine, trial & error is probably the only method here.

There shouldn't be any spikes in the SSA circuit, since 15 V potentials are build-up slowly because of 1000 uF elcos charging characteristic (dV/dt). Bias currents in all SSA stages rising slowly just because of charging mechanism, so there's probably some other mistake present, to burn your BJT-s.
If compare between those 3 pairs, which would be better ? (or I should buy tons of them for pile-stocking ^^)
Yeah, I think I will go for 1360/3423, its from 'quite' reliable source i think. Its a e-bay online electronic store, and alot of promising trades it made.
About the spike, it isn't generated from the amplifier design. Its due to main power (240V AC) surge/spike, So it last only less than 0.1s, so it only over-volt positive side of power rail, thus destroying my positive transistor.
I'm getting better Vce transistor for VAS, but what about input stage ?
(does changing input cascode transistor to one of those above will prevent destruction ? or make use of regulated rail with zener is better ?)
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Old 9th April 2012, 12:21 PM   #2364
sonnya is offline sonnya  Denmark
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Originally Posted by Lazy Cat View Post
You "lucky ba..." you are at least one step ahead , poor me

You're just and say nothing, probably having a lot of fun

Andrej
It was more the time of the Day (past midnight) you postet updates. !!!
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Old 9th April 2012, 12:33 PM   #2365
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Originally Posted by hahfran View Post
That works but not quite in the desired direction. The even harmonics are reduced, but odd harmonics are at best unaffected or even increase.
I'll give the topology below a try.
Of course the triple output stage has now to be inverting.
Thats a interesting combination, does it work ???

BTW I think youre going in the wrong direction, dont try increase vas gain, rather keep it the same or even reduce it.
What you do is use a baxandall pair for vas, push pull like used by yamaha since 1980s. With this youll get below 10 ppm distortion at 1Khz and about 20 ppm at 20khz, this with triple output stage, the gain only around 45 db and openloop Fc close or exceeding 20khz. Performance can be further improved using emitter peaking techniques on the imput pair but then becomes more complex. As for compensation only additional low pass filter needed to raise gain margin which could be on the low side although phase should be around 90. This is without input cascode, the cascode will make stability very difficult to obtain but you can try. The input transistors have very low voltage swing accross them, so cascoding has little effect, the main contributer here is vas that has to supply all voltage gain. For just 6 transistors this kind of performance cant be beat.
Youll find this scheme used in a Jap amp (for Jap market only) of the early 2000s, ask them who designed it

Last edited by homemodder; 9th April 2012 at 12:47 PM.
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Old 9th April 2012, 03:12 PM   #2366
hahfran is offline hahfran  Germany
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I ahve not tested that yet beacause runnig to lots of simulations with even more difficult simulated speaker with passive crossovers I found that the EF output is less sensitive to
nasty low impedance with high phase shifts than the triple cascades. Although EF produces almost twice the thd it is more smooth with such loads. This becomes strikingly evident when the fb is grounded, no fb from the power stage. Thus follows compensation and such is easier with EF. No wonder that most commercial amps prefer EF.
Although I am weary of Spice models in this respect- the Gummel Poon model is very simple - I'll test now with model of thermaltrak BJT.
The EF works nice with the Vas op set to 40 mA .
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Old 9th April 2012, 03:58 PM   #2367
hahfran is offline hahfran  Germany
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This is achieved by Vas at 24 V p out without fb at complex load
Fourier components of V(outvas)
DC component:1.20654

Harmonic Frequency Fourier Normalized Phase Normalized
Number [Hz] Component Component [degree] Phase [deg]
1 3.300e+02 2.455e+01 1.000e+00 -179.98 0.00
2 6.600e+02 5.630e-02 2.293e-03 -108.39 71.59
3 9.900e+02 6.281e-02 2.558e-03 168.17 348.16
4 1.320e+03 2.369e-03 9.650e-05 -170.86 9.12
5 1.650e+03 2.340e-03 9.529e-05 111.09 291.07
6 1.980e+03 1.257e-03 5.118e-05 176.13 356.11
7 2.310e+03 8.078e-04 3.290e-05 96.33 276.31
8 2.640e+03 6.874e-04 2.800e-05 145.42 325.40
9 2.970e+03 4.207e-04 1.713e-05 69.61 249.60
Total Harmonic Distortion: 0.343872%

the odd harmonics are still too high but anyway it can be achieved that this spectrum remains constant ( relatively) over the frequency range up to 15 khz and that is good.

The choice of the upper cascode BJTs has in the simulation no effect at all as long as VCEo is above 50 volts. However, and unexpected , the choice of the lower cascode BJTs has tremendous effect. I assume a problem with the Spice models is likely. It is hard to explain why small signal BJTs from Fairchild should perform much better then
from Motorola. I will use MPSA43 / 93 because I have a bag of these.
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Old 9th April 2012, 04:09 PM   #2368
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Quote:
Originally Posted by hahfran View Post
I ahve not tested that yet beacause runnig to lots of simulations with even more difficult simulated speaker with passive crossovers I found that the EF output is less sensitive to
nasty low impedance with high phase shifts than the triple cascades. Although EF produces almost twice the thd it is more smooth with such loads. This becomes strikingly evident when the fb is grounded, no fb from the power stage. Thus follows compensation and such is easier with EF. No wonder that most commercial amps prefer EF.
Although I am weary of Spice models in this respect- the Gummel Poon model is very simple - I'll test now with model of thermaltrak BJT.
The EF works nice with the Vas op set to 40 mA .
The input stage I mention will probably do 40 50 ppm distortion with only a EF. No more than 10 ma needed accross vas, 3 ma accross input transistor of baxandall pair.
The triple EF is less troublesome in real life than what simulators seem to suggest, try find a Jap amp not using them..........
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Old 9th April 2012, 05:06 PM   #2369
hahfran is offline hahfran  Germany
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Originally Posted by homemodder View Post
The input stage I mention will probably do 40 50 ppm distortion with only a EF. No more than 10 ma needed accross vas, 3 ma accross input transistor of baxandall pair.
The triple EF is less troublesome in real life than what simulators seem to suggest, try find a Jap amp not using them..........
I do not quite get what you mean could you provide a sort of schematic ( w/o values) ? To be clear what I mean is double EF instead of triple CFP. I think i will make a PCB for test imho the compensation and feedback must be optimized on a real device.
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Old 9th April 2012, 07:03 PM   #2370
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Originally Posted by sonnya View Post
It was more the time of the Day (past midnight) you postet updates. !!!
Yeah I was busy testing CCS-s, many of them ...

Hey sonnya and MiiB, at which phase of an amp R&D are you? All the parts of schematic settled, confirmed and tested? Listening tests in progress? Please say something, since I'm very curious Lazy Cat.
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