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1
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by shasam on Today at 08:20:37 AM »
I am going to use a 2*15V transformer, and NJW0281G (NPN) / NJW0302G (PNP) output transistors, in CFP configuration, for 8Ω output.
Datasheet can be found here : https://docs.rs-online.com/41ca/0900766b813e9eb0.pdf


For now, my calculations are :

Total DC power supply  =  2 * (Veff *√2 - 2Vdiode) = 2 * (15 * √2 – 2) = 38,5V

Output power =  (power supply - loss)² / (8*Rload) = (38,5 – 5)² / (8*8 ) = 17W (I found similar result in LTSpice)

Output peak current = (power supply - loss) / (2*Rload) = (38,5 – 5) / (2*8 ) = 2,1A

Output peak power  = (power supply - loss) * Output peak current =  (38,5 – 5) * 2,1 = 70,35W


With Rod Eliott rules-of-thumb previously quoted :
Quote
"Having discounted the idea of any 'rules-of-thumb', I'm going to give you one anyway . Let's assume that you want to deliver 100W into 8 ohms, so you need a power supply with ±42V rails (I'm going to ignore losses here). The amp must also be able to drive nominal 4 ohm loads, so expect the minimum impedance to be 3 ohms. Worst case (resistive load) dissipation is therefore ...

I = V / 2 / R = 21 / 3 = 7 Amps
P = V / 2 * I = 21 * 8 = 168 Watts (peak)
This accounts for the resistive part of the load, and as we saw above, the reactive part of the load causes dissipation to double. Just like second breakdown, we aren't interested in the average dissipation - this influences the size of heatsink needed, but not the transistor's safe area. Therefore, Ppeak will be ...

Ppeak = P * 2 = 168 * 2 = 336 Watts"
(from https://sound-au.com/soa.htm)

That give me : I = (Vrail / 2) / R = (19.25 / 2) / 8 = 1.203125A
Ppeak_resistive = Vrail / 2 * I = (19.25 / 2) * 1.203125 = 11.58...W(peak)
Ppeak_reactive = Ppeak_resistive * 2 = 23.16...W(peak)


Max dissipation for NJW0281G / NJW0302G is 150W at 25°, 0W at 150°
Derating = 150W / (150° - 25°) = 1.2W/°C

Maximum_case_temperature = (Pmax_at_25° - Ppeak_reactive) / derating + 25°
= (150 - 23.17) / 1.2 +25 = 130°C


I have use the formula given here for average worse dissipation : https://www.updatemydynaco.com/documents/Class_B_Amplifier_Dissipation_Calculations.pdf
Pdiss_worst_case =  Vps² / (19.75 * RL) = 38,5² / (19,75 * 8 ) =  9,38...W
I don't really understand the calculation here. I have use it because it give me the worse result of all the calculation I have read online. What do you think about it?

Heatsink_max_thermal_resistance = (Maximum_case_temperature - Ambiant_temperature) / Pdiss_worst_case = (130 - 50) / 9.5 = 8.42... °C/W
I don't really know what to use for ambiant temperature, I have use 50°C here. I will use bigger heatsink to limit the max temperature lower than 130°C too.


I tried to make it as clear as possible. What do you think about it, please? Could you spot some mistakes? Should I take bigger margin?

Thanks a lot!
2
I dont see any pictures???
3
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by shasam on Today at 04:19:03 AM »
Thanks a lot Phatt!

I try to une rules of thumb too, but different ones look to give really different results sometimes (for heatsinks calculation for exemple, I find results from simple to double with different rules of thumb I have found).

I won't try current feedback for this one. Like you said I try to keep it simple (and I still find it not so simple...).
Maybe for a next project.

I am not familiar with polyfuse, I need to learn more about them, but this look really interesting!  :cheesy:

I have noticed you use different heatsinks for the driver transistors. Do you find the thermal tracking to be better like this?
4
Hello,

I am new here, looking for someone that may be able to help me first identify, and then maybe figure out a way to fix the circuit board on a small portable amp I have had since I was a kid.

Attached are a couple of pictures.  It has 1/4" input on the left side, 9v mini power in on the back, and 6 C-cell battery holder inside, and a 1/4" speaker out on the back.



(hope that's how you attach files...)

With 9V in I can measure voltage part way through the board, but the voltage get lost in the middle. 
Speaker does produce some hum/static with power on and touching the input jack.
Guitar signal in does not make it to the speaker, but pushing audio from a iphone into the in jack does make it out of the speaker but very quietly. Adjusting the amp volume in this case distorts the signal but does not increase it's volume.

Any insights appreciated.

Best regards,


5
Amplifier Discussion / Re: Peavey VSS-20 amplifier with chorus noise problem
« Last post by Jazz P Bass on September 27, 2020, 10:13:08 PM »
Have you checked the 'digital +9vdc supply'?
It should be pretty free of any ripple voltage (Vac).
6
Amplifier Discussion / Re: Peavey VSS-20 amplifier with chorus noise problem
« Last post by jpcar on September 27, 2020, 08:35:30 PM »
I’ve replaced U7 (with a new Xvive reissue MN3007) and the hiss remains.
Hmm. What to do next.

Should I tackle Q6, Q7?
7
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by phatt on September 27, 2020, 06:39:26 AM »
Oppsy forgot the Test board pic. :-[
Probably not the ideal test setup but when on a budget one has to be inventive.
Most of this is recycled El stuff I've collected over time.

As I've done a few times, I did again test to see if Current FB was worth the fuss but just did not hear any magic so gave it a mis. It's those two 5watt Resistors on the side, near the Grey output jack.
Not in picture but I used a light bulb limiter for every new tweak,, It saved me a couple of times during this experiment.  ;)
Phil.


8
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by phatt on September 26, 2020, 10:16:13 PM »
Hi Shasam,
                If you want the indepth analysis there are far greater minds here that can explain the finer points.
I was just a frustrated muso guy who got fed up with the crap that was being sold that never delivered so I bent my brain and read a whole lot of books and started building my own gear.
Most of my knowledge is just basic *Rule of thumb* thoughts I have gleaned from books and later  wonderful people (via the WWW) like Rod Elliot and many other great people who have shared there knowledge.

My rules of thumb;
1/ KISS (keep it simple stupid)
Don't get caught up with teck heads who want to analyze the black holes in amp design. i.e Current FB is it worth it? Well some claim a big difference others say nay,, only you can make that call  xP
My observation,,,It likely helps if the Amp is working at high volumes.
At bedroom levels I doubt if any ears could hear a dramatic difference. 
You have to use your nose (Intuition) to decide if something tecky is worth the extra effort (& often expense).

Yes all done in preamp,, I don't even use a guitar amp, lol.
It's an old SS Laney keyboard amp,, all the mojo tricks are in my pedal board so the Amp is just the power driver.
I have 3 Valve Amps but I use the SS Laney when playing live, it's just so versatile as I can play my El Acoustic, Strat or keyboards all through one Amp. 8|

I've lost count of all the circuits I've built in the last ~35 years and I've built a lot of disasters.  :-[ over that time I've designed about 4 success circuits that work really well (Some are on this site)

The Laney rebuild I'm posting for you to show the BBoard testing setup of a power amp
It's very basic and likely not an ideal bench test setup but well worth the effort.
As you can see in the pics the laney only had a small Tx and when using the Keyboards the supply could not keep up. I rebuilt the entire PSU and power amp section. The 3 Ch preamp section is basic but functional. (Hey it does have a Tank Reverb which is nice)
You can see in the pic the difference from the original back panel and the rebuild.

I've added the Schematic for you as that may help to understand the pictures.
This uses over rated devices as I've found with my basic rule of thumb knowledge. :P
As long as the transistors are way above what is needed then less chance of failure.
(i.e the 2SC5200 is a 250V-15A-150W device) They are the 3 main things to look out for.

As explained in Art of Electronics the whole idea of these DC coupled amp designs is to keep the DC Q point of *Output & the bases of Q1,Q2 at close to Zero as possible*,as I've noted on my Schem.
The Voltage and Temp measurements were taken from test circuit not a simulation.

Current limit is done with a trick little device on the output,a PTC (Poly Thermal Capacitor or just PolyFuse)
Hifi geeks might not like them but saves all the extra PCB space.
Short circuit testing was done with amp at full power on the test circuit pictured about 20 times for at least 10 Seconds.
The Poly Fuse will trip at ~110Watts, The colour will darken when they get close to trip.
Absolutely brilliant little device. :dbtu: :dbtu: :dbtu: :dbtu:

Pleae note;; This was 5 Years back I may have slightly changed something since then,, but it's all very close to how it was built.

Re bandwidth,
I used 250pF at C6 to help limit HiFreq. (Use 250V cap)
C4 sets the low Freq roll off,, smaller values can help roll off excess bass.
100uF there is likely of little use for guitar freq.
Have fun,, Phil.



9
Amplifier Discussion / Re: Peak dissipation in class B output stage
« Last post by shasam on September 26, 2020, 10:04:27 AM »
Thanks Phatt!

How would you limit the bandwidth please? With bigger Miller capacitor? Or in feedback network? I was thinking to limit it in the preamp section.

Some of the greatest guitar sounds ever heard were produced by what would now be considered a very bad design. :lmao:

I am aware of that, but without going to digital, i prefer take a modern approach, with tone and distorsion independant of volume control (OK, speakers and ears are still going to react differently with volume change ;) )


As for design help if you can find a copy of "Art of Electronics" it's well written for those of us who are less gifted at complex maths. :duh
Thanks! I have something similar, in french (my native langage), but this one look more complete.

As noted in the ESP page you quoted, anything running below +/-35VDC is not likely to give much trouble and that covers a basic amp of 40~50Watts so 15 ~20 Watts is fairly safe.
That's the reason why I begin with this, I already have collect bigger transformers for future projects :)


I'll try and dig up some old pictures of my testing circuits to give you ideas.
I would really appreciate it, thanks a lot!
10
Tubes and Hybrids / Re: 12 v tube preamp
« Last post by flester on September 25, 2020, 07:49:43 AM »
Should I have tuned the guitar? Of course

Sent from my SM-A505FN using Tapatalk

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