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and now for something completely different part 3


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2SK170 and 2SJ74 are the singles for those. And there are the Linear Systems versions (LSK389 and LSJ109 or variants).

Of course the majority of the original Toshiba parts on ebay, aliexpress and even some suppliers (dalbani and their ilk) are fakes.

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Close to finish casing my balanced CFP amp. Waiting for a 4-pin XLR socket. This one runs on 26VDC rails and 220mA bias current.  I am posting an unfinished amp because I want to report that

So when I got my case I got pretty upset with the looks of the power switch: So I search around the house for what could be used to improve this and found and unused aluminum kitchen drawer

Another CFA is alive and well. Sorted out the issue with the bias when using the SS/ZF switch by doing the following (board rev 1.2): - Remove the 4 ZF jumpers near the 600pF - Increase R44/

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The 2sk389/2sj109 have unusual packages so the chances of fakes are probably quite low. For TO-220 its a different story. In the uk  on ebay I have had fake 2sa1486 from littlediode, fake 2sa1968 from nikko, fake 2SJ79  from littlediode_components (same seller as littlediode). I strongly recommend if you are going to spend money on original Japanese transistors you buy both a dca75 transistor identifier and a DUOYI DY294 transistor breakdown voltage tester.  If you put a fake into a circuit the consequences can be catastrophic and cause considerable damage not to mention a danger to your life. For example the fake 2SA1486 had around 200 volts LESS breakdown voltage then they should have had and the 2sj79 did not even have the correct pin out and would fail as a dead short at almost any voltage if wired up as 2sj79.... The other problem is that ebay only give you a few months to return purchases. I did not get the DUOYI DY294  tester until about 9 months after I purchased the transistors (I simply did not realise fakes where a thing. I purchased the dca first and it identified the 2sj79 pintout as not consistent with the spec sheet... I thought the dca75 was miss identifying. Later I got the DY294  after reading about fakes on the T2 build pages and found the dca75 was correct) . Although I contacted ebay explaining that the seller was selling fraudulent components AND presenting a clear safety risk ebay said they could do nothing....  I contacted paypal.. again they refused to do anything and the sellers are still selling the same fake parts. So now I test everything. Luckily I did not lose a lot of money but I don't want to think about what would have happened if I had built an original blue hawaii with those components....  The dca75 is also useful for troubleshooting if you have a component failure. I had a 10m90s die on a golden reference HV board which caused a chain of destruction in my blue hawaii. The dcs75 was very useful for quickly identifying bad transistors. The hawaii was back up and running the next day.  Although the dca75 is not designed for in circuit testing, if you have one channel fail or you are super paranoid after completing a new build you can always compare test results with the other channel. (its not totally foolproof compared to desoldering the components and testing out of circuit) but it can save a lot of time. Obviously you don't want to use the DY294  in circuit because injecting high voltages into a circuit is not a good idea.

One word of caution, the DY294 cant be used to breakdown test depletion mostfets since by their nature they are conducting by default and so register as having a very low breakdown voltage. The DY294 can also be used to match and test high voltage zeners which can be useful. The sockets on the DY294 are not that great, but its trivial to replace them with colour coded leads with test clips on the ends, this also allows you to test smd parts using sot23 adaptors etc. 

Edited by jamesmking
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The transistors I got performed up to spec and looked similar to known genuine parts. Even the legs had slight discoloration characteristic to old electronics. 

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I bought some 2SJ109BL from a guy named Gary at DIYAudio. Something expensive. Once at home checked and everything was perfect according to the readings he had made and Toshiba's specifications.

Luck? Maybe .... but if you don't want to play Russian roulette, better the Linear Sis 2SC174 / 2SJ74.

 

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  • 4 weeks later...

Running my CFA at 200mA bias in ZF mode, works perfectly but when I try to go over to SS mode it acts funny.

Bias jumps up way high when only the SS jumpers are engaged, is this normal behaviour? Using the latest boards.

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Your last posts with gerbers were a page back, on Jun 11 and Jun 13. So the Jun 13 board is the latest (same filenames)? Or are those boards the same? Jun 13 post also has updated schematic.

Edited by Pars
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I have boards but I haven't put them together yet. Can you just switch between ZF and SS, aren't the component values and placements different so that you have to choose a configuration up front?

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Maybe my post was a little ambiguous. Right now have it at 200mA in ZF. I shut it down and "jump" the amp to SS. When I power it up  I get something like ~300mA bias.

My ultimate question is: should it act like this? Seems cumbersome adjusting bias up and down between the different modes.

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no it should not act like this. maybe it is oscillating in one mode.

one person had a problem similar to this and fixed by adding 500pf B to C on the output transistor pair. Maybe the output transistors have changed over the years as i still have 100's of older ones.

Edited by kevin gilmore
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Another CFA is alive and well. Sorted out the issue with the bias when using the SS/ZF switch by doing the following (board rev 1.2):

- Remove the 4 ZF jumpers near the 600pF

- Increase R44/R49 to 50K

- Increase R50/R51 to 4.2K

- Remove C9/C10

Set at 190mA, plays beautifly in both modes. A must-have as reference in the amp collection. Thanks to KG for developing this amazing circuit!

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  • 2 weeks later...

Hello! 

I am going to try to build the Uber amplifier for Raal Sr1a. But Kevin said that 

Quote

So correctly done, one would do the Uberamp2-Raal edition

What should be changed in the scheme to drive 0.2 ohm load better?

Is it the last version ? 

Thanks in advance. 

uberamp2.pdf

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Thank you very much!

About the bass filter - I think I'll add it to the feedback, just need to calculate conditions of stability properly. 

Just a two questions(sorry if they are stupid): 

1. How much transistors I need to purchase ?) I mean if they should be matched with each other ? For example: I need to get 4*9 2SA1943 (for balanced configuration). But how should I match them for such a parallel connection? And  how much should I buy them ?

Asking just because they are not cheap 🙂  and as a fan of tubes I have no experience with matching transistors.

2. The uberamp2 works in class A or AB ?

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you can do up to 3 amps of class A bias with big enough heatsinks.

The one feedback resistor, you can make into a resistor that is in series with a resistor that is in parallel with a cap.

pick the numbers you want for appropriate bass boost.

once in a while you get a bad transistor or low hfe etc, so always buy more than you need.

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you can do up to 3 amps of class A bias with big enough heatsinks.

Does it mean that if I would raise loudness level so much so that amplitude of output current would be more than 3A(or 6A in balanced configuration - with 2 boards per channel), then It would work in AB?

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once in a while you get a bad transistor or low hfe etc, so always buy more than you need.

 

Ok, It means that I do'nt need to buy x*3 number of transistors. Thanks 🙂 

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  • 4 weeks later...

Another CFA3 here to be built:

The transformer i used is from Antek with 200VA 25V for each channel.

Still waiting for the rectifier bridge.d which probably will arrived tmr. Hope i can power it up tmr ASAP, can`t wait to enjoy it.

BTW, any help about the trimmer resistor near the VCC+- terminals. What`s it used for?

 

20201013175526.thumb.jpg.64f9d9e16449bf39b8865b02dbf97426.jpg

 

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1 hour ago, Pars said:

Now there is a HC approved overkill transformer!

The two trimpots on the GRLV by the output terminals are for balancing out the output voltages.

Thx Pars. Actually, i did`t ask the question clearly😑 My apologize. It`s the one in CFA Board near the VCC+- terminals.

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