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Thread: Tweaking an 80s preamp

  1. #1
    Join Date: Apr 2008

    Location: Cheshire, UK

    Posts: 2,753
    I'm Clive.

    Default Tweaking an 80s preamp

    I have a Phillips Black Tulip 22AH280 preamp. It's quite a sophisticated circuit, not typical Phillips of the era - infact I believe it was designed and built in Japan. I've looked at the circuit, I don't want to make any major changes but some capacitors leap out at me as being candidates for improving:

    Phono stage output: a series pair of 10uF electrolytics making them bipolar. Replace with one 4.7uF polyprop.

    Linestage input: there's an NPN transistor fed by a 470nF electrolytic and a PNP fed by another 470uF electrolytic with the capacitor in reversed polarity. Could I replace these capacitors with polyprops or as I suspect is polarity important here? I'm confused by this configuration.

    Linestage output: a series pair of 10uF electrolytics making them bipolar. Replace with one 4.7uF polyprop.

    The service manual with the circuit is at:
    https://www.vintageshifi.com/repert....ice-Manual.pdf

    I'm thinking the series capacitors in particular could be changed, I suppose suitable film capacitors weren't available in the 80s or were very expensive. Is this wise?

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    TT 1 Trans-Fi Salvation with magnetic bearing + Trans-Fi Terminator T3Pro + London Reference
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  2. #2
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 5,168
    I'm Alan.

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    Very wise, yes I would replace all with PP.

  3. #3
    Join Date: Apr 2008

    Location: Cheshire, UK

    Posts: 2,753
    I'm Clive.

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    Quote Originally Posted by Firebottle View Post
    Very wise, yes I would replace all with PP.
    Hi Alan, thanks...can you shed any light on the two 470nF caps at the input side ie why the are in opposing polarities? I understand why the 10uF caps are configured as they are but the 470nF ones mystify me - that's not a difficult thing to do! I'm concerned that polarity matters at the input side for some reason I don't understand - so would PP caps be ok in that position?
    TT 1 Trans-Fi Salvation with magnetic bearing + Trans-Fi Terminator T3Pro + London Reference
    TT 2 Garrard 301 with NWA main bearing + Audiomods Series Six 10.5" + Ortofon 2M Mono SE
    Digital Lindemann Limetree Network Bridge / Chord Qutest
    Preamp Hagerman Cornet 2 phono stage / Temple Audio Suppercap Phono Stage / John Chapman's Slagleformer-based AVC-1 / Transcendent Sound Sub Buddy
    Power Amp Temple Audio Monoblocks with Supercap LPS / 300B WE91 SE Monoblocks / Firebottle EL84 PSE
    Bass Amp & DSP Behringer iNuke NU3000DSP x 2
    Speakers 1 Bastanis Sagarmatha Duo with twin baffleless 15" bass drivers per side
    Speakers 2 MarkaudioSota Viotti One

  4. #4
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 5,168
    I'm Alan.

    Default

    Yes it's a split supply buffer stage, so the small bias voltage on the transistors is going to be positive on the top one and negative on the bottom one, with respect to ground.

    Another good reason to replace with PPs, electrolytics aren't best when they only have a small polarising voltage.

  5. #5
    Join Date: Apr 2008

    Location: Cheshire, UK

    Posts: 2,753
    I'm Clive.

    Default

    I get it now. Thanks, I'm now confident non-polar caps will be fine in that position. I didn't want to get it wrong!
    TT 1 Trans-Fi Salvation with magnetic bearing + Trans-Fi Terminator T3Pro + London Reference
    TT 2 Garrard 301 with NWA main bearing + Audiomods Series Six 10.5" + Ortofon 2M Mono SE
    Digital Lindemann Limetree Network Bridge / Chord Qutest
    Preamp Hagerman Cornet 2 phono stage / Temple Audio Suppercap Phono Stage / John Chapman's Slagleformer-based AVC-1 / Transcendent Sound Sub Buddy
    Power Amp Temple Audio Monoblocks with Supercap LPS / 300B WE91 SE Monoblocks / Firebottle EL84 PSE
    Bass Amp & DSP Behringer iNuke NU3000DSP x 2
    Speakers 1 Bastanis Sagarmatha Duo with twin baffleless 15" bass drivers per side
    Speakers 2 MarkaudioSota Viotti One

  6. #6
    Join Date: Apr 2008

    Location: Cheshire, UK

    Posts: 2,753
    I'm Clive.

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    I swapped the electrolytics in the signal path for PP and also what was probably a polyester type for PP too. The change in sound is dramatic, how much is down to better a far type of capacitor and how much is down the electrolytics being 40 years old is hard to know...but it's an awful thing to have electolytics in the signal path...3 sets if I include the phono stage. The preamp is sounding darned good now.
    TT 1 Trans-Fi Salvation with magnetic bearing + Trans-Fi Terminator T3Pro + London Reference
    TT 2 Garrard 301 with NWA main bearing + Audiomods Series Six 10.5" + Ortofon 2M Mono SE
    Digital Lindemann Limetree Network Bridge / Chord Qutest
    Preamp Hagerman Cornet 2 phono stage / Temple Audio Suppercap Phono Stage / John Chapman's Slagleformer-based AVC-1 / Transcendent Sound Sub Buddy
    Power Amp Temple Audio Monoblocks with Supercap LPS / 300B WE91 SE Monoblocks / Firebottle EL84 PSE
    Bass Amp & DSP Behringer iNuke NU3000DSP x 2
    Speakers 1 Bastanis Sagarmatha Duo with twin baffleless 15" bass drivers per side
    Speakers 2 MarkaudioSota Viotti One

  7. #7
    Join Date: Oct 2012

    Location: The Black Country

    Posts: 5,168
    I'm Alan.

    Default

    Excellent.

  8. #8
    Join Date: Mar 2009

    Location: Nottingham

    Posts: 319
    I'm Ian.

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    Just bought one of these !! came up at a price I couldn't resist. Any other upgrade suggestions. Do you know what the switched outputs are - for MFB speakers ?

  9. #9
    Join Date: Apr 2008

    Location: Cheshire, UK

    Posts: 2,753
    I'm Clive.

    Default

    Quote Originally Posted by Mr.Ian View Post
    Just bought one of these !! came up at a price I couldn't resist. Any other upgrade suggestions. Do you know what the switched outputs are - for MFB speakers ?
    The circuit diagram suggests the switched output is for headphones - in addition to the main front mounted headphone socket.

    On the rear there's the main 2V output, the switched 2V output (phones) and a 3.6V output / +10db output.

    Certainly swap out any polarised caps in the signal path. The improvement was significant. Mine is / was a European version and was wired for 220V, it's a bit fiddly to get to the mains wiring but it's a good idea to switch it to the 240V tap on the transformer, if you have the same setup.
    TT 1 Trans-Fi Salvation with magnetic bearing + Trans-Fi Terminator T3Pro + London Reference
    TT 2 Garrard 301 with NWA main bearing + Audiomods Series Six 10.5" + Ortofon 2M Mono SE
    Digital Lindemann Limetree Network Bridge / Chord Qutest
    Preamp Hagerman Cornet 2 phono stage / Temple Audio Suppercap Phono Stage / John Chapman's Slagleformer-based AVC-1 / Transcendent Sound Sub Buddy
    Power Amp Temple Audio Monoblocks with Supercap LPS / 300B WE91 SE Monoblocks / Firebottle EL84 PSE
    Bass Amp & DSP Behringer iNuke NU3000DSP x 2
    Speakers 1 Bastanis Sagarmatha Duo with twin baffleless 15" bass drivers per side
    Speakers 2 MarkaudioSota Viotti One

  10. #10
    Join Date: Mar 2009

    Location: Nottingham

    Posts: 319
    I'm Ian.

    Default

    Hi Clive what make of caps did you use in the end anything special?
    Ian

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