[Folsom]

I am still going to get some more panasonic capacitors for power supply. Then I need a few Black gates for the crossovers. Jantas for the power supply as filters. Then I am going to have to grab some fancy opamps. I will probably solder connection the wire in the crossovers later, at least the signal wire.

[mgalusha]

What is it? Gainclone?

[Folsom]

What is it? Gainclone?

Yup. 3875 Gainclone.

I am going to be making it a very nice one. I plan to use some Exodus RCA jacks, Cardas one twist copper binding posts, Cardas hook up wire, vampire cast copper signal wire, and no pot or any thing on the amplifier itself.

The pre-amplifier will use the same wire, a Black Velvet pot unless I can find a worth attenuator for under $50 or some thing. It will also use exodus RCA jacks. I choose Exodus because they are the only remotely affordable ones with high copper content. Also this pre-amplifier will have a power output for a Bugle for later, or what ever else runs off of 15 + or - .

I am going to most likely be building my speaker wire and RCA's out of Belden 89259, Bullet plugs, and perhaps some spades. I have been planning on doing the combo 89259 with some solid stuff RCA's with shielding over the top for that matter. Solid on the ground I determined would be best.

Also I am building a two dual socket box for plugs. I am making all my own wires out of 14 guage woven design (2 negaitve, 2 postive, 1 ground). The ground will go over the top of the ferite clamp at each end. The box has two really nice 20a copper, nickle plated sockets. Hubble and Bryant ends will be used.

Lastly I might build a Hagerman USB dac.... That will be about it for now.... Until I get a nice summer job then some nice FR125 speakers might be in order.

[mgalusha]

Cool. It will be interesting to learn how it all turns out.

An alternative to the Black Velvet pot is the TKD although it's a bit more expensive at $61.95 (Parts Connexion). They are very nice and have excellent tracking. I've built several preamps using these and they are pretty close (IMO) to a stepped attenuator.

[Folsom]

I am a bit worried my transformer may not work...

You need two channels for sure right? I know you can use one recifier board.

The transformer has output of X2-----24-----X1 . The 24 is the center tap I suppose, and they are connected togather via a bridge on the hookup part between the two terminals.

If this does not work I am going to be a little upset because my alternatives, as I understand it, will be get another or buy a freaking expensive torodial that likes to make vibrations.

[mgalusha]

The center tap is 1/2 the voltage across the secondary. To use it with a bridge rectifier and provide a bipolar output, you ground the center tap and connect the secondary leads to the AC connections of the bridge. You will then have a + and - relative to ground. Of course you'll still need filter caps and all the rest.

If your transformer is rated for 24V, center tapped then it's normally noted as 12-0-12 with 0 being the center tap.

After rectification this will give you about 17V on each rail. Maybe a little more depending on the regulation of your transformer and load. Hope this helps.

[mgalusha]

BTW, one of the boards in your photos looks like a Marchand PS10 power supply. It's wired in the fashion described above.

[Gordy]

I've used 5 Avel transformers and haven't had a vibration/hum problem with any, but have certainly read of problems...

Dan Wright has a novel (to me anyway) mounting system for his toroidals in the MW-9 preamps.  He uses a swivel rubber gasketed(?) post so that the tranny is suspended and doesn't contact the chassis or pass vibrations to the tubes.  I was athunk'n that you could use two single holed rubber stoppers to form an axle at the hole of the transformer, then mount the stoppers to the chassis with a bolt through the stopper holes.  The transformer is now suspended and doesn't contact either the bolt or chassis. Rather drastic, but should work if your concerned about vibration...

[Folsom]

The center tap is 1/2 the voltage across the secondary. To use it with a bridge rectifier and provide a bipolar output, you ground the center tap and connect the secondary leads to the AC connections of the bridge. You will then have a + and - relative to ground. Of course you'll still need filter caps and all the rest.

If your transformer is rated for 24V, center tapped then it's normally noted as 12-0-12 with 0 being the center tap.

After rectification this will give you about 17V on each rail. Ma ...

In other words I have the completely wrong transformer.

[randytsuch]

Do you have a meter? (you should, if you are going to DIY)

If you do, you can check the AC voltage output of the transformer.
If the transformer puts out two close to equal voltages between the three wires, you should be able to use it, but you need to see what the AC voltage is.

Multiply the AC volts by 1.4 to calculate what the DC volts will be.

The DC needs to be in the voltage range allowed for a 3875, which is between 20 and 84 VDC.  Most people like to run their GC's are lower voltages, closer to 20 VDC, but you get less power at lower voltage rails.

Datasheet is here
http://www.national.com/ds/LM/LM3875.pdf

Good luck.

Randy

[Folsom]

Do you have a meter? (you should, if you are going to DIY)

If you do, you can check the AC voltage output of the transformer.
If the transformer puts out two close to equal voltages between the three wires, you should be able to use it, but you need to see what the AC voltage is.

Multiply the AC volts by 1.4 to calculate what the DC volts will be.

The DC needs to be in the voltage range allowed for a 3875, which is between 20 and 84 VDC.  Most people like to run their GC's are lower voltages, clos ...

Yeah I need a multimeter or some thing.... I know how to use one, that is not hard at all. I understand transformers for the most part, but this one is not labeled in such a way, as I learned.

I am going to be upset if it is only 12v per channel. Assuming the center tap can be split via removal of the connector.

At 24-28v I am just fine for 8ohm or 4ohm speakers so I will have decent output.

I was hoping I could get this sorted out with out a multimeter!

[randytsuch]

If you are going to keep at DIY, then you really need a decent DVM, so I would just break down and get one. (easy for me to say   )

I always put the PS together, then check it's voltage output before I hook it up.

The other thing with a chipamp is that you should check the dc offset out of the chip's output before you connect it to your speakers.

Randy

[rez]

Randy,
What do you use as a load for the secondaries when you check their voltage.  I'm pretty new to this but will have to be making just such measurements soon.
Thanks
Roman

[JoshK]

Randy's suggestions are good ones.

Roman,
Check the voltages with it unloaded and you can check it with a resistor across (higher watt one) and it will give you an idea of the regulation.  But open is close enough to get the picture.

[randytsuch]

Randy's suggestions are good ones.

Roman,
Check the voltages with it unloaded and you can check it with a resistor across (higher watt one) and it will give you an idea of the regulation.  But open is close enough to get the picture.

Thanks Josh,

Roman, Josh just said what I would have, just measure it unloaded.  It will drop a little when you load it, how much depends on the transformer rating.  But, I just check the ps voltage with no load, just to get an idea of what the voltage will be.  With the transformers I use, I don't see much voltage drop under load

Randy

[rez]

Josh and Randy,
Thanks for the replies!  I hope to get my SKA underway soon and, albeit in a different amp, DOS's concerns about getting the voltages and connections right at the trafo and ps echo'd my own.  Using a 300VA Plitron with 35V secondaries.
Roman

[Folsom]

Yeah I got the wrong transformer..... It is useless to me. Even if I could fake two channels I will only get 12v.....

[Folsom]

Cool. It will be interesting to learn how it all turns out.

An alternative to the Black Velvet pot is the TKD although it's a bit more expensive at $61.95 (Parts Connexion). They are very nice and have excellent tracking. I've built several preamps using these and they are pretty close (IMO) to a stepped attenuator.

What about Marchands 24-step attenuator... $75 for the kit. Expensive to me, but not that much more than $61.95 from Canada.