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Old 7th June 2006, 12:50 AM   #1
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Location: Dallas, Texas
Default relay volume control - POP!

Hello,

I've made a PIC-controlled output attenuator per the Aleph P 1.7
schematic for my, well, for my Aleph P 1.7 clone. Funny how that
works.

It's pretty good in operation - but there are maybe 3 places
where there are audible pops in transition from one volume
setting to the next. The worst (surprise !!!) is when the msb
transitions.

Anyone got any advice on how to fix this?

I've thunk of a few things myself but it's a bunch of time to
try these out one by one until I get something that is good.

1) go to 0 volume as an interim from one volume setting to the
next. I'm thinking maybe 5 ms or so. Just enough to get rid
of overlaps. I can adjust the delay using my controller with
very little effort - it's just another download.

2) cascade the changes up one bit at a time when making a
change up and cascading the changes down one bit at a time
when making a change down.

Thanks for any help in this regard.

OK OK so I know now there will be some interest in what I did.

My amp is a 2-chassis job with the controller and power supplies
in one chassis and an analog-only chassis. There is no digital
noise in the analog chassis because the only time any digital
circuitry changes in that chassis is when a control input changes.
The digital control in the analog chassis is totally isolated from
the analog circuitry. They are connected at the transformer
primaries only and a 1k resistor to chassis ground in the power
supply chassis so that the digital common doesn't float WRT
ground.

I basically copied the entire analog part of the Aleph P 1.7
schematic with the following exceptions:

1) since I don't have the Pass Labs program to control the
amp, I made my own digital control subsystem. That part is
all mine - the board, the SW, etc. I used a PIC16F684 as the
controller, I have 8x 8-bit S-P converters to create a 64-bit
output field from the PIC, and I have encoders as inputs for
volume, balance, and select. I also have a mute input switch.
I have a turn-on output mute delay, volume is 8 relays
controlled from a 64 level lookup table with volume steps
chosen as 4 piecewise linear transfer functions, and I have
left and right volume 7-segment displays that are also driven
by lookup tables in the PIC. It almost looks professional.
hahahaha

2) I substituted 3300 uF caps for all the 1000 uF caps in the
power supply system.

3) I added 2x 10 uF WIMA film caps on all power rails in the
amps.

Other than that, it's an Apleph P 1.7 clone.

So now for the next bit that everyone wants to know. How it
sounds. To set the stage, I've been using a BoSoZ for a few
years now. All I can say is WOW. The P 1.7 is far more revealing
and fine. It finds all sorts of stuff I've never heard before.
And it drives better down low than the BoSoZ does. I find that
the soundstage has a lot more depth, or that it now has depth.
And probbably because my BoSoZ uses an input attenuator and
the P has an output attenuator, the P is spooky quiet. You can
set the volume to very high levels, press play, and then BLAM,
on a tune with a dynamic start, the sound just hits you in the
chest. You have no clue that it is on its way before it hits. Truly
dynamic.

My system so far: (it's a journey....)
Aleph 2 clone monoblocks
Aleph P 1.7 clone
Classe CDP0.3 CD player
home grown "maxx/utopia" mutt speakers
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Old 7th June 2006, 06:16 AM   #2
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You most probably have a change in DC offset when switching the volume. If you can eliminate that, you will lose the thump. I don't think going to zero between switches will cure it.

Jan Didden
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Old 7th June 2006, 09:00 AM   #3
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Quote:
Originally posted by wayne325

. . . audible pops . . .
Do you hear them even with the diodes across the magnet coils?

Regards
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Old 7th June 2006, 02:28 PM   #4
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jh6you: There are diodes across the coils. #1 - I can't see why this
would matter to the relay outputs. If there are no diodes, then it
would kick back the controller, not the amp. #2 - There are only
3 step transitions where you can hear any pop at all. These are NOT
the max transition points (that is, the highest number of relays is
changing value) but rather they are when the msb or 2nd msb change
value. It's a good point but I can't see that it is the problem.

janneman - there is no DC offset. The output is AC coupled and the
attenuator is after the caps. Also - why then would it only be 3 steps
where there is a pop ? So again, good point and thanks but that's
not it unless I don't understand what you mean.

The problem, as I understand it, is that for maybe 1 or 2 ms or so, the
msb relay is connecting before other relays are disconnecting - meaning it
goes to almost full volume for a short period of time, then settles
at the correct volume. This is heard as a loud popping noise from the
speakers.

Any other takers or am I getting this wrong ???

W.
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Old 7th June 2006, 04:36 PM   #5
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wayne325,

>> The worst (surprise !!!) is when the msb transitions.

You could Mute on these transitions! only a few milliseconds to avoid the Pop!

Regards.

Alain.
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Old 7th June 2006, 04:50 PM   #6
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Perhaps using a different type of relay with a stack that opens fast will solve your problem?

If you have old phone relays for example, you can use a tab bender to set the make-break ratio of the relay which will set how far the armature must move before the contacts open and close.

I know this is a bit of work, but if your problem is caused by relays not opening in time, I would suggest that as a means to rid yourself of the problem at its root.

PS: Have you tried any other relays in your circuit?
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Old 7th June 2006, 04:56 PM   #7
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Quote:
Originally posted by wayne325
jh6you: There are diodes across the coils. #1 - I can't see why this
would matter to the relay outputs. If there are no diodes, then it
would kick back the controller, not the amp. #2 - There are only
3 step transitions where you can hear any pop at all. These are NOT
the max transition points (that is, the highest number of relays is
changing value) but rather they are when the msb or 2nd msb change
value. It's a good point but I can't see that it is the problem.

janneman - there is no DC offset. The output is AC coupled and the
attenuator is after the caps. Also - why then would it only be 3 steps
where there is a pop ? So again, good point and thanks but that's
not it unless I don't understand what you mean.

The problem, as I understand it, is that for maybe 1 or 2 ms or so, the
msb relay is connecting before other relays are disconnecting - meaning it
goes to almost full volume for a short period of time, then settles
at the correct volume. This is heard as a loud popping noise from the
speakers.

Any other takers or am I getting this wrong ???

W.

If there is no DC offset there can be no pop. Even with AC coupling, the bias current from the amp input will come from the amp input and flow "backwards" through the attenuator to charge the AC coupling cap to a certain DC. Depending on the switching that DC will also be switched and that gives the plop. Especially when you switch the msb because there the change is max.
Try to measure the DC at the input pin (after the cap, no signal) then switch the msb. What is the difference?
Do you have a schematic of that input stage you can post?

Jan Didden
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Old 7th June 2006, 07:27 PM   #8
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Jan,

"If there is no DC offset there can be no pop"

I believe that if you connect the output of a preamp to a power amp
for 3 ms at fulll volume, then return it to normal, it will sound like a
"pop" or a "bang".

This is an output attenuator, not an input attenuator. It's the attenuator
from the Aleph P 1.7.

Alain, have you had this problem before and solved it by muting for a
second ?

I'm proposing to do this:
detect need for input change
set all relays to 0
wait x ms
set all relays to new volume setting.

What I'm concerned about is that when the relays all get set to their
new value, do I still get a pop - or do I get pops in different places on
the map?

I don't want to mute at volume changes because that means the mute
relays will take a pounding. Maybe this isn't a problem ?

Clearly, I can't change relays to fix this problem. The time and cost
would be huge.

W.
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Old 7th June 2006, 07:35 PM   #9
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Hi,
Relays have different make/break times, and typically, the latch off time is shorter than the latch on time by a few milliseconds (1 to 2 ms, check your relays' datasheet). Then, when changing from one volume combination to an other one, you'd have to first activate the relays that will be going from off to on, leaving the previously activated relays on, and a few millisecond later (corresponding to the difference between the lach on and the latch off), you'll have to switch off all the relays that need to be off... Doing this, the moment when relays actually switch on and off should be the same, and the POP should be gone...
Dunno if I'm clear here... But since your attenuator is C driven, it's quite easy to implement using a parametrable time out interrupt to do this, and you can modify the timeout delay to match your relays' specifications...

Hope this helps
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Old 8th June 2006, 02:18 AM   #10
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Cheff,

OK so what you've described got me thinking. What I'm going to try
first is a mix of what I was going to do originally and what you described
in your post.

So here's the plan:

Volume is at setting X
input change detected
Set all relays to 0
one by one from lsb to msb, set relays to new setting with 1 ms between.
Volume is now at setting X+1 or X-1.

Example:

01111111 start value
input change = "add 1" (because this is the most interesting case)
00000000 (set to 0)
wait 1 ms
00000000 (load new lsb which is still 0)
wait 1 ms.....
00000000 (load new bit 1 which is also 0)
etc
00000000 (load new bit 6 which is also 0)
wait 1 ms
10000000 (load new bit 7 = msb which is 1)
and voila - no pop.

But there will be an 8 ms silence. I wonder what that sounds like.

I'll let you all know how it goes.

W.
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