Pre amp

justshoemarkjustshoemark
in DIY
Hi all
On a passive pre what step attenuator to use.
10k 50k or 100k.
Many thanks

Comments

  • sovereignsovereign
    I'd Waite for the clever blokes to pitch up here, but in the meantime you will probably find it is dependant on your power amp. I know for my LDR passive pre 10k was far too low and it was 100k I needed in the end
  • justshoemarkjustshoemark
    Hi thanks for that. What power amp do you have?
    Many thanks
  • uglymusicuglymusic
    I'd like to try a Passive with my SECAs, as I've more often used passives than actives.

    It would also free up the Listen to use as a nice headphone amp. Col tells me the headphone amp is Class A (the pre- is Class A/B). All I need is a pair of headphones!
  • justshoemarkjustshoemark
    Yea I'm interested in the pre they are doing just can't seem to get hold of one.
    Col said they are realy good.
  • uglymusicuglymusic
    The Listen is a now-defunct TQ product. Col's new pre, as far as I can see, is a completely different beast.
  • justshoemarkjustshoemark
    That's what he said anyone no how much they are.
  • sovereignsovereign
    justshoemark said:
    Hi thanks for that. What power amp do you have? Many thanks
    Hi Mark
    One of Colins 40W Class A SECA
  • AntiCrapAntiCrap
    justshoemark said:
    That's what he said anyone no how much they are.
    Ask Sondale
  • justshoemarkjustshoemark
    Thanks col I have ask but no reply. He is prob busy.
  • SpevSpev
    Did anything ever happen with the pre that was mentioned back in june/july 2015?  I remember getting in touch to try and buy a board but then forgot about it and didn't hear anything.  Is that the pre that's being mentioned here?  (still interested...)
  • justshoemarkjustshoemark
    Yea think it might be that one.
  • sondalesondale
    The PreAmps are currently being made and tested by Colin and yours truly. The smaller one Q20P is sounding very good but is wanting a few case changes before being released.

    The M50P is awaiting its case and then comes the build / fit and test; early tests are again very favourable.

    As to price, until we have an exact cost on the build side I am not really prepared to release a figure.

    The preamp that we played around with last year underwent a few changes to become the M50P.

    No boards for any of the EWA products will be released for DIY and I do not see us creating a kit preamp only kit SECA amplifiers.

     
  • justshoemarkjustshoemark
    Thank you for the info
    May I ask you the difference between the to pre's?
    And I no you can't rush perfection but just wondering a time scale.

    Many thanks Justin
  • PACPAC
    Here's one I made earlier:

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    The pot value is dependant upon both your source output impedance and power amp input impedance, and the pot acts like a voltage divider.  Your source likes to see a load of between 10 to 20 times its output impedance.  eg if you have something with an output impedance of say 500 Ohms, the pot value should be about 10K worst case but to confuse things, this isn't the actual rating of the pot but the sum of the worst case which is about the rated value/4 (-6dB point), therefore a 50K pot would be an appropriate value in this case.  If your input impedance for your power amp was say 100 KOhms, then your output impedance from a 50K pot will be 50/4 plus the full value of the source output impedance, so 13KOhms.  This should work ok, even though the 10 times minimum rule of thumb ratio of output to input impedance hasn't quite been met.

    Get it wrong and you have too heavy a load for the source to drive.  If your source impedance is too high for the pot value or the power amp input impedance, then you can still use a passive but you'll need an active buffer to allow the source to drive the pot.  If the value of the pot is too low and the capacitance opf the interconnect too high, you'll kill dynamics and rob HF.  Get it right and you have the nearest thing to a straight bit of wire with the benefits in SQ this can bring (ie low distortion and full frequency response).  The last thing to remember is that there are several ways to effect the earthing. I like to use individual switched earths for low noise.  Others use star earthing and others might prefer ground plane earths.
  • uglymusicuglymusic
    That's a nice looking thing, Paul. Did you build it for your own use, or is it a 'production' item?
  • justshoemarkjustshoemark
    That looks nice. I see what your saying with the loads how do you find out the load of a power amp.
    Many thanks.
  • sovereignsovereign
    Nice one Paul.

    Looks about as complicated as a pre amp needs to be.  
    Good luck!
  • PACPAC
    uglymusic said:
    That's a nice looking thing, Paul. Did you build it for your own use, or is it a 'production' item?
    It was built for my own use with a Radford STA25 Dave, but I had several enquiries after building it to make similar ones for others, which I'm happy to do.

    justshoemark said:
    That looks nice. I see what your saying with the loads how do you find out the load of a power amp. Many thanks.
    Contact the manufacturer or look on the specifications details for your amplifier where input load ought to be stated (along with input sensitivity).
  • uglymusicuglymusic
    PAC said:
    uglymusic said:
    That's a nice looking thing, Paul. Did you build it for your own use, or is it a 'production' item?
    It was built for my own use with a Radford STA25 Dave, but I had several enquiries after building it to make similar ones for others, which I'm happy to do.


    Thanks Paul. I'll give it some thought. I know what I'm doing next with my system, but after that I've got to decide if to scratch the itch of finding out what a Passive sounds like with the SECAs.
  • justshoemarkjustshoemark
    Thanks Paul. We'll I'm using a musical fidelity f19
    Il have to look at the spec see if I can find it out.
  • PACPAC
    Spec sheet wont list it....you'll need to contact MF to find out.  Input sensitivity is 750mV I think so no gain needed.  As long as input impedance is ok, a passive will work fine.  Just look at your source components.  If all digital front end, just use a 10K or 10K pot.  The source output impedance will be low and there's few power amps that a 10K or 20K pot wont drive.  Use Log pots, not linear pots.
  • DocfosterDocfoster
    A handsome passive there, Paul. :-)
  • justshoemarkjustshoemark
    Thanks Paul that's good imfo there.
    I've just managed to buy an albarry pre on eBay. Didn't mean to realy just put a lucky bid on.

    What do people think. Is passive better or any difference to passive pre?

    Many thanks justin
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