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Old 2nd December 2010, 11:13 PM   #1
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Default DIY BJT buffer design

Hi

This is my version of the class A BJT transistor buffer from Building a buffered Gainclone chip amp. ("Simple class A discrete buffer circuit"). I've omitted the DC blocking capacitor as my power-amp circuits (LM3886 units) already have them on their input.

Will four 1.5uF MKT capacitors be sufficient for decoupling? It will be powered by a regulated +/-15V supply.

Other than that, just running this design past people more expert than myself (there are a lot of you, trust me) to see if my version is sane and workable before building it.

Regards...
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Old 2nd December 2010, 11:19 PM   #2
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Thought of another question: does it make any sense to separate power and signal grounds like I've done?
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Old 4th December 2010, 05:04 PM   #3
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Hi
I assume you simulated this circuit or at least adapted from something successful,
but why the 1 amp BC639? Sure, this a class A output stage but what do you have
in mind as a load and your planned quiescent current? If you need high output current
capacity and hence will have high quiescent current and dissipation, the T0126 version
BD139 is a lot easier to arrange cooling for. OTOH, if this is a marginal rating issue,
BC327 or 8 are no slouch at this type of duty.

A simple buffer will not have many layout issues provided your supplies are well filtered
for the current required. Bypass requirements are pretty much related to that too.
Not sure that placing the power input terminals central and running the 15V rails between
the in and out signal terminals is a good idea, though. Generally, keep them well apart.

Good luck and watch pinouts if trying substitutes!
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Old 5th December 2010, 07:57 AM   #4
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Quote:
Originally Posted by Ian Finch View Post
Hi
I assume you simulated this circuit or at least adapted from something successful,
but why the 1 amp BC639? Sure, this a class A output stage but what do you have
in mind as a load and your planned quiescent current? If you need high output current
capacity and hence will have high quiescent current and dissipation, the T0126 version
BD139 is a lot easier to arrange cooling for. OTOH, if this is a marginal rating issue,
BC327 or 8 are no slouch at this type of duty.

A simple buffer will not have many layout issues provided your supplies are well filtered
for the current required. Bypass requirements are pretty much related to that too.
Not sure that placing the power input terminals central and running the 15V rails between
the in and out signal terminals is a good idea, though. Generally, keep them well apart.

Good luck and watch pinouts if trying substitutes!
Thanks for comments. As it's driving a chip-amp, the load is small, way way below, as you suspect, the 1 amp 639s specification. The choice of BC639 was dictated by what is locally available in retail stores. There are many other choices but that unit was the only one the original article at Decibel Dungeon mentioned that I can source locally, so at least I know it works without any major problems. I may get into trying different BJTs in the future (in case it turns out I don't have a life after all - you never know). Your point about the supply rails going close to the signal lines is an excellent one - thanks for pointing it out.

Cheers
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Old 5th December 2010, 09:37 AM   #5
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Hi,
from my experience the 1.5F caps should be way sufficient for local onboard buffering of the supplies.
Especially when there is only an intermediate load to drive like 10k.
Wouldn't run the buffer on much lower load impedances than that though without increasing the standing current, as the max out voltage before (current limit induced) clipping is 0.6V/R5*R_load.

Also take a look at your voltage-regulators in combinination with the buffercircuit, and what else may be running off them.
Voltage-regulators are amps too, so they have a chance of oscillating too if not treated with care.
Agreed, its not very likely - haven't whitnessed an oscillating voltage-regulator-IC myself so far.
But I also don't know what your voltage ragulation looks like. Hence the hint

Regards
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Old 5th December 2010, 10:18 AM   #6
Bonsai is offline Bonsai  Taiwan
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R3 and R9 seem like very high value for a bipolar input. Are you feeding this circuit from a DC coupled source?

The current source emitter load is around 5mA. This means, assuming an hfe of 300 (my guess), the base current of Q1 and Q4 will be circa 17uA. Across a 100k bias resistor this is a voltage drop of 1.6V - which translates directly into loss of headroom. Ugly.

Most source equipment (CD player, iPhone/iPod)now days will easily drive 10k, and 5k is usually no problem.
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Old 5th December 2010, 04:18 PM   #7
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Hi
I had a look at the Simple Discrete Buffer Circuit you referred to on the the Decibel Dungeon forum. Now I understand how the original poster came up with BC637 actually but it seemed he was throwing in absolutely anything that came to hand and could only have assessed the results by ear.

The BC 547 would have been the appropriate choice. Really, that was one sure way to the bin for the project when you would still still have sub-optimal results after countless guesses at what combinations of parts might sound better before the smoke escapes.

Bonsai gives you a good starter to designing a properly working circuit there. I would follow that up with the aim of learning to design your own electronic circuits and make part selections properly. Then you will be streets ahead of that guy's guessometry method and a lot happier with the sound.
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Last edited by Ian Finch; 5th December 2010 at 04:22 PM.
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Old 5th December 2010, 08:02 PM   #8
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Quote:
Originally Posted by Bonsai View Post
R3 and R9 seem like very high value for a bipolar input. Are you feeding this circuit from a DC coupled source?

The current source emitter load is around 5mA. This means, assuming an hfe of 300 (my guess), the base current of Q1 and Q4 will be circa 17uA. Across a 100k bias resistor this is a voltage drop of 1.6V - which translates directly into loss of headroom. Ugly.

Most source equipment (CD player, iPhone/iPod)now days will easily drive 10k, and 5k is usually no problem.
10K is more reasonable for R3/R9?

I expect the input will be a CD player or the output of my PC's Xonar Essence sound card, so, no, not DC coupled.

Thanks for all the info!

Quote:
Originally Posted by krachkiste View Post
Hi,
from my experience the 1.5F caps should be way sufficient for local onboard buffering of the supplies.
Especially when there is only an intermediate load to drive like 10k.
Wouldn't run the buffer on much lower load impedances than that though without increasing the standing current, as the max out voltage before (current limit induced) clipping is 0.6V/R5*R_load.

Also take a look at your voltage-regulators in combinination with the buffercircuit, and what else may be running off them.
Voltage-regulators are amps too, so they have a chance of oscillating too if not treated with care.
Agreed, its not very likely - haven't whitnessed an oscillating voltage-regulator-IC myself so far.
But I also don't know what your voltage ragulation looks like. Hence the hint

Regards
Thanks. I'll post my regulator circuit today and provide a link - it's already been under scrutiny in the Power Supply forum but I haven't posted my last revision there yet.

Do you recommend changing R5?

Quote:
Originally Posted by Ian Finch View Post
Hi
I had a look at the Simple Discrete Buffer Circuit you referred to on the the Decibel Dungeon forum. Now I understand how the original poster came up with BC637 actually but it seemed he was throwing in absolutely anything that came to hand and could only have assessed the results by ear.

The BC 547 would have been the appropriate choice. Really, that was one sure way to the bin for the project when you would still still have sub-optimal results after countless guesses at what combinations of parts might sound better before the smoke escapes.

Bonsai gives you a good starter to designing a properly working circuit there. I would follow that up with the aim of learning to design your own electronic circuits and make part selections properly. Then you will be streets ahead of that guy's guessometry method and a lot happier with the sound.
Thanks. I disliked the pick and mix randomness of transistor selection too (as my experience/education levels increase, I keep finding things on that site I don't like the look of). The simplicity of this circuit is its attraction, especially as more popular JFET circuits often use difficult to find components.

I just looked up the 547 and can get it locally through AZtronics. Looks like one of those parts available everywhere.

Thanks for the help!
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Old 5th December 2010, 10:14 PM   #9
godfrey is offline godfrey  South Africa
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Try get the BC547B (or C). They have better Hfe (i.e. current gain) than the BC547A. See the datasheet here for how the grading works.
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Old 6th December 2010, 07:02 PM   #10
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Quote:
Originally Posted by random007 View Post
I keep finding things on that site I don't like the look of. The simplicity of this circuit is its attraction, especially as more popular JFET circuits often use difficult to find components.
If you haven't already visited this popular site, do - often!
Elliott Sound Products - The Audio Pages (Main Index)
Project 37 (The DOZ preamp) might look a little familiar in its topology and good to learn from.
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