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Old 15th April 2006, 12:58 PM   #1
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Default ROOKIE AMP: 80W / 4 Ohms

...grrrrr... !!!!
During my last move the transport guys probably dropped my container from the forklift !! Lots of stuff broken...
Also my NAD receiver T750 . I checked through it and noticed that this beast was designed to be unserviceable. You really have to disassemble everything until you can access the solder side of the main PCB !!
....hm, the more I examined that receiver the less I liked it... tons of signal conditioning circuits in the audio path...
And a really badly damaged PCB. From visual inspection I found lots of broken tracks!!! Really a lot, - ugly!
OK, I did not need the dolby surround anyway. Also here in China I do not need the radio, because I prefer european WEB radio here.

Clear decision: Put the circuits to scrap and treasure some valuable Japan transisors. Use the enclosure and PSU (+/-35V). DIY a simple amp. ( Class D amp project for Subwoofer postponed... )

It should be simple, but audiophile. I checked through my component assortment and found tons of BC550/560.
Some BC141/161. Some BF757/760. And some 2SC2922 / 2SA1216 !!!
Planning my Rookie Amp... I call it Rookie, because of its simplicty.
Preliminary schematic is attached, feel free to comment. I would bias the input stage at 6mA. The VAS at 50mA
and output stage at 130mA. The frequency compensation is something, which I will have to evaluate on the first proto.
The schematic is just pointing out some options of lead and lag compensation....
I am also considering freewheeling diodes at the output transistors, but I am not fully convinced about their necessity.

Looking forward to your laughter
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Old 15th April 2006, 01:12 PM   #2
AndrewT is online now AndrewT  Scotland
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Hi Choc,
what are the diodes b to output doing?
Should they be c to output?
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regards Andrew T.
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Old 15th April 2006, 01:25 PM   #3
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Hi Andrew,
yes the freewheling diodes, would be from C to output, or C to E.
Are you convinced that they would be a must?
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Old 15th April 2006, 01:28 PM   #4
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...uhps, now I understand your question.
The diodes from B to output are for current limiting.
As soon as the voltage drop across the emitter resistor exceed 1.4V they will short the VAS to output. ... OK, may be a 500mA type would be better than the 1N4148....
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Old 15th April 2006, 01:33 PM   #5
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input stage is forced to work with low Vbc ?
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Old 15th April 2006, 01:43 PM   #6
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Quote:
Originally posted by darkfenriz
input stage is forced to work with low Vbc ?


Yes, why not?
They provide high gain, low noise, DC rail is +/-35V only.
Input stage transistors are forced to 35V only.
Losses in each input transistor is around 200mW, which is pretty fine if you press them onto a metal plate. Not OK? Do I miss out something?
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Old 15th April 2006, 01:58 PM   #7
Eva is offline Eva  Spain
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You are going to suffer both very poor PSRR and uncontrollable bias in the second stage with this input stage. I think you should rethink the design a bit

Tip: One of the keys to get low distortion in amplifiers with symmetric supply rails is to keep the input-output "substractor" stage as isolated from supply rails as possible. Even the Cbc capacitances of the input transistors in the range of a few picofarads play an evil role here (I have measured it) as the error signal introduced appears on the output multiplied by the open loop gain without a chance for the circuit to correct it, so coupling the input signal directly to the rails with resistors is not a great idea. You may consider cascodes, though.
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Old 15th April 2006, 02:13 PM   #8
anatech is offline anatech  Canada
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Hi ChocoHolic,
The input stage current will be very temperature dependant, and therefore the Vas stage as well. A little rethink in that area might help.

-Chris
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Old 15th April 2006, 02:20 PM   #9
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Eva / Ana:
...I am hoping that PSSR should be acceptable because, the biasing resistors will be very high (several meg Ohm) compared to the driving impedance.

For thermal I am using the szikali Vbe multiplier, which would overcompensate the output stage and by this hopefully can compensate the negative influence of drifting input to the output.

Don't you think, "it could work" ?
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Old 15th April 2006, 02:27 PM   #10
anatech is offline anatech  Canada
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Hi ChocoHolic,
It might work on a "one-off" basis. I wouldn't trust it for production. I am worried that the input stage will make the entire amp unstable with temperature / voltage variations.

Build it and see. You can always redesign the input stage if it turns out to be a little flaky.

-Chris
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