Naim Nap 250 (135) specs

dinstam · 28th July 2011, 07:28 AM

Hello,
does anyone know
1) what the dc offset of the Naim Nap 250-1 should be?
2) what is the slew rate of the Naim Nap 250-1 ?
I have searched over the net and I cannot find these specs.

Regards to all

sakis · 28th July 2011, 04:01 PM

offset for any amplifier should be a total sweet zero... slew rate now at this age of amplifier shouldent be a serious consern ...obviously a machine like that should have more serious problems than a bad slew rate ...

kind regards sakis

dinstam · 28th July 2011, 04:49 PM

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

sakis · 05:42 PM

hmmm that is an interesting remark ...i have never repaired a Naim Nap 250 so far but one is coming to my shop for repair/upgrade tomorow i will take a look and post my findings here .

kind regards sakis

sakis · 28th July 2011, 05:42 PM

( first look at the schematic )
i dont like quasi circuits ...as a person i am a cubist so i like symmetric circuits .I could be tempted to convert it in a typical complementary but this could also apply if the amplifier was mine ...

more tomorow then ...

sakis

ingenieus · 09:18 PM

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.

lohk · 28th July 2011, 09:30 PM

Only the very first NAP250s have used the BDY58 - since the early eighties Naim uses their own transistor (a fast switching transistor - presumably made by Semelab)

dinstam · 29th July 2011, 06:37 AM

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.

sakis · 29th July 2011, 05:57 PM

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

Salas · 29th July 2011, 06:02 PM

Is it the old version with the 4 screw type bridge diodes on a panel between Tx and PCBs with the power transistors on cabling and back panel? About 25mA bias and ~20mV offset?

28th July 2011, 07:28 AM	#1
dinstam diyAudio Member Join Date: Dec 2005 Location: Serres	Naim Nap 250 (135) specs Hello, does anyone know 1) what the dc offset of the Naim Nap 250-1 should be? 2) what is the slew rate of the Naim Nap 250-1 ? I have searched over the net and I cannot find these specs. Regards to all

28th July 2011, 04:01 PM	#2
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	offset for any amplifier should be a total sweet zero... slew rate now at this age of amplifier shouldent be a serious consern ...obviously a machine like that should have more serious problems than a bad slew rate ... kind regards sakis __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

28th July 2011, 05:20 PM	#4
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	hmmm that is an interesting remark ...i have never repaired a Naim Nap 250 so far but one is coming to my shop for repair/upgrade tomorow i will take a look and post my findings here . kind regards sakis __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr Last edited by sakis; 28th July 2011 at 05:42 PM.

28th July 2011, 05:42 PM	#5
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	( first look at the schematic ) i dont like quasi circuits ...as a person i am a cubist so i like symmetric circuits .I could be tempted to convert it in a typical complementary but this could also apply if the amplifier was mine ... more tomorow then ... sakis __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

28th July 2011, 08:57 PM	#6
ingenieus diyAudio Member Join Date: Apr 2011	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. Last edited by ingenieus; 28th July 2011 at 09:18 PM.


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28th July 2011, 04:49 PM	#3
dinstam diyAudio Member Join Date: Dec 2005 Location: Serres	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

28th July 2011, 09:30 PM	#7
lohk diyAudio Member Join Date: Apr 2001 Location: Austria, near Linz	Only the very first NAP250s have used the BDY58 - since the early eighties Naim uses their own transistor (a fast switching transistor - presumably made by Semelab) __________________ /alohka/

29th July 2011, 06:37 AM	#8
dinstam diyAudio Member Join Date: Dec 2005 Location: Serres	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.

29th July 2011, 05:57 PM	#9
sakis diyAudio Member Join Date: Jan 2006 Location: nea makri athens greece	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 __________________ SERVICE ΙΑΠΩΝΙΚΩΝ ΜΗΧΑΝΗΜΑΤΩΝ ΗΧΟΥ www.eastelectronics.gr

29th July 2011, 06:02 PM	#10
Salas diyAudio Chief Moderator Join Date: Oct 2002 Location: Athens-Greece	Is it the old version with the 4 screw type bridge diodes on a panel between Tx and PCBs with the power transistors on cabling and back panel? About 25mA bias and ~20mV offset?

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