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Old 22nd November 2004, 09:00 PM   #1
squadra is offline squadra  Netherlands
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Question relay based attenuator

Hello,

Since there is no Amplifiers/Pre-amps forum, and the attenuator will be used with my chipamp, I am posting this here

Attached is the principal schematic of my relay based attenuator.
I have drawn only 4 relays i.o. the 13 relays needed, but that is because the cad tool's interface was & .

I want relays i.o. the normal switch most people use for 2 reasons:
- being lazy I need a remote.
- being lazy I need a remote.

Click the image to open in full size.
This is how it is supposed to work:
There are 3 relays in the diagram connected to the multi-tap transformer, each of these relays is connected to 2 taps on the transformer. By activating one of these 3 relays 2 taps are connected to the 4th relays. This 4th relay is used for selecting either of these 2 'active' taps.

The relays are activated by the 74595 IC (through a driver not shown on the schematic)
The 595 can be controlled using only 3 wires, and by 'daisy chaining' the 595's a sufficient number of outputs is available.
Using opto-couplers the 595's are isolated electrically from the microcontroller board, and only when the volume is changing there is digital activity.
As a result there should be almost no spikes or other nasty things near the analog signals.

Per channel I would need a single board, using the daisychaining feature there is no real limit to the number of channels or taps on the transformers (for me it would be 2 channels and 24 taps transformers).

Does all this look like a project that could result in something good?
I can't find any flaws, but that probably only indicates that I'm not searching hard enough for them

Thanks for your input,

Peter
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Old 23rd November 2004, 01:56 AM   #2
mcs is offline mcs  Denmark
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Default Re: relay based attenuator

Quote:
Originally posted by squadra
Attached is the principal schematic of my relay based attenuator.
I have drawn only 4 relays i.o. the 13 relays needed, but that is because the cad tool's interface was & .
I'm working on the same thing, as you may have seen here: Anyone familiar with this Lite remote volume kit?
But I use resistors (and six relays) instead of a transformer.

Quote:
This is how it is supposed to work:
There are 3 relays in the diagram connected to the multi-tap transformer, each of these relays is connected to 2 taps on the transformer. By activating one of these 3 relays 2 taps are connected to the 4th relays. This 4th relay is used for selecting either of these 2 'active' taps.
That way the signal will pass through two relays - some will say that's very bad

Quote:
The relays are activated by the 74595 IC (through a driver not shown on the schematic)
The 595 can be controlled using only 3 wires, and by 'daisy chaining' the 595's a sufficient number of outputs is available.
I built a similar board a couple of months ago. You can see a bad photo here: http://stiftsbogtrykkeriet.dk/~mcs/SowVol2.jpg (the one at the bottom). I used an Allegro chip instead of the 595s. That way you can get 32 outputs with relay drivers using one chip instead of 8. The board has drivers for two channels (64 relays) and 16 aditional relays using the 595 and ULN chips on the right.

Quote:
Using opto-couplers the 595's are isolated electrically from the microcontroller board, and only when the volume is changing there is digital activity.
Why? The clock signal will be off (i guess) when you are not changing positions. So I don't see what the optocouplers will achieve?

Best regards,

Mikkel C. Simonsen
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Old 23rd November 2004, 04:59 AM   #3
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Why not just use a volume control chip like the PGA2311?
256 steps, 0.5dB/step, SNR of -114dB (IIRC), simple SPI interface to the micro, which can be daisy-chained.

I'm putting 3 of them in my 6-ch home theatre amp.
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Old 23rd November 2004, 12:35 PM   #4
mcs is offline mcs  Denmark
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Quote:
Originally posted by TwoSpoons
Why not just use a volume control chip like the PGA2311?
I hope you mean the 2310?

The reason I don't like the 2310 a lot, is that you need a buffer on the input (if you read the datasheet), you also get an op. amp. on the output. That's two op. amps. that are not required in a transformer/resistor attenuator. You also get more distortion from the CMOS switches in the chip than you get from relays.

The attenuators I make can be made in any impedance, any number of steps (I think 64 is more than enough) and they can handle large signal swings without problems.

Best regards,

Mikkel C. Simonsen
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Old 23rd November 2004, 06:13 PM   #5
squadra is offline squadra  Netherlands
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Default Re: Re: relay based attenuator

Quote:
Originally posted by mcs

I'm working on the same thing, as you may have seen here: Anyone familiar with this Lite remote volume kit?
But I use resistors (and six relays) instead of a transformer.
That's cool, but I would like to use the transformers because they apparently have higher dynamics on low volumes.
With my solution there is no step where 2 relais need to switch their state.
Your site looks nice, btw.
Too bad you didn't use a PIC, that could have made it easier for me

Quote:

That way the signal will pass through two relays - some will say that's very bad
Some people can even hear differences in the direction a cable is connected in, even when an audio signal is alternating in it's nature

Quote:

I built a similar board a couple of months ago. You can see a bad photo here: http://stiftsbogtrykkeriet.dk/~mcs/SowVol2.jpg (the one at the bottom). I used an Allegro chip instead of the 595s. That way you can get 32 outputs with relay drivers using one chip instead of 8. The board has drivers for two channels (64 relays) and 16 aditional relays using the 595 and ULN chips on the right.
I'm not going to try to solder that chip on a pcb
The 595 & driver ic's are very cheap and I can get them in the shop-around-the-corner.

Quote:

Why? The clock signal will be off (i guess) when you are not changing positions. So I don't see what the optocouplers will achieve?
That is true, and exactly why I will use the optocouplers
The clock on the digital board isn't stopped, and this way there is a very high degree of separation between analog and digital boards. Well, that's my aim anyway


Quote:

Best regards,

Mikkel C. Simonsen
Cheers,

Peter
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Old 23rd November 2004, 06:15 PM   #6
squadra is offline squadra  Netherlands
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Quote:
Originally posted by TwoSpoons
Why not just use a volume control chip like the PGA2311?
256 steps, 0.5dB/step, SNR of -114dB (IIRC), simple SPI interface to the micro, which can be daisy-chained.

I'm putting 3 of them in my 6-ch home theatre amp.
Because I wanted to check if these transformer based attenuators sound better as some people claim, and it is a nice, fairly easy project.

Cheers,

Peter
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Old 23rd November 2004, 06:25 PM   #7
jwb is offline jwb  United States
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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.
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Old 23rd November 2004, 08:33 PM   #8
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I do mean the 2311.

The THD+N is quoted as 0.0002% at 1khz. Thats going to be completely swamped by the distortion in the LM4780.

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.

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.

A trafo with less than 0.0002% distortion is going to be a costly beastie, silicon is much cheaper and I'm on a budget .

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?
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Old 23rd November 2004, 09:05 PM   #9
mcs is offline mcs  Denmark
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Default Re: Re: Re: relay based attenuator

Quote:
Originally posted by squadra
I'm not going to try to solder that chip on a pcb
The 595 & driver ic's are very cheap and I can get them in the shop-around-the-corner.
You can get them in a DIL version as well

I haven't had a "shop-around-the-corner" for 10 years anyway...

Best regards,

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Old 23rd November 2004, 09:20 PM   #10
mcs is offline mcs  Denmark
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Quote:
Originally posted by jwb
1) Use relays with an electrostatic shield. The relays I used couple a lot of noise into the signal.
I have made a small testboard with different relays. I have had no noise problems at all with those (and I don't think they have shields).

Quote:
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.
I have 6 resistors max. so the problem should be smaller.

Quote:
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.
I always use an 7805 - I have a lot of them

Quote:
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.
Depends a lot on the type of relay. A common type like the Omron G5V needs about 100mA as far as I remember...

Quote:
Originally posted by TwoSpoons
I do mean the 2311.
Why not use the 2310 with the added headroom and (i assume) lower real-life distortion?

Quote:
The THD+N is quoted as 0.0002% at 1khz. Thats going to be completely swamped by the distortion in the LM4780.
That distortion is probably not the worst-case distortion...

Quote:
A trafo with less than 0.0002% distortion is going to be a costly beastie, silicon is much cheaper and I'm on a budget .
But transformer distortion may sound a lot nicer than silicon distortion

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?
Who are you talking to here?

Best regards,

Mikkel C. Simonsen
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