50W amp

[zeus_threat]

Hi i've simulated and prototyped this amp. Seems to work fine . Any comments for improvement are welcome

Thanks

[lineup]

Originally posted by zeus_threat

Hi i've simulated and prototyped this amp.
Seems to work fine .
Any comments for improvement are welcome

Looks all perfect, to me.
I think you could build this amplifier.
Maybe an upgrade to TIP35/36 or MJE15024/25 in output stage.
It was long ago since really good amplifiers stopped using 2N3055 / TIP3055.
I guess you used them only for simulation.
TIP35 and TIP36 are my favourites, if you do not want to use TO3.
They are still lower price, but can take considerable powers.



Input stage is 0.3/0.3 mA and the balance is 50-50 =good.
Second stage, VAS is about 4 mA.
Using BC546 and BD140 ?????
Of course you should change BD140 to BC556.
At only 120 mWatt ( 4mA x 30 Volt ) BC556 can still work alright.
An interesting alternative may be BC639 - BC640 for this second stage.
They are 1.0 Watt TO92, while BC546/556 is only 0.5 Watt.

Because you use BD140-BD139 as Drivers for output
you can do good with TO92 in VAS stage, if they can take 60 Volt.

Personally I would increase the minimal current in Q7/Q10 drivers.
This is the current thorough R11 ( 1 K ).
I know most would use lower current, like you.
I would make this current ~10-50mA!!!, for example use R11= 100 Ohm.

----------------------------------------

Finally, you have a very good amplifier circuit there.
Some small adjustments only.
and you can start putting it together- In Real.

Will you use PCB .. or will you try hard wiring
= solder components directly to each other?


lineup

======================================
Datasheets PDF download
TIP35 & TIP36 data - 125 Watt Power Transistors
http://www.bourns.com/pdfs/tip35.pdf
http://www.bourns.com/pdfs/tip36.pdf

[Gigapod]

Hi zeus_threat,
Thanks for posting your schematics with easy-to-find components.
I have a question:
what are C2 R16 R18 for?
Thank you.

[edl]

Hi,

Nice work, I prefer Lin topology too.

About modifications broadly speaking I agree with Lineup.
So:
- scale down R11 to 100/220 ohm
- bypass Q5 with a 47nF cap
- omit R1 and R2, or change them to 100 ohm (they need to be: R=26/I(mA) )
- use a resistor + a trimmpot instead of R7 for proper adjust of bias (20-100mA)
- if amp is stable without R17 (it WILL be stable!), then omit that
- bypass supply with a 10uF-470uF elko, and with a 100nF-470nF cap
- scale down input cap to 2.2uF, or increase C2 to 220uF (time constant in the feedback need to be higher than at input)
- use BC639 or BD139 instead of BC645 in the VAS stage (Q6)
- i wouldn't upgrade output transistors. TIP2955 and TIP3055 will do the job fine.

[edl]

Originally posted by Gigapod
Hi zeus_threat,
Thanks for posting your schematics with easy-to-find components.
I have a question:
what are C2 R16 R18 for?
Thank you.

Hi,

C2 is for blocking DC in the feedback.
R16 is the part of the feedback loop. It determines the gain (it's 27dB), and with C2 it forms a high-pass filter.
R18 is for setting back ground loop.

[edl]

On the margin: +/-30V isn't enough for 50W-RMS on 8 ohm.
28Vp, + 5V (voltage drop on the filter cap) + 5V loss in circut and by rectifiing = about 37V.

[Gigapod]

Thank you very much edl for your very clear explanation, and thank you also for your PS voltage analysis.

What do you call "Lin topology" ?

Can you explain your other suggestions (reason, improvement expected)?

[edl]

Hi Gigapod,

So let's start.

Lin topology? In audio this topology [differential amplifer (long taled pair-LTP) + voltage amplifier stage (VAS) + driver and output stage] is called 'Lin". Sorry, I don't know where this name came form.
This bulidup remindes me to the early RCA designs from the USA. Maybe "Lin" was a designer at RCA corporation?

About R11: the alue determines the idle current of the driver transistors (I=1,2/R). 10mA is OK, so R11 need to be about 100 ohm.

About bypassing Q5: sometimes the Vbe multiplier (formed by Q5+R6+R7) tends to oscillate. A small cap stops oscillation.

About VAS transistors: at 35V supply Q6 will dissipate 0,15W. That causes that the transistor will be 100°C hot at 50°C ambient temperature. That's not enough safe for me.

[lineup]

Originally posted by edl

- use BC639 or BD139 instead of BC546 in the VAS stage (Q6)
- i wouldn't upgrade output transistors.
TIP2955 and TIP3055 will do the job fine.

Yes, think edl is very right here.
He did also look a bit closer than I did at your circuit.
Consider his advice. Try them out in your simulations, if you can.

Also after checking the output TIP2955/3055 vs. TIP35C/36C
I have to admit, TIP2955/3055 might work as well.
Maybe even slightly better at moderate powers
due to possibly a bit higher gain, HFE, than TIP35 has.

The only small advantage for TIP35 is lower heat resistance.
1.0 C/W versus 1.4 C/W for TIP2955.
This means TIP35 will work at lower temperature than TIP2955/3055
at higher power. If same size heatsink.
But if you use a bit larger heatsink for TIP2955/3055
there will be no difference.
And the margin for overheating will be good.

But for more power = more heat, you may want to use TO3 devices.
Like the MJ15024 / MJ15025 ( MJ15022 / MJ15023 ) I mentioned.


Regards
lineup

[edl]

You're right, Lineup!

Cheers,