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Old 25th August 2002, 11:23 PM   #21
lohk is offline lohk  Europe
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Hartmut,

thank you for the detailed explanation of your mods.

1. Re-routing of the VAS anti-oscillation cap: Sounds interesting; the orginal Naim schematic is clearly standard 70/80ies here. But this mod will change the feedback behavier perhaps. I will try that with my next amps.

2. Anti-oscillation styrophene caps from VAS output to rails:
There aren't any, there have never been any. I checked all my fotos, my notes and schematics - here you have been missleaded somehow.
The mentioned caps are part of a R/RC phase-correction networks, which are situated in the base line of the driver transistors. They have never been connected to the rails, why should they.

To quote the constructor Julian Vereker (from his big interview in HIFI+ Issue4 Dec1999 / Jan2000):
"... Back when I was trying deperately to get the NAP200/250 to work, I remember someone showing me a modular American amplifier - Spectrasonics, I think it was called. I looked at the circuit and noticed a couple of resistors and capacitors which seemed to be different from anything I'd seen in other designs. This got me thinking, and then I realised that of course the positive half of the amplifier and the negative half of the amplifier are bound to be different, but they share a common feedback loop, so you have to get the two halves accurate in terms of phase gain, or you'll never make the design stable - and there were these little phase-correcting networks doing just that. ..."

These networks (different on both halves) are therefore a part of the design an cannot be changed without changing the whole thing - it will definitely be a different amp afterwards.
(Only the bigger (old) amps have these networks, using the 250/135/180 boards, and the (old?) 140, not the 90 and the Nait)

Bias:
I always measured some bias, making them AB amps anyway. But Julian Vereker actually did care about something else:
(quote from a naim forum archive):
"... All Naim power amps are class B and they have as low a bias as we can manage, just a few milliamps.
There are two reasons for doing this, in order to make a good push/pull power amp, the two halves need to match very closely since there is only one common feedback loop - this applies whatever the 'class' of the amp. If one achieves this degree of precision then one only needs a very small bias current. ..."

I personally run my AB amps with some bias (up to 100mA), I feel they sound better this way. But it maybe only, because I cannot match (output) transistors as close as possible.

I always found the Naim design as stable as other good amps, and to integrate the LS cable within the design is a very clever idea. Just the cable manufacurers probably do not like that idea so much. And you do not have to use Naim cable: 2 x 4mm2 stranded loose cables, 3 to 4 times per meter slightly twisted together, will do the job too (maybe not with the same sound quality ...).

Klaus
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Old 26th August 2002, 02:43 AM   #22
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An often ignored problem with regard to amplifier stability is the presence of a non-minimum phase, (RHP), zero generated by multiple signal paths from input to output.

An RHP zero has the magnitude response of an LHP zero, (i.e: it 'breaks up'), but the phase response of an LHP pole, (i.e: phase response 'breaks down', or tends to - 90 degrees in the limit).

This undesirable zero can be moved to the LHP by connecting a small resistor in series with the miller compensation capacitor(s), or moved to infinity by placing a capacitor in parallel with each of the often-used series resistors to the output stage.

This is the purpose of the RC networks leading to the output stage in McBrides 'Naims'. Shunt compensation by loading the second, (transimpedance), stage to ground, or supply rails, (same thing at signal frequencies), is an exceedingly bad idea with respect to second stage distortion...




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Old 26th August 2002, 09:53 AM   #23
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Default Re: Bias settings

Quote:
Originally posted by ALW


I respectfully suggest that either the amps you were testing were faulty / maladjusted, or you were measuring incorrectly, since all Naim amps, irrespective of o/p device, have some bias current. Why would Naim fit a bias pot otherwise?

There's a resistor in series with the o/p that can be mistaken for the emitter resistors

I always set mine for min THD.

A.
Hi A.

I have measured a Naim 160, a Nait 1, and my NAP140 so far, and yes, I know that serie resistor, too. Adjusting bias on that series resistor would blow the outputs, surely ;-)

regards,
Hartmut
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Old 26th August 2002, 09:33 PM   #24
ALW is offline ALW  United Kingdom
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Default Bias stuff

Quote:
I have measured a Naim 160, a Nait 1, and my NAP140 so far, and yes, I know that serie resistor, too. Adjusting bias on that series resistor would blow the outputs, surely ;-)
I guess it would - bit of a DC problem

You have though, without a doubt, maladjusted amps, I have a Nait1 sitting above me as I type, and it's been serviced by Naim a few years ago, and definitely has standing bias.

The two 110's I used to own were also biased to low levels. Douglas Self comments on bias in his amp design articles - there is an optimum bias point, above which distortion increases due to the gm doubling effect of both devices being one simultaneously.

Whether this is also the reason for Naim's low bias setting is something I'm noit sure about, but setting bias for min THD results in a very low standing bias, relative to most amps.

Andy.
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Old 27th August 2002, 08:16 PM   #25
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Quote:
Originally posted by lohk
Hartmut,

2. Anti-oscillation styrophene caps from VAS output to rails:
There aren't any, there have never been any. I checked all my fotos, my notes and schematics - here you have been missleaded somehow.
The mentioned caps are part of a R/RC phase-correction networks, which are situated in the base line of the driver transistors. They have never been connected to the rails, why should they.

Klaus
Hi Klaus,

I just checked my 140, you are right, there are no anti-oscillation caps like described in my further email. Maybe I got confused with some Naim clones (maybe German Dr.Jazz from Cologne).

regards,
Hartmut
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Old 27th August 2002, 09:24 PM   #26
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Lightbulb Top tip

JV said: "...This got me thinking, and then I realised that of course the positive half of the amplifier and the negative half of the amplifier are bound to be different, but they share a common feedback loop, so you have to get the two halves accurate in terms of phase gain, or you'll never make the design stable..."

For anyone striving to design audiophile quality PP amps, especially using feedback, this recognition of asymmetric circuit behaviour is PROFOUNDLY IMPORTANT.
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Old 7th September 2002, 10:12 AM   #27
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Default NAIM NAP 250

Original scematic for a Naim nap250 amp
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Old 7th September 2002, 10:21 PM   #28
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Default NAIM NAC32

NAIM pre NAC32 schematic
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Old 7th September 2002, 10:26 PM   #29
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Default NAC32

NAIM32 pre
part2
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Old 7th September 2002, 10:29 PM   #30
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Default NAC32

NAIM pre schematic
part3
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