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#51 | |
diyAudio Member
Join Date: Jul 2007
Location: Upper Hutt
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Quote:
If so the gain bandwith product would differ in each output half, and the amplifier. There would be competing poles. You cannot compensate for dual poles so in that situation the amplifier would be unstable even with no input. I don't see this amplifier achieving the claimed THD spec of <0.02% unless the outputs are epitaxial devices. |
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#52 | |
diyAudio Member
Join Date: Nov 2018
Location: Sibiu
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#53 | |
diyAudio Member
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
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Quote:
People that are willing to pay $3300 for a NAD on Ebay (last week asking price) are preserving the delusion of "quality".
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey PV-1.3K, MMA-81502, Herald RA88a mixer, Steinway console, Wurlitzer 4500, 4300 Last edited by indianajo; 19th January 2021 at 10:28 PM. |
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#54 | |
diyAudio Member
Join Date: Jul 2007
Location: Upper Hutt
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Quote:
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#55 |
diyAudio Member
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
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In post 17 I suggest .47 ohm emitter resistors. I used .51 in my AX6. There are successful epitaxial transistor amps with .33 ohm, .22 ohm, and possibly even .1 ohm emitter resistors.
You can see pictures of how I flew the wiring from driver board to output transistors on the heat sink in this AX6 post #212: Retro Amp 50W Single Supply - Page 22 - diyAudio Keeping the OT wires 1" away from each other, and keeping the temperature sense wires 2" away from output transistor wires, are key IMHO avoiding oscillation. You'll see my base & emitter wires were 4" long so I could flip the driver board over for work on each side. The high current stuff is IMHO, not sensitive to position or length inside the RF free case. Of course, the NAD has no temperature sense diodes or transistor to cause oscillation with the VAS. Note nigel7557 or whatever has had trouble with oscillation in Maples (?) 1970's amps that came with homotaxial transistors. His solution was to put 10 ohm 1 watt resistors in the base line to the output transistors. I did that as a damage limitation during meltdown practice, when I replaced the 1970 RCA transistors (burned ) with NTE60's (MJ15003 white box?) epitaxials in my dynakit ST120. I replaced the wires from the driver board to the bases of the transistors over on the heat sink, with 10 ohm 1 watt resistors. Flying right out in the air. If the OP gets oscillation, I suggest that is the next step. As I suggested in post 14.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey PV-1.3K, MMA-81502, Herald RA88a mixer, Steinway console, Wurlitzer 4500, 4300 Last edited by indianajo; 20th January 2021 at 12:21 AM. |
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#56 |
diyAudio Member
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Yes, also look at this thread: NAD 3020 (& related NAD output stages) . Does the OP there seem familiar?
This same problem with the 3020 series NAD design and the simple emitter resistor solution with suggested resistor values have been covered here time and again (particularly around holiday time ![]() Last edited by Ian Finch; 20th January 2021 at 12:26 AM. |
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#57 |
diyAudio Member
Join Date: Jan 2010
Location: Jeffersonville, Indiana USA
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BTW, my 50 w retro AX6 amp,with single pair NTE60 output transistors & .51 ohm wirewound emitter resistors (added), a 70 v rail, put out 24 VAC for 5 seconds at a time on SP2-XT "8 ohm" speakers. That is 72 watts. Test done with Simpson 266-XLPM analog voltmeter which measures average AC voltage. I do have heat sinks on the drivers & vas, which Apex doesn't specify on the AX6. VAs was a GE D44R4 from the '70s and drivers were On semi MJE15028/29. All 30 mhz Ft TO220 transistors. Idle bias current was adjusted to 20 ma, so there was no output transistor switching.
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Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey PV-1.3K, MMA-81502, Herald RA88a mixer, Steinway console, Wurlitzer 4500, 4300 Last edited by indianajo; 20th January 2021 at 12:35 AM. |
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#58 | |
diyAudio Member
Join Date: Jul 2007
Location: Upper Hutt
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Quote:
The RCA hometaxial was rated at 150W and used in consumer and industrial products and probably within the military. There would be need to service older appliances and equipment relying on these in workplaces etc. You would have a situation of supply and demand and hoarding by the bigger players whom NAD as a relative infant would never have been able to compete with. Last edited by mjona; 20th January 2021 at 05:41 AM. |
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#59 |
diyAudio Member
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> The RCA hometaxial was rated at 150W
RCA 1966 (hometaxial) 2N3055 was rated 115W. This part shows ~~1.3Vbe at 5 Amps Ic, so "acts like" 0.14 ohms internal emitter resistor due to significant parasitics. 2N3771/3 was rated 150W. Last edited by PRR; 20th January 2021 at 05:55 AM. Reason: bad number |
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#60 | |
diyAudio Member
Join Date: Jul 2007
Location: Upper Hutt
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