Pressing on with the sim to test my hypothesis - so far all results are from running positive rail PSRR. This is the better one, when I've understood this I'll move on to the negative rail.

Installing a hefty RC on the positive rail which only leaves the two output emitter follower stages (driver & output) exposed to the ripple gives 110dB below 80Hz. This seems too good - I rather suspect the models now, particularly for the output devices. Perhaps these transistors have no output (collector-base) capacitance?

So it looks as if it might not be the output stage, rather the TIS (or VAS) which is the culprit. However protecting only this stage from the ripple while having the input stage exposed gives a very poor plot (power supply gain above a few kHz) so this approach looks invalid. Anyway going back to the detail of the plot - with only the driver and output stages seeing ripple, we have about 61dB PSRR at 20kHz, from the +ve rail. Moving the driver collector to the clean supply, the performance gets amazing (too amazing so the models must be suspect) - 126dB below 1kHz and 107dB at 20kHz.

So rather looks like my 'output stage is the culprit' hypothesis has taken a fatal hit. That is until I verify what the output transistor models really are. Stay tuned.....

OK I am gradually getting the hang of understanding PSRR better, let's see how my explanation goes.

After fixing up the imbalanced current mirror (two resistors in the emitterss - one got changed at some point) now the PSRR in the normal 26dB closed loop gain maxes out at 65dB below 1kHz. This was the anomaly I had been scratching my head over for a few hours. That's because I expected to see it climb as shown in the DS PSRR graph, reaching 103dB minus the closed loop gain of 26dB or about 77dB. But in fact its 12dB short of this.

My understanding of how this discrepancy arises is that those PSRR plots are for the open loop case and are referred to the input. In such a case the input signal is obviously very small, there's very high OL gain at LF. So what dominates the PSRR is the supply rejection of the input stage. This is rather akin to noise figures being dominated by input stage noise in high gain configurations.

However in the CL case with only 26dB of gain the output signal is much smaller and so tthe output stage PSRR can dominate the figure - which is what I think is happening. When feedback is applied the limiting factor is no longer the input stage's PSRR rather the output stage's. The figure of 65dB seems remarkably similar to that achieved by the LME49600 buffer. Obviously that one is output stage limited as its only an output stage

So if this understanding is correct then the usual chip amps do not in fact offer better PSRR at lower freqs (than the TDA8566) as I had previously imagined going by their PSRR plots. Also what follows from this (if its correct) is there's precious little point in filtering the small signal stages (as can be done on the TDA7294 for example) because the damage is being done at the output stage. So this would mean chipamps do not in fact lose out to discrete amps where the small signal stages can be filered to an arbitrarily high level - both are in fact output stage limited in practice.

Yep, sounds interesting ... send it along when you reckon you've got it sorted ...

Thanks,
Frank

Where's the evidence they care about the sound? The website is pure FFT koolaid from what I can see. Cyril Hammer being quoted on the WBF thread is talking BS about amplifier propagation delay, doesn't hold out much hope he has a clue about amp design, at least not ones with feedback.

Yes - it seems to me that its all a question of 'what's the low hanging fruit?'. If you listen to people like CopperTop (oops, sorry Groucho) or Tim on WBF, seems they consider the low hanging fruit to be passive speakers. Make 'em active first they say! I used to hold that view, but my current journey has found the lowest hanging fruit to be the DAC. Then the amp. Only now I've found out how hard it is to get a decent PSU for an amp do I consider going active. That's for the amp's benefit though, which is not a mainstream reason as far as I can see - most will say its because passive XOs are the lowest hanging fruit.

Having said that I realize on reflection that I haven't got the timeline quite right. Looking back on this blog, I started out with active speakers and found poor implementation - particularly grounding. Then I went on to power supply filtering. Then I worked on DACs, still feeding my active speakers. I only went back to passive speakers when I wanted to develop amps because of not wanting to build more than two channels at a time for SQ evaluation and also not to rely on questionable transparency active XOs. It was only then that I discovered that passive speakers did not suck as badly as I'd previously thought.

Incientally on amps, today I've made some progress on simulating PSRR in LTSpice, something we touched on when comparing chipamps. I have a discrete schematic of LM1876 which matches the OL gain and PSRR in the DS for the LM1876. Its based on the schematic shown in the DS, with some minor tweaks. For example I need 20pF comp capacitance rather than the 10pF shown. There's also a transistor upside down in the first current mirror. What is most interesting is - I do not get PSRR as high as shown in the DS when I have a closed loop amp. It follows the HF plot very closely but the PSRR referred to the output tops out at 51dB below 10khz and stays stuck there as the freq goes down. Wanna play with the schematic?

Maybe won't be necessary - I've found an unbalanced current mirror is at least partially responsible for this, work in progress...

Got to knock off the big'uns before the littl'uns show their faces, . This is a round and round and round game, it's easy to ditch, dismiss a tweak at one stage because its impact is swamped by a "bigger" problem ... only to have it become crucial when the system is more "resolving", .

"Vicious" recordings are a good way to spot whether there is some subtle interaction going on, which is not obvious with cleaner material ...

Yes everything matters, but not everything matters equally. I had the same 0603 resistors for I/V duty and figured if I could hear the noise (and I could with these shunts in the DAC PSU) then maybe I could hear it in the I/V. So I swapped them out for thin-film. Can't say I've noticed a difference. Whereas 'received wisdom' is that I/V resistors are inordinately sensitive to construction quality. Perhaps my system is just not sufficiently resolving

A bit of the ol' everything matters, yet again ...

There's a small twist to the story of the poor sounding active filter - it was inadvertently built with noisy shunt regs. I have found the resistors I used have oodles of excess noise (0603s really aren't recommended unless the high stability thin-film types). So I'm modding the shunts to take thin-film (I happen to have a couple of reels) and for the other voltage setting resistor whose value I don't have in thin-film, I'm substituting a 1206 type thick film which are more benign. Then I'll have another listen....

Which model did you get? The 0ana60 seems like the basic one, it goes up from there. (and I mean, way up!)

A few months back I was looking for the Hakko or a Weller. Unfortunately the Weller ones seemed considerably overpriced here in Greece. I then searched for Hakko with no luck. The internet had much better prices, but the shipping costs most of the time were ridiculous. I ended up buying an ERSA which was on sale at a local hardware store, and never regretted it.
Still going strong with no issues and I really like the timer safety stuff.

As an example, the original circuit specifies +18 V DC power. To obtain this, you would use a 18 or 20 V Zener, and a transformer with 18, 20, 22, or 24 VAC secondary.

I bought a Hakko 936 many years ago and have never regretted it. The only regret I had was not buying one sooner!! The FX-888 was it's replacement (and now the FX888D)

I've not used the newer models but I have never been anything but 100% happy with my 936.

Tony.

I think the Hakko FX-888 is no longer a current model, it is replaced with the FX-888D with a digital display and push button controls.

I picked up one of these... not bad for what I'm doing.

Hakko FX-888 Soldering Station: Amazon.com: Industrial & Scientific

i have the wes51 and i love it

The sound review is online.

Hi great job thank you, can you clarify the voltage with the zener ? I am a bit noob and my English are not so good, so a more detailed explanation would be appreciated.
Regards

Looking forward to your results. Even though I'll probably never buy them.

No I am not going to be much more specific in this instance as I didn't listen intently enough to understand what the colourations were. I just didn't enjoy listening to this circuit as much as the earlier passive one. But I can give you a hint from the kinds of music I listened to - on piano at the top end of its register, some notes sounded too right to be true (like hyper-real) and others missed the mark. Leonard Cohen's voice wasn't as sexy, brass lost some of its intense bite and became more electronic--y. This could easily be caused by having resonances in the power supply impedance at particular frequencies but its just speculation.

The original reason I did the active filter was to get a more portable (smaller size primarily) solution but when I'd optimized the power supplies with enough caps for low impedance at HF I realized there was practically no size advantage after all. So passive wins on all fronts but ease of manufacture. Probably I'll find someone to wind the coils with sufficient precision so even this will get sorted...