About make an audio power amplifier.

[Nikolai Petrenko]

Hi Nikolai,
I have captured a Rolls Royce pre amp PSU in EAGLE now. Just doing a bit of twiddling and checking and will be posting soon.

spec

Thanks!

 

[spec]

​

ERRATA
(1) Note should read: REPEAT COMPLIMENT OF 15V SUPPLY HERE TO FORM -15V SUPPLY (BC546=BC556, BD139=BD140)
(2) C2 should read 4m7F
(3) R22 should read 4K7
(4) Title along lower schematic should read: 02CSEL02_ETO_NIKOLAI_PREAMP_DESCRETE PSU_Iss01-00_2016_03_20_sh1~N

NOTES

DATA SHEETS

 

[Nikolai Petrenko]

View attachment 98346
​

ERRATA
(1) Q5 should read BD138 not BD139
(2) Q7 should read BD138 not BD139
(3) C2 should read 4m7F
(4) R22 should read 4K7
(5) Title along lower schematic should read: 02CSEL02_ETO_NIKOLAI_PREAMP_DESCRETE PSU_Iss01-00_2016_03_20_sh1~N

NOTES

DATA SHEETS

Thank. I think BD140 instead of BD138, because BD140 are available.

 

[Nikolai Petrenko]

You know: output short-circuit are serious threat to amp, especially with careless person like me . So I have modified Rod Elliot's circuit, add compressor circuits to limit output current. Any mistake?

I hope that Rod Elliot will not sue me because of mod the circuit .
Ah, will MJE340 and MJE350 replace BD139 and BD140 in above circuit well?

 

[spec]

Thank. I think BD140 instead of BD138, because BD140 are available.

Well spotted Nikolai.
BD139= NBJT
BD140=PBJT
spec

UPDATE 2016_03_21: have corrected original schematic.

 

[spec]

You know: output short-circuit are serious threat to amp, especially with careless person like me . So I have modified Rod Elliot's circuit, add compressor circuits to limit output current. Any mistake?
View attachment 98355

15K resistors should be 1K5
1N4001 would not be necessary. 1N4148 would be OK (only 12 mA max)
spec

 

[spec]

Ah, will MJE340 and MJE350 replace BD139 and BD140 in above circuit well?

Can fit in Q4 position OK. But neither transistor (BD140 or MJE350) good here for the voltage amplification stage (VAS). Only 12mA collector current max so why fit a medium power transistor which has compromised hfe and ft in this critical position.

Not enough current capability for two output driver transistors though.

spec

 

[Nikolai Petrenko]

15K resistors should be 1K5

Yes, my foolish again, this is the reason why I am almost get bad mark of exam .

Can fit in Q4 position OK. But neither transistor (BD140 or MJE350) good here for the voltage amplification stage (VAS). Only 12mA collector current max so why fit a medium power transistor which has compromised hfe and ft in this critical position.

Not enough current capability for two output driver transistors though.

So did you want to say TIP41 and TIP42 will be better to place in Q4, Q5, Q6?

 

[spec]

Yes, my foolish again, this is the reason why I am almost get bad mark of exam .

Everybody makes mistakes, especially me. A major mistake is no problem. It is the subtle mistakes that cause mayhem.

So did you want to say TIP41 and TIP42 will be better to place in Q4, Q5, Q6?

TIP41 and TIP42 would be OK for the two driver transistors.
The VAS transistor is one of the most critical transistors in any audio amplifier of that architecture. It is this single transistor that provides all the voltage gain in the amplifier and needs to have good linearity and high ft at currents of 0 to 12mA. This is radically different to the requirements of the output drivers.

spec

 

[audioguru]

15K resistors should be 1K5

Hee, hee. 15k would limit the current in each output transistor to about 60A!

 

[Jony130]

add compressor circuits to limit output current. Any mistake?

Yes I see one. Notice that the VAS current (Q4) is not limited and this would cause a big problem. So your circuit will only "protect" Q6 and Q8.
This is why almost none use this type of a protection circuit. For example you can fix this by adding an emitter resistor into Q4 and two diodes from Vcc to Q4 base.

 

[spec]

Yes I see one. Notice that the VAS current (Q4) is not limited and this would cause a big problem. So your circuit will only "protect" Q6 and Q8.
This is why almost none use this type of a protection circuit. For example you can fix this by adding an emitter resistor into Q4 and two diodes from Vcc to Q4 base.

Nice one Jony

spec

 

[Nigel Goodwin]

For example you can fix this by adding an emitter resistor into Q4 and two diodes from Vcc to Q4 base.

Are you perhaps a little confused?, the two diodes (along with an emitter resistor) would be at the bottom side of the circuit, creating a constant current source in place of the two 3.3K resistors.

I don't think adding the two diodes to the top would help sound quality?.

 

[Jony130]

the two diodes (along with an emitter resistor) would be at the bottom side of the circuit, creating a constant current source in place of the two 3.3K resistors.

Yes, sure we can replace the bootstrap circuit and use constant current source instead. But it does not solve the problem with lack off current limitation in Q4.

I don't think adding the two diodes to the top would help sound quality?

No, first add Re resistor into the circuit. And the value of this resistor is Re = Vd/(2*IcQ4) = 0.7V/(2 * 5.3mA) = 0.7V/10.6mA ≈ 68Ω but let us pick 47Ω ohms. Next we add two diodes from Vcc to Q4 base. So, for normal operation this two diodes are OFF.

 

[audioguru]

An emitter resistor added to the Voltage Amplifier transistor will reduce open loop gain and increase distortion.

 

[Nigel Goodwin]

An emitter resistor added to the Voltage Amplifier transistor will reduce open loop gain and increase distortion.

It'll certainly reduce open loop gain, but will it increase distortion? - as it's providing local negative feedback, and thus 'reducing' distortion. As it reduces the need for quite as much overall feedback, it probably leaves distortion pretty much the same?.

 

[audioguru]

The class-A voltage amplifier produces even harmonics distortion that is reduced with the emitter resistor but the output stage produces odd harmonics distortion that needs all the negative feedback it can get to reduce its distortion.

 

[Nikolai Petrenko]

Update version, I think will be better:

 

[spec]

Here is a schematic showing the principle of protecting the VAS and the output transistors without intruding on the fundamental operation of the amplifier. The transistor in the VAS base will add a very small amount of distortion but it will not be significant in relation to the overall performance of the Elliot amp.

spec

 

[Nikolai Petrenko]

Hmm, I have thought about use FM to transmit audio but FM receivers are too complex, so I choose AM instead. New problem raising: MPF102 are not available. Will 2N5484 or 2N5486 or 2SK170 replace MPF102 in the circuit well?
**broken link removed**