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Old 5th June 2008, 10:31 PM   #1
sekanix is offline sekanix  Spain
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Lightbulb Simple and tested bipolar transistor amp

Hi!

I'm new on this site, i just discovered it yesterday when searching information about class-D amplification and for my surprise it's full of interesting information about amplification and many projects... So i feel that i need to contribute something. I was born in Russia and 10 years ago aprox have builded an interesting amp, the schematics is not mine, but taken from a very good book (Bolotnikov and Itaev) about HIFI solid state amplification. Now you can find this circuit on internet, but always in russian and with russian semiconductors. Well, now i live in Spain and builded again this amp using the most popular and available transistors here.

I think, increasing the voltage and changing the drivers (BD139 and 140) you can experiment and increase the output power. I already have working this amp on proto board (photo attached) and it's sounds better than any integrated STK-based cheap junk...

If you decide build this amp you will not need to make any adjust, only if you use other type of drivers and final transistors a bias should be adjusted for them changing the type of D1 diode or connecting more diodes in parallel or serial.

The main characteristics of this device are:

- Power source: +/- 36V 2A for every channel

- THD aprox 0,07% at 50W 1Khz

- Signal/Noise rate 89dB (my unit noise is unnapreciable)

- Standby current 80 - 100mA

- Input impedance 10Ko

- Frequency response 20 - 50000 Hz

If you find any way to improve this circuit or have any question -> mail

Godd Luck!!!
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Old 6th June 2008, 12:16 AM   #2
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It seems that the BC547 are pushed very near their limit of 45 Volts. I'd rather use BC546 instead.
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Old 6th June 2008, 12:28 AM   #3
x-pro is offline x-pro  United Kingdom
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There are several errors in the schematic you've posted. Diodes D2-D5 should be connected other way and not connected to the output node. Unless this is corrected the amp will just blow up instantly. On top of this deadly error input and feedback electrolytics are connected in a wrong polarity.

x-pro
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Old 6th June 2008, 01:06 AM   #4
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The electrolytics seem allright to me. The 4.7 uF at the emiters of the LTP sees about half of the negative rail, so it is correct, the plus side to ground.
The 47 uF in the negative feed-back sees zero volt. No problem. And the 100 uF boot-strap also sees zero volt on the output rail, and about half rail in the negative side. OK for me, plus up and minus down.
Diode D1 inded is reversed. It is used to bias the output in pure class B. Connecting it that way, will blow the output in a microsecond (or less).
Diodes D2...D5 seem to protect the bases of the output stage from reversed polarty in case of saturation, and are shown the correct way.
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Old 6th June 2008, 02:17 AM   #5
lineup is offline lineup  Sweden
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Besides the diodes wrong way ( D2-D5) it is not too bad.
I mean the general idea has been seen working many times.

I would probably lower the Voltage a bit.
+- 36 Volt for a single pair of MJ15002/01 is not needed.
They wouldnt be to cope with so much.
Say when a loudspeaker have dips from 8 Ohm downto like 5 Ohm.
Not if you are unlucky
.. read: Some of your girlfriends happens to turns up the volume for FULL when not 100% sober.

For 1 pair of output devices, I would go for smoething like +-25 VDC.
Less heat, gives better performance.

Also if you lower, you may keep BC547/557, without any hazards.
But when you can consider 45 V transistors, the low-noise variants BC550C/ BC560C are much to prefer.
'C' transistors, instead of 'B' have ihgher gain. Gives less base currents, and so less output DC-offset issues. Easier to get 0.001 Volt at output terminals

With the current +-36, the 2 input transistors share not perfectly.
Each will not have close to 50% of the current ( 2.60 mA ) coming from the tail resistors ( 6k8 + 6k8 ).

You should try to get something like 1.25 mA in the 680 Ohm resistor at upper left.
Probably you should use 560 Ohm here, instead of 680 Ohm.

It is very important that BOTH Input transistors have exactly same currents.
48-52% is okay. But for example
1.00 mA vs. 1.60 mA is actually = 38-62% ... !!!!


Regards, Lineup, from Lineup Audio
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Old 6th June 2008, 08:18 AM   #6
sekanix is offline sekanix  Spain
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Quote:
Originally posted by whoandcar
The electrolytics seem allright to me. The 4.7 uF at the emiters of the LTP sees about half of the negative rail, so it is correct, the plus side to ground.
The 47 uF in the negative feed-back sees zero volt. No problem. And the 100 uF boot-strap also sees zero volt on the output rail, and about half rail in the negative side. OK for me, plus up and minus down.
Diode D1 inded is reversed. It is used to bias the output in pure class B. Connecting it that way, will blow the output in a microsecond (or less).
Diodes D2...D5 seem to protect the bases of the output stage from reversed polarty in case of saturation, and are shown the correct way.

Hi... sorry for D1 mistake made when drawing the diagram, y will attach a corrected schematics this evening (for Spain). Also some photos of the proto amp, that actually are working fine, but with only +/- 30V (I dont have other transformer...)

Thank for input transistor type suggestion, i will change it for BC550. I used BC547B because y don't have others now... so let's go to the store
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Old 6th June 2008, 08:18 AM   #7
sekanix is offline sekanix  Spain
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Quote:
Originally posted by x-pro
There are several errors in the schematic you've posted. Diodes D2-D5 should be connected other way and not connected to the output node. Unless this is corrected the amp will just blow up instantly. On top of this deadly error input and feedback electrolytics are connected in a wrong polarity.

x-pro

My friend, the only really "deadly" error I made was D1 inversion, and it was a errata.

Regards.
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Old 6th June 2008, 09:20 AM   #8
timpert is offline timpert  Netherlands
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Hi,

You have pretty much a bog standard amp here, built successfully millions of times by people around the world. One note though: using only one diode (D1) to bias your output stage will mean that your transistors are completely turned off at idle, giving rise to quite some crossover distortion. Your feedback loop will deal with this to a certain degree, but some idle current in the output stage will eliminate the problem at the source. As your amp is nearly identical to Rod Elliott's project 3A, I'd suggest implementing the Vbe multiplier in his design and make the bias adjustable. Take a look at it here:

project 3A

[edit] Make sure the transistor used in the Vbe multiplier is thermally coupled to the output transistors, i.e. on the same heatsink.

Regards,
Jurgen

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Old 6th June 2008, 09:26 AM   #9
AndrewT is online now AndrewT  Scotland
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The input filter (10uF) will allow a significant AC voltage to develop across the NFB filter (47uF).
I recommend that the 47uF be changed to 330uF or even 470uF 16V. Add a pair of diodes across the NFB to limit the reverse voltage to 1.4Vdc. Consider adding a parallel cap of about 1uF to the NFB & input filters.

I agree with Xpro.
The bases of the LTP will sit at a small negative voltage. Both these filter caps should be reversed.
Quote:
Make sure the transistor used in the Vbe multiplier is thermally coupled to the output transistors, i.e. on the same heatsink.
no. The Vbe multiplier must be coupled to the drivers in the Shiklai (CFP) output stage.
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Old 6th June 2008, 09:36 AM   #10
sekanix is offline sekanix  Spain
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Quote:
Originally posted by timpert
Hi,

You have pretty much a bog standard amp here, built successfully millions of times by people around the world. One note though: using only one diode (D1) to bias your output stage will mean that your transistors are completely turned off at idle, giving rise to quite some crossover distortion. Your feedback loop will deal with this to a certain degree, but some idle current in the output stage will eliminate the problem at the source. As your amp is nearly identical to Rod Elliott's project 3A, I'd suggest implementing the Vbe multiplier in his design and make the bias adjustable. Take a look at it here:

project 3A

[edit] Make sure the transistor used in the Vbe multiplier is thermally coupled to the output transistors, i.e. on the same heatsink.

Regards,
Jurgen


The D1 diode in the original design was 2 diodes... but I deleted one during adjusting the bias experimentally. No distortion is apreciated at low volume levels.

Thank you all for the feedback about this amp, I will take into account some of them (like transistor type change and 680R replace to 560R in order to balance the input stage, so when I return home today I will check the operation of it...

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