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  1. Old Comment
    bear's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    ok, so moving the buffer next to the "pot" eliminates the cable and then drives the cable with a very low Z, removing the impedance that is generating the noise? But is the cable the offending element, or would it still generate noise if the "pot" were inside the amp, AT the input stage??

    And, also, isn't the input to the buffer the same (in essence) high Z input??

    Seems confusing to me.
    permalink
    Posted Today at 01:07 PM by bear bear is online now
  2. Old Comment
    rjm's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    Absolutely. Worst case is when the wiper perfectly divides the resistance in half. This is usually around the 2-3 o'clock position, so it is rarely encountered in practice... but above 11-12 o'clock the impact can often be heard.
    permalink
    Posted Today at 07:50 AM by rjm rjm is offline
  3. Old Comment
    bear's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    so ur saying midrange of the theoretical pot is the most problematic for this noise issue...?
    permalink
    Posted Today at 03:40 AM by bear bear is online now
  4. Old Comment
    rjm's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    A 47k volume potentiometer at maximum is just a 47k resistance to ground. There is no series element, so the output impedance is defined by what the stage before it consists of (in parallel with 47k). It is essentially removed from the circuit - no negative influence - but at the same time it's function (adjusting the volume) is also defeated.
    permalink
    Posted Today at 02:48 AM by rjm rjm is offline
  5. Old Comment
    bear's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    If you turn the volume all the way up...?
    It looks like another resistor in parallel with the input resistor on the "amp", no? This would have what sort of negative effect... and I'm a bit unclear how the input to a buffer looks much different than the input of a power amp, unless you are saying the interconnect is going to generate that noise, is that what's being said?

    The problem with a "TVC" is that it is a transformer, and with that comes all the issues of using transformers... having looked at the waveforms coming off some of the highly regarded ones, I've not opted to go that way.
    permalink
    Posted Yesterday at 11:37 PM by bear bear is online now
  6. Old Comment
    rjm's Avatar

    Quick note on line level output currents

    Hi Jan,

    Thanks for your comment.

    I have read passionate arguments insisting that dB should not be adulterated to dBv or dB(V) since the expression is unitless and 20 dB is still 20 dB irrespective of whether we refer to voltage (10x) or power (100x). So I tend to adhere to that convention.

    The existence of pro-audio standards like dBu muddies this of course, and since we are discussing line output levels in absolute terms I'll add a note above.
    permalink
    Posted 22nd July 2016 at 12:09 AM by rjm rjm is offline
  7. Old Comment
    jan.didden's Avatar

    Quick note on line level output currents

    Good summary. I only want to comment that your dB values seem to actually be dbV - dB below 1V. Since there are other 'dB values' like dBu (ref. 1mW @ 600 ohms) and dBr (ref some explicitly stated ref) I think this should be mentioned.

    Jan
    permalink
    Posted 21st July 2016 at 10:37 AM by jan.didden jan.didden is offline
  8. Old Comment
    rjm's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    I am just wondering how many beers that's after...

    The diagram has been corrected to show the correct output impedance of the attenuator.

    Also, I agree that there is no need to worry as long as the driving impedance is below 220 ohms, 47-100 ohms being pretty typical in line stages anyway. If you have a 10k or smaller pot., and never turn up the volume very high it's likely almost always within that safe range. The only downside you face then is the low input impedance.
    permalink
    Posted 20th July 2016 at 12:45 PM by rjm rjm is offline
    Updated 20th July 2016 at 10:07 PM by rjm
  9. Old Comment
    Calvin's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    Hi,

    "A Rant on ..." is just the right phrase for this little treatise.
    It just highlights possible flaws, thwarfs possible advantages, adds a failure at a decisive but rather inconspicuous point and is based on a single briefly observed circuit design, just after the fashion of īDonīt disturb my opinions with factsī or īI donīt care if my test procedure is unsuitable as long as it can support my prejudice.ī
    So letīs see if thereīs some flesh to munch ...
    · "sure enough the system noise increased ..."
    - The omittance of the Buffer didīt change the gain structure of the system, hence it didnīt change the noise gain and the noise gain structure at all.
    The only change been the cable connecting point to the power amp moving from ībehindī (or with) the buffer to ībeforeī (or without).
    Seen from the VC the input impedance of the Buffer was only replaced by the input impedance of the power amp plus cable.
    If we now assume a 1m long cable run as rather electrically or noise-wise negligible -or model it as bunch of stray parameters- and assume same impedances for the buffer, resp. the power amp input, than theresīs no reason to assume a higher noise value for the buffer-less mode.
    In fact just the opposite applies as the bufferīs internal noise is omitted with.
    · "Noise is usually induced as a current..."
    - This is correct so far. As it is mainly related to the base/grid/gate-current of the input devices itīs obvious that bipolars with their much higher input currents suffer more from current noise than FETs or Tubes.
    When in the discussed system the noise actually increased it could only be due to different input stage impedances and topologies of the buffer, resp the power amp, or radio interference due to indecent shielding of the cable.
    In other words, if the VC īseesī a say 47k impedance it doesnīt care at all noise-wise if that impedance is directly at its output pin or at the end of a 1m cable run (it may give bandwidth issues ... but thatīs not the point here)
    · "For a given speaker efficiency and cartridge sensitivity, the gain is fixed. If you don't have a line preamp with gain, the gain must be added to the phono stage or the power amplifier."
    Though theoretically true it doesnīt apply so in praxis.
    Ideally the gain structure is as such that You apply only so much gain as required.
    Now allmost all modern sources supply for output voltage levels of ~6dB to 10dB more than needed to drive the power amplifier into clipping.
    Hence attenuation of the signal is required already.
    Due to increasing tolerances in Potis towards higher attenuation values, lower attenuation values would be preferrable.
    rem: Probabely the only true reason for high priced Potis is their lower inter-channel tolerance at high attenuation values.
    Adding another gain stage/Buffer only makes matters worse.
    Itīd add fully unneccessary gain, noise and THD.
    · "From an impedance standpoint it is more susceptible to noise pickup than even the cartridge output!"
    - Correct, but thatīs the reason we use decent shielded cables ... to not pick up radio noise.
    And what impedances are we talking about anyway?
    The output impedance of a classical Poti varies between 0 and 1/4th of its nominal value - not 1/2 as falsely suggested in the Sketch. (47k-Pot --> 0R-11k75 ... which brings Zeff down to 9k4 instead of 15k6, already fulfilling the rule of thumb of > 1:5 sender-output-impedance : receiver-input-impedance ratio).
    Almost all commercial preamps feature output resistors between ~47R and 470R to reduce capacitive sensitivity and/or to apply for muting etc. etc.
    So, whats the big differnce anyway, compared to the 0R-2k5 of a more appropriate 10K-Poti??
    - From a noise standpoint it is always best to put the highest gain to the front.
    Only if radio interference is to be expected a bufferīs low output impedance may be helpful, but certainly not another gainstage.
    A gain-stage only made sense if weīd like to drive the cable with higher voltage levels to increase the signal-to-(interference)noise-ratio on the cable-run itself.
    But that required a dedicated attenuator at the power amps input or a power amp with correspondingly lower gain on the other hand.
    Use decent cabling and You donīt have to cure the system with questionable measures in first place.

    -- The only real reason that could speak against passive drive is impedance mismatch.
    This could lead to bandwidth issues.
    If the load impedance the VC sees is sufficiently high and(!) the upper bandwidth limit remains well above the audible range, than thereīs no reason -technically or acoustically- speaking against passive VC.
    This claim asks for VCs with low nominal resistance value (10k or less for Potis).
    The effect of varying output impedance of simple Potis -and as such varying bandwith and possibly too high output impedance can be omitted with VC-circuits featuring constant and lowish output impedance.
    Iīve done this in my Preamp and it works perfectly (switched resistor network, bit-weighted, ~1k).
    A optional cable-buffer can be switched unaudible into and out of the signal path on-the-fly via IR-control, allowing for reliable comparison.
    Over more than 10 years of continuous use it showed that only in severe cases, like cable runs of considerably more than 10m (unbalanced!) an/or input impedances considerably lower than 5kOhm, that the buffer could make a positive difference.
    With more conventional setups the passive mode was generally the preferred choice ... not because of better sound (only the soundstage seemed a tiny bit smaller with the buffer included), but simply to keep unneccessary parts/stages out of the signal path).

    jauu
    Calvin
    permalink
    Posted 20th July 2016 at 08:03 AM by Calvin Calvin is offline
  10. Old Comment
    abraxalito's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    You have so far omitted considerations related to the need for a power supply for the buffer. How did you power your 'absolutely brilliant' buffer?
    permalink
    Posted 19th July 2016 at 10:24 PM by abraxalito abraxalito is offline
  11. Old Comment
    anatech's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    Sure. A buffer is flat in impedance and frequency response. The transformer type does matter in quality, but so does everything else. My experience has been that a TVC can be good, but a nice buffer stage can be absolutely brilliant! Interactions at higher frequencies can be difficult to control using a TVC, and depends greatly on the source impedance. A real buffer is high Z input and normally low Z output no matter the source impedance. The buffer is usually less expensive too. Gimmie another reason to go electronic - I'm there already!

    -Chris
    permalink
    Posted 19th July 2016 at 10:01 PM by anatech anatech is offline
  12. Old Comment
    abraxalito's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    Do you have any evidence/reasoning to support the notion that the trafo is sub-optimal compared to the buffer? Surely it would depend on the quality of the buffer, no?
    permalink
    Posted 18th July 2016 at 10:49 PM by abraxalito abraxalito is offline
  13. Old Comment
    anatech's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    Even the transformer is sub-optimal compared to the normal active buffer to drive the cables and input section of the amplifier. So while you are quite right in that the TVC is the best option if you are forced down that road, there are better ways to perform that function.

    Even the name "passive preamplifier" is utter nonsense.
    permalink
    Posted 18th July 2016 at 01:59 AM by anatech anatech is offline
  14. Old Comment
    rjm's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    Yes, at least that way you harness the voltage usually thrown away in the attenuator and put it towards lowering the output impedance...
    permalink
    Posted 16th July 2016 at 08:12 AM by rjm rjm is offline
  15. Old Comment
    abraxalito's Avatar

    A Rant on Why Passive Preamps are Totally Stupid

    A transformer (TVC) to me is the only sensible passive pre-.
    permalink
    Posted 16th July 2016 at 07:19 AM by abraxalito abraxalito is offline
  16. Old Comment
    rjm's Avatar

    AT-HA5000

    None I'm afraid, sorry.

    Just looking at the circuit alone - unless they completely fluffed the power supply it should sound far better than just decent. Of course with a price tag to match...
    permalink
    Posted 16th July 2016 at 02:17 AM by rjm rjm is offline
  17. Old Comment

    FM tuner for Jitter analysis

    I found info for calibrating this setup here: http://www.keysight.com/upload/cmc_u...?&cc=US&lc=eng Pages 28 and 29. I'll set the stuff back up and try the calibration suggested to see what I get.
    permalink
    Posted 14th July 2016 at 09:20 PM by 1audio 1audio is online now
  18. Old Comment
    Tam Lin's Avatar

    High Sample Rate PCM – II

    Because the corrections are based on dynamic analysis.
    permalink
    Posted 13th July 2016 at 10:23 PM by Tam Lin Tam Lin is offline
  19. Old Comment
    abraxalito's Avatar

    High Sample Rate PCM – II

    A lookup table corrects for static distortions but what if the errors are dynamic ones?
    permalink
    Posted 13th July 2016 at 01:37 AM by abraxalito abraxalito is offline
  20. Old Comment

    AT-HA5000

    Hi !
    I am very interested in your project. Are there any news ?
    Moreover I see your following comment

    Quote:
    .. got me thinking again about the Audio Technica AT-HA5000 which is something of a benchmark in its class ...
    Do you have any information or direct experience about the actual sound quality of the original AT-HA5000 amp ? how good is it ?
    Thanks a lot indeed for any information.
    Kind regards, gino
    permalink
    Posted 7th July 2016 at 06:49 AM by ginetto61 ginetto61 is offline
    Updated 7th July 2016 at 06:52 AM by ginetto61
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