[noveltone]

Hi there everyone,

This is my first post on this forum so let me first say hello! I'm happy to find such an active DIY site. In terms of experience, I have built a CMoy amp, refurbished an old Sansui amp (with much help found at AudioKarma), and lately built a STEPS. It's the STEPS I'd like to ask you all about.

It basically works fine and it's not outrageously noisy, but a huge step above the batteries on the CMoy. When I turn the amp up over halfway the hiss reveals that it has what seems like an AC hum to it. So I went back and tested it again according to Tangent's procedure and discovered something strange about the ripple current before the regulator. Depending on what way I have the DMM leads I either get 0 mV or 820 mV AC across TP3 and TP5. The DC here is 61.7 V, ending up at 18 VDC after all is said and done. By my reckoning the 820 mV unregulated AC is way too high and could be the source of the noise. What do you all think could be the source of this?

I checked the solder joints at the smoothing caps and they are fine. I checked most of the others with the same result. Any help you could offer would be appreciated.

Thanks,

Collin

 

[heatmizer]

Give us some close up pictures. Where did you measure 61v dc. When you measured 820 mV what scale was the meter on.

 

[noveltone]

Hi,

I can't get images just yet, but I can tell you that I measured the 61VDC to ground after the rectifiers and smoothing caps, and after C8. That measurement is concurrent with 18V final output with no amp load. To answer your other question, My meter is auto-ranging, so the 820mV really read as 0.820 VAC. Does that help?

Collin

 

[tangent]

First, I wouldn't trust such a meter's results very far. It's clearly being confused by the DC component when in AC mode, which means they haven't even bothered making the AC meter AC-coupled. Another bad sign is that it displays millivolts as a fraction of volts, suggesting that its accuracy is poor below 1 V. All in all, this sounds like a meter made for checking house wiring, car batteries, and suchlike. Not that you can't use such a meter for electronics, but you have to keep a low limit on what you expect from it. It's quite possible that 820 mV is just as wrong an answer as 0 mV in this case, and your hum is coming from somewhere else.

Second, why are you making the regulator drop 43 V? Even with a meager CMoy load on it, your dissipation has to be a good half watt or so. You're not going to be able to put much more load on it without having to worry seriously about cooling.

 

[Polaris111688]

Also, I think 820mV of ripple would destroy the opamps in the CMoy. I had a bad experience with my first build ever 3 years ago. I used a crappy wall-wart that had too much ripple, which killed my CMoy. Since your STEPS works with your CMoy, I doubt it's producing that much ripple.

 

[n_maher]

Just to second what Tangent said, you really need to reconsider this setup given how much voltage you're dropping. I can't imagine that your long term goal is to use this PS with the Cmoy and odds are whatever you do have planned for it is going to draw a lot more current and make quick work of that regulator.

 

[noveltone]

It's true that my meter is not too great. It's not the cheapest meter Radio Shack had one bit it's no Fluke either. If it can't make accurate measurements below 1V, then the bias and DC-offset adjustments I made to my large speaker amp are probably suspect as well. hmm...

I certainly appreciate everyone's feedback.

I'm unclear on something, however. I'm still learning my theory here, so please bear with me. Because the STEPS is a single-voltage PS, does the fact that my transformer has two 18V windings (+18V and -18V)mean that it is really putting out 36V because that's the differential? It seems so now, but I previously thought I had to get that one to get 18V at the output. All my measurements seem to point towards 36V as my real voltage. The only reason, therefore that I CAN get 18V output is because of the adjustable resistance of the trimpot, right?

If that's the case I think I will use a 12V(+/-) transformer I have, since 24V is more in line with the opamp. It sounds like I'm lucky something hasn't destroyed itself yet! n_maher, my goal was indeed to power the CMoy or maybe a phono stage down the road, but I think I got screwed up on the transformer voltage.

Thanks everyone,

Collin

 

[Pars]

If your transformer is actually two sep. secondaries (0-18V x2) then the proper way to use it for this application is not in series, but with the secondaries in parallel for 18V. You are mistaken to think of this as a +18V -18V transformer, even though that is what they are most often used for. You could even just use one of the secondaries if it is high enough current for your application.

If it is a center tapped transformer (18-0-18), then you would only use 1/2 of the secondary (18-0) if you wanted 18Vac from the transformer. This would give you something like 25.4Vdc less the diode drops for the bridge rectifier (~1.4Vdc), so around 24Vdc before the regulator.

 

[tangent]

Quote:

Originally Posted by Polaris111688 /img/forum/go_quote.gif I think 820mV of ripple would destroy the opamps in the CMoy.

Erm...I really don't see how.

Op-amps will tolerate all kinds of horrible power, as long as it's not too high and the right polarity. Ripple shouldn't do anything more than pollute the output signal unless it's so bad that the polarity of the rails is swapping. With 18 V output, that means there would have to be over 9 Vp-p of ripple.

Quote:

Originally Posted by noveltone If it can't make accurate measurements below 1V, then the bias and DC-offset adjustments I made to my large speaker amp are probably suspect as well. hmm...

Maybe, but I don't mean to say that the meter's useless below 1 V. For two closely-spaced measurements where you're just trying to match one channel to another, such a meter would probably be better than nothing. Hardly ideal, but better than nothing.

Its value is most severely compromised when you're trying to make judgements on absolute measurements, like in this instance, rather than relative ones.

Quote:

my transformer has two 18V windings

If you're using one of the recommended transformers on the actual STEPS board (as opposed to DIYing something and just calling it a "STEPS") then the secondaries are in series. There's no other way to do it on the STEPS board without cutting traces and such.

This means 36 Vac under load, before rectification and filtering. The actual AC voltage is higher under light load, and rectification and filtering makes it go up even more.

My power supply parameter estimator exists so you don't get surprised by this sort of thing. Hint.

 

[noveltone]

OK.

Tangent, your power supply calculator is very useful. Not understanding that the transformer secondaries were connected in series (and what that meant) caused me to be misled by the faulty information I put into your matrix. garbage in garbage out. I am in fact building the STEPS on your PCB with no mods, in the Hammond case, with the other recommended parts, you get the idea. The transformer is an Amveco 15VA.

Pars, I think I understand what you're saying. From what I can tell the CMoy draws so little current it would probably do fine using just one of the two 18V secondaries. That leaves me lots of options, if true, and that's great. Since I'm already up to eyeballs in new info, I think I'm going to go stock on this one and swap out the transformer. This is really great, though, because I just learned a bit about transformers and why Tangent decided to make his PS the way he did. Thanks for all the info, folks.

Collin

 

[noveltone]

Hey I just wanted to conclude this thread by saying that I sucked it in and bought a Fluke 179 meter. Where I was getting 820 mV AC ripple before, unreliably, I'm now showing 10-11 mV rock solid. Also, the switch to the 12-0-12 (rather than 18-0-18 as before) transformer is making life cooler for the regulator and less noisy overall. I still need to switch out a resistor but all seems well. I'm a Fluke convert now. Thanks for everyone's help.

Collin

 

[wquiles]

Quote:

Originally Posted by noveltone /img/forum/go_quote.gif I'm a Fluke convert now. Thanks for everyone's help.

Good equipment really helps, but the advice from the experienced folks in this (and other) forums is even more helpful

Congrats on your first Fluke!