F5 power amplifier

[Variac]

To repair rotted wood in boats we use something called "Penetrating Epoxy"
It flows like water. I don't know if it would damage the insulation on transformer wires (I doubt it ) West Marine has it.

[Melon Head]

This perfectly illustrates one reason why not to use an under rated transformer.
The time, cost an effort to fix a buzzing transformer is a lot more than forking out a few more dollars for a higher rated transformer, which would avoid the problem all together in the first place.

[jacco vermeulen]

Originally Posted by Variac

something called "Penetrating Epoxy"

Mark,

whether it's West or any other brand, penetrating epoxy is the plain deal with two shots of extra solvent.

For transformers and all other coils : 3M Scotchcast 280
Best way to use is with vacuum injecting and autoclave curing.
For the autoclave thing, just bang it in the oven at 120C or build a fancy DIY one, lots of examples on the web.
Same story for vacuum injecting, it is easier than one might think.
(long time ago me had a sabbatical at an airplane factory where they did prepreg resin/kevlar cure stuff in huge autoclaves)

[samoloko]

Originally Posted by scranton

Just the one transformer buzzes. I can hear it faintly from 3 feet away. And that unit's speaker has a very minor hum that I can only hear withmy ear on the thing and I hear it from start up. The transformer develops the buzz an hour after startup. The other monoblock makes no chassis noises and the speaker is perfectly quiet.

About this time I did do a mod on my cd player. Bypassed the output caps per Kit Ryan's .
suggested mods in Audio xpress. Could this be related? Cd is attached to B1 that is attached to F5. Thanks for help.

could you replace transformers and see

[labjr]

Originally Posted by thanh1973

This perfectly illustrates one reason why not to use an under rated transformer.
The time, cost an effort to fix a buzzing transformer is a lot more than forking out a few more dollars for a higher rated transformer, which would avoid the problem all together in the first place.

250 VA per monoblock doesn't seem like under-rated transformers. The stereo F5 has a 300VA transformer. Though this seems like a reason to choose a brand of well-made transformers. I would think Avel transformers are good quality.

[Melon Head]

I am not suggesting that in this specific case that an under rated transformer was used.
But this is exactly what will happen if you use an under rated transformer.

Hi scranton what voltages and bias current are you using in your F5 amp?

[Renron]

Originally Posted by jacco vermeulen

Mark,

whether it's West or any other brand, penetrating epoxy is the plain deal with two shots of extra solvent.

For the autoclave thing, just bang it in the oven at 120C or build a fancy DIY one, lots of examples on the web.

Great idea, great results too. But.....
DON'T USE YOUR HOUSEHOLD OVEN FOR THIS! Toxic fumes result and will contaminate the interior of the oven. An excellent alternative would be a small toaster oven or a DIY oven as Jacco suggested.
Your "other half" would be P!ssed.


Quote ""an amp so simple, even a cave man could build it"
I screwed mine up and am getting help on the rebuild from Steve Eddy. Thanks Steve!

Ron

[tinitus]

Originally Posted by labjr

Though this seems like a reason to choose a brand of well-made transformers. I would think Avel transformers are good quality.

Very true
But having hum in one speaker indicates there may be another small problem as well
Significant or not, I dont know
It plays fine as is
Good
Though I have had one experience with a certain amp
It sounded fine, and I had no idea that it was unstable and unreliable
But when the printboards on one channel was literally on fire, I knew
Unreliable amps can be very dangerous

[scranton]

Thanks Tinitus and others: I reversed channels and disconnected signal and output and the transformer still buzzed after warmup. With no load, I did the adj again and I see the DC offset more unstable than before, fluctuating 8-10 mv around zero (Is negative DC offset an issue?) The voltage on R11 and 12 is steady at .585v which I adjusted slightly to keep offset above zero. I do think I have two problems: instability due to earlier mistakes and a transformer.
The heatsink comment earlier struck home. AT 6X9X3 per amp I was concerned about using them in 2 converted Bogen PA chassis(pl). For a novice it works so well. I assumed they would get hot as hell if inadequate. I can touch both of them as long as I want. Does the heat instead get absorbed/dissipated by the mosfet and source resistors? R11 and 12 look overheated. I used the fabric-like insulators.
Thanks for all the tips/ With a set of great PD boards and parts I am going to rebuild and get another transformer and contemplate Conrad heatsinks. Thanks
I don't think I will repot the transformer
But if it can be done, whatever it is, someone here will know.

[daly41k]

I was thinking of building a five channel F-5 for a home video application, has anyoned done this? I have a couple of 2kva @ 45 volt trannies left over from another project. I could use one and of course I would have to step down the voltage but I can do it. Feed it multiple power supplies for each channel and I think the 25-50 watts would be enough. The speakers already have powered subs so I am just pushing the tweeters and mids in the front and the back speakers don't need much power. As always the case and heatsinks are the big issues. I have a couple of AX140's I built based on the AX100 thread but these are way toooooo big and run too hot for a living room application. Actually too good to waste on home video. Just curious as winter is coming up and I have to stay busy. dave