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From: Jon Kirwan on 26 Jan 2010 15:57
I'd like to take a crack at thinking through a design of an
audio amplifier made up of discrete BJTs and other discrete
parts as an educational process.

I imagine this will be broken up into three sections; input
transconductance, transimpedance VAS, and output driver. But
other arrangements (such as combining the VAS and output
driver using a signal splitting BJT) would work for me, in
learning.

I said "BJT" and "discrete" but I'm also open to the idea of
using BJT pairs, such as the BCV61 and BCV62. In the case of
current mirrors, that may make sense. But not high-priced,
elite and/or hard to get, or obsolete. And no FETs. This is
to be about learning to design with BJTs.

SMT vs through-hole isn't an issue for learning about a
design, I suppose. If I need to build up some section and
test it with a signal, I'll probably want to do it quickly
and without having to buy services every step of the way. So
I may 'dead bug' SMT parts to get there. (The basic idea
here is to learn, not to make something tiny.)

Although I have some other applications, right now I'd like
the target use to be as a computer speaker system (not unlike
those dirt cheap, sadly almost all of them 10% THD, systems
sold today into this market. Except that I'd like to work
through the design on my own, from start to end.

Given what I understand right now from a very short search on
the topic, the input should be taken as a maximum of 1.0Vrms
and the input's load should appear to be something like 10k
ohms. If someone knows different from that, I'll accept the
criticism and change that spec.

I'd like to consider a tone control and a volume control to
be included.

Output is to be into a small 8 ohm speaker. With that
maximum 1.0Vrms at the input and the volume control set to
maximum the wattage into 8 ohms should be around 10 watts.
Since human hearing won't tell much difference between 8
watts and 12 watts, this is a bit of a sloppy spec and I'm
open to anything in the area of 5-20 watts... though I'm
really wanting to keep the rail voltages down to something
modest and the BJTs not having to tolerate hugish Vce.

Now that I say this, an odd idea comes to mind because the
CFL light bulbs include two TO-220 BJTs that can handle quite
a high Vce on them. I could cannibalize those. But to be
honest, I'm still not needing high watt outputs. So there's
no reason to think about scavenging such parts.

I would like to design it to work into 4 ohms as a margin
bound and not as a design goal, but even 5.6 ohms would be
acceptable.

I'm not looking for this to be done quickly, either. If it
takes months of only occasional back-and-forth, I'm fine with
that. Also, I expect to do my work and don't expect someone
else to hand-hold me from complete ignorante to complete
enlightenment. :) I just need someone to slap my face when
I say something terribly stupid and/or point in a truely
useful direction when I need it. Or else someone who is
wanting to explore this with me and willing to work for it.

Is anyone here willing to consider a sincere discussion?

Jon
From: Jon Kirwan on 26 Jan 2010 16:00
On Tue, 26 Jan 2010 12:57:13 -0800, Jon Kirwan
<jonk(a)infinitefactors.org> wrote:

>... ignorante ...

.... ignorance ...

Jon
From: David Eather on 26 Jan 2010 18:04
Jon Kirwan wrote:
> I'd like to take a crack at thinking through a design of an
> audio amplifier made up of discrete BJTs and other discrete
> parts as an educational process.
>
> I imagine this will be broken up into three sections; input
> transconductance, transimpedance VAS, and output driver. But
> other arrangements (such as combining the VAS and output
> driver using a signal splitting BJT) would work for me, in
> learning.
>
> I said "BJT" and "discrete" but I'm also open to the idea of
> using BJT pairs, such as the BCV61 and BCV62. In the case of
> current mirrors, that may make sense. But not high-priced,
> elite and/or hard to get, or obsolete. And no FETs. This is
> to be about learning to design with BJTs.
>
> SMT vs through-hole isn't an issue for learning about a
> design, I suppose. If I need to build up some section and
> test it with a signal, I'll probably want to do it quickly
> and without having to buy services every step of the way. So
> I may 'dead bug' SMT parts to get there. (The basic idea
> here is to learn, not to make something tiny.)
>
> Although I have some other applications, right now I'd like
> the target use to be as a computer speaker system (not unlike
> those dirt cheap, sadly almost all of them 10% THD, systems
> sold today into this market. Except that I'd like to work
> through the design on my own, from start to end.
>
> Given what I understand right now from a very short search on
> the topic, the input should be taken as a maximum of 1.0Vrms
> and the input's load should appear to be something like 10k
> ohms. If someone knows different from that, I'll accept the
> criticism and change that spec.
>
> I'd like to consider a tone control and a volume control to
> be included.
>
> Output is to be into a small 8 ohm speaker. With that
> maximum 1.0Vrms at the input and the volume control set to
> maximum the wattage into 8 ohms should be around 10 watts.
> Since human hearing won't tell much difference between 8
> watts and 12 watts, this is a bit of a sloppy spec and I'm
> open to anything in the area of 5-20 watts... though I'm
> really wanting to keep the rail voltages down to something
> modest and the BJTs not having to tolerate hugish Vce.
>
> Now that I say this, an odd idea comes to mind because the
> CFL light bulbs include two TO-220 BJTs that can handle quite
> a high Vce on them. I could cannibalize those. But to be
> honest, I'm still not needing high watt outputs. So there's
> no reason to think about scavenging such parts.
>
> I would like to design it to work into 4 ohms as a margin
> bound and not as a design goal, but even 5.6 ohms would be
> acceptable.
>
> I'm not looking for this to be done quickly, either. If it
> takes months of only occasional back-and-forth, I'm fine with
> that. Also, I expect to do my work and don't expect someone
> else to hand-hold me from complete ignorante to complete
> enlightenment. :) I just need someone to slap my face when
> I say something terribly stupid and/or point in a truely
> useful direction when I need it. Or else someone who is
> wanting to explore this with me and willing to work for it.
>
> Is anyone here willing to consider a sincere discussion?
>
> Jon

Yes, I've done a few amps... Truthfully it would be nice to do something
useful and it dovetails well with something else I am doing.
From: Jon Kirwan on 26 Jan 2010 18:19
On Wed, 27 Jan 2010 09:04:02 +1000, David Eather
<eather(a)tpg.com.au> wrote:

>Jon Kirwan wrote:
>> I'd like to take a crack at thinking through a design of an
>> audio amplifier made up of discrete BJTs and other discrete
>> parts as an educational process.
>>
>> I imagine this will be broken up into three sections; input
>> transconductance, transimpedance VAS, and output driver. But
>> other arrangements (such as combining the VAS and output
>> driver using a signal splitting BJT) would work for me, in
>> learning.
>>
>> I said "BJT" and "discrete" but I'm also open to the idea of
>> using BJT pairs, such as the BCV61 and BCV62. In the case of
>> current mirrors, that may make sense. But not high-priced,
>> elite and/or hard to get, or obsolete. And no FETs. This is
>> to be about learning to design with BJTs.
>>
>> SMT vs through-hole isn't an issue for learning about a
>> design, I suppose. If I need to build up some section and
>> test it with a signal, I'll probably want to do it quickly
>> and without having to buy services every step of the way. So
>> I may 'dead bug' SMT parts to get there. (The basic idea
>> here is to learn, not to make something tiny.)
>>
>> Although I have some other applications, right now I'd like
>> the target use to be as a computer speaker system (not unlike
>> those dirt cheap, sadly almost all of them 10% THD, systems
>> sold today into this market. Except that I'd like to work
>> through the design on my own, from start to end.
>>
>> Given what I understand right now from a very short search on
>> the topic, the input should be taken as a maximum of 1.0Vrms
>> and the input's load should appear to be something like 10k
>> ohms. If someone knows different from that, I'll accept the
>> criticism and change that spec.
>>
>> I'd like to consider a tone control and a volume control to
>> be included.
>>
>> Output is to be into a small 8 ohm speaker. With that
>> maximum 1.0Vrms at the input and the volume control set to
>> maximum the wattage into 8 ohms should be around 10 watts.
>> Since human hearing won't tell much difference between 8
>> watts and 12 watts, this is a bit of a sloppy spec and I'm
>> open to anything in the area of 5-20 watts... though I'm
>> really wanting to keep the rail voltages down to something
>> modest and the BJTs not having to tolerate hugish Vce.
>>
>> Now that I say this, an odd idea comes to mind because the
>> CFL light bulbs include two TO-220 BJTs that can handle quite
>> a high Vce on them. I could cannibalize those. But to be
>> honest, I'm still not needing high watt outputs. So there's
>> no reason to think about scavenging such parts.
>>
>> I would like to design it to work into 4 ohms as a margin
>> bound and not as a design goal, but even 5.6 ohms would be
>> acceptable.
>>
>> I'm not looking for this to be done quickly, either. If it
>> takes months of only occasional back-and-forth, I'm fine with
>> that. Also, I expect to do my work and don't expect someone
>> else to hand-hold me from complete ignorante to complete
>> enlightenment. :) I just need someone to slap my face when
>> I say something terribly stupid and/or point in a truely
>> useful direction when I need it. Or else someone who is
>> wanting to explore this with me and willing to work for it.
>>
>> Is anyone here willing to consider a sincere discussion?
>>
>> Jon
>
>Yes, I've done a few amps... Truthfully it would be nice to do something
>useful and it dovetails well with something else I am doing.

Oh, my gosh! This is great to hear!!

Could you tell me what you know about the outputs of a PC
sound chip/board? As I gather the details right now, the
idea of 1Vrms max into a load of 10k ohm may be right. Do
you know any of the details?

Thanks,
Jon
From: David Eather on 26 Jan 2010 19:11
Jon Kirwan wrote:
> On Wed, 27 Jan 2010 09:04:02 +1000, David Eather
> <eather(a)tpg.com.au> wrote:
>
>> Jon Kirwan wrote:
>>> I'd like to take a crack at thinking through a design of an
>>> audio amplifier made up of discrete BJTs and other discrete
>>> parts as an educational process.
>>>
>>> I imagine this will be broken up into three sections; input
>>> transconductance, transimpedance VAS, and output driver. But
>>> other arrangements (such as combining the VAS and output
>>> driver using a signal splitting BJT) would work for me, in
>>> learning.
>>>
>>> I said "BJT" and "discrete" but I'm also open to the idea of
>>> using BJT pairs, such as the BCV61 and BCV62. In the case of
>>> current mirrors, that may make sense. But not high-priced,
>>> elite and/or hard to get, or obsolete. And no FETs. This is
>>> to be about learning to design with BJTs.
>>>
>>> SMT vs through-hole isn't an issue for learning about a
>>> design, I suppose. If I need to build up some section and
>>> test it with a signal, I'll probably want to do it quickly
>>> and without having to buy services every step of the way. So
>>> I may 'dead bug' SMT parts to get there. (The basic idea
>>> here is to learn, not to make something tiny.)
>>>
>>> Although I have some other applications, right now I'd like
>>> the target use to be as a computer speaker system (not unlike
>>> those dirt cheap, sadly almost all of them 10% THD, systems
>>> sold today into this market. Except that I'd like to work
>>> through the design on my own, from start to end.
>>>
>>> Given what I understand right now from a very short search on
>>> the topic, the input should be taken as a maximum of 1.0Vrms
>>> and the input's load should appear to be something like 10k
>>> ohms. If someone knows different from that, I'll accept the
>>> criticism and change that spec.
>>>
>>> I'd like to consider a tone control and a volume control to
>>> be included.
>>>
>>> Output is to be into a small 8 ohm speaker. With that
>>> maximum 1.0Vrms at the input and the volume control set to
>>> maximum the wattage into 8 ohms should be around 10 watts.
>>> Since human hearing won't tell much difference between 8
>>> watts and 12 watts, this is a bit of a sloppy spec and I'm
>>> open to anything in the area of 5-20 watts... though I'm
>>> really wanting to keep the rail voltages down to something
>>> modest and the BJTs not having to tolerate hugish Vce.
>>>
>>> Now that I say this, an odd idea comes to mind because the
>>> CFL light bulbs include two TO-220 BJTs that can handle quite
>>> a high Vce on them. I could cannibalize those. But to be
>>> honest, I'm still not needing high watt outputs. So there's
>>> no reason to think about scavenging such parts.
>>>
>>> I would like to design it to work into 4 ohms as a margin
>>> bound and not as a design goal, but even 5.6 ohms would be
>>> acceptable.
>>>
>>> I'm not looking for this to be done quickly, either. If it
>>> takes months of only occasional back-and-forth, I'm fine with
>>> that. Also, I expect to do my work and don't expect someone
>>> else to hand-hold me from complete ignorante to complete
>>> enlightenment. :) I just need someone to slap my face when
>>> I say something terribly stupid and/or point in a truely
>>> useful direction when I need it. Or else someone who is
>>> wanting to explore this with me and willing to work for it.
>>>
>>> Is anyone here willing to consider a sincere discussion?
>>>
>>> Jon
>> Yes, I've done a few amps... Truthfully it would be nice to do something
>> useful and it dovetails well with something else I am doing.
>
> Oh, my gosh! This is great to hear!!

Don't get too excited, my brain has been badly scrambled. I often miss
things on a first pass - don't be afraid to check - I won't take offense.

>
> Could you tell me what you know about the outputs of a PC
> sound chip/board? As I gather the details right now, the
> idea of 1Vrms max into a load of 10k ohm may be right. Do
> you know any of the details?
>
> Thanks,
> Jon
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