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.
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.
Re: Re: 'Trimming' phono-pre's output with pot/attenuator
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.
sreten said:
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.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.
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.
Tim__x said:
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.
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.
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.
AndrewT said:
Hi Andrew,
I assumed that a pot wired as a potentiometer (wiper to signal) presents a constant load to source? IOW - a10K pot wired as potentiometer and the ADC's 10K would parallel to an unchanging 5K (and a 20K to 7.5K) - is this right?
The phono is a Gram Amp 2SE, for which unfortunately there are no detailed specs re. output impedence and loading, although I contacted Graham Slee and he said unhesitatingly that 10K is no problem at all, so I guess it's got a reasonably robust output.
Tim__X is saying that the presence of a buffer-stage on the Calf's input means it is tolerant of high source impedences, so I impliclty took that to mean that the quoted 10K input-impedence is simply for confirming compatibilty with sources as a load.
I guess I'll have to experiment with some cheap components and find out for myself what works.
A.
I think you'll find it's not as critical as it seems. Generally the output Z of (transistor) preamps are fairly low.
AndrewT's comments re bass droop may be true if the output capacitor(s) of the preamp are small-valued, but again this is not usually the case, i.e. 10uF would probably be typical and that would give you a pretty low f3 regardless (almost) of any reasonable load..
Giving it a go with what parts you can gather is probably the best path to take!!
Cheers!
AndrewT's comments re bass droop may be true if the output capacitor(s) of the preamp are small-valued, but again this is not usually the case, i.e. 10uF would probably be typical and that would give you a pretty low f3 regardless (almost) of any reasonable load..
Giving it a go with what parts you can gather is probably the best path to take!!
Cheers!
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