Helo Sakis and thank you for your kind reply.
The question arises from the schematics of the amp which I have seen. It seems that the input differential amplifier does not have equal currents through each leg. This affects both the dc offset and the slew rate. In fact it seems that slew rate is incresed at the expense of higher offset.
Here is the circuit from Neil McBride's site.http://www.neilmcbride.co.uk/output-amp2.pdf
Cheers
The question arises from the schematics of the amp which I have seen. It seems that the input differential amplifier does not have equal currents through each leg. This affects both the dc offset and the slew rate. In fact it seems that slew rate is incresed at the expense of higher offset.
Here is the circuit from Neil McBride's site.http://www.neilmcbride.co.uk/output-amp2.pdf
Cheers
I own an old NAP250. What's the dash one about? As far as DC offset goes, it's the same as for any other amp - as little as possible.
Slew rate? Never measured it. Probably nothing special. If you ask me, the thing is a little overrated. And I say that in the nicest possible way.
PS: a simulation of the NAP250 circuit in LTspice shows 2V/us for rising edge and 4V/us for the falling edge of a square wave. However, this is not quite accurate because I used MJ802s as output devices. A SPICE model for the long defunct BDY58 is nowhere to be found.
BTW, what would the best and brightest suggest as a replacement for the BDY58 when repairing a dead 250? My take on the MJ802 is that there _might_ be a problem with HF stability, but it's just a hunch.
Slew rate? Never measured it. Probably nothing special. If you ask me, the thing is a little overrated. And I say that in the nicest possible way.
PS: a simulation of the NAP250 circuit in LTspice shows 2V/us for rising edge and 4V/us for the falling edge of a square wave. However, this is not quite accurate because I used MJ802s as output devices. A SPICE model for the long defunct BDY58 is nowhere to be found.
BTW, what would the best and brightest suggest as a replacement for the BDY58 when repairing a dead 250? My take on the MJ802 is that there _might_ be a problem with HF stability, but it's just a hunch.
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Hello Ingenieus an lohk,
Thank you for your replies.
The dash is meant to discriminate between the previous model and the new Nap 250. I am not qualified to suggest replacements for BDY58 transistors. But, take a look in Modifying Naim Audio power amplifiers. (A very good site).
Indeed, all of my simulations (TINA basic) show a problematic behavior at high frequencies. Bode plots indicate that phase reaches (-180o ) before gain drops below 1 (dB=0). Maybe that is why Naim used special transistors. More knowledgeable members may clarify this.
Thank you for your replies.
The dash is meant to discriminate between the previous model and the new Nap 250. I am not qualified to suggest replacements for BDY58 transistors. But, take a look in Modifying Naim Audio power amplifiers. (A very good site).
Indeed, all of my simulations (TINA basic) show a problematic behavior at high frequencies. Bode plots indicate that phase reaches (-180o ) before gain drops below 1 (dB=0). Maybe that is why Naim used special transistors. More knowledgeable members may clarify this.
i am still working on the naim 250 that came in the shop today and will post details of my work and findings till monday ...
From a first look is a terrible machine and sorry to say it has things that i dont like ...
obviously poor choise of LTp and also eventhough thermal coupling was very very possible to do the never did .....Beyond sound improvement from matched LTP thermal coupling will also provide extra stability ...
I am just started so give it a bit of time and see how it goes
Kind regards sakis
From a first look is a terrible machine and sorry to say it has things that i dont like ...
obviously poor choise of LTp and also eventhough thermal coupling was very very possible to do the never did .....Beyond sound improvement from matched LTP thermal coupling will also provide extra stability ...
I am just started so give it a bit of time and see how it goes
Kind regards sakis
sakis, its the "primitive" circuit that gives the amplifier its character, the more evolutionary advanced ANY amplifier is, be it tube, or transistor, ie the more complex it gets, the less character and perhaps musicality it becomes.
perhaps read that as imperfections in the measured performance, and distortion and poor slew rates.
you can hear it in valve amps too, the best are triodes directly heated, as they become more advanced in years later, they also become more perfect and less good sounding imnsho.
they had it right first time around with triodes, you can hear amplifiers get cleaner and more characterless as they get longer down the design era, ie later build, or younger in years and more advanced circuitry. kills the sound character.
perhaps read that as imperfections in the measured performance, and distortion and poor slew rates.
you can hear it in valve amps too, the best are triodes directly heated, as they become more advanced in years later, they also become more perfect and less good sounding imnsho.
they had it right first time around with triodes, you can hear amplifiers get cleaner and more characterless as they get longer down the design era, ie later build, or younger in years and more advanced circuitry. kills the sound character.
The bolt-on original was sweeter than the olive version for one. There are a couple of 10uF and 47uF Philips lytics on the 40V regulators to surely renew plus a couple of 5.6kohm resistors that get brownish on them, use 2W. Also the main filter caps need check naturally, and the DO-4 diodes usually blow erratically after many on off cycles in the course of years. Those are better to replace with 2x 35A bridges. It does better in the bass and gets safe. My experience when I had fixed some bolt-on original NAP-250 at least. That one uses a 2 diode and centre tap double secondary floating PSU, no short between the mid point and chassis is affordable.
Hi,
All Naim Amp's I dealt with had minimal ofset (a few mV).
Feck All?
It is a very slow Amp, compared to more modern designs. It also must be used with a naim pre, which include steep lowpass filters, which avoid challenging the poweramplifiers slewrate.
Naim Amplifiers (including preamps) are interesting, in how they utilise almost unmitigated 1970's circuitry and instead concentrate on layout and powersupplies...
Ciao T
All Naim Amp's I dealt with had minimal ofset (a few mV).
Feck All?
It is a very slow Amp, compared to more modern designs. It also must be used with a naim pre, which include steep lowpass filters, which avoid challenging the poweramplifiers slewrate.
Naim Amplifiers (including preamps) are interesting, in how they utilise almost unmitigated 1970's circuitry and instead concentrate on layout and powersupplies...
Ciao T
mine yes it is, includes also a VI limmiter ( so much for "hi end " equipment ) and about a gozilion of tantaliums
Gosh this machine have a million things "that we dont do ".... !!
Even the cables that conect output transitors to the amplifier boards are twisted all together BC and E
zobel has been boiling in the past on the particular amp so either its a oscilation thingy or high freq instability or wrong calculation for the zobel ( features 8.2 resistor with 220nf cap ) quite big i think ...
procedure will be
replace if possible most of the tantalium inside
upgrade /match/thermal couple LTP
add bypass and decoupling
rewire outputs
search behaviour of zobel after all
remoove the VI limmiter ( costumers request ) ( mine also ha ha ha )
beef the idle a bit
that will be arround the amp
power supply i will see kinda later
kind regards sakis
Gosh this machine have a million things "that we dont do ".... !!
Even the cables that conect output transitors to the amplifier boards are twisted all together BC and E
zobel has been boiling in the past on the particular amp so either its a oscilation thingy or high freq instability or wrong calculation for the zobel ( features 8.2 resistor with 220nf cap ) quite big i think ...
procedure will be
replace if possible most of the tantalium inside
upgrade /match/thermal couple LTP
add bypass and decoupling
rewire outputs
search behaviour of zobel after all
remoove the VI limmiter ( costumers request ) ( mine also ha ha ha )
beef the idle a bit
that will be arround the amp
power supply i will see kinda later
kind regards sakis
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ok then !!!
.... no romance for me though .... i will just do my best about it the way i think its i see no point on preserving those "character mistakes " now days
oh yes most important !!!!! remove the blody XLR from signal add a pair of nice RCA ....and even more importand remove the stupid XLR FROM MAINS POWER SUPPLY
NOW DAYS THERE IS NO WAY TO APPROVE A MACHINE WITH XLR PLUG FOR MAINS ....
kind regards
sakis
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Hello to all and thank you for replying.
I do not own a NAP 250. Looking at the schematics it seemed a rather easy implementation for DIY.(!?)
But, running some simulations I saw that the input LTP is not “balanced”. (Here again is the schematic http://www.neilmcbride.co.uk/output-amp2.pdf).Q1 passes a lot more current than Q2. The simulated dc offset was in the range of 50 mV. Also, for a similar amp made by Avondale audio they instruct that the “output dc offset should not be more than 50 mV” (http://avondaleaudio.com/power-amplifier-module-ncc200/)(http://avondaleaudio.com/power-amplifier-module-ncc200/).
When I change (in simulation of the schematic from McBride) the value of R4 to ~1200 R, the circuit balances better and the dc offset drops below 5 mV. So, why did the designer chose to use an “unbalanced” input LTP? Is it possible that this due to the quest for better slew rate? Which leads to my naive question:
Slew rate is calculated from the Miller capacitor and the tail current of a balanced input LTP. Is the calculation valid also for unbalanced input LTPs?
Cheers to all
PS: As I have said, simulation shows a high frequency instability. How did the amp survive in the market for such a long period? Is this due to the special transistors used by the manufacturer?
.
Well Sakis, the interesting story is, that this simple, generic amp has been in the market for ~30yrs.
I do not own a NAP 250. Looking at the schematics it seemed a rather easy implementation for DIY.(!?)
But, running some simulations I saw that the input LTP is not “balanced”. (Here again is the schematic http://www.neilmcbride.co.uk/output-amp2.pdf).Q1 passes a lot more current than Q2. The simulated dc offset was in the range of 50 mV. Also, for a similar amp made by Avondale audio they instruct that the “output dc offset should not be more than 50 mV” (http://avondaleaudio.com/power-amplifier-module-ncc200/)(http://avondaleaudio.com/power-amplifier-module-ncc200/).
When I change (in simulation of the schematic from McBride) the value of R4 to ~1200 R, the circuit balances better and the dc offset drops below 5 mV. So, why did the designer chose to use an “unbalanced” input LTP? Is it possible that this due to the quest for better slew rate? Which leads to my naive question:
Slew rate is calculated from the Miller capacitor and the tail current of a balanced input LTP. Is the calculation valid also for unbalanced input LTPs?
Cheers to all
PS: As I have said, simulation shows a high frequency instability. How did the amp survive in the market for such a long period? Is this due to the special transistors used by the manufacturer?
HMMMM may be with a serious zobel ??? ha ha ha
if simulation shows a high frequency instability if you see the construction of the amplifier you will feel sure that this is also adding more stabilty issues to the amplifier .
Dont be surprized that an amplifier that now days looks kinda bad survived in the market for such a long period.... other terrible equipment have done similar tasks .... see at marketing ...expectations from UK ( made in UK = good ) and reputation builted through the years from NAD or other manufacturers ...
if simulation shows a high frequency instability if you see the construction of the amplifier you will feel sure that this is also adding more stabilty issues to the amplifier .
Dont be surprized that an amplifier that now days looks kinda bad survived in the market for such a long period.... other terrible equipment have done similar tasks .... see at marketing ...expectations from UK ( made in UK = good ) and reputation builted through the years from NAD or other manufacturers ...
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