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Old 15th April 2002, 09:31 PM   #21
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Talking NAD3020

I agree! And to make it a little more tempting.....

http://www.gmweb.btinternet.co.uk/nad3020pwrhr.htm

H.H.
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Old 15th April 2002, 09:35 PM   #22
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Geoff,

Sorry, my post was a little unclear. From the September 1996 Update pages:-

"As I commented, at the time, the design gave a somewhat lower distortion if the hFE of Tr1 was greater than that of Tr2. This caused the output circuit to act as an amplifier with an active collector load rather than an output emitter follower with an active emitter load"

Perhaps the 200 vs 160 hfe's was too great a difference and might have been self defeating and therefore not allow the 2N3055's to give their best? All I know is that to my humble (and of course subjective) ear'oles, the MJ's sound better in the amplifier. Bias current, however, must come into the sonic equation because curiously (and maybe someone has an explanation for this, if we really want to flog the subject to death) I tried the same substitution in a DoZ Headphone amplifier with my Senn 580's and they sounded worse, so I put the 2n3055's back in and the magic came back. The main difference is the headphone rail is half that of the power amp (23V vs 40V) and it's biased at 330mA per channel instead of 1.7A for the power amp.

PS Geoff, Thanks for the great website by the way.
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Old 15th April 2002, 10:21 PM   #23
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That's right!!!

Thanks Geoff, the class-A website is indeed great.

Thanks to you and Rod Elliot I started this hobby and went on building a class-A power amp.

greetings, Thijs

PS did any of you ever see a Bedini 25/25 schematic.. an amp with such a lovely name much be a lovely amp.. I haven't found anything yet...
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Old 15th April 2002, 11:05 PM   #24
Geoff is offline Geoff  United Kingdom
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Sidney

No need to apologise, it's not the first time I've seen this comment in a forum post. I think the paragraph from the 1996 article that you quoted is a little misleading (perhaps due to some editorial interference) and that it should be read in conjunction with the table in the original 1696 article.

Another problem with the 1996 article is the suggestion that Tr1 be replaced with a Darlington to ensure that its gain is greater than that of Tr2. When I tried this five years or so ago, the only result was severe oscillation. I didn't bother to try and find a cure but just reverted to matched output transistors.

A possible explanation for your experience when trying the MJs in the headphone amp is the gain characteristics of the two devices. The performance of the JLH circuit is greatly influenced by the gain of the output transistors, as indicated in Table 2 in the original article. From measurements I have taken, the gain of a 2N3055 is around 280 at an Ic of 0.3A compared with 120 at 1.5A and less than 100 at 2A. Conversely, all the MJ/MJL devices that I have tested have had a gain that progressively decreases as the collector current is reduced from 2A to 0.1A

Thanks for your kind comments about my website, and thanks for yours as well Thijs.

Geoff
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Old 15th April 2002, 11:40 PM   #25
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Default 3055...

Dear Thijs,

2N3055's are very old design, yes. And they exhibit poor linearity, I cann't agree whith you. On my desk, the Motorola Power Data Book, I can see : Typical Dc current gain 150 @ 0.25 A , 80 @ 1 A , 50 @ 2 A. This causes important, audible harmonic distortion, at all frequencies.

The low current-gain-bandwith product (2 Mhz) causes very unpleasant intermodulation distortion at high frequencies.

Modern power transistors, specially designed for audio, are more expensive, but unquestionably more linear and fast, and give obvious inprovement of sound reproduction, specially with present sources, which can have noticeable energy in the high frequencies.

Regards, PL.
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Old 16th April 2002, 01:27 AM   #26
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Default Lab In ?

In the NAD 3020, what is the purpose of the "LAB IN" input ?

Regards, Eric.
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Old 16th April 2002, 02:25 AM   #27
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Default more 3020 stuff

More 3020 info, http://www.angelfire.com/ca/rchau/audio.html#2
and other stuff too.

Regards, Eric.
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Old 16th April 2002, 06:37 AM   #28
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Default I'm also working on JLH96

I got my JLH96 project working some time in January this year. I went with newer output transistors right from the start. I use Sanken 2SC2837... Newer transitors indeed need steps to make the whole amp stable.

Additionally I have made one more bigger change until now. I have thrown out C4 (the el.cap. from feedback to ground). To compensate for the current that starts to flow through R6 I paralleled R1 and RV1 with 5030 ohm of resistance (4.7k + 330ohm). This left the DC offset pretty much where it was and it needed only minor adjustment..... I know, I know you guys think that it's not a good move as the DC offset will begin to drift with amp warming up. Well it does, but it is in order of 30-40mV total and I don't consider it to be of any concern.

The sound changed a lot more than I had expected. It was almost of the same magnitude as I experienced by removing electrolytics from output of Marantz CD67. One of the strenghts of JHL amp often described is how it plays piano music. Well that improved even more. The soundstage after the mod is even more open and a lot of air and freedom......

Anyway, I have some ideas to try out in coming months, one of them being a mission to get rid of Tr5 current source and C5. One idea being to substitute it with JFET current source....

I have had a plan to write all my experiences down and provide solid measurements and send it to Geoff to put on his site (we talked about that possibility with Geoff at the end of last year and he was willing to do that), so I don't want to ramble too much at the moment as I want it to be more tought through and I hope it will be helpful for other JLH owners/potential builders as a source of some ideas

Ergo

PS. I'm very happy with these amps already, but the tweakers hart demands improvements...
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Old 16th April 2002, 09:09 AM   #29
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Dear P.Lacombe,

You are right. I read the D. Self book again last night and I was terribly wrong! Sorry about that. The 2N3055 does indeed have large beta drop in the 1.2A range. As I remmebered it was not so severe but after reading it again I must appologize. However the gain that is lost at higher current may not be there at all when using the mj15003 since it has lower gain to begin with. So the 2N3055 will provide more gain=feedback=less distortion at the most importend sound levels. I must say that I am very curiuos about the new transistors, but I am still not convinced: measurements and listeing trials will guve the final awnser.


Dear..ergo..

come on!!! show us the good stuff!!

greetings,
thijs
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Old 16th April 2002, 09:16 AM   #30
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Dear P.Lacombe

The low current-gain-bandwith product make the amp stable without a dominating compensation cap. Since no turn on or off is needed the output device may not not to be faster?

Is there any point in using faster devices when it means instaling compensation components to slow things down?


I always see such cap around diff input stages which use voltage feedback. Sometime the input capacitance of MOSFETS takes care of that but isn't that the same priciple?

greetings,
thijs
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