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Old 2nd December 2002, 01:05 AM   #1
aborza is offline aborza  United States
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Lightbulb Greening a Classic?

There has been a bit of discussion on improving the Digi125 but no one seems to have picked up the gauntlet. Well, I am no expert, but I think I have followed what others have suggested in this Forum. If I can get some feedback I will do up the board and publish it.

I need to know if I screwed up the biasing arrangement with the changes I have made.

Also did I bias the caps to ground from the feedback loop properly?

The extra diodes are there for experimentation.

Is there a good place to put a pot to adjust offset? Value?

Any suggestions are welcome.

Well, here it is.......... and thanks.
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Old 2nd December 2002, 01:43 AM   #2
halojoy is offline halojoy  Sweden
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Default I can see no fault in this

It should work resonably well
especially with regulated supply.
The currents in the input pair
are calculated to acheive equally sharing of current.

You can split the 2k21 resistor, above the 4 diodes,
into two resistors,
and put a bootstrap capacitor
from between the resistors to the output.

Or replace that resistor with a current source,
but then you lose the simplicity
of the design.

I maybe should increase the currents in the input pair.
0.3mA to drive the 15mA in the BD139
is not much.
1/10 would make the current 1.5mA
Something like 1-2 mA would be my choice
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Old 2nd December 2002, 04:29 AM   #3
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In addition to the bootstrapping mentioned above, this
amp seriously needs a few parts. (Did I just say that???)

1) Some sort of input resistance to prevent parasitic oscillation
with the source impedance. Happens all the time.

2) An output RC network to prevent oscillation with the load.
Also happens all the time

3) Flyback diodes across outputs - You never Know.

4) Decouple the V+ on the diff pair's current source to get
the ripple out unless your supply is regulated.

5) You will really need some adjustability on the bias.
4 diodes is never going to give you the exact value, and you'll
need better than that.

Do all these things and you won't be sorry.

Do all these things and you'll have an HK Citation 12.



Oh, I forgot. You also need some resistance between the
emitter of the drivers to bias them up. A hundred ohms
shoud do it.

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Old 2nd December 2002, 10:14 AM   #4
djk is offline djk
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Ditch the bias on the feedback caps, all it will do is inject hum. If you really want to try this use a battery and a 1M resistor.
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Old 2nd December 2002, 10:35 AM   #5
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Quote:
Originally posted by djk
Ditch the bias on the feedback caps, all it will do is inject hum. If you really want to try this use a battery and a 1M resistor.
Don't you think it will work to just insert a 1M resistor between
the voltage divider and the caps? The caps themselves should
provide sufficient decoupling to get rid of the hum I'd guess.
If not, then a zener across the resistor to ground in the voltage
divider should help.
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Old 2nd December 2002, 04:01 PM   #6
aborza is offline aborza  United States
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Ok,

I am taking y'alls advice. And I really appreciate it. Thanks.

Is this a board project? With the help of TOAO (Nelso Pass, The One And Only) and the rest of this forum's super contributors we may end up with a great sounding simple amp that was designed by the combined efforts of this board's posters. There is a lot of learning and experience here that can be leveraged into a great project. Let's keep it up. End of pep talk!

Here is the latest schematic.

Nelson, is this what you meant for biasing the drivers?

Because I am not one of those with a lot of knowledge and experience in the field, I am at al loss for figuring the proper DA resistors to get the DA current up and also keep proper bias on the VAS.

Also the bias circuit needs resistor values.

Help!

BTW with the RC filters for the front end dropping the front end V rails, will the amp be able to fully drive the Output to the output's rails less the output VBE drops?

Anybody here willing to put this amp into a simulator and help tweak it up?

In any event here is the latest revision
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Old 2nd December 2002, 04:48 PM   #7
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Well, I would reduce the 3K01 filter resistors down to
something like 33 ohms, and the negative one needs to
be at the right of the 59 ohm resistor.

Your 470 uF bootstrap cap (-) terminal needs to attach to
the output, not the negative input.

Personally, I don't think you need the 150 pF input or the
extra 1K input resistor.

Also personally, I would ditch the two 470 uF back to back
polar caps and the DC bias network and replace them with
just one such cap with a film cap in parallel and a 22k
resistor also in parallel to bleed off DC.

The bias transistor ??? values could be 6.8K on the CB, and
a 5K pot BE.

The bias to the diff pair could be around 33K, and the
resistance off the input drain in the region of 220 ohms -
adjust either of these for DC offset.

Output swing? With the bootstrap it will be much higher
than it was.

At this point the open loop gain is low and stability is probably
high enough that you can consider reducing the 150 pF
lag cap to between 0 and its current value, looking for
oscillation as you do so.
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Old 2nd December 2002, 05:35 PM   #8
halojoy is offline halojoy  Sweden
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I think you should couple the bootstrap
capacitor to the Output of the amp
not to the inverted input

well, Nelson covered that also
I see it now
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Old 2nd December 2002, 11:01 PM   #9
aborza is offline aborza  United States
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My thanks to Nelson, Halojoy, dik and Christer. With their help the project has moved along.

Below you will find the third installment of the saga. Indeed, if you squint at the schematic real hard, you should see the beginnings of a PC Board!

The schematic has been shuffeled a bit for thinking about the PC Board layout. Please check me out for errors. I have made a bunch so far.

I moved the +V 33R resistor to the right side of the bootstrap resistors so that the VAS gets quiet DC on both ends. Is that ok?

The 22R resistor/Jumper on the collector of the DA is there for PC Board layout ease. I understantd the resistor will not hurt. If it does a jumper can be used. That resistor is a spare anyway.

The 500R pot on the other leg of the DA is for offset adjustment. A cermet multi-turn should be just fine, don't you think?

Point A on the schematic connects to point A. I did it this way for clarity and to show that in the practical amp, point A will be the positive speaker lug on the amp. Any thoughts on this technique?

You will note that I have used caps that are overkill. But this allows the builder to use the same caps throughout the amp. If bought in bunches (like at DigiKey) they will be cheaper and there is less chance of error for a beginning builder.

I am sure there is more that can be done. But, since this is the first time I have attempted such a project (design rather than straight diy) I have no clue what should be done next.

When I get feedback on this version I will post the schematic again. Then tweak, tweak, tweak. Someone might try to model it on their simulator for shaping it up before I do the PC Board.

So, here is version three:
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Old 2nd December 2002, 11:04 PM   #10
aborza is offline aborza  United States
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Oops,

I forgot to ask, should there be a film cap paralleled with the Bootstrap cap?
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