ETI-466 300W Amplifier

[auric]

Hi:

I am wondering if anyone has experience and success building the ETI-466 300W Amplifier:

http://users.otenet.gr/~athsam/power_amp_300w.htm
or
http://www.alphalink.com.au/~cambie/ETI466Web.htm

I've read a couple of threads on here already and would like to hear from other members who have experience with this design.

I've been working on the module for some time now and after some research on the internet, it appears mine is suffering from high frequency oscillaion in the output/driver stage.

Anybody experience the same problem with this design? Any ideas on how to correct it?

Thanks,

Derek

[EWorkshop1708]

Try some small caps on the driver transistors between base and emitter. a 1nf cap or 500pf cap should work.

[auric]

Hi:

Thank you for your reply.

Do the addition of the capacitors on the driver transistors alter the frequency response / sonic qualities of the amplifier?

I'm guilty of a few things that may have contributed to the HF oscillation.

1) The emitter resistors are "no name" brand and probably exhibit some induction.

2) I didn't have access to a metal shop to custom fabricate PCB mounted heatsink. Instead I used off the shelf heatsinks extending the power transistors off the pcb.

I assume you have built one of these before. What is your opinion of amplifier?

Best,

Derek

[anatech]

Hi Derek,
Many were build years ago. Many burned up years ago. Try to reduce the gain in the output stage, bypass the outputs right where they are.

I don't consider it a great amplifier, but get yours working and you built an amplifier! It's still worthwhile.

-Chris

[Tony]

Hi,

this amp is just like the Tigersaurus amp designed by Daniel Meyer, of Southewest Tecnical Products, only the ouput trannies used the MJ15003/15004 in the ETI466.

this amp like the Tigersaurus packed a lot of punch in the bass end, and is likewise prone to oscilations. personally i did not find this amp good in the mids and highs...

the output stage has a voltage gin of about 4, and this is because the front end is fed off a much lower voltage so that the vas output swing is not enough to drive the amp to full power.

perhaps a 4.7 ohm resistor to the base of the output power trannies may help, although i have not tried this as i have retired all of my tigersaurus amps.

btw, these amps are good for PA duties, for HI-Fi i will stay away from them.

[bigpanda]

Hi Derek,

Why do you need a fancy heatsink? All you need is a L al. bracket, a bit of patient, skill and sweat + a small drill press.

[anatech]

You have no idea how many of those crossed my bench over the years. They are not reliable enough for PA duty.

Try and get it going, and bypass the heck out of the supplies. Due to the output stage having gain, you really do not want to mount the outputs off-board.

-Chris

[auric]

Firstly many thanks for everyone's gracious assistance!

I'm from Toronto, Canada and this amplifer was featured in an 1980 ETI article titled simply "The Brute" with no further reference to the designer or author of the article. I've been struggling with it for a LONG time without much success.

By chance I discovered some references online to the ETI-466 which turned out to be the SAME amp! This eventually led me to this forum.

Some additional questions:

1) Do the addition of the capacitors on the drive transistors alter the frequency response/sonic qualities of the amplifier?
2) Is this thing supposed to run "warmly" even without signal? Its not hot but warm.
3) Do you need two caps, one across Q7 and Q9?

I added only one to Q9 and it tamed the amp. All of the high current draws disappeared!

BUT, I also needed to add a 150 K resister in parallel to 2K2 bias pot to bring it into range to where I could get a 1 volt drop across the 10 ohm test resistors that are put in place of the fuses during the set-up procedures.

Any thoughts?

Thanks again!

Derek

[anatech]

Hi Derek,
First, read the notes. Inductive emitter resistors were a big problem. You will need to look at the output waveforms both under load and not. Look for oscillations. That will be your guide. I feel that because the outputs are not on the circuit board, your amp may tend to be unstable.

-Chris

[clem_o]

Derek,

It'll be hard to verify the stability of the amp without an oscilloscope. As others have already posted (and based on Tony's comments) if its anything like the Tigersaurus, you may want to drive just the LF speakers with the amp - any HF oscillations tend to take out tweeters in domestic loudspeakers real quick...

:-)


Cheers!


ps: will the capacitors alter the frequency response - yes, but it will still be audibly better than an unstable amp!