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  1. Old Comment
    fas42's Avatar

    What's the PSRR of an emitter follower?

    Straight off the top of my head, use Audacity to create a 20Hz square wave, the level of which is near full scale; then add a HF signal of appropriate frequency on top which causes the summed output to reach maximum amplitude, or a touch below - see what the distortion behaviour of that is ...
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    Posted 17th April 2015 at 01:37 AM by fas42 fas42 is online now
  2. Old Comment
    abraxalito's Avatar

    What's the PSRR of an emitter follower?

    I've been wondering what might be the best measurement to capture output stage PSRR issues. As you say perhaps not FFT as the average effect is going to be low but the interference is impulsive. With audio signals having high crest factors the signal isn't in the problem area (near to the rails) for very long.
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    Posted 16th April 2015 at 11:23 AM by abraxalito abraxalito is offline
  3. Old Comment
    fas42's Avatar

    What's the PSRR of an emitter follower?

    Nice detective work there, Richard! The point about PSRR being dependent on the instantaneous output level is important, I have noted that some people run tests of a low level HF signal riding on the back of a LF waveform that approaches clipping - and transient excess distortion occurs at the peak values of the LF input. In some sim's I've also noted similar behaviour - time domain glitches are where the action is, and these may easily be missed or hidden in FFT analyses.
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    Posted 16th April 2015 at 10:51 AM by fas42 fas42 is online now
  4. Old Comment
    Mark Whitney's Avatar

    Are PSRR plots all they're cracked up to be?

    Compare the supply currents with a 10kHz signal to a 10kHz plus 100Hz signal.
    Or the same test comparing a filtered and a unfiltered audio signal.

    You could also add DC to each of the outputs and force class A operation.
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    Posted 16th April 2015 at 10:15 AM by Mark Whitney Mark Whitney is offline
    Updated 16th April 2015 at 10:20 AM by Mark Whitney
  5. Old Comment
    abraxalito's Avatar

    Are PSRR plots all they're cracked up to be?

    Mark do you mean because this is an active system the HF isn't riding on the LF in the same amp, so the OPS is changing halves more frequently? If so good point that I'd not considered before, I need to reflect on it.

    Yep - bridged amps see half the impedance. When I was driving the tweeter directly that wasn't with a bridged amp though - just a normal TDA7265. Thanks to some dodgy engineering on some other speakers I tried trafos - one model is using TDA8947 (which is bridged) and fed into 4ohm units. If not for that I'd not have discovered trafos!
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    Posted 16th April 2015 at 09:17 AM by abraxalito abraxalito is offline
    Updated 16th April 2015 at 09:19 AM by abraxalito
  6. Old Comment
    Mark Whitney's Avatar

    Are PSRR plots all they're cracked up to be?

    A few things that may influence the PSRR:

    By reducing the low frequencies there is an increase of the output switching between the two rails and therefore an increase of HF rail current.

    Add to that the low impedance (2ohm) seen by each of the bridged IC. The bypass caps and IC are working harder than with a normal 8 ohm load and full audio signal.
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    Posted 16th April 2015 at 09:08 AM by Mark Whitney Mark Whitney is offline
  7. Old Comment
    abraxalito's Avatar

    47 Labs 0347 "Treasure" stereo amplifier

    Links to 47 Labs site no longer point to anything.
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    Posted 16th April 2015 at 04:56 AM by abraxalito abraxalito is offline
  8. Old Comment
    abraxalito's Avatar

    Are PSRR plots all they're cracked up to be?

    Thanks for the question. The reason that the roughness on HF is related to the PSRR is because it reduces with more capacitance on the rails. More caps means lower supply ripple. No other changes were made than adding more low ESR caps close to the chip. I have no other explanation but that its related to power supply noise and the chipamp's ability to reject that. I got a similar effect with my TDA8566 - HF roughness went down from adding a lot (30 or so) of 10uF ceramic caps across its supply pins.

    On your second point perhaps you could turn up KSTR's work in a similar vein - he convinced me that this is a reasonable method to eliminate the negative rail supply issues. He also reported improved SQ when he implemented his idea. So what about my explanation is lacking for you?
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    Posted 16th April 2015 at 12:13 AM by abraxalito abraxalito is offline
  9. Old Comment
    rjm's Avatar

    Are PSRR plots all they're cracked up to be?

    A couple of quick comments.

    1. What makes you think that the roughness you hear (HF distortion?) should be related to the power supply ripple rejection figures? Sorry I don't follow the logic.

    2. Your last reply "...no signal-related current is flowing between local 0V and VEE meaning there can be no ripple." sets off alarm bells. I'm not saying you are wrong, just that your explanation does not convince.
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    Posted 15th April 2015 at 11:39 PM by rjm rjm is offline
  10. Old Comment
    rjm's Avatar

    I've been cloned!

    Thanks for the suggestion. To be honest I'm not out to stop the sale of the module. All I would ideally ask for is attribution.

    I think I will contact the seller, though I doubt it will come to anything.
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    Posted 15th April 2015 at 11:28 PM by rjm rjm is offline
  11. Old Comment
    abraxalito's Avatar

    Are PSRR plots all they're cracked up to be?

    Its important if your circuit configuration is the usual balanced one around ground yep. I've bypassed this issue though by creating a local 0V from a couple of TL431s in series (because the max per device is 30V and I want 31-37V depending on my power supply) and referencing that local 0V directly to pin4 (VEE) of the LM4766. So no signal-related current is flowing between local 0V and VEE meaning there can be no ripple. Therefore only the positive PSRR matters in my configuration.
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    Posted 15th April 2015 at 02:21 PM by abraxalito abraxalito is offline
  12. Old Comment

    Are PSRR plots all they're cracked up to be?

    Forgive me if this is an obvious question, but isn't the -PSSR just as important as +PSSR in a split supply device? Particularly because it's only 50% of what +PSSR is at 20K?
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    Posted 15th April 2015 at 01:16 PM by MrSlim MrSlim is offline
  13. Old Comment
    fas42's Avatar

    Fix it time for PC monitor speakers

    Things have moved on - I have yet another laptop to use as my main m/c, and this is looking promising in terms of the internal sound - the built in speakers don't obviously misbehave, it will be worthwhile exploring a bit.

    I'll start another blog entry detailing what I find ...
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    Posted 15th April 2015 at 07:37 AM by fas42 fas42 is online now
  14. Old Comment
    abraxalito's Avatar

    Are PSRR plots all they're cracked up to be?

    Good questions Frank.

    I'm working on part 2 right now and went back to first principles - that is the PSRR of a single transistor operating as an emitter follower. At least that's a common feature of all the opamp and chipamp output stages which aren't rail-to-rail types.

    Yes to your question - PSRR in a closed loop (feedback) circuit is going to vary according to gain - more gain means poorer PSRR. Tom's ultra-high loop gain amp has proven that - but his poor results in THD for the 'bare' LM3886 on a non-bench PSU is one of the things which prompted me to look into this rather deeper. A quick finger in the air shows Tom's THD+N (0.003%) with naked LM3886 is more than 10dB worse than would be expected from the PSRR plot of the chip.
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    Posted 15th April 2015 at 06:18 AM by abraxalito abraxalito is offline
  15. Old Comment
    fas42's Avatar

    Are PSRR plots all they're cracked up to be?

    ... if that PSRR plot is to be believed ...

    The point being, how does one extrapolate those numbers, tested in a certain fashion, to all situations in a working setup? Does the PSRR behaviour alter when used at different gain settings - I did some sim's quite some ago where I used a fully modelled, through rather basic opamp circuit, and the PSRR behaviour at normally used closed loop gains was not as expected! Also, does the PSRR behaviour alter when used at different loadings?

    So much is assumed - only a final, working circuit in the configuration that will be used, that is rigorously tested in a fully debugged jig should be trusted, IMO, if one wants numbers, .

    Curious how you going to check this out in LTspice, if a fully valid model for the opamp is not available ...
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    Posted 15th April 2015 at 06:11 AM by fas42 fas42 is online now
  16. Old Comment

    The best sounding I/V resistor

    Ive heard about I/V resistor so many times. Why not try this?
    Find out, which is the lowest R possible at the DAC output, then take the minimum value as a fix resistor and a trim in series and listen, where it sounds best. There are fantastic trims out there, now. Ive tried that at another place. But be aware about the solder direction of both! The trims are 1202 from Texas components... if you found the sweet spot, you can take a fix value again or the combination...depending on what you like best (and I cannot predict the result)
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    Posted 13th April 2015 at 03:09 PM by Scheff Scheff is offline
  17. Old Comment
    kevinkr's Avatar

    I've been cloned!

    Have you considered filing a complaint of IP infringement with eBay? They do actually tend to take these complaints seriously.
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    Posted 13th April 2015 at 02:22 AM by kevinkr kevinkr is offline
  18. Old Comment

    Schitt's flagship DAC uses a not-for-audio DAC chip.

    Would be interesting to see how it does in the Jurgen Reis test.
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    Posted 4th April 2015 at 04:22 AM by Quip Quip is offline
  19. Old Comment
    wlowes's Avatar

    6C33C OTL Amp Comes to Life

    Thanks Ryanj
    Now paired with your Distinction TDA1541a board there is some amazing sound coming from these beasts!
    Walter
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    Posted 30th March 2015 at 12:13 AM by wlowes wlowes is offline
  20. Old Comment
    triode_al's Avatar

    Stock Status

    you should find a european partner
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    Posted 28th March 2015 at 10:39 AM by triode_al triode_al is offline
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