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Old 22nd February 2019, 07:29 PM   #181
Vrystaat is offline Vrystaat  South Africa
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Well, I have recapped and fixed a NAD214 which is a nice amp, the P3A beats it easily on all fronts, and I actually liked the Nad a lot !
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Old 23rd February 2019, 02:17 AM   #182
Ian Finch is offline Ian Finch  Australia
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There's so many NAD models, different design types and groupings, that we can't easily compare or evaluate them unless we have the model to hand or we're expert enough to understand what the designers intended from their topologies and component types/values.

I'm already at a disadvantage as I'm no expert and I haven't heard one in use but the 214 design has a CFA topology, regulated front end power supplies and an EF triple output stage with no tricks so I'd expect a very clean, low distortion sound with lots of grunt, as needed for bridged mode operation. There's a lot of capability and comprehensive protection there but not much is on offer for someone seeking audiophile quality sound. I can understand how a P3a, even the published website version we have all seen and copied, would beat this and many other, more expensive commercial models, on sound quality.

P3a may appear to contradict every current audio design maxim in its old school simplicity but that's where it's strong appeal as a DIY project and performer originate.
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Old 11th March 2019, 02:29 AM   #183
eliasib is offline eliasib  Venezuela
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Long time ago I wanted to build this wonderful amplifier. I upload some photos of it..I
thank Stephie B., my friend, for helping me get original transistors for the input stage. Getting some old reciclying parts, at last! I finished the amp which differs from Rod Elliott's only by adding a current mirror to the differential pair collectors. I supplied the amp with +/-40, using C5200 and A1943 in the output stage, I left bias current at 30 mA for each transistor maybe bias is high but temperature through them kept normal. Rod does not recommend the Vbe multiplier is attached to the main heatsink since overcompesation can occur causing crossover distortion due to the complementary feedback pair or Sziklai pair used in the output stage. The sound is very good.
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Old 11th March 2019, 04:42 AM   #184
johnego is offline johnego  Indonesia
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Quote:
Originally Posted by eliasib View Post
I left bias current at 30 mA for each transistor maybe bias is high but temperature through them kept normal.
I think the recommended value is 50-70mA? Then 30mA is probably the minimum. If you think you need more current, you have to reduce the driver's collector resistors from the 220 Ohm, and prepare a small heatsink for the driver.
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Old 11th March 2019, 12:32 PM   #185
eliasib is offline eliasib  Venezuela
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Hi johnego for your reply.. I think 70mA would be high enough to destroy the output transistors.
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Old 11th March 2019, 12:48 PM   #186
jacques antoine is offline jacques antoine  France
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Originally Posted by eliasib View Post
Hi johnego for your reply.. I think 70mA would be high enough to destroy the output transistors.
Mine is used daily and has been biased at 75mA for the last 5 years with 1943/5200 outputs. I would reduce to 2x35V though.
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Old 11th March 2019, 01:47 PM   #187
eliasib is offline eliasib  Venezuela
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Hi Jacques, but you measure bias current between collectors of both Power transistors?
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Old 11th March 2019, 02:53 PM   #188
johnego is offline johnego  Indonesia
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Quote:
Originally Posted by eliasib View Post
Hi johnego for your reply.. I think 70mA would be high enough to destroy the output transistors.
What do you think is the bias current of class A amplifiers? Mine is above 70mV across 0.47 Ohm output resistor. I think that's around 150mA?
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Old 11th March 2019, 03:06 PM   #189
Marc Vi is offline Marc Vi  Netherlands
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The bias current of a class A amplifier is above 1,0 A !!
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Old 11th March 2019, 03:58 PM   #190
jacques antoine is offline jacques antoine  France
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Quote:
Originally Posted by eliasib View Post
Hi Jacques, but you measure bias current between collectors of both Power transistors?
You can measure bias current in a number of ways.
1. use the amp reading of your PSU if it has one.
2. remove one fuse (see original schematics) and measure intensity across the fuse terminals with a multimeter.
3. Like Johnego suggested: read voltage across one of the output collector resistors and calculate intensity with Ohm's law.
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