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Old 23rd November 2004, 10:32 PM   #11
squadra is offline squadra  Netherlands
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Quote:
Originally posted by jwb
I built a relay attenuator for a headphone project and it was very fun. I will just share what I learned from that project. The design works great and functions as a wonderful attenuator but man, it is noisy. If I were to implement the design again (and I might), here's what I would do:

1) Use relays with an electrostatic shield. The relays I used couple a lot of noise into the signal.

2) Use low-noise resistors. Because my design is a series attenuator, with 16 resistors in the signal path, the resistor noise adds up easily.

3) Use a regulated power supply. The amount of current required to drive the relays and electronics turned out to be quite significant. I initially had used a simple zener and pass transistor regulator, but that wasn't enough regulation and it overheated anyway.

Don't listen to anyone who says you can't drive a relay with a '595. I drive the relay straight off a '595 without any problem.
Build another version, it is fun and also nice to discuss

1+2: How much noise is generated by the resistors, and how much by the relais?

3: There will be a regulated power supply, probably using a simple 7805.

BTW, I'm not going to skip the drivers, they are cheap and it could be a problem if i have a pcb created and the current required is too high for the 595's.

Quote:
Originally posted by TwoSpoons
I have an input buffer anyway - INA163 - because I'm doing balanced inputs to try to kill the hum loops that inevitably form when connecting several pieces of equipment together.
Wouldn't the isolation provided by a transformer be very useful for that?

Quote:

Good luck with the relays - make sure you pick signal relays, as the power relays require a decent 'wetting' current to keep the contacts clean.
The relay I have in mind is Zettler's ZA832, a small gold plated relay.
Over here they are about 3 euro each (and cheaper in larger numbers)
Quote:

Edit: Just noticed something - the input impedance seen by your source is going to change according to which tap you pick. Is this OK for you?
The input impedance of the chip amp is about 100k, if the transformer is ideal then the impedance as seen by the source is 100k or higher.
My source has buffered outputs, but even without buffer I think all should be well.

Peter
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Old 24th November 2004, 12:22 PM   #12
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Downloaded a demoversion of OrCAD, now this is the complete schedule of one channel.
Although I couldn't save it like this, apparently too many nets.
Click the image to open in full size.
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Old 24th November 2004, 01:23 PM   #13
mcs is offline mcs  Denmark
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It looks fine to me. But why don't you just use one relay for two consecutive tabs? Like 0 and -2dB? I think that would make the wiring easier.

I'll draw a dual-channel, single-chip version of that schematic later today (if you don't mind...)

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Mikkel C. Simonsen
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Old 24th November 2004, 06:18 PM   #14
mcs is offline mcs  Denmark
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Here's my attempt at a schematic: http://stiftsbogtrykkeriet.dk/~mcs/RelVol2_sch.gif

Useable for both resistor and transformer attenuators...

The PCB layout will have to wait till after dinner

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Old 24th November 2004, 09:23 PM   #15
squadra is offline squadra  Netherlands
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Quote:
Originally posted by mcs
It looks fine to me. But why don't you just use one relay for two consecutive tabs? Like 0 and -2dB? I think that would make the wiring easier.
I don't use 2 tabs because there is a difference in the switching.
When we use 4 relays there are 6 possible positions:
Code:
Pos Rel1 Rel2 Rel3 Rel 4
 0    1     0     0     0
 1    0     1     0     0
 2    0     0     1     0
 3    0     0     1     1
 4    0     1     0     1
 5    1     0     0     1
As you can see switching between 2 positions requires 2 relais to change their state (except from pos. 2<->3, in that case only relay 4 is (de-)activated)

In your proposal we have:
Code:
Pos Rel1 Rel2 Rel3 Rel 4
 0    1     0     0     0
 1    1     0     0     1
 2    0     1     0     0
 3    0     1     0     1
 4    0     0     1     0
 5    0     0     1     1
In this case either we have 1 relay changing state (i.e. from pos. 0<->1) or 3 relays changing state (i.e. from pos 1<->2)

There is no difference in complexity of the pcb, the transformer wires are long enough and they will be connected on the pcb right beside the relays.

Quote:

I'll draw a dual-channel, single-chip version of that schematic later today (if you don't mind...)
I don't mind at all, the more the merrier
Quote:

Best regards,

Mikkel C. Simonsen
Cheers,

Peter
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Old 24th November 2004, 10:00 PM   #16
mcs is offline mcs  Denmark
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Quote:
Originally posted by squadra
[B]I don't use 2 tabs because there is a difference in the switching.
OK. But I have made attenuators where up to 6 relays change state at the same time (using one right now), and that's no problem.

Quote:
There is no difference in complexity of the pcb, the transformer wires are long enough and they will be connected on the pcb right beside the relays.
Yes, but if you use resistors my option is easier (unless I've overlooked a layout option). It looks like everything will fit on a single-sided euroboard...

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Old 24th November 2004, 11:57 PM   #17
mcs is offline mcs  Denmark
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Everything did fit on a Euroboard - just.

Here's the layout:
Click the image to open in full size.

I don't think it can be made a lot smaller/simpler than that... And when using a transformer either switching arrangement can be made using this board design I just realized.

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Old 25th November 2004, 06:19 PM   #18
squadra is offline squadra  Netherlands
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Quote:
Originally posted by mcs
Everything did fit on a Euroboard - just.

Here's the layout:
I don't think it can be made a lot smaller/simpler than that... And when using a transformer either switching arrangement can be made using this board design I just realized.

Best regards,

Mikkel C. Simonsen
Nice, but I didn't check the traces
I've read that the relays should be at least 5mm apart from eachother to prevent them from interfering.
Maybe that's not true for all dil relays?

You could also connect the 4 unused outputs to relais for input switching.
That would lead to a 1 IC preamp wih 4 inputs and passive attenuation

Cheers,

Peter
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Old 25th November 2004, 11:11 PM   #19
mcs is offline mcs  Denmark
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Quote:
Originally posted by squadra
Nice, but I didn't check the traces
I didn't get any DRC errors, so if the schematic's OK then...

Quote:
I've read that the relays should be at least 5mm apart from eachother to prevent them from interfering.
Maybe that's not true for all dil relays?
I have read that also. I also once found a datasheet that explained why. The manufacturers cannot guarantee that the relays work correctly if you operate them on low voltages, submit them to shock and strange temperatures etc. all at once, unless you keep that distance. For normal use you can place them a lot closer - and I have done so in the past.

Quote:
You could also connect the 4 unused outputs to relais for input switching.
That would lead to a 1 IC preamp wih 4 inputs and passive attenuation
Yes, if only there was more space left on the PCB...

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Old 25th November 2004, 11:43 PM   #20
squadra is offline squadra  Netherlands
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OK, I also have been playing around with the pcb design package.

I have created 1/2 a board.
To get a complete channel you would assemble 2 boards and daisychain them.
Placing them on top of eachother allows me to build a really compact housing.

The pcb is 80 * 50 mm (about 3" * 2").
IC1 = ULN2803
IC2 = 74595 in SMD package
C1 = 100nF
Rel1..7 could be Zettler AZ832 or similar

Top layer:
Click the image to open in full size.

Bottom layer:
Click the image to open in full size.

Silkscreen:
Click the image to open in full size.
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