Choosing attenuator for MF X-10D Clone - diyAudio
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Old 8th July 2013, 08:38 PM   #1
thommy is offline thommy  United Kingdom
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Default Choosing attenuator for MF X-10D Clone

I plan to add a stepped stereo volume attenuator to my X-10D tube buffer, making it into a single source preamp hooked between a pair of class d amps and my usb dac.

I know to insert the attenuator at the input instead of the 1Meg resistor just in front of the 22nF input cap on the X-10D board however, there are a variety of different values available for the stepped pots, ranging from 10k upto 1000k.

Which one do I choose? I guessed at the 1000k in order to match the resistor at the input stage but am not sure about this.
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Old 9th July 2013, 12:11 PM   #2
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Check the input impedance of your class D power amps.

you should be able to use a volume control between the X10-D and power amps.
10K or so volume will probably be OK if the power amps are 20K or more.
Or user a linear pot of 8-10 TIMES the value of the power amp input impedance (this 'skews' the linear law to something similar to 'audio taper', but normally gives much better channel matching)
I would recommend this order as switch-on transients from the valves could pop your speakers and valve noise getting to the power amps will be reduced with the volume control in this position.

Last edited by blu_glo; 9th July 2013 at 12:14 PM.
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Old 9th July 2013, 11:39 PM   #3
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Thanks! The input impedance is listed as '2x10Kr', is that likely to be ok?
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Old 9th July 2013, 11:44 PM   #4
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Thanks! The input impedance is listed as '2x10Kr', is that likely to be ok?
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Old 10th July 2013, 04:07 AM   #5
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Quote:
Originally Posted by thommy View Post
Thanks! The input impedance is listed as '2x10Kr', is that likely to be ok?
10K is a little on the low side and means you really must stick with a lower value however you should be fine with 10K attenuator.

It will still skew the "audio" law somewhat but shouldn't be an issue.

I noticed rather later that you are actually considering stepped attenuator, if so ignore my rambling about pot laws and matching as it is not such a issue on such attenuators.

Last edited by blu_glo; 10th July 2013 at 04:30 AM.
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Old 10th July 2013, 06:45 AM   #6
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"MF X-10D Clone" - do you have cct diagram for this?
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Old 11th July 2013, 12:01 PM   #7
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No but it's ebay item 121141388295 - I couldn't make the hyperlink work or even post the full URL here. I have sent the seller a message.

It will handle a couple of line level inputs via an autoswitch which I guess will all output the standard 2mv (?) level. The circuit apparently loses about 5% gain on the way through and has no provision to add any more, so on a pair of 250w mono block amps that's an output loss of 12.5w per channel. That may reduce the chance of the amps becoming overloaded. This project will be for a bar soundsystem and so it would be good if I could be sure that nobody could blow it up by cranking up the volume knob to '11'...

Each amp has the impedance 2x10kr but I may be able to have that altered before they ship if its essential. I don't understand enough about this to see why the input impedance wouldn't be correct for a straight through preamp handling a standard line level signal.

Last edited by thommy; 11th July 2013 at 12:09 PM.
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Old 12th July 2013, 09:03 AM   #8
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This might help...
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Old 12th July 2013, 11:55 AM   #9
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Quote:
Originally Posted by thommy View Post
No but it's ebay item 121141388295 - I couldn't make the hyperlink work or even post the full URL here. I have sent the seller a message.

It will handle a couple of line level inputs via an autoswitch which I guess will all output the standard 2mv (?) level. The circuit apparently loses about 5% gain on the way through and has no provision to add any more, so on a pair of 250w mono block amps that's an output loss of 12.5w per channel. That may reduce the chance of the amps becoming overloaded. This project will be for a bar soundsystem and so it would be good if I could be sure that nobody could blow it up by cranking up the volume knob to '11'...

I don't understand enough about this to see why the input impedance wouldn't be correct for a straight through preamp handling a standard line level signal.
What do you mean by "autoswitch"? Rotary type?

Line level inputs work at approx. 300mV rms, but can be anything from less to 8V maximum (from op-amp circuits). CD for example normally has an absolute maximum of 2-2.5V rms most DAC chips are geared toward this level.

According to original unit manual, circuit originally had a gain of 1.1x so it should boost the signal but only very slightly. You have not provided the component values so impossible to tell if anything has been changed.

"Each amp has the impedance 2x10kr but I may be able to have that altered before they ship if its essential."

input impedance? It doesn't really matter but should be greater than that?
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Old 12th July 2013, 12:35 PM   #10
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No, there's a 4 input board available which looks for the presence of at least a 200mV signal and then switches over.

Haha, the guy selling the pairs mentioned it was a no gain circuit, with a little loss. Maybe I just need to buy all the bits to find out whether or not it will work. I've asked him again for the component values.

My main concern with this output level vs imput imedance question is to make sure that I am not feeding the amps too low a signal, making them more inefficient and reducing overall audible output.

With reference to the stepped attenuators, do the differing impedance values offer response curves for the knob?
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