Passive Preamp Project - Help?

ambiophonics
ambiophonics Posts: 726
edited April 2015 in DIY, Mods & Tweaks
I'm in the process of building a passive preamp to try out in my system. I have most of the parts ordered including a ladder type stepped attenuator but am having a hard time understanding how to properly choose the value of the stepped attenuator. It seems its a trade off between possibly loading down the source or high frequency roll off. I don't understand all of the math involved in figuring this out despite extensive reading online. Anyone care to help shed some light on this issue? I'm happy to elaborate on my specific system specs and what I've read/found so far.
2 Channel - Polk SDA-2BTL, Carver TFM-35, Peachtree iDAC, Qobuz streamed via Episode Lynk using bubble UPnP server
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  • mhardy6647
    mhardy6647 Posts: 33,899
    edited April 2015
    edit: I'm sorry, bur my knee-jerk answer wasn't adequately correct enough to be helpful.

    Did you look at this design? http://diyaudioprojects.com/Misc/DIY-Stepped-Attenuator/
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  • EndersShadow
    EndersShadow Posts: 17,593
    DIYAudio may be a better place to post... lots of DIY guys there......

    I'm sitting on a cool little DIY Pass Pre-amp myself with a stepped ladder resistor volume knob. Its gonna go into the closet for later as I've got a nice older HT Pre-amp incoming so I've got volume control.
    "....not everything that can be counted counts, and not everything that counts can be counted." William Bruce Cameron, Informal Sociology: A Casual Introduction to Sociological Thinking (1963)
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  • EndersShadow
    EndersShadow Posts: 17,593
    FYI....

    http://app.audiogon.com/listings/tube-passive-pramplifier-attenuator-48-steps-2015-03-10-preamplifiers-72310-ar
    "....not everything that can be counted counts, and not everything that counts can be counted." William Bruce Cameron, Informal Sociology: A Casual Introduction to Sociological Thinking (1963)
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  • ambiophonics
    ambiophonics Posts: 726
    mhardy6647 wrote: »
    edit: I'm sorry, bur my knee-jerk answer wasn't adequately correct enough to be helpful.

    Did you look at this design? http://diyaudioprojects.com/Misc/DIY-Stepped-Attenuator/

    That's funny, I saw that post earlier and left a comment - just got a detailed response on the guys site and a separate personalized email response from him. Seems like a very nice guy.
    2 Channel - Polk SDA-2BTL, Carver TFM-35, Peachtree iDAC, Qobuz streamed via Episode Lynk using bubble UPnP server
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  • mhardy6647
    mhardy6647 Posts: 33,899
    Looked like he knew what he was talking about, as well.

    Impedance matching is is the big trick to passive "preamplification" (at least, as I understand it). I'm curious -- did you consider using a tapped transformer or autoformer for attenuation rather than a stepped resistor-based attenuator? See, e.g., www.intactaudio.com
    (that's the route I went, FWIW -- the autoformer volume control seems more transparent to my ear)
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  • ambiophonics
    ambiophonics Posts: 726
    The autotransformers look great but they're a bit outside of the budget for this build. I'm only going to spend about $100 total on this build just so I can see if it really makes a noticeable difference.
    2 Channel - Polk SDA-2BTL, Carver TFM-35, Peachtree iDAC, Qobuz streamed via Episode Lynk using bubble UPnP server
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  • mhardy6647
    mhardy6647 Posts: 33,899
    If you have enough drive from your source component for the power amp and if the impedance match from source to power amp is good... it will :- )
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  • ambiophonics
    ambiophonics Posts: 726
    edited April 2015
    Here it is! Still needs some labels for the controls and rear panel but it's ready to go otherwise. The stepped attenuator took forever to arrive from Taiwan and then was not the same type I ordered. I ordered a ladder type 100k with KOA 1% resistors and was sent a series type 100k with Vishay SMD precision resistors. I was given a refund for the part the sent me so I decided just use that for now. I have inputs for CD and Phono plus a home theater bypass input that doesn't go through the volume control. The 3 position switch chooses between the three inputs. If anyone is interested I'll include the resistance measurements of each volume position below. Seems pretty closely matched and much better than the volume pot I removed from the TFM-35 which was pretty inaccurate in the lower range.

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    Left Channel

    99.6k
    99.1k
    98.9k
    98.7k
    98.5k
    98.1k
    97.7k
    97.2k
    96.5k
    95.7k
    94.7k
    93.4k
    91.8k
    89.6k
    87.2k
    83.9k
    79.6k
    74.5k
    68.3k
    60.1k
    49.1k
    36.1k
    20.0k
    00.2ohms

    Right Channel

    99.6k
    99.1k
    98.9k
    98.7k
    98.4k
    98.1k
    97.7k
    97.1k
    96.5k
    95.6k
    94.7k
    93.4k
    91.8k
    89.6k
    87.2k
    83.9k
    79.5k
    74.4k
    68.3k
    60.0k
    49.1k
    36.1k
    20.0k
    00.2ohms
    2 Channel - Polk SDA-2BTL, Carver TFM-35, Peachtree iDAC, Qobuz streamed via Episode Lynk using bubble UPnP server
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  • mhardy6647
    mhardy6647 Posts: 33,899
    It looks absolutely terrific.
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  • ambiophonics
    ambiophonics Posts: 726
    Everything is plugged in now and it sounds really transparent - most noticeably things sound really good at low volume, excellent imaging, tight deep bass. I've only heard a few tracks so far and can't crank it up tonight.
    2 Channel - Polk SDA-2BTL, Carver TFM-35, Peachtree iDAC, Qobuz streamed via Episode Lynk using bubble UPnP server
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  • mhardy6647
    mhardy6647 Posts: 33,899
    "Transparent" is what the passive attenuators do, IME. There's the aforementioned (potential) trade-offs of no additional gain stage nor impedance matching (buffering) but the relative merits of passive vs. active just have to be evaluated empirically, I'd opine.

    Have fun!
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  • steveinaz
    steveinaz Posts: 19,538
    I have a Placette passive (vishay) and love it. Transparency off the charts, and outstanding linearity at any level.
    Source: Bluesound Node 2i - Preamp/DAC: Benchmark DAC2 DX - Amp: Parasound Halo A21 - Speakers: MartinLogan Motion 60XTi - Shop Rig: Yamaha A-S501 Integrated - Shop Spkrs: Elac Debut 2.0 B5.2
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