[sprocket87]

Hey all,
 
I'm working on my first amp design. I want to do a combination RIAA-equalized phono preamp and headphone amp. I want to be able to listen to my turntable in a discreet area without having a large stereo receiver around.
 
The goals for my project are:
 
- Small, portable enclosure
- Able to run off wall power or batteries
- Built in battery charger
- Flexibility for future modifications
 
My basic design is a combination of this mini phono preamp: http://www.diyaudio.com/forums/analog-line-level/79545-mint-tin-phono-amplifier.html
 
With Tangent's CMoy: http://tangentsoft.net/audio/cmoy-tutorial/
 
I'm using several other Tangent circuit designs, including his virtual ground channel PSU and battery trickle charger.
 
I've got hand-drawn schematics for most of it, but for now I just want to share my power supply schematic and see if I'm on the right track. I'm VERY new to electronics but I'm trying to take the design process slow to make sure I understand what everything does and why. My dad is an electronics whiz and has given me a lot of great info and guidance so far, and I'm trying to apply that to my first solo build.
 
If you have time to look this over and give me some feedback that'd be great! 
 
Power Supply for combo amp
Full size: http://i.imgur.com/QTrul.png
 

 
I'll post the actual amp schematic at some point soon... 
Thanks for looking,
 
Jesse

 

[nikongod]

This looks like a fun project!
 
I would advise against the whole "one box" thing, particularly since you said you wanted upgrade flexibility. Although its possible to build a one box like you want upgrading it is generally the worlds biggest PITA. Additionally you cant sell off the old bits to fund the new one (without re-casing them and sinking more money and effort into something you may just want sold at that time - which makes working on it many times more frustrating than working on something you are interested in while your working on it.)
 
With that out of the way, it should not be too dificult to build what you are looking for in 2 small boxes. You should be able to find enclosures that fit UNDER the TT fairly easily. I think its easy to justify a big amp that fits under the TT this way. Perhaps a PPA in a short enclosure (or put the TT on small riser blocks) for example.
 
If your sticking with the Cmoy, go 1 step up and build a CHA-47.

 

[sprocket87]

 
Thanks for the reply! What you said about splitting the boxes makes perfect sense -- but I still don't wanna
 
I can see the wisdom of it but I'll probably reserve that for later builds. I have a "vision" for this one, for the enclosure, some frills, etc. I really would like it to be one combined kit. And I think it is good for me to design it this way as I've got to think about the design more than if I was just soldering someone else's schematic part-for-part. I have to know how to tie the circuits together, and to power them appropriately, etc.
 
Besides, the two major circuits are a VERY basic phono preamp and a VERY basic headphone amp. I don't foresee trying to sell them off. If nothing else this will remain a keepsake of my very first build
 
The "flexibility" part was more aimed toward being able to add on frills later, like VU meter/lights, perhaps EQ, etc. 
 
I'm not familiar with the CHA-47, I'll check it out. I was originally planning to do the Pimeta but it looked a little more complex than I wanted to bite off quite yet.
 
Care to look over my PSU schematic and see if there is anything obviously wrong with it?

 

[porthillsbomber]

Hi, I can't see the point in the first regulator other than spreading some of the heat dissipation for the 317. Also I'm not really sure how powering a circuit off a battery being charged by a constant current would perform. I think you need another diode to come from D1 to Vin to the rhs of D2 to power the circuit when the battery is charging and if you leave in the first regulator you might as well just use the circuit in the AMB mini^3.
 
The batteries are NiMH correct? (6 cell)

 

[sprocket87]

Quote:

Hi, I can't see the point in the first regulator other than spreading some of the heat dissipation for the 317. Also I'm not really sure how powering a circuit off a battery being charged by a constant current would perform. I think you need another diode to come from D1 to Vin to the rhs of D2 to power the circuit when the battery is charging and if you leave in the first regulator you might as well just use the circuit in the AMB mini^3.
 
The batteries are NiMH correct? (6 cell)

Thanks for the feedback. I'll answer your Q's as best I can:
 
- The reason I put the LM7818 in place was to act as a linear regulator since whatever 24V wall transformer I end up using will likely be a cheap unregulated one. This was based off some info on Tangent's site (source). If you're saying this is unnecessary and redundant, awesome! Will the LM317 regulate the same way?
- The battery trickle charger was taken from Tangent's Pimeta Tweaks page (about halfway down under "NiMH Trickle Charger"). I drew my schematic the same way he drew his schematic (see pg 2, power supply). Are you saying this layout will not simultaneously charge and power the circuit? I assumed it would since (1) Tangent said so 

 and (2) it's supplying 18V to the battery and to the circuit. But my understanding of how a voltage regulator works is limited.
 
- I'm not familiar with the AMB mini^3 design. I'm trying to keep this as simple as possible but still keep good, clean voltage input, as well as trickle charge my batteries.
- Yes, I'm planning to use 6-cell NiMH
 
 
Here's the rest of my circuit (this time in high contrast black!). I went ahead and laid out both channels just because. Thoughts?
 
Full-size version: http://i.imgur.com/hD22r.png
 

 
Thanks!!

 

[porthillsbomber]

Yes in your current configuration the 7818 is redundant I believe. The lm317 will charge and power the circuit simultaneously.
 
The rest of the circuit looks fine although I don't know what all those caps on the feedback loop do.

 

[sprocket87]

Quote:

Yes in your current configuration the 7818 is redundant I believe. The lm317 will charge and power the circuit simultaneously.
 
The rest of the circuit looks fine although I don't know what all those caps on the feedback loop do.

 
OK, thanks very much.
 
I assume you mean in the phono preamp stage, and those are part of the RIAA equalization network (vinyl records are EQ'd a specific way when mastered and won't sound right without this). 

 

[porthillsbomber]

Yip those were the ones I was meaning. Ok that makes sense then. Looks like a good project I hope it goes well.

 

[sprocket87]

Thanks! All the components have been ordered so hopefully I'll be able to start laying it out soon.

 

[tonal]

As far as I can see from the schematic you are using the op-amps in the wrong way.
You should reverse the + (non inverting) and - (inverting) input pins because the way
they are layed out will lead to oscillation (instead of negative feedback you are using positive).
Also think about placing the reference (ground) on the middle of the batteries instead of using
rail splitter and a BUF634 to drive the ground.
So if you're going for 14.4V (12x1.2V) split the batteries on two groups of six.
And it might not be possible to use it while charging it...

 

[sprocket87]

 
Back from the dead! I took a break from electronics learning for awhile since we had a baby right around the time I posted this, so I've not had much time. But now I'm ready to get cracking on this amp. Nothing much has changed, any newcomers want to give it a once-over and offer some tips?
 
Thanks,
 
Jesse