[tourtelot]

I am getting ready to build a PPA and I am building the TREAD PS first. I have access to lots of nice regulated wall-wart DC supplies mostly in the 12VDC range but some up to about 20VDC. It will be a VERY stock PPA with the AD8610 chips biased into Class A as recommended.

Would it be better to use a smaller supply, maybe 17VDC, regulate it to 15VDC and use 16V caps or to use the 20VDC supply, regulate it to 18VDC and use the 25V caps?

Or to not be cheap and go buy a regulated supply capable of giving me 24VDC into the PPA?

Thanks.

D.

 

[rds]

The TREAD can give you up to 32V DC using a 24 VAC wallwart. Why would you use a regulated power supply in front of it?

The AD8610 can only handle up to 26V DC. A supply voltage somewhere close to that limit would be a good idea (ie 24V).

 

[tourtelot]

I tried Tangent's estimator and I don't see a way to get more out than I put in. It always returns the error "no regulation. . ." Certainly new enough at the math part to be doing something stupid, but. . . .

D.

 

[tourtelot]

I am trying to spec a primary power supply for my TREAD. As per the above discussion, now I am really confused. Would someone be kind enough and suggest a nice supply (AC or DC) for running the TREAD to a good voltage to run my (yet to be built) PPAv.2? Thanks.

D.

PS. If you feel like helping lessening my confusion over Tangent's estimator values and what the above poster said about getting 32VDC out of a TREAD using 24VAC, that would be even better.

 

[tangent]

The main thing to worry about when reusing wall warts out of the junk bin with a virtual ground based amp like the PPA is to make sure they're isolated. This is covered in the PPA parts selection guide.

As for your confusion about not seeing how to get more DC voltage out of a TREAD than you put in with AC is that you're probably changing the first setting at the top of the calculator from "raw AC" to "DC wallwart". The first setting is suitable both for naked transformers and AC-AC wall warts. The second is only for unregulated DC-output wall warts, in which case there is much less "free" voltage, that being only that due to the voltage rise of a transformer not under full load. The third setting, for regulated wall warts, gives you zero "free" voltage, it having been fully regulated out already.

 

[tourtelot]

First off, thanks for the reply. But. . . .

Sorry that I am being so dense, but I still get "no regulation" if I try and put less AC volts in than the DC volts out. Well, within +/- a volt or so. To get a 24VDC out to the PPA, I seemingly need to find an AC/AC wall-wart with a weird output voltage. A common 24VDC (nominal) AC supply gives me the "the voltage drop across the regulator is more than needed to maintain good regulation; choose a more appropriate transformer, or raise the regulator’s output voltage by at least 0.5 V to reduce the drop across the regulator" error for 24VDC out to the PPA. If I am using 25V caps in the PPA, it would seem to be best to leave the Vin at 24VDC. An additional +.5VDC in to the PPA is getting pretty dicey, right?

So obviously, I am unclear about the concept here; I can't imagine finding a 28VAC transformer, right?

On the other hand, the 20VDC power-supply is from a compter. Lots of current, probably pretty clean DC output, 20.7VDC unloaded. Seeming perfect for a 15VDC in to the PPA. Two-wire AC in. It just means a capacitor swap for 16V caps. Will I hear the difference with the spec'df op amps, either the BB or the AD?

Just trying to tie the theory and the practice together here.

Thanks.

D.

Hopefully this doesn't read "stoopid" like the "What mic should I use" questions that I see all the time in my other forums<bg>.

 

[tourtelot]

Ahhh. Maybe a typival AC/AC tranformer's unloaded voltage is MUCH higher than I would have guessed. In the example, I see a 34VAC unloaded voltage for a 24VAC output. Is the unloaded spec on a data sheet? Or do I buy and measure?

D.

 

[rds]

The 34V is the rectified voltage, it is not AC. The rectifier makes the voltage more or less DC. To find the retified voltage use: (transformer output)*sqrt(2). In this case that's 24*sqrt(2) = 34V

 

[tourtelot]

After the TREAD rectifier? So find the AC value for the transformer, work backwards from needed 24VDC into the PPA?

D.

 

[rds]

Yeah, when you're choosing the transformer you should start with the DC output voltage you want and then figure out what VAC you need.

Remember although you have 34V rectified the regulator will drop a minimum of about 2V. So your maximum output voltage with a 24VAC wallwart will be about 32V.

 

[tourtelot]

So, a 15VAC wall-wart would be 21.2VAC "unloaded" which would allow 20VDC out of the TREAD into the PPA. Did I do that right?

D.

 

[guzzler]

Quote:

Originally Posted by tourtelot /img/forum/go_quote.gif So, a 15VAC wall-wart would be 21.2VAC "unloaded" which would allow 20VDC out of the TREAD into the PPA. Did I do that right? D.

No and no. The 15VAC from the AC adaptor is 15V RMS (wikipedia page). The peak-to-peak voltage is 21.2V. So, when you rectify this you get 21.2V (also you need to subtract 1.4V for the drop across the rectifier diodes). The LM317 requires Vin to be at least 1.25V higher than Vout, so the highest you'd probably get out of this setup would be 18V (21.2-1.4-1.25-[a bit more for safety, don't forget ripple]).

 

[tourtelot]

>rds The 34V is the rectified voltage, it is not AC. The rectifier makes the >voltage more or less DC. To find the retified voltage use: (transformer >output)*sqrt(2). In this case that's 24*sqrt(2) = 34V

Tangent's Power Supply Parameter for the TREAD says that a nominal 15VAC, unloaded at 21.2VAC will be able to deliver 21VDC to the PPA. Ripple of around half a volt. "Sane" it says.

Doesn't that conflict with the statement above? Maybe I am just "not getting it" but I guess that the end result is that I am looking for a AC supply to use with a TREAD PS to run my PPA at around 20-24VDC. I plan on using 25V caps.

 

[rds]

Quote:

Originally Posted by tourtelot /img/forum/go_quote.gif Tangent's Power Supply Parameter for the TREAD says that a nominal 15VAC, unloaded at 21.2VAC will be able to deliver 21VDC to the PPA.

Can you link us to where it says that? Because that doesn't make much sense to me. You'll get about 21V rectified, but then you need to regulate that, so it's going to be under 20V for sure when it leaves the TREAD.

 

[tomb]

Quote:

Originally Posted by tourtelot /img/forum/go_quote.gif >rds The 34V is the rectified voltage, it is not AC. The rectifier makes the >voltage more or less DC. To find the retified voltage use: (transformer >output)*sqrt(2). In this case that's 24*sqrt(2) = 34V Tangent's Power Supply Parameter for the TREAD says that a nominal 15VAC, unloaded at 21.2VAC will be able to deliver 21VDC to the PPA. Ripple of around half a volt. "Sane" it says. Doesn't that conflict with the statement above? Maybe I am just "not getting it" but I guess that the end result is that I am looking for a AC supply to use with a TREAD PS to run my PPA at around 20-24VDC. I plan on using 25V caps.

You need to burn 5V at the LM317 to gain the highest ripple rejection. The filter caps and other features of the TREAD design lets you push that to about 3V or so, but 5 is still better. That's what rds is referring to - you need to burn Volts through the LM317 (can you say, "heat sink?") in order to get the optimum regulation and ripple rejection.

Just a warning: Tangent's calculator may say "sane" (assuming you've input correctly), but half a volt of ripple is intolerable for a quality headphone amp power supply. You'll probably even hear a hum, but even if you don't, a direct comparison with an amp powered by something under 1mV ripple will be night and day in sound quality - faster, tighter, more detail, etc., etc. I could be wrong, but I suspect you may be interpreting that "half a volt of ripple" incorrectly.