Preamp with features!

jwb · 18th February 2009, 05:07 AM

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

h_a · 18th February 2009, 07:21 AM

Quote:

I like a remote control.

Ja, me too!

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

Have fun, Hannes

fotios · 18th February 2009, 10:49 AM

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

Bonsai · 18th February 2009, 12:18 PM

looks cool - nic e simple and elegant.

don't use atmel . . . . they may not be around in 6 months time.

h_a · 18th February 2009, 02:50 PM

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!

jwb · 18th February 2009, 03:00 PM

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.

jwb · 18th February 2009, 06:47 PM

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.

fotios · 18th February 2009, 07:18 PM

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

jwb · 18th February 2009, 08:02 PM

In a latching relay the contact itself is a permanent magnet.

SQLGuy · 18th February 2009, 08:32 PM

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).

18th February 2009, 12:18 PM	#4
Bonsai diyAudio Member Join Date: Aug 2006 Location: Taipei, Taiwan	looks cool - nic e simple and elegant. don't use atmel . . . . they may not be around in 6 months time. __________________ bonsai http://www.hifisonix.com/

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18th February 2009, 10:49 AM	#3
fotios diyAudio Member Join Date: Feb 2007 Location: ΔΡΑΜΑ - North Greece	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

18th February 2009, 06:47 PM	#7
jwb diyAudio Member Join Date: Mar 2002 Location: San Francisco, USA	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.

18th February 2009, 08:02 PM	#9
jwb diyAudio Member Join Date: Mar 2002 Location: San Francisco, USA	In a latching relay the contact itself is a permanent magnet.

18th February 2009, 08:32 PM	#10
SQLGuy diyAudio Member Join Date: Jan 2009	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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