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Old 26th June 2015, 07:08 PM   #1
bjoras is offline bjoras  Norway
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Default Doxa 70 Signature Series Mk2 and BD245C/BD246C

Hi! My first post in this excellent forum. A lot of valuable info from many clever enthusiasts and kind people! Thanks!

I burned one channel in my Doxa 70 Signature Series Mk2 push-pull amp , all of the emitter resistors, one PNP, one NPN of 5 in parallel and the PNP of the driver pair and its collector resistor. There is an incorrect schematic circulating on the Internet, which also has been discussed in this forum:

diyAudio

... and also discussed here:

Doxa 70 Clone

I visited the designer a couple of days ago, who happens to live in my town. He told me these transistors went out of production many years ago. He was sorry not being able to help me. He used to order thousand of TI's BD245C/BD246C and sorted them in groups. He's stock went empty several years ago.

I'm aware of other brands and replacements. I'm just worried that the excellent sound in this amp will degrade, replacing the burned transistors. The designer told me that there is no feedback in this amp, making it even more important to match the transistors. Somebody probably will sugest replacing all the transistors in both channels with new replacement. Maybe, but Lauvland (the designer) have no experience with other then the TI transistors. This amp is designed by listening, not by reading transistor datasheets.

That's why i ordered Locki_Z's (Intelligent Curve Tracer 3.0 release) curve tracer. My hope is that I eventually get matching curves, by ordering transistors from different sources. Looks like they are hard to find. Looks like most sources are chinese.

The resistors shouldn't cause any problems. This old amp has 5% emitter resistors. The tolerance is not critical, according to Lauvland.

I would really like to get any advice from you! Do you think I will succeed following this route? I really love the sound of this amp, and cannot stand the thought, that I never will get it back.

Best regards
Bjørn

Last edited by bjoras; 27th June 2015 at 08:18 AM.
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Old 27th June 2015, 01:15 PM   #2
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--your assumptions are wrong ( type of TRS versus listening impressions )
--you need to have a better understanding of how amplifiers work
--The transistors you are talking about are rubbish
--you are mixing listening impressions with technical analysis ( very wrong )

In reality
Only European designers used/use so slow transistors , that is valid since 1968 , So the perceived result might be nice to your ears but in reality and in real amplifier designing those type of transistors have been abandoned many many years ago ....

Option one
Look in the market for any transistor of today that is close to original specs ,from a trusted manufacturer of your choice ...Local store or ebay is not an option
Buy as many as you can , sample first , match closely , replace all , never mix match old and new parts and you will come up very very close to the original result

Option 2
Get real semis , rugged and fast enough for a decent audio amplifier ,understand what compensation is and does ,alter accordingly ,invest money to be able to evaluate an amplifier NOT the transistors that its made off and expect as a result to have a real amplifier in your hands ...

Kind regards
Sakis
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Old 27th June 2015, 06:12 PM   #3
bjoras is offline bjoras  Norway
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Thank you very much for your advices Sakis :-)

The reason I'm concerned about using replacements, is this commment:

Quote:
Do NOT replace the original BD254 and 246 with "better" japan versions!!!
Keep the originals!!
Remember that the constructor of DOXA, Knut Lauvland, have done a lot of testing to find the best possible transistors for the amp. And this does not always meen the ones with the best spec.

In early versions of the DOXA amps, Lauvland used fast and linear transistors like 2sA1227 and 2SC2987. But these were changed to BD245 and 246 by the ear-test!!
Doxa 70 SE Amplifier

... and

Quote:
Hi, I`m new. I must say a few words. Five transistors are in paralel and one transistor is driver for this five. If you need change only driver transistor you don`t need change and other five. Don`t put any other transistor except BD or TIP. I listen this amp before seven years and I never hear better amp before.
Doxa 70 SE Amplifier

Maybe I should use TIP35C/TIP36C, recommended In the same thread?

Best regards
Bjørn
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Old 27th June 2015, 07:14 PM   #4
mkc is offline mkc  Denmark
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Hi Børn

Perhaps you should try contacting these guys and see if they can help you?

TRANSISTOR, TO-3P-ROHS-KONFORM | Elektronik Lavpris ApS

PNP 100V 10A 80W TO218 | Elektronik Lavpris ApS

Mogens
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Old 28th June 2015, 04:26 AM   #5
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Quote:
Originally Posted by bjoras View Post
....Maybe I should use TIP35C/TIP36C...
I believe TIP33/34 are closer, if not equivalent, and both in production by ST and On Semi.
Focus on matching, such as you get with new product supplied in tubes by reputable manufacturers as this will save time and money in learning the hard way - trying to match dissimilar products, batches and losing die sequence in the wafer. 5 devices used in parallel are likely matched only to ensure current sharing, because they will be lightly loaded and running with good linearity for what normally make awful audio transistors.
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Old 28th June 2015, 05:34 AM   #6
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Ahhh ! The Doxa 70. Real wild design , symmetrical "rush cascode" .
https://mrevil.asvachin.eu/amp/topologies/rush/
Actually looked at this one to make another IPS. Notice there is no
compensation whatsoever. The Cob of the (slow) triple EF is the compensation.
Disrupt that with a faster device and there goes stability and SQ.

So the reason for the relatively fast rush and the dog ar$e slow OPS
is to compensate the rush. This is why the original designer stock piled
so many original devices.

I tried running the stock Doxa IPS directly to a modern triple - nope !
Had to first run it through a modern VAS (with compensation) , the
dominant pole had to be set somewhere , and the fast OPS's was too high.

OS
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Old 28th June 2015, 08:37 PM   #7
bjoras is offline bjoras  Norway
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Thank you all for much appreciated replies

Looks like TIP33C/TIP34C is the way to go. Seem there is consensus that this transistor is nearest match to BD245C/BD246C.

It seems that well known trusted sources have problems delivering the BD's. Looks like the Bournes versions are available to a certain degree. But they are easy to find on places like Shop bd245c online - Buy bd245c for unbeatable low prices on AliExpress.com. You'll find both Bournes and MEV brands. They are cheap, but you never know if they're fake. Still I'm tempted trying some.

I wonder if I should order 20, 40 or even 100 of each kind to get descent matching Ordering 100 from a source with good reputation easily get more expensive, than buying another second hand amp of the same kind.

Maybe I should order some samples from different sources and check them on the curve tracer, which I hopefully receive in mid July (locky_z's mentioned above).

Best regards
Bjørn
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Old 29th June 2015, 03:12 AM   #8
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Would you buy this spec. TIP33 from China?
(at least KEC are honest about differences - most generic part datasheets are just copied over )
Hint: look at Hfe, Ft and think of stability in light of Ostripper's comment. See KEC TIP33C datasheet link Here:
TIP33C Replacement Parts & Analogues | All Cross Reference Guides
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Old 29th June 2015, 07:53 AM   #9
bjoras is offline bjoras  Norway
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Thanks Ian!

I am tempted to try BD samples from China, since they are scarce elsewhere. To me, it looks like the Bournes brand BD's from China, are the same they still offer from good well known sources.

I'm not convinced this will work. They might be fake or far from the original TI spec.

But I'm not convinced that the TIP's will work either. They may also be far from the TI BD spec.

Your link and comment about that look interesting. That might help me. I'll take a closer look at that!

A norwegian member of this forum, kindly offers me 5 pairs of the original TI BD's. The price is steep though. I also think that 5 pairs are to few to get a descent match? One of the drivers (same transistor type) is also burned, so I need a match for PNP/NPN on that one to. I tend to think that I have to replace the transistors in the channel that still works too. Then I need to replace 12 pairs. Don't I need at least twice of that amount or even more, to get a descent match?

Thanks again, Bjørn
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Old 30th June 2015, 02:05 AM   #10
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The problem faced with Chinese copies - or rather substitutes, is as I have tried to show. Despite type marking, the example was a substitute type, not a copy, by Korea Electronics Corp. It has characteristics more like audio transistors such as 2SD1047 and a larger case too. It's really a decent, high gain, high frequency audio type that will almost certainly oscillate in the Doxa circuit. You would need to redesign the amplifier to use such a part.

The quantity needed for matching can't even be guessed, since you don't know what the batches are made up from. They could be sorting rejects or leftover product from factory parts bins, new stock loosely bagged or taken directly from a manufacturers boxing, in sequence. These are very different samples with the chance of a match ranging from very good to very unlikely.

Otherwise, Id stay with plan B and buy a used working amplifier, assuming it had not suffered the same fate in the past. Look after it and don't fit huge or exotic caps or risk shorts, as the design obviously had issues - don't invite trouble.
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