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Old 29th August 2006, 06:35 PM   #1
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Default 'Trimming' phono-pre's output with pot/attenuator

Hello All,

by 'trim' the phono's output, I mean cutting no more than -3 to 6 dB, not full attenuation to mute.

The reason for wanting to do this is so I can connect it directly to a decent stand-alone analogue/digital convertor, which has no level adjustment, for recording puposes.

Most discs stay well under 0dB on the ADC's meters, but a few 'hot' ones (not least 12" 45's) clip by a good few dB.

Interconnet will be DNM Reson, which is exeptionally low capacitance (amazingly, no more than 20 pF per meter).

The ADC's input-impedence isn't particularly high ("greater than 10Kohms"), but given the pot (say, 20K?) wouldn't be turned far enough to create an unreasonably high output-impedance , could I get away with connecting it as a simple potentiometer without losing any bandwith or dynamics from the source, or introducing noise?

thanks,
A.
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Old 30th August 2006, 07:50 AM   #2
AndrewT is offline AndrewT  Scotland
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Hi,
are you running the phono cartridge straight into the ADC?
or
using a phono into phono pre-amp (RIAA) then into the ADC?
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Old 30th August 2006, 01:30 PM   #3
sreten is offline sreten  United Kingdom
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Default Re: 'Trimming' phono-pre's output with pot/attenuator

Quote:
Originally posted by Aleksunder
Hello All,

by 'trim' the phono's output, I mean cutting no more than -3 to 6 dB, not full attenuation to mute.

The reason for wanting to do this is so I can connect it directly to a decent stand-alone analogue/digital convertor, which has no level adjustment, for recording puposes.

Most discs stay well under 0dB on the ADC's meters, but a few 'hot' ones (not least 12" 45's) clip by a good few dB.

Interconnet will be DNM Reson, which is exeptionally low capacitance (amazingly, no more than 20 pF per meter).

The ADC's input-impedence isn't particularly high ("greater than 10Kohms"), but given the pot (say, 20K?) wouldn't be turned far enough to create an unreasonably high output-impedance , could I get away with connecting it as a simple potentiometer without losing any bandwith or dynamics from the source, or introducing noise?

thanks,
A.
Possibly be easier to add an attenuation switch to the
ADC, say - 10dB, making its input range more flexible.

/sreten.
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Old 31st August 2006, 04:40 PM   #4
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Default Re: Re: 'Trimming' phono-pre's output with pot/attenuator

Quote:
Originally posted by sreten


Possibly be easier to add an attenuation switch to the
ADC, say - 10dB, making its input range more flexible.

/sreten.

The ADC has -10 and -16 dBV jumpers, and I've got it on the more sensitive setting, which matches it to the phono amp pretty well. I'd prefer to hit -2 or -3dB every time, but given 24-bit files, there's a fair bit of latitude for adding gain digitally if the recordings are too far under.

Obviously I can't fix clipping though.

By attenuator-switch, do you mean a simple switchable voltage divider, like a 1 position stepped attenuator?

You've actually given me an idea there! - A 2 or 3 position switch would probably be enough for reasonable matching of different discs.
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Old 1st September 2006, 01:30 AM   #5
clem_o is offline clem_o  Philippines
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3-position switch is a good idea, or, use a 5K or 10K stereo pot.

The output impedance of your phono preamp should be able to handle the impedance reasonably, and any series resistance introduced by the pot should be alright with the ADC...

Cheers
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Old 1st September 2006, 03:20 AM   #6
Tim__x is offline Tim__x  Canada
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Is this a commercial ADC, or a DIY one?

If the later, is the input of the actual ADC IC buffered? Many ADC's have a moderately nonlinear input impedance and require a low impedance source for the lowest distortion. If it is buffered feel free to use any reasonable attenuator you want. If it's not buffered, check the datasheet and see if the manufacturer recommends a buffer.
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Old 1st September 2006, 07:30 AM   #7
sreten is offline sreten  United Kingdom
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Hi,

the obvious thing to do is wire the jumper positions to a switch.

/sreten.
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Old 1st September 2006, 08:37 AM   #8
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Quote:
Originally posted by Tim__x
Is this a commercial ADC, or a DIY one?

If the later, is the input of the actual ADC IC buffered? Many ADC's have a moderately nonlinear input impedance and require a low impedance source for the lowest distortion. If it is buffered feel free to use any reasonable attenuator you want. If it's not buffered, check the datasheet and see if the manufacturer recommends a buffer.
THe ADC is a late model Midiman Flying Calf, and as it happens, the manual *does* actually specifically mention that it has input-buffers, so that's very useful info, thank you.

What had me worried was that it also specifys that input impedence of "greater than 10K ohms" which implied that impedence matching was an issue , but this is presumably for the benefit of knowing source loading..

regards,
A.
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Old 1st September 2006, 08:47 AM   #9
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Quote:
Originally posted by Tim__x
Is this a commercial ADC, or a DIY one?

If the later, is the input of the actual ADC IC buffered? Many ADC's have a moderately nonlinear input impedance and require a low impedance source for the lowest distortion. If it is buffered feel free to use any reasonable attenuator you want. If it's not buffered, check the datasheet and see if the manufacturer recommends a buffer.
Forgot to say - given the ADC's '>10K' input, I was thinking of using a 20K pot to allow the phono-stage to see a 7.5K load.

I thought perhaps using a 10K pot (and hence 5K load) might be pushing it a bit.

Does that make sense to you?

thanks,
A.
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Old 1st September 2006, 09:27 AM   #10
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
Hi,
no.
The calf has an input impedance of greater than 10k. It would be normal to source this from about 1k0 or less.
A 10k pot before the calf will show a maximum of 2k5 (10k / 4) when set to -6db and less than 2k5 for all other attenuation settings. 2k5 may be tolerable if it is located very close to the calf AND if the calf has a small value HF filter on the input. Treble response varies with attenuation setting.

Now looking at your source end, if you use a 10k pot then the source sees about 10k//input Z of the calf when set to -0db and this impedance rises to 10k when set to full attenuation.

The worst case is 10k//10k and becomes a 5k load for the source.
Is your source less than about 500r? Has it got a DC blocking cap? When loaded with 5k, does it give a reasonable bass response. Bass rsponse varies with attenuation setting.

If you can vary the output of the phono pre-amp it will give you a more consistent performance.
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