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-   -   Replacement transistors for leach amp (http://www.diyaudio.com/forums/solid-state/16293-replacement-transistors-leach-amp.html)

Bosium 11th June 2003 01:12 AM

Replacement transistors for leach amp
 
Hi, I was wondering if I could test my leach amps with 2n3055's and mj2955's instead of the mj15003's and mj15004's? I was planning on reducing the rail voltages to +-50VDC instead of the nominal +-58V-62V to compensate for the 3055's and 2955's max Vce of 60Vdc.

It's just that these parts cost about an eighth of the price of the 15003's and 15004's. I'm not planning on using them in the actual application.

I can't see a practical problem with this, can you?

Thanks, your input is much appreciated.

Gareth

JOE DIRT® 11th June 2003 01:46 AM

I think you will be fine doing that...people critisize the 3055`s but I still enjoy them for power supplies and amplifiers

DIRT®

andy_c 11th June 2003 02:43 AM

With a +/- 50V supply, you'd need transistors with a breakdown voltage of 100V minimum. DC-wise they'll only see 50V, but when the output is driven near the rail, one Vce will approach zero and the other will approach the difference between the positive and negative supplies.

Bosium 11th June 2003 07:02 PM

Hi, thanks for the reply.

Would it be alright if I use them for just to test that nothing blows up on powerup?

I know about the 100R resistors, but i've had so many transistors blow up on me i am wary.
I have been plagued with counterfeit mj15003 transistors, and i can't seem to find a source.

I informed the biggest source around here (Communica) about the counterfeit transistors they were selling after finding out after almost 2 years of struggling that such things even existed. They were very helpful and 'sorry', and told me they took the stock off the shelves and i'd have to wait a few months for new stock. I eventually tried to buy some more from them a few months down the line when they told me there was stock again, only to find they were counterfeit AGAIN!

I told them there and then and i wanted to keep the fakes as proof but they wouldn't let me keep them. I tried to get hold of the general manager for the branch, a Mr Dawood, but it seems he was never 'in ', and he still hasn't phoned me back despite several messages.

I think this is disgusting. It shows that he knew they were selling fake transistors but is too cowardly to face me, a humble EEng student at the local technikon. btw, I'm in Cape Town, south Africa if anyone is interested.

I wish there was a way for me to make more of a stink about this. It's absolutely appalling.

anyway, /end rant.

thanks for the help.

Gareth

JojoD818 12th June 2003 10:26 AM

Hi Bosium,

Maybe you should be double checking the placement of your parts on the pcb first. Check if the transistors pins are in the correct position. Check everything in the pcb just to be sure.

At least you will only have to worry about fake transistors for the outputs. :cannotbe:

jojo

Bosium 12th June 2003 10:39 AM

shot man,

The boards do work, i soldered two 100R resistors from the output to either side of R36 and shorted Q7 to test the boards without the output stage and they draw about 3ma per channel quiescently.

I think they work ok, i tested them on my scope and everything seems peachy. it's just the output that i've been struggling with for ages now.

Gareth

StuartH 12th June 2003 11:49 AM

Hi

If I remember correctly there is a distributor in Cape Town known as Falcon electronics, or some permutation on that.

You could also try AVNET in Capetown (021) 689-4141

Otherwise RS will definately have the components, although at far higher cost.

Hope that helps.

cheers,
Stuart

skaara 12th June 2003 12:57 PM

You could use mj15024 and mj15025 but pay attention u wont get counterfeit devices..

How do you mean "they didnt let you keep them"? Werent the transistors already yours? And even if they werent, they cant keep you from buying them if they are selling them to anyone else..

Bosium 12th June 2003 10:53 PM

hi again

thanks for the help stuart, i'll give them a buzz. I know that RS has them but they're like 60 bucks each. ouch. communica is probably still selling them for R10 or so each, which is way too cheap for these devices. Strange that the MJ15004's i bought two years or more ago all are original and none are blown, it seems the problem lies with the NPN devices.

I chatted this out with some important people at cape tech and they said that purchases from communica by the tech would be curbed. I even saw a whole box of BD140's bought with the intention of use for pracs which all had an hfe of 0. I wish i could do something about this, it's downright fraud. Maybe i'll phone head office.

Anyway, at this stage i guess i'm more worried about completing the 2year old project than seeking recompense from communica.


Skaara, i mean that the second batch, that i thought would be fine by now were counterfeit again. They took them away from me in the front shop and credited me, saying they couldn't sell them to me, but when i phoned the b@stards the next day i was told they were in stock! talk about cheek and complete and utter disregard for their clients.

Gareth

AJT 13th June 2003 12:11 AM

Bosium,
in 1992, i assembled a leach super amp using 2n3055/mj2955 as outputs. i downsized the rail voltages from +/- 85 to +/-50 volts to be able to use 56,000/63volts caps on the power supply rails. i also reduced the value of zener bias diodes for the input diff amp, while maintaining the bias current values for each stage...i even used a torroid power transformer which i handbuilt...the amp is still alive to this day...the performace i'd say is just as good, although at lower power level....so go ahead and use them 2n3055's just remember to lower the rail voltages...to bad i never get my hands on the leach low tim amp plans, it is only later(about a few weaks back) that i stumbled opun leach again in the interent, and i am very glad to meet him again....


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