First discrete amp, Need help with NTE 390, 391, 375, 398, and BD140, 139 project - diyAudio
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Old 23rd December 2008, 12:55 AM   #1
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Default First discrete amp, Need help with NTE 390, 391, 375, 398, and BD140, 139 project

Those are all headed my way.
In the process of purchasing some little caps to finish up a project, I got all fascinated over the possibility of building a transistor amp.

I was thinking that those new parts could make some classic sounds like NEC's original B618, D588 with A1567, A794 as seen in the Technics, circa 1975, opening day product line.

Problem: The modern equivalent for half of that was missing, so I had to go with the new manufacturer's suggested complimentary part. The two smaller NTE made no sense to me (television parts?), so I got a backup plan from ONsemi (audio parts).
For pre, I also got several of every transistor listed as seemly in the Decibel Dungeon discrete buffer project as well as some not-so-close equivalents to C828.

Unfortunately, that was as far as my plan got me. Oops!

I'm in need of a PCB or a photograph of a seemly way to wire the power amp on perfboard.

Is there any chance that there's a newbie help document with remarkably understandable documentation? Yes, a resistor goes there and here and everywhere else, and yes I can assemble a kit. Boring! I want to know, and neither a slew of indecipherable documents nor a kit will do it. Help?
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Old 23rd December 2008, 02:30 AM   #2
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Looks like you already have the VAS and drivers (BD139/140) for a lower power (+- 45V) Self "blameless" design..

I don't know about those NTE parts, except they cost 10 times
as much as better Mouser.com parts (KSA1845/KSA 992).

With your BD's and 10$ in better parts (NJW 1302/0281)outputs..
$1.40 apiece ..,one could have a real good 60-80W discrete amp.

Click the image to open in full size.

[Real simple amp above]

For simplicity, leave out Q3/4-R3/4.. replace with 1k resistors,
Q6/8/9/10 can be your BD139/40's.. just 1 pair NJW1302/0281
and a 30-0-30 tranny + rectifier/caps , your good to go..

OS
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Old 23rd December 2008, 05:30 AM   #3
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Is this good so far?
Attached Images
File Type: jpg frugalamp.jpg (86.0 KB, 1670 views)
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Old 23rd December 2008, 08:09 AM   #4
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Hi danielwritesbac,

I dont want to discourage you, but I have have to tell you to rather stay well away from NTE parts, if you dont believe me, crack open one of those output transistors.
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Old 23rd December 2008, 08:00 PM   #5
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Default Swapped for OnSemi parts

I would not wish to use NTE parts that are worse than Onsemi's parts. Since there was a backorder, I was able to change my order.

8 each of
bd140
bd139

4 each of
njw1302
njw3281
njw0281
njw0302
mje15032
mje15033

As you can see, I was a bit confused over which output transistors to purchase, so I got those possibilities covered. The order changes did not significantly increase the expense. Thanks guys!!!

One thought, is that I would like to drop the gain down to just enough to produce full power at the output without overloading the input--the minimum advisable gain. That should help avoid putting extraneous amplification on aberrations introduced by Onsemi and myself.

The photographs so kindly contributed show a seperate placement on the amplifier board for the 100uF caps. So, I'll need your recommend on some 100uF, 100v caps--whatever model you use for seemly, clear, results.

And, is there a way to purchase a PCB for this project?

Thanks again!
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Old 23rd December 2008, 08:48 PM   #6
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Those onsemi parts are rather good, no problem there, NTE use to have ok parts back in the 80s but they have since slipped Im afraid, ostripper should be able to help with a board.
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Old 23rd December 2008, 11:26 PM   #7
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Ah, welcome Dan, master of the chip amp realm, now set to reek havoc on the world of discrete design!!

OS has a nice amp here - a fine project for you to cut your teeth on.
Good luck and have fun.
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Old 24th December 2008, 02:11 AM   #8
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I'll be making a layout for this amp soon...(near the new year)
simple as possible..

Quote:
By DWB -One thought, is that I would like to drop the gain down to just enough to produce full power at the output without overloading the input--the minimum advisable gain. That should help avoid putting extraneous amplification on aberrations introduced by Onsemi and myself.
22 K for r2/12 will bring the gain down for a smaller amp ,(40-50v)
rails. you might want to adjust r10 ,as this sets LTP (input differential) current . For a 45V PS ,270-300R will give you
1.8-2 Ma for each LTP.

Quote:
By DWB -100uF, 100v caps
Any will do.. panasonic's or nichelon ( I think I spelled that right)
This amp is so simple I built it on a plug in experimenters board.
first get the CCS working (Q5/LED) then build the whole amp
except for the outputs. Tack 2 47r resistors from emitters of Q9/10
(make sure you heatsink them!!)
to output for a small 10w amp to verify proper operation..
NO burnt OP devices . Then adjust r20 about half way
(500R)...all ready for your OPS (NJL0281/03020).

These amps have NO noise, almost no "turn on thump"
you will be surprised..(read any of Douglas self's books on
amplifier design for theory on amp ..google it.)
OS
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Old 24th December 2008, 07:21 AM   #9
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Quote:
Originally posted by MJL21193
Ah, welcome Dan, master of the chip amp realm, now set to reek havoc on the world of discrete design!!

OS has a nice amp here - a fine project for you to cut your teeth on.
Good luck and have fun.
Wonderful post!! Havoc indeed. I laughed so much when I read the first part. Well, I did recently manage to turn the full range forum upside down, with a little help from Leslie Gore to provide the Sunshine, Lollipops, and Rainbows, and a garage with less roof, no doors. The thread was so interesting and turning criticism into constructiveness was fantastic.

It took many years for me to learn how to make scalable speakers that I can get real music from, and a year to figure out op amps, so I don't expect to be able to do some near-magic with discrete design for quite some time.

Right now I'm looking for points of reference.
For instance, I'm having a bit of trouble comprehending the current mirror thing, I don't understand Q8 at all, and my handmade FM radios are great at tuning in anything except FM.

The discrete amplifier design that I really want to make is quite similar to the Frugalamp. I mean, its so close!! Right now, getting that close would be a very good thing.
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Old 24th December 2008, 07:46 AM   #10
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Quote:
By DWB -For instance, I'm having a bit of trouble comprehending the current mirror thing, I don't understand Q8 at all
Hi, Daniel.. The current mirror (Q3/4) is an optional component
that balances out the current used by the differential (Q1/2).

You can use them or not (only 2 - $.07c components). They
might reduce distortion from .05% to .005% at 1KHZ...

Q8/6 is the VAS (voltage amplification stage) , it is what
gives you that big 80v p-p wave at the output stage..
C14 (68pf cap) Is Cdom, it keeps you from building an oscillator
and it's value determines the high frequency characteristics of
the amp.

And finally Q7 and its associated components Bias the
drivers (Q9/10) which in turn bias the outputs, Q11/13..etc.
Q7 should be on the heatsink with the outputs to
feedback thermal compensation...

These are just a little harder than IC amps as component choices
are more demanding (resistor values in particular).. but
this circuit is very forgiving and with average care you
should have no issues...
OS
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