New Push Pull Buffer Approach for special Kind of THD - MOSFET & BjT - New for me - diyAudio
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Old 2nd April 2010, 02:13 PM   #1
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Default New Push Pull Buffer Approach for special Kind of THD - MOSFET & BjT - New for me

Today I get an integrated amplifier for repair. It is a NAD 310. The reason was quickly find: burned PCB and broken solder points arround the zener's for the baxandall equalizer stage.

But curious and new for me is the output power buffer topology:
1) one N-Channel MOSFET in the positive half (Philips/NXP BUK-555, TO-220)
2) one PNP BjT in the negative half (Sanyo 2SB817, TOP-3)

What kind of push pull output buffer is this (so called ?????) ? Who knows more about this, e. g. AES papers and other amplifier models, where such power buffers are in use?

Schematic for NAD-310 you will find here:
Schematic Diagram - Free Schematics Download (highest resolution)
http://img526.imageshack.us/img526/9...d310schxb3.jpg
NAD 310 - need some info

Last edited by tiefbassuebertr; 2nd April 2010 at 02:15 PM.
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Old 2nd April 2010, 02:43 PM   #2
stoc005 is offline stoc005  United States
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Capacitor coupled to the speaker (UCK!), single supply amplifier. If this is SOTA, I give up. Smells cheaply built to me. No RIAA preamp. I guess vinyl is dead.
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Old 2nd April 2010, 03:13 PM   #3
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Quote:
Originally Posted by stoc005 View Post
Capacitor coupled to the speaker (UCK!), single supply amplifier.
Definitely better than serial relais contact against dc protect, even by use of cheap caps.

in my previous post the schematic URL was wrong:
Nad Download Page - nad-310-integrated-amplifier-schematic.pdf || ABELtronics
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Old 2nd April 2010, 06:12 PM   #4
stoc005 is offline stoc005  United States
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And exactly what is C225 (33p) and R255 (100K) doing for us?? Why is some of the (high frequency??) left channel, via these two parts, fed back into the right channel input(Q203 emitter)?? Why the cross channel feedback??
Didn't get me wrong, if you are happy with the sound, keep using it. It's just that I suspect there are "better" amplifier out there, IMHO. I listened to a Heathkit AA-15 that had a 'lytic for DC blocking in series with the speaker and was a single supply amp for years. I was happy with it. The NAD circuitry is just not really revolutionary.
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Old 3rd April 2010, 01:34 AM   #5
AKSA is offline AKSA  Australia
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Tief,

This topology measures badly (high even order), but sounds very good.

A quasi version of this approach is also extant from the same era. N type mosfet at pos rail, pnp driver and npn output at neg rail. This sounds very good, too.

Give it a good listen once you fix it. You will be very surprised.

Hugh
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Old 4th April 2010, 08:02 AM   #6
ontoaba is offline ontoaba  Indonesia
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It is buffered class A with positive feedback. The designer trying to create good buffer with mosfet. The bottom side has linear topology and the upper side just hot resistor. Then buffered, then FB placed, just it, I think.
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Old 4th April 2010, 02:49 PM   #7
balaboo is offline balaboo  United States
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Like Hugh (Mr.AKSA) said, this is a very musical sounding topology - I use it for my guitar amps, except that I use NPN bjt and IRFP9140. Not too bad for music listening, but fabulous for electric guitar or vocal usage !
And the output capacitor helps prevent blown speakers when playing "live" ...
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Old 4th April 2010, 09:04 PM   #8
ontoaba is offline ontoaba  Indonesia
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Most positive feedback amplifier has its disadvantage at high frequency, but this is might different because of the buffer has faster and separately follow the stage before. Lower side BJT gaining the current source, and upper mosfet gaining that hot resistor.
It was good in past, but it is not interested any longer because of high THD and hot VA stage.
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Old 6th April 2010, 09:15 PM   #9
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Quote:
Originally Posted by AKSA View Post
Tief,
This topology measures badly (high even order), but sounds very good.
Hugh
Yes, in case of low idle current between 30mA and 100mA (badly measuring results still present, independend of the topology).
If I choice an idle current of 1,3A, this topology measures very good (see attachement - please ignore the device numbers from my schematic because it's not the same than by orig NAD schematic).


onto aban: Where you have discover the PFB (positive feedback) ? I find only a NFB network (negative feedback - consists of 820R/10R/1MF for AC and 3K3 for DC).

stoc005: integrated RIAA head amp by an integrated amp is a very bad solution for various reasons. The royal way is still an external RIAA device like this:
NAD Electronics :: PP-2 Phono Preamplifier
Cheap and high quality at the same time.
Attached Files
File Type: pdf C ...-A-NAD 310.ckt.pdf (22.0 KB, 181 views)
File Type: pdf C ...-A-NAD RIAA MM-MC.ckt.pdf (55.9 KB, 106 views)

Last edited by tiefbassuebertr; 6th April 2010 at 09:32 PM.
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Old 7th April 2010, 06:38 AM   #10
ontoaba is offline ontoaba  Indonesia
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hi thief, you have improved its simulation result, but 1.3A is about to think another way, it is too hot for cheap amps.
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File Type: png untitled.PNG (40.1 KB, 599 views)
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