GC Preamp Suggestions

[Gcollier]

I am looking to build a multichannel preamp for a 5+ channel gainclone, this will be for home theater use. My plan so far is as follows:

Source Selector: switch ripped from an old parallel port switch (easily modified...plenty of channels!)
Digital pot for each channel: DS1802
Input Buffer: BUF634
Gainstage: OPA637

this is similar in design to this post: http://www.diyaudio.com/forums/showt...061#post197061

I have used the DS1802 before and am quite satisfied with the sound, and also with the ease with which it can be implemented.
I have not worked with input buffers before but figure this would make a good first gain stage to feed into the OPA637. I have never used OPA637 before either but I have done plenty with OPA134.

This will most likely be feeding 5+ channels of parallel LM4780.

Anyone who wants to help with the this design please feel free....unless you are going to suggest tubes

Thanks

[TwoSpoons]

I'm currently building something very similar. I'm doing differential inputs with the INA136, volume control with the PGA2311, and diff output to drive bridged 4780s with OPA1632. Input selection is with relays, using the double pole trick to eliminate crosstalk.

The whole thing is going to be run by a Cygnal C8051F320 microcontroller (because thats what I'm using at work). I'm putting a 16x2 LCD on the front for input / volume indication. Channel by channel trim, for each input, stored in Flash on the micro. Might also try my parental lock idea - a PIN code limits the volume to a settable level. Just so teenagers can't have loud parties while you're out!

[Gcollier]

Originally posted by TwoSpoons
I'm currently building something very similar. I'm doing differential inputs with the INA136, volume control with the PGA2311, and diff output to drive bridged 4780s with OPA1632.

Wow! Sounds like one heck of a project! I was also looking at PGA2311 but since I have no experience with PIC programming I am relegated to DS1802...which really isn't that bad! I must admit the idea of a PIN to lock out the volume control is interesting...I've got 4 year old triplets...imagine the wild parties when they are 16...yikes Anyway if you have any pics/schematics of your project , or suggestions for caps/resistors etc to use in the signal path I'd be interested. I'm trying to keep the price point down, but still wan't reasonable quality.

[steenoe]

I did quickly put together a preamp like CarlosFM's in the above mentioned thread. I was amazed with the sound That was surely a very nice experience! It was a bit too analytic, for my taste though. Everything is so clearly cut, that I have never heard anything like it. One advice, take CarlosFM's saying about the OPA's likes capacitance seriously. I had 100uF right at the pins of the OPA's. I do not think tube guys find this one very musical though! This is not based on long time listening, just based on comparison with my normal setup. (BosoZ)
Have a look at my perfboard.

Steen.

[steenoe]

This is the PSU feeding the circuit above.
It was all thrown together because I wanted to test CarolsFM's ideas! I have to admit it was a great thing

Steen.

[steenoe]

I wanted to test CarolsFM's ideas!

Hate to quote myself, BUT this is actually Pavel Macura's circuit.
Sorry Pavel. Known as PMA on this forum. But CarlosFM did bring it to our attention, on a bigger scale Hope none is offended

All the best
Steen.

[Gcollier]

Originally posted by steenoe

Hate to quote myself, BUT this is actually Pavel Macura's circuit.
Sorry Pavel. Known as PMA on this forum. But CarlosFM did bring it to our attention, on a bigger scale Hope none is offended

All the best
Steen.

I'll have to do a search on this circut. I'm not sure what to think about this circut sounding so "precise" I guess that could be a good thing since this is intended to be a home theater preamp. I actually used a CMOY headphone amp based on an OPA134 to drive an LM3875 gainclone and found that something "magical" happened with the sound...of course it could have just been the fact that I was bypassing the dreadful opamp in the portable CD player I was using as a source.

[scone]

I have not worked with input buffers before but figure this would make a good first gain stage to feed into the OPA637

Is there any reason to do this instead of using the OPA637 to feed the buffer? I am trying to understand preamp design since I'm planning on building one of my own. I typically see the 637 feeding a buf634, which acts as a current amplifier I think. I figure this is because the 637 has low output current? What would be the advantage of putting the buffer first?

[skyraider]

I think the magic here is the result of the precision of 627 and current capability of 634.

I build my preamp according to carlos recommendations, and yes, this is good. The bypass is important to make sure no oscillation happens, which practicaly ruins a good opamp performance. Although overall I prefer tube preamp, this is the best SS preamp there is.

neat work, steenoe!

[steenoe]

Here is the link to the original circuit:
http://www.pha.inecnet.cz/macura/buffer_en.html
I have bought two of Pavel's really cute Pcb's, and will actually build one to keep for myself. I intend to use as much of CarlosFM's
recommendations as possible on the small boards. One will be used as Linestage the other for a headphone amp. I am planning on making a RIAA also, so that this will end up as a complete mini preamp.

Steen.