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Old 18th February 2009, 05:07 AM   #1
jwb is offline jwb  United States
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Default Preamp with features!

I got tired of having those minimalist preamps with no features. I like a remote control. I like gain matching between inputs. I like balance. I like LED displays. So I started designing a new preamp, the design of which is about 80% complete. The basics are:

Two channels (dual mono)
3 balanced inputs
6 unbalanced inputs
2 balanced outputs (for bi-amplification)
CS3318 programmable gain attenuator/amplifier
OPA1632 line drivers
Choice of DC, AC, and transformer coupling on the inputs
Remote control
8-character 5x7 LED display

Analog stuff is powered from super-regulated +/-9V supplies, with separate secondaries for each channel. Digital 3.3V power comes from a separate transformer and power supply.

I'm just starting to come to grips with the amount of drawing necessary, but I have the power supplies, the input controls, and the preamp channels mostly drawn. I'll post them up here when I complete each bit. Here's one channel, although the symbols for the single-ended inputs run off the bottom. If you can't read it, a PDF is also attached. And the preliminary bill of materials is also attached for reference.

Link to BOM
Link to PDF
Attached Images
File Type: png channel.png (94.9 KB, 1879 views)
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Old 18th February 2009, 07:21 AM   #2
h_a is offline h_a  Europe
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Quote:
I like a remote control.
Ja, me too!

What kind of microprocessor are you going to use? An atmel?

Have fun, Hannes
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Old 18th February 2009, 10:49 AM   #3
fotios is offline fotios  Greece
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Hi jwb

If you have not already thought... and because you will use a MCU... take a look in latching type relays with double coils (like set and reset) for use as input selectors. There are two options of almost same quality. Panasonic DS2E and Omron G6AK. Beneffit of latching type it is obviously their mechanical armature to keep the contacts in place without the need of continuous operation of coils, thus absence of any EMI in the input signal.

Fotios
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Old 18th February 2009, 12:18 PM   #4
Bonsai is offline Bonsai  Taiwan
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looks cool - nic e simple and elegant.

don't use atmel . . . . they may not be around in 6 months time.
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Amplifier Design and Construction for MUSIC! http://hifisonix.com/
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Old 18th February 2009, 02:50 PM   #5
h_a is offline h_a  Europe
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Quote:
don't use atmel . . . . they may not be around in 6 months time.
Ouch

That would hurt as I only own an atmel-programmer

Anyway, that wouldn't be the first occasion using obsolete parts

Have fun, Hannes

EDIT: by the way, thanks fotios for the tip - a very elegant suggestion!
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Old 18th February 2009, 03:00 PM   #6
jwb is offline jwb  United States
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Quote:
Originally posted by fotios
Hi jwb

If you have not already thought... and because you will use a MCU... take a look in latching type relays with double coils (like set and reset) for use as input selectors. There are two options of almost same quality. Panasonic DS2E and Omron G6AK. Beneffit of latching type it is obviously their mechanical armature to keep the contacts in place without the need of continuous operation of coils, thus absence of any EMI in the input signal.
I considered using latching relays but I rejected the idea because I don't like that the contacts themselves are magnets. That seems like a big drawback. I'm not driving the relays directly off the microprocessor. I'm driving them through '595 shift registers, which in turn drive a common emitter NPN. Also I've taken care to design the +3.3V supply to have .01Ohm impedance out to 200MHz. I _think_ it should be pretty quiet. I hope!

And yes I was planning to use an AVR since that's what I know how to program best.
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Old 18th February 2009, 06:47 PM   #7
jwb is offline jwb  United States
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Note also that any thermal EMF across the relay should appear as common mode noise at the input, and the CMRR of this circuit is ridiculously high.
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Old 18th February 2009, 07:18 PM   #8
fotios is offline fotios  Greece
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Quote:
Originally posted by jwb


I considered using latching relays but I rejected the idea because I don't like that the contacts themselves are magnets.
From those that i know, the coils are electromagnets and not permanent magnets in all types of relays. When the supply is turned off, then the coils becomes no magnets furthermore. There is not any physical magnet inside, only a coil arround a ferite. In latching type, the action of electromagnetic coil is momentary, and 5ms of activating it is more than enough for the mechanism to latch the contacts. There is not the need for further supply the coil with voltage to keep the contacts in place.

Fotios
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Old 18th February 2009, 08:02 PM   #9
jwb is offline jwb  United States
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In a latching relay the contact itself is a permanent magnet.
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Old 18th February 2009, 08:32 PM   #10
SQLGuy is offline SQLGuy  United States
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Correct. And it's not the contacts themselves that are magnetic anyway. The contacts are carried on metal strips which are themselve attached to a steel plate. It is this plate that is spring-loaded and actuated by the electromagnetic pole piece of the relay.

Better relays sometimes even have gold plated silver contacts (certainly non-magnetic).
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