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PostPosted: 14 May 2011, 07:23 
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Joined: 15 Jan 2011, 10:53
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Location: Darmstadt, Germany
FenderBender wrote:
Somtimes I don't like the test circuits from the datasheets. Like on some linear regulators, they suggest like a 1uF filter cap....Now, I'm no electrical engineer, but I think 1uF might be a tad small. I use like 5600uF, just sayin. It seriously only costs like $0.75 more anyway. I guess if you were mass producing, but still...

If your working with linear regulators keep in mind to use the right type of capacitor. Actually the type of capacitor can be more important than the mere value. So if the datasheet tells you to use a 1uF tantalum cap (like for the LM317), go with it. You can still use a larger one, but stay with tantalums or you have to use considerably larger aluminium electrolytic types (at least 25uF for the LM317). Even with the aluminium electrolytic capacitors the performance of your circuit can depend on the series resistance of the device.
5600uF seems a little to much to me, but that's just me.
FenderBender wrote:
I'll integrate a Tillman preamp right onto the amp board, but I'll put the PSU close to the AC recepticle to reduce any AC noise from getting into the amp circuit. I'm thinking maybe 2x 4700uF 25V and a simple bridge. 12VAC transformer. Should it be regulated at 15V perhaps? Since the power draw of a TDA2003 shouldn't exceed 1A, perhaps I could slap a low dropout regulator on there. No?

Using a toroidal or shielded transformer can reduce power supply noise quite a bit. A shielded and separate enclosure for the power supply is also helpful. Don't forget to ground the case.
If the size of the power supply enclosure is not important to you, try to use capacitors with a higher rating. Switching the transformer on and off can produce large transient voltages, killing your 25V electrolytics.
Using a 12V transformer for 15V regulated output can cause you some trouble. The voltage from the transformer drops as the current rises and might put your regulator out of regulation.

Hope I could help.


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PostPosted: 14 May 2011, 12:55 
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Joined: 18 Sep 2010, 18:43
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I'd use a regulator with a dropout of about 0.5V. So if I had roughly 17VDC from a 12VAC transformer, shouldn't that be enough? Perhaps not. I'll have to look it over again.

I used Panasonic FC on my power supplies.

Yes, I'd like to use toroidal, but they all seem quite expensive even on the low power. A 2A toroid @ 12V is like $25, which is a bit too much for me. Perhaps if I was trying to build a super HiFi amp, but for a simple guitar amp, I'm not sure exactly how necessary that would be. But you know better than I do.


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PostPosted: 14 May 2011, 15:21 
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Joined: 09 May 2011, 20:48
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Sheesh guys. I'm sorry. I made a different account because I kept on getting incorrect password no matter how hard I tried and apparently it saved my info on my other computer. Should have made that clear. Anyway, I'm not sure which account is even working half of the time.


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PostPosted: 14 May 2011, 15:26 
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Joined: 18 Sep 2010, 18:43
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Just consider this closed before I confuse anyone else.


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PostPosted: 15 May 2011, 05:01 
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Joined: 15 Jan 2011, 10:53
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Location: Darmstadt, Germany
TheLaw wrote:
I'd use a regulator with a dropout of about 0.5V. So if I had roughly 17VDC from a 12VAC transformer, shouldn't that be enough? Perhaps not. I'll have to look it over again.

I used Panasonic FC on my power supplies.

Yes, I'd like to use toroidal, but they all seem quite expensive even on the low power. A 2A toroid @ 12V is like $25, which is a bit too much for me. Perhaps if I was trying to build a super HiFi amp, but for a simple guitar amp, I'm not sure exactly how necessary that would be. But you know better than I do.

Did you measure 17VDC under load or free running? To be completely sure, try to measure the transformers output voltage while connected to a load. Of course, the load should be as high as you estimated for your whole circuit to get realistic values. Also include the rectifier bridge and filter capacitors in this setup. If you have access to a scope have a look at the output waveform. The output ripple after the filter capacitor might still be too much to keep the regulator in regulation.

Panasonic FCs are great capacitors.

If the toroidal transformers are too expensive you can always get similar performance with shielding: use a separate, shielded (= metal) enclosure for the power supply, put a bare PCB between the regulator circuitry and the transformer, and connect both to ground.


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PostPosted: 16 May 2011, 08:14 
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Joined: 08 May 2009, 08:20
Posts: 918
Location: Duesseldorf, Germany
Hi guys ;-)

Caps: The Wima caps are very accurate in my opinion. I use them where i can. On tube amps, i like to use the Orange Drops,
because they seem to be a bit softer sounding (spelling?!?). I built an EL84SE with Wimas and the Drops. The Drops really had
a little Vintage touch (dunno why and i can not validate this).

Power supply: With a regulator, somewhat about 4700uF and 1000uF rated at 35V surely would be enough. I think there's
no need for huge filtering if you plan a regulated supply. The ICs usually provide a good ripple rejection by them self.
A 1Ampere supply seems a little bit low. The chip can handle up to 3.5A thus i'd rather go for a 2A supply regulator (78S15),
because the amp should distort soft and not go into clipping early. OK - must be tested first.
Transformer, filter cap, bleeder, regulator, diode, noise cap, buffer cap - ready to go...

The components are ordered and they arrive mid of this week. I think i can start with it on Friday..

Have fun ;-).


PS - Danke für die Hilfe, Ferdinand!

_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 16 May 2011, 11:11 
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Joined: 09 May 2011, 20:48
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Where is the noise cap get placed in the power supply, and where does the buffer cap go? The bleeder is right after the main filter caps? My PSUs are usually rather primitive. If you find the time, would you care to do a really quick schematic? (Nothing fancy) It seems that you actually know what you are doing.


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PostPosted: 17 May 2011, 12:19 
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Joined: 15 Jan 2011, 10:53
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Location: Darmstadt, Germany
FenderBender wrote:
Where is the noise cap get placed in the power supply, and where does the buffer cap go? The bleeder is right after the main filter caps? My PSUs are usually rather primitive. If you find the time, would you care to do a really quick schematic? (Nothing fancy) It seems that you actually know what you are doing.


In the datasheet for the LM350 you will find a pretty nice schematic on page 6 (http://www.national.com/ds/LM/LM150.pdf). Just follow the application hints on pages 5 to 6 to get a good explanation of the schematic and build your circuit accordingly. You will have to include a heatsink for the regulator to work properly.
The LM350 is a close cousin of the LM317 and therefore provides nice regulation and low noise. If you use separate enclosures for the power supply and amp include a bypass capacitor at both ends of the connection between them.

I would not use excessive output capacitance with a voltage regulator, as the stored charge can deliver very high currents under fault conditions and therefore destroy the regulator IC. To attenuate high frequency noise in the power supply rails, add a 100nF ceramic capacitor in parallel to your large reservoir electrolytic cap.

If cost is a concern for you try to use classic ICs like the LM317 and LM350, as they are usually cheaper than more modern, low dropout regulators. This, of course, only applies if the higher power losses and the higher voltage differential needed are not important.

Hope I could help.


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PostPosted: 19 May 2011, 18:45 
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Joined: 09 May 2011, 20:48
Posts: 35
The latest PSU I built was purely from some old FCs/films I had laying around.

Fake schematic of it:

2A Diode Bridge -> 5600uF -> 100nF MKP4 -> 5V Reg -> 4700uF + 4700uF

Stupid?

I don't know. Just playing around.


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PostPosted: 21 May 2011, 16:12 
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Joined: 08 May 2009, 08:20
Posts: 918
Location: Duesseldorf, Germany
It is really nothing special. A very basic circuit. Usually a film / foil capacitor (C3) is mounted right after the output to catch up some
noise and transients. C4 (buffer cap) acts like a buffer for the load. It provides some extra current (if needed) and
it helps to stabilize the regulation. D1 protects the regulator against overvoltage at the output. An LED or bleeder
drops the voltage after the circuit is switched off. The 78xx regulators don't like any voltage higher than the input voltage.
Attachment:
15V_reg_sup.GIF
Using a high capacitance at the regulator output actually isn't a good idea.
The charge from the output capacitors may destroy the chip. That's why a protection diode is used.

I've started with the amp circuit a few days ago. Some fine tuning must be done now, but it works fine. It's also a fun project ;-).
I would not suggest to use a pre amplifier. The TDA2003 is designed to operate with an input voltage of about 50mV.
The pickups can deliver up to 2volts, so i am using a buffer and some gimmicks for the sound and a volume control...
Which kind of sound would you like to have? More clean or with an option from clean to overdrive / distorted / fuzzy?

Have fun!


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_________________
Cheers,
Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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