Question on wiring a full wave bridge rectifier on my amp(s)

jonwb · « **on:** 17 Sep 2005, 07:14 pm »

Hey gang,

I'm in the process of making a few upgrades to the power supply on my amplifiers (two mono amps). I'm following some of the design guidlines listed here. Currently my power suplly basically looks like the diagram shown below:

except I don't have the .1uF cap across the power switch and my caps are 26,000uF each for the + and - rails.

On the proposed dual rectifier setup, I'm curious how they'd be wired specifically.

Here's a basic picture of the type of rectifier I'm referring to:

On the package the odd-oriented terminal is marked '+' and I know the opposite corner is '-'. So the '+' termnal goes to the positive rail and the '-' terminal goes to the negative rail. And, both of my hot leads from the transformer go to the opposite corners of the rectifier. Here's a photo:

So, when using a seperate rectifier on each rail I'd figure for the positive rail rectifier... one of the "hot" leads from the transformer would go to the '+' terminal and "neutral" wire from the transformer would get tied to the '-' terminal.

On the negative rail I'm a little unsure... would the "neutral" wire from the transformer be tied to the '+' terminal on the rectifier and the '-' terminal get tied to the other "hot" lead on my transformer? Like this:

scottnixon · « **Reply #1 on:** 17 Sep 2005, 07:49 pm »

The way you've marked up the dual bridge is wrong and it will likely blow
chunks out of something or at least dead short and pop fuse, big damage will occur.

Occam · « **Reply #2 on:** 17 Sep 2005, 08:08 pm »

Jon,

Although you seem intent on killing yourself, or at least blowing up your amp, please don't do it.

First, examine the 1st schematic, and examine its meaning. Look at the picture of the solid state brigde you've posted. The brigde terminals are labeld (s),(-),(s),(+) for good reason [those 's's shoud be rotated 90degrees] and understand how diodes work. The electrons flow out the anode,actually where 2 anodes meet, the bases of the triangle which connects to the negative rail (electrons have a negative charge) and the positive rail connects to the point of the bridge where the two cathodes connect, the lines at the tip of those trianges. The AC from the transformer windings connect to those rotated 's's, where an anode and cathode meet.

Therefore, you've labeled the second schematic arse backwards. What you've labeled the (+) and (-) terminals should be rotated (s) terminals, and the rightmost terminals should be (+) and the leftmost, (-).

1. I'm chastising you speaking as someone who has 'blowed up' more equipment and been blown accross the room more times than I care to think about.

2. You really should buy a copy of an ARRL Handbook at a flea market, or a similar text.

3. Call me at the phone # I'll PM to you.

jonwb · « **Reply #3 on:** 17 Sep 2005, 09:41 pm »

Paul, thanks for the offer to help. I'm here to learn... that's what this circle is for correct?

jonwb · « **Reply #4 on:** 18 Sep 2005, 06:59 pm »

Quote from: Occam

First, examine the 1st schematic, and examine its meaning. Look at the picture of the solid state brigde you've posted....

What you've labeled the (+) and (-) terminals should be rotated (s) terminals, and the rightmost terminals should be (+) and the leftmost, (-).

Yeah, that makes sense... Here's another mark-up (where (on the lines going off to the right) the top line coming out is +Vcc, the middle 0 and the lower -Vcc):

Does that look better? Also (assuming I have one 'hot' lead going to each rectifier and the 'neutral' line going to both (split)) is it important which lead from the tranny goes to which 's' terminal?

thanks again

ScottMayo · « **Reply #5 on:** 19 Sep 2005, 01:00 am »

Quote from: jonwb

Paul, thanks for the offer to help. I'm here to learn... that's what this circle is for correct?

Yes, and I applaud your sense of adventure. Make sure your insurance is paid up though. Making mistakes in the power supply of an amp can hurt.

Ask yourself why you are doing this. I'm not saying there's never a legitimate reason to upgrade a power supply; there's plenty of mid-fi gear that could do with better power. But they could also usually do with better output devices, tighter tolerances on parts, and a lot of other things that matter more than the power supply. And for high end gear, the power supply is going to be more than adaquate ten times out of ten. So while I can see the appeal of rebuilding gear, starting with the power supply of an amp for someone a little fuzzy on how bridge rectifiers work, might be just a little ambitious, and probably very unnecessary.

If you're going to carry on, let me suggest two things. First of all, the power supply of most amps doesn't go inert as soon as you turn them off. Caps can hold a big charge for a surprisingly long time. If you turn an amp off and reach right in, you can get unlucky. Unlucky doesn't include dead unless you are very unlucky indeed, but it's something to consider. Learn how to safely discharge a cap. The end-of-a-screwdriver approach, while popular, is unwise.

Second, practice the technican's rule: when working on a power supply, one hand stays in your pocket. Always. If it's in your pocket it can't brainlessly grab the anything grounded or oppositely charged, so it can't conduct high voltages up your arm, across your chest, and down your other arm. I'd told there's nothing more embarrassing than going into cardiac arrest and collapsing face first into the same gear that just stopped your heart.

Seriously, guy, when I was 15 I was taking apart guitar amps and table radios, and I lived to tell about it, but it's not because I was smart about it. Home made electroshock therapy made me what I am today - learn from that example.

Occam · « **Reply #6 on:** 19 Sep 2005, 01:48 am »

Scott - Sound advice indeed. And as Jon said, one of the major rationals for naming this circle 'The Lab', is to not only aid the DIYer, but to further the practical and theoretical understanding of what is behind the modification audio components.

Frankly, I'm unsure of what real benefits, other than the lessening of stresses on the bridges, accrue by going from a single to dual bridges. Pesonally, my impractical preference is eliminating the use of overwound, unshielded toroidal power transformers. IMO, a blight upon audio whose only benefits are their constrained magnetic fields.

But unsaid, is a more sinister motive for my involvement as moderator of this Lab board. And that is the reintroduction of Darwinian forces into our society.......

There are a number of posters here, certainly not Jon, and who tend not to participate on the Lab, prefering to pontificate on all things technical, without the least benefit of any quantitative efforts, who I'd very much like to encourage to start modifying their own equipment.

JoshK · « **Reply #7 on:** 19 Sep 2005, 02:05 am »

I think you'd be better off bypassing the bridge with 22nf caps then adding another bridge. Then if you still wish, better the PSU caps. I might disagree with Scott regarding improvement in the PSU not being much help, or less of a positive impact then other things in the amp but certainly he has very sage advice when it comes to safety.

jonwb · « **Reply #8 on:** 19 Sep 2005, 10:19 pm »

Thanks for the feedback guys. Yeah, its probably a little ambitious for someone of my limited electrical experience to be rebuilding the power supply on my amplifiers. (Then again tackling these as my DIY first speaker project was probably a little ambitious as well).

Anyway... with the help of some clarifications from Paul I finished rewiring amp #1 last night. One of the biggest challenges w/ this sort of thing is fitting all the new parts w/in the package space you have. I re-checked my connections many, many times. I put the part of the cover on that shielded me from the big caps. I pulled the two fuses from the circuit board on the amp that come off the +Vcc and -Vcc inputs (you can seem them here) figuring if something was screwed up it wouldn't fry my amp (and I could make sure readings were good before powering up the amp itself). I got a power strip, so I had "remote" power switch capability.

So when I flipped the power strip switch, the breaker that's in the power switch on the amp 'tripped' and I turned off the power strip right way. Well, that didn't work as I hoped. No other noise or smoke at all, just the power switch going click. Rechecked the wiring, everything looked good, so I decided to call it a night. No sense triying again since I couldn't see anything to do differently.

Quote from: JoshK

I think you'd be better off bypassing the bridge with 22nf caps then adding another bridge.

I agree Josh, I think my next step will be to just reconfigure to go with a single bridge. That'll greatly simplify the wiring. I already have the diodes on the bridge bypassed w/ these.

Quote from: Occam

There are a number of posters here, certainly not Jon, and who tend not to participate on the Lab, prefering to pontificate on all things technical,

Yeah, you won't catch me doing that. Just one crazy, head scratchin' question after another...

Quote from: ScottMayo

Learn how to safely discharge a cap. The end-of-a-screwdriver approach, while popular, is unwise.

While I may be a little a little "fuzzy" on the whole bridge rectifier thing. Those giant caps scare the crap out of me.

I make sure they are good and dead, w/ a bleeder resistor, before I fiddling around in there.

Thanks again guys, I'll keep you posted...

jonwb · « **Reply #9 on:** 20 Sep 2005, 04:03 am »

Well that wasn't so bad... It took about an hour to ditch the dual rectifier setup and redo/finish my mods using a single bridge. I turned it on via the same method as last time except this time no tripping power switch. Figured that was a good sign. Measured the rail voltage on both sides +77V and -77V just like the doctor ordered. Put the amp back togther and checked the DC offset (it came in around 10-12mV). Took the leap of faith and hooked up the amp to a speaker and ran some music through it. Hey, it works!

So, in the end I did the following:
1) added a .1uF cap across the power switch (film and foil)
2) added a pair of .1uF caps across the output(s) from the tranny (same kind as above)
3) replaced the bridge rectifier w/ a fresh 400V 25A unit
3) bypassed each of the diodes in the bridge w/ a 22nF cap (4 total)
4) bypassed my two large pre-existing 26,000uF caps (one for each rail) each w/ a .47uF film and foil, a .01uF PPT Theta AudioCap and a 5K ohm 5W resistor.

Having only one amp done makes drawing any conclusions on the sound difficult (that and a bunch of virgin caps), so I'll hold my opinions until I've finished the other amp and they've been run-in a little.

Thanks again for the help. I'll keep you posted.

Jon

jonwb · « **Reply #10 on:** 22 Sep 2005, 12:01 am »

Well mission accomplished! Finished the other amp up last night and spent a couple hours listening. I'm sure the caps will benefit from more break-in, however, right off the bat the amps did sound noticably better. The best way I can describe it is a richer/fuller sound. The highs are altogether more open and detailed, but w/ no harshness. Bass seems a little "tighter" but the bulk of the improvement is in the mids and upper octives. Certainly worth the trouble, and the $45 in parts.

Prior to the mod I was contemplating if these would be "long-term" amps for me. I liked them, but wasn't quite as satisfied w/ the sound as I wanted to be. You know... just now quite there. Now, I think we'll be enjoying their company for a while.

Thanks again,
Jon