Preamp: General Idea

[Devil_H@ck]

As my JLH '03 is soon to be finished, I was thinking of building a preamp. This could come in handy since my JLH hasn't got a volume knob .

I searched on the forums, but didn't find anything, I found a lot of small circuits which I could use in a preamp, but no complete preamps. Well, actually, that's not totally true, I found one great design and I have bookmarked it, but I haven't got access to that PC right now.

Anyway, I decided to design my own preamp, I figured that with a little help from you guys, it should be do-able.

Here are a few idea's that came to mind (I might forget some, I'll add things in the process of designing the preamp):
- Input: about 8 (cd, radio, surround [more info below], tv, pc, aux1, aux2, aux3)
- LCD menu
- digital volume control (decent attenuators are way out of my budget)
- infrared remote
- ability to choose between RCA & XLR (for front channels only)
- ability to switch between amp & headphone amp (muting other surround channels in the process should these be in use)
- selection/menu buttons

So, this is what I came up with:
- Everything will be controlled with a microchip
- Volume control with a PGA2310. I will need 4 of these, one for front, one for middle, one for rear, one for sub and center
- Input selection through MPC507, this features 8 stereo inputs.
If surround is selected the according channels will be unmuted, else they will be muted through the PGA2310.
- XLR output with DRV134
- LCD control with µC
- buttons with a resistive network, µC will figure out what button was pressed
- Buffering with a BUF634
- If needed XLR inputs will be added with an INA134

What I don't know yet:
- How to switch between RCA/XLR, as far as I can see the MPC507 is for input selection only, not for output selection. I need to find a 1 channel in -> 2 channel out selection chip. The only chip I have found so far was one from Analog, but it had horrible THD ratings (0.3% if I remember correctly), so that won't do any good to the sound.
- What microchip to use? I don't know anything about microchips, but I'm willing to learn. I'll probably follow electronics next year at university, so I'll be able to use this knowledge in the future.
- Am I using to much buffers? If so, where should they be placed? I think it's a good choice to add one after the PGA2310, but should I add one in front of the RCA/XLR switching chip or add one in front of the XLR/RCA outputs?

As you can see, a lot of questions. I'd really appreciated if someone helped me with a couple of things.

Here's a "schematic" I drew up in Word:

[iCebReakEr409]

Elektor's "Audioschaltungen der Spitzenklasse" used CMOS Logic Circuits at their maximum supply range for mulitplexing, etc. The technical details were pretty good (THD, stereo seperation, ...)....and the sound, too.

They used 4051, 4029, 4510, 4028, 4093 CMOS

The preamp had 8 inputs, too They also used those cmos circuits for volume control.

[Devil_H@ck]

Hm, well, actually, I'm sure I can learn everything I need to know for this with some help of books etc., but the only thing that I don't find is a 2 input -> 1 output chips, this is a demultiplexing chips right? I only found some general CMOS chips not meant for audio at all.
If I really can't find one, I guess I'll just use 2 relays, but I want to try making an all chip preamp that still sounds great.

So, no one who knows about an audio demultiplexing chip?
Oh btw, I've searched and read a bit about digital pots. It seems that there are quite a lot on oppinions on those chips, eg. PGA2310, Cyrus chip, ... Are there any new chips that are really good and thatcould be used for this?

[sobazz]

You should maybe take a look at Wolfson WM8816. It does not have an integrated gainstage, so you're free to choose between different IC opamps and discrete designs. Some people on this board even considered a tube stage as an option.

[Werner]

Was going to recommend the 8816 too, even though not from experience, from from word of mouth from someone in the audio industry.

Not to discourage you, but university is not going to teach you how to build a preamp. Far from it.

I would also recommend you to start with a simpler preamp, no logic, processors, LCDs, just the circuits and power supplies that you need. It is already hard enough to make something sound decent (*) without 10% of Intel's annual output on board.

(* Unless, of course, you have 10% of Intel's annual profits on board too.)

[Werner]

Ow, I forgot. Sonny Andersen may release modules for 8816-based preamps to the DIY community in the near future.

standard daughterboard size for an unsiversal preamp

[iCebReakEr409]

Originally posted by Devil_H@ck
I only found some general CMOS chips not meant for audio at all.

You have to look for the analog cmos like the 4051 mentioned above: "4051 8-Channel Analogue Multiplexer/Demultiplexer ". There are other analog chips, too. I haven't found a good overview, only this one: http://www.itis.spb.ru/win/tech/cmos4000.htm - search for "analog" on that site.

I don't think they would have called the book like "top class audio circuits" if those cmos chips wouldn't sound good .You need to buffer those chips with inverted op amps, though, to get very good results.

Just a suggestion. The book is already some years old, so there might be better chips. But they achieved 0,005% < THD @ 20kHz/1V with NE5532 opamps.

[blu_line]

Douglas Self has started with some articles on Analog muxing with CMOS devices in "Elecetronics World"
Interesting read !
I do it with relays.


For the processor i would take a PIC 16F877 or like.
I2C interface, RS232, SPI and etc plus enough pins to control a decent LCD. An loads of program space for nice features like IR ctrl and etc !

I am using a 16F84. But that is just enough to do basic features.

Definatly go for diff out !

grtz

Simon

[Devil_H@ck]

Originally posted by sobazz
You should maybe take a look at Wolfson WM8816.

Thanks, I'll take a look at it.

Originally posted by Werner
Not to discourage you, but university is not going to teach you how to build a preamp. Far from it.

I know, actually I was refering to the µC programming.

Originally posted by Werner
I would also recommend you to start with a simpler preamp, no logic, processors, LCDs, just the circuits and power supplies that you need.

It's just me, I like to do things the hard way. Besides, I took a good look at the datasheets of the chips I've found so far and it looks quite simple to control them.

Originally posted by iCebReakEr409
You have to look for the analog cmos like the 4051 mentioned above: "4051 8-Channel Analogue Multiplexer/Demultiplexer ".

I'll check it out.

Originally posted by blu_line
Douglas Self has started with some articles on Analog muxing with CMOS devices in "Elecetronics World"

Could I find this somewhere on the internet? I don't really have access to english electronics magazines.

Originally posted by blu_line
For the processor i would take a PIC 16F877 or like.
I2C interface, RS232, SPI and etc plus enough pins to control a decent LCD. An loads of program space for nice features like IR ctrl and etc !

Sounds great and I noticed that type of chip a lot while browsing the net.

Btw, this is almost exactly what I want, I guess I'll just steal the idea's I like and add/edit what I don't .

[Werner]

"I know, actually I was refering to the µC programming."

They won't teach you that, either. Especially if you persue a degree in informatics or computer science engineering. Maybe if you go for electronics. Just maybe ...

Where are you (going)? I mean, degree and place?


"far and it looks quite simple to control them."

Of course. But they might wreak havoc with the analogue components in the same box.

Fine if you want to build a microprocessor-based system that happens to be a pre-amp. Less nice if the end-result should be a good-sounding preamp before anything else.

You pick you choice.


BTW, Sonny Andersen's modular kit also has display and microcontroller parts.