Amplifier virgin here...seeking input.

colonelhogan · 21st December 2009, 02:41 AM

Hello All,

A little introduction first. I Have been lurking for a while, reading and learning, finally signed up and now am about to undertake my first audio build ever -- a gainclone using double lm3875 chips. I am a Mechanical engineering student, so I have a good understanding of circuits and such, but lack a lot of the theoretical meat behind the inner workings of components.

with that said, I am in need of a transformer (s) to power my project. I am assuming that a 24 volts RMS (in the secondaries) is a good value, as it will (theoretically) give you 24*sqrt(2) = just under 34 volts, minus a bit for losses.
I need to know a good place to get a transformer...I have been looking for old trashed stereos and such that have beefy power supplies in them, but have found nothing as of yet. I need to know a good place to buy one for as little money as possible (college student here

), if I fail in finding one free.

I have gotten almost everything else I need from ripping apart old monitors, printers, ect...but powerful transformers are hard to come by.

any input would be appreciated, maybe you guys know some secret places to check for old electronics to mine parts from.

Thanks guys!

star882 · 21st December 2009, 02:19 PM

Find an old PC power supply and rework the output circuit to give you 24v.

chingyg · 21st December 2009, 03:21 PM

Yes reworking old computer supply funded me a few projects. See Low voltage dual supply from old computer PSU for my way of doing it.

dave_gerecke · 21st December 2009, 03:42 PM

I would agree with the reworking of the PC power supply, but he will colonel will need two of them, and be able to ground the positive of one output and negative of the other, since the LM3875 is designed for a bipolar supply. Alternatively you can make some changes to the circuit to allow it to run off a single ended supply, but I don't remember the details. It should be in the datasheet of the LM3875. I would suspect that if a single ended supply is going to be used, then the output voltage of the power supply would need to be higher than 34VDC.

Peace,

Dave

P.S. where are you located colonelhogan. If you are in the USA, and are willing to pay for shipping, I may be able to scrounge up a suitable transformer in my collection of old receivers, etc. You could also check out Apex Jr.Home Page. I haven't bought from Steve, but he seems to have a good reputation with the other members here.

chingyg · 21st December 2009, 03:52 PM

you dont need 2 power supplies, you can use the +12 and -12 line of 1 power supply, just need to beef up the -12V line a little.

dave_gerecke · 21st December 2009, 04:14 PM

Then you won't be getting full output capablities of the amplifer chip. That is if you are using the PC supply as designed with +/-12v out on each of the 12v channels. There also has to be consideration for the current demand of the chip. While a PC supply may be rated for 300+ watts, most of this goes to the +5v or +3.3v channels due to the demand that is imposed by a computer. The +/-12v rails are provided mostly for communications signals, and subsequently have a lower current capablility.

Peace,

Dave

chingyg · 21st December 2009, 04:18 PM

oh yeah thats right to, but still computer psu is a lot cheaper and well beginner friendly.

colonelhogan · 21st December 2009, 04:19 PM

I was looking at these units from Skycraft Surplus:
24 vac Transformer 4 amp

4 amp, 24 volt transformer. two of them would give me the +/- 35 volts I would like to have. Is 8 Amps (192 watts) enough? I would think it would be, the lm3875 is a 56 watt amplifier, but it probably dissipates around 100 watts at full power (just a guess), but I'll rarely run it at full power.

dave_gerecke · 21st December 2009, 04:32 PM

Hello again, I agree with chingyg and star882 about the price difference, especially because you can find old PC supplies for free if you look around a little.
I did a little quick calculations; if colonelhogan is using an 8 ohm speaker and is looking to get the 56w continuous out of it, he will need around 22 VDC @ around 3 amps per LM3875. The 22vdc would be for each rail, so +/-22vdc rails.

Peace,

Dave

Minion · 21st December 2009, 05:12 PM

Colonelhogan , That transformer won"t work , not unless you get 2 of them , You need a Transformer that has 2 x 24v secondaries or one that is 48v CT a single 24v secondary won"t do it for you ..... Check out Apex JR in the Vendors section , I think he has a suitable transformer in a $20 range ....

Cheers

21st December 2009, 02:41 AM	#1
colonelhogan diyAudio Member Join Date: Dec 2009	Amplifier virgin here...seeking input. Hello All, A little introduction first. I Have been lurking for a while, reading and learning, finally signed up and now am about to undertake my first audio build ever -- a gainclone using double lm3875 chips. I am a Mechanical engineering student, so I have a good understanding of circuits and such, but lack a lot of the theoretical meat behind the inner workings of components. with that said, I am in need of a transformer (s) to power my project. I am assuming that a 24 volts RMS (in the secondaries) is a good value, as it will (theoretically) give you 24*sqrt(2) = just under 34 volts, minus a bit for losses. I need to know a good place to get a transformer...I have been looking for old trashed stereos and such that have beefy power supplies in them, but have found nothing as of yet. I need to know a good place to buy one for as little money as possible (college student here ), if I fail in finding one free. I have gotten almost everything else I need from ripping apart old monitors, printers, ect...but powerful transformers are hard to come by. any input would be appreciated, maybe you guys know some secret places to check for old electronics to mine parts from. Thanks guys!

21st December 2009, 02:19 PM	#2
star882 diyAudio Member Join Date: Mar 2007	Find an old PC power supply and rework the output circuit to give you 24v. __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!

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21st December 2009, 03:21 PM	#3
chingyg diyAudio Member Join Date: Sep 2009 Location: South Africa	Yes reworking old computer supply funded me a few projects. See Low voltage dual supply from old computer PSU for my way of doing it.

21st December 2009, 03:42 PM	#4
dave_gerecke diyAudio Member Join Date: Dec 2007 Location: Vermont	I would agree with the reworking of the PC power supply, but he will colonel will need two of them, and be able to ground the positive of one output and negative of the other, since the LM3875 is designed for a bipolar supply. Alternatively you can make some changes to the circuit to allow it to run off a single ended supply, but I don't remember the details. It should be in the datasheet of the LM3875. I would suspect that if a single ended supply is going to be used, then the output voltage of the power supply would need to be higher than 34VDC. Peace, Dave P.S. where are you located colonelhogan. If you are in the USA, and are willing to pay for shipping, I may be able to scrounge up a suitable transformer in my collection of old receivers, etc. You could also check out Apex Jr.Home Page. I haven't bought from Steve, but he seems to have a good reputation with the other members here.

21st December 2009, 03:52 PM	#5
chingyg diyAudio Member Join Date: Sep 2009 Location: South Africa	you dont need 2 power supplies, you can use the +12 and -12 line of 1 power supply, just need to beef up the -12V line a little.

21st December 2009, 04:14 PM	#6
dave_gerecke diyAudio Member Join Date: Dec 2007 Location: Vermont	Then you won't be getting full output capablities of the amplifer chip. That is if you are using the PC supply as designed with +/-12v out on each of the 12v channels. There also has to be consideration for the current demand of the chip. While a PC supply may be rated for 300+ watts, most of this goes to the +5v or +3.3v channels due to the demand that is imposed by a computer. The +/-12v rails are provided mostly for communications signals, and subsequently have a lower current capablility. Peace, Dave

21st December 2009, 04:18 PM	#7
chingyg diyAudio Member Join Date: Sep 2009 Location: South Africa	oh yeah thats right to, but still computer psu is a lot cheaper and well beginner friendly.

21st December 2009, 04:19 PM	#8
colonelhogan diyAudio Member Join Date: Dec 2009	I was looking at these units from Skycraft Surplus: 24 vac Transformer 4 amp 4 amp, 24 volt transformer. two of them would give me the +/- 35 volts I would like to have. Is 8 Amps (192 watts) enough? I would think it would be, the lm3875 is a 56 watt amplifier, but it probably dissipates around 100 watts at full power (just a guess), but I'll rarely run it at full power.

21st December 2009, 04:32 PM	#9
dave_gerecke diyAudio Member Join Date: Dec 2007 Location: Vermont	Hello again, I agree with chingyg and star882 about the price difference, especially because you can find old PC supplies for free if you look around a little. I did a little quick calculations; if colonelhogan is using an 8 ohm speaker and is looking to get the 56w continuous out of it, he will need around 22 VDC @ around 3 amps per LM3875. The 22vdc would be for each rail, so +/-22vdc rails. Peace, Dave

21st December 2009, 05:12 PM	#10
Minion diyAudio Member Join Date: May 2006	Colonelhogan , That transformer won"t work , not unless you get 2 of them , You need a Transformer that has 2 x 24v secondaries or one that is 48v CT a single 24v secondary won"t do it for you ..... Check out Apex JR in the Vendors section , I think he has a suitable transformer in a $20 range .... Cheers

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