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Old 12th November 2004, 09:03 PM   #1
kneadle is offline kneadle  United States
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Default Rectifier Bridge Assembly Difficulties

Hello friends,

I'm trying to build this preamp published by Nick Whetstone for my BrianGT GC--which works great. Unfortunately, I'm experiencing some difficulty with the initial PSU stage, and I've determined that at least one problem is in the bridge.

When I assembled the entire first stage of the PSU, designed for +/-21VDC, upon testing it, I ended up with either 12 or 24VDC; I don't know which because the power resistors at the front end of the PSU got so hot that they began to burn the breadboard.

So, as Nick instructed, I disconnected the PSU before the smoothing cap to measure immediately after the bridge. So the circuit essentially consists of two resistors for the hot leads connected to these four diodes . I hooked up the hot leads of my Tx, which, although rated at 18VAC, was producing 21VAC. The hot leads went to the resistors. My DMM leads went from one output rail to one of the N leads of the Tx.

The rectifier is producing exactly +/-9.3VDC, but the resistors are not hot at all.

I've checked and checked and checked the diode arrangement, and I can't imagine where I've gone wrong.

What's wrong with my bridge?

Thanks,
Dave
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Old 1st December 2004, 07:19 AM   #2
hjelm is offline hjelm  Sweden
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Hi,
I think the diodes are backwards in the link you attached.
Shouldn't the color band of the diode be on the positive side of the circuit? I am guessing that either the regulators are shorted or the electrolytic cap is conducting like a short in the backwards direction. Measure the DC from the rectifier bridge and connect the positive side to the correct part of the following circuit.
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Old 1st December 2004, 02:18 PM   #3
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Ack...

Besides the diodes being obviously backwards, that circuit has a lot of crap. So much you can't even redesign it to be good, you might as well start over from scratch.

Tim
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Old 1st December 2004, 02:27 PM   #4
kneadle is offline kneadle  United States
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Quote:
Originally posted by Sch3mat1c
Ack...

Besides the diodes being obviously backwards, that circuit has a lot of crap. So much you can't even redesign it to be good, you might as well start over from scratch.

Tim
Well, I managed to build it, and it sounded good till one of my joints broke, which resulted in a burned up transistor; otherwise, I wouldn't know what's wrong with it.

So the diagram shows the rectifier diodes in backwards, eh? Well, that should explain my difficulties earlier (I posted this nearly a month ago. It took two days for me to figure out that I should build the rectifier bridge "blind," then attach neg to neg and pos to pos rails). Someone should let decdun owner/operator Nick (Nuuk) know.

Dave
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Old 1st December 2004, 02:39 PM   #5
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Quote:
Originally posted by Sch3mat1c
Ack...

Besides the diodes being obviously backwards, that circuit has a lot of crap. So much you can't even redesign it to be good, you might as well start over from scratch.

Tim
Yes, the diodes are the wrong way round, but as for the comment on the circuit being rubbish, I cannot see anything fundamentally wrong with it. It's basically the datasheet circuit!

What do you suggest then?
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Old 1st December 2004, 02:53 PM   #6
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Quote:
Originally posted by Sch3mat1c
Ack...

Besides the diodes being obviously backwards, that circuit has a lot of crap. So much you can't even redesign it to be good, you might as well start over from scratch.

Tim


Try to be positive and offer some solution instead.
Try also to understand what the thread starter is after.



As pointed out, this circuit is a basic LM317/337 regulator has no crap but this isn't the same as the circuit can't be improved. If you mean crap = not Spitzenklasse, you are right about crap, compared to what?
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Old 1st December 2004, 04:15 PM   #7
Nuuk is offline Nuuk  United Kingdom
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Quote:
Besides the diodes being obviously backwards, that circuit has a lot of crap. So much you can't even redesign it to be good, you might as well start over from scratch.
I'm ever-willing to learn Tim, nevermind improve my hi-fi. So could you post a better circuit for us please?
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Old 5th December 2004, 09:23 AM   #8
AndrewT is offline AndrewT  Scotland
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Hi.
has the circuit for the PSU been changed since this thread started?
I have looked at it today & the diodes seem OK.
Starting from the neg cap conn. both the lower diodes point towards the AC conns & then both the upper diodes point to the pos cap conn.
What's wrong with that?
The buffer is a constant current sink emitter follower, this should work ok. it runs at 5 to 6mA giving an output current of about +-5mA peak.
Personally I would opt for a larger output cap, propolyene not ester nor mylar.
regards Andrew T.
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Old 5th December 2004, 10:59 AM   #9
kneadle is offline kneadle  United States
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I believe Nick did change the picture since this thread started.

In addition, some posts from this thread have disappeared somehow. I finished installing the preamp on 3 Dec, and it sounds fantastic.

Thanks for describing the circuit for me; now I know what it is I built.

Dave
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Old 5th December 2004, 12:00 PM   #10
Nuuk is offline Nuuk  United Kingdom
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Yes, I can confirm that the circuit has been corrected now!

Quote:
Personally I would opt for a larger output cap, propolyene not ester nor mylar.
I usually stick to 4.7 uF for these DC blocking caps but I found that 2 uF bypassed with a 0.1 uF works very well on this circuit. I always prefer polypropylene as well!

I wondered where those other posts went to as well. I got an email notification that there was a new post to this thread but when I clicked on the link I only found my previous post.
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