Cyrus One/Two volume pot... linear??? - diyAudio
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Old 12th October 2012, 01:18 PM   #1
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Default Cyrus One/Two volume pot... linear???

After receiving yet another Cyrus amplifier to service and finding it had been mishandled by Parcelforce, I was heartbroken to find that the most damage had been taken by the volume pot... The customer is currently filing a claim with Parcelforce before I continue work on the amplifier.

The volume pots in these amplifiers are pretty darn rare. In fact the only way I've found to replace them (and keep the amplifier aesthetics as original) is to take one from another amplifier!

Click the image to open in full size.
And this is what Parcelforce did...
Click the image to open in full size.

So, 10KBX2
10K = Resistance
B = LINEAR?
X2 = Dual gang

Is this not a bit strange, using linear in a volume position? There seems to be no added circuit to make this linear pot logarithmic so why did Cyrus do this?

And if anyone knows a good way to replace the volume pot (balance completely not essential) and keep the original knob setup?
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Last edited by bleachershane; 12th October 2012 at 02:08 PM.
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Old 12th October 2012, 02:12 PM   #2
AndrewT is offline AndrewT  Scotland
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Can you measure the vol pot, even just one track, to confirm it is linear or some audio type taper?

Set to 0, 270 and the geometric middle 135, rotations and measure the resistances.

A lin can be chosen because generally the track to track matching is better with dual lin.
That maintains correct balance as the vol pot is rotated.

There may be a log law faking resistor in there.
eg.
a 10k pot feeding an input with a 1k5 Rin value will give a sort of log law over the lower 3/4 of the rotation. The top 1/4 becomes a sort of S shape taper.
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Old 12th October 2012, 03:56 PM   #3
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Here is the measurements of the potentiometer taken whilst in circuit, this would show up any fake log resistors used (there are none that I can identify on the board itself)

Click the image to open in full size.
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Old 13th October 2012, 01:59 AM   #4
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"B" taper was originally described as a double-end log. pot, pretty much as you measured.
I won't guess why it was done this way and the service manual doesn't specify the part but usually they found application in balance or Baxandall tone controls.
HiFi Engine has a readable service manual for Cyrus 1
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Old 13th October 2012, 02:15 AM   #5
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Double-end logarithmic pot? Now that's new to me, but makes sense in relation to the graph. Thanks for the answer... I wish they hadn't messed around with pot law codes so much!

Have all the service manuals, but as you found they didn't state the exact part specification.

I wish they hadn't used such a bloody awkward part to replace!
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Old 13th October 2012, 02:52 AM   #6
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Quote:
Originally Posted by bleachershane View Post
I wish they hadn't used such a bloody awkward part to replace!
Exactly! On reflection, I think the plan was to give the impression of LOTS more power available as the sound is subjectively loud quite early in turning up the volume. There is only so much you can do to impress buyers with a 25W amplifier!

My Cyrus 5 is not so quick to rise in volume but uses a flashy digital control so I guess that there, the law is coded in the (OEM?) chip.

If ever an "A" taper was available for such a complex assembly, I can't see why it wouldn't substitute if the owner agreed. It just means he uses more of the scale and gets finer adjustment. I've even replaced wafers in some unobtanium pot assemblies but that is just a labour of love, taking hours to complete and test.
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Last edited by Ian Finch; 13th October 2012 at 03:00 AM.
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Old 13th October 2012, 08:40 PM   #7
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Your reply makes complete sense... Although perfectly powerful for me (I quite like the Cyrus One) I can see that they would've wanted to give an impression of more power being on tap with the slightly sly use of this type of pot... which I never knew existed!

In my own Cyrus I would rather replace this for something like an Alps Blue Velvet, the problem with that as you may know, is coupling the pot shaft with the knob in a Cyrus One/Two. Now, if I could find a reliable, reasonably simple way to do that, I would be a very, very happy man!

The customer is starting his claim with Parcelforce before I begin any further work. I have ONE solitude original volume pot in a Cyrus Two which I use as a donor for repairs...

Now, replacing the track wafer... interesting, but must be a bloody nightmare!

Still, work a few hours of experimenting!
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Old 14th October 2012, 05:29 AM   #8
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Originally Posted by bleachershane View Post
In my own Cyrus I would rather replace this for something like an Alps Blue Velvet, the problem with that as you may know, is coupling the pot shaft with the knob in a Cyrus One/Two. Now, if I could find a reliable, reasonably simple way to do that, I would be a very, very happy man!...
Don't despair, I have read a Cyrus 2 thread here about the ALPS RK27 transplant...seemed a good result but of course, you lose balance control....I'm searching again.
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Old 14th October 2012, 06:43 AM   #9
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Default ALPS transplant

Ah, here it is in a concise form as about the best result I saw. Kinda miss Sonusthree around here.
potentiometer for cyrus2
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