Error in D. Self-Riaa?

[h_a]

So I finally find again some time to toy with a minimalistic phono pre and like the simplicity of the bootstrapped variant of the H.P. Walker phono stage presented by Douglas Self in his book on small signal audio design (p185). I attach the original schematic as it is to my knowledge not available online elsewhere (mods please let me know if that is copyright troublesome).

Punching my own variant of it into SPICE, I find the RIAA equalization (C3/C4/R5/R6) of the circuit (the same happens for the original unmodified circuit) is lacking precision, not to say it is far off at 20 Hz with 6dB or so.

Calculating the RIAA time constants* the values are indeed far off:

C3xR5=5940us (should be 3180us)
C4xR6=68us (should be 75us)
(R5||R6)x(C3+C4)=281.7us (should be 318us)

Curiously the same values are also used in the 2-transistor variant on the page before.

Using adapted values like 1nF/3.6nF/75k/883k bring back the accuracy, both in the equations as well as in simulation; (as these values are not available, one would select more suitable real-life values like 820+10pF/3nF/89.9k/1.06M or so).

As my prototype will be finished only in a few weeks, I can't check the actual circuit yet.

I can't see something wrong with my calculations or understanding of the circuit, but as I respect Mr. Self very well, I wonder if I'm missing something?


Hannes

*nice little exercise, see equations for the inverting RIAA-network at
Website of Wayne Stegall - Phono Equalization Calculations

[DF96]

Have you seen the classic RIAA paper by Lipschitz? In many RIAA networks, few of the CR time constants directly equal the RIAA values, as the components interact. You can get into trouble by comparing a (possibly wrong) RIAA network with a (possibly wrong) inverse RIAA network. Much better to compare simulated/measured response with a proper calculation, as the calculation will be free of assumptions about network interactions.

[SY]

Two things:

1. This is presented as a "typical" circuit from the '60s, and indeed it is. RIAA conformance was somewhat loose, and this EQ network uses standard R and C values, compromising absolute accuracy. That's good for cost and manufacturability, though.

2. The time constants are more complicated than that- you treat them as if they don't interact, but in fact they do. See Stanley Lipshitz's definitive paper in JAES (and a simpler version in Audio Amateur) about how to do those calculations correctly.

edit- Crosspost with DF96, sorry.

[h_a]

Thanks for pointing me to the interaction problem and the reference to the Lipshitz-Paper! I will see whether I can get access to it.

Any case, it appears strange to me that also the simulation does not give the correct results; I would assume interacting impedances should be treated properly in SPICE. Otherwise SPICE would be pretty pointless for calculating filter responses.

Hannes

[DF96]

Spice will usually correctly simulate what you give it. Are you comparing gain with the formula, or inserting an inverse network with perhaps the same mistakes as the network you are testing? Quite a lot of published/commercial RIAA preamps get it wrong, so I would not be in the least bit surprised if some inverse networks are wrong too.

[h_a]

I'm simply checking the -20dB at 20Hz versus 1kHz and +20dB at 20kHz relative to the amplitude at 1kHz. That simple property is not fulfilled.

Meantime I was lucky and could obtain the Lipshitz-paper; it does not appear to do so much different, it seems that particularly the resistor/cap ratios change significantly, while the time constants are still calculated in the same way (for this particular network).

We'll see, I'll give it a good read tomorrow

[DF96]

Many early and/or discrete designs obtain the 3180us LF shelf from a combination of network values and 'insufficient' loop gain i.e. the network compensates for the lack of gain. This means that the circuit will be sensitive to the current gain of Q2, as that helps set the open loop voltage gain. Self's output bootstrap will have changed things a little too. What degree of accuracy does he claim for it? This complication does not appear with an op-amp active circuit or a passive circuit.

[Bonsai]

SY and DF96 are absolutely correct about the time constants. The network components interact so you cannot use simple R.C time constants. The classic paper on the subject is by Lipshitz and on my website I have a write up and an RIAA calculator based on his equations.

Lipshitz also addresses the issues arising when using feedback RIAA amplifiers where the loop gain is insufficient, which for the most part is what you will get with simple 2 and 3 transistor circuits.

[h_a]

The network components interact

Lipshitz notes a 8-18% influence due to the interaction, not the 6dB that I see in simulation.

By the way, thanks for the hint to your article. I know it already and find it an interesting writeup! However, I find it irritating that your spreadsheet claims to work with R3=0, but always displays non-zero (and even negative) values for R3, which is inconsistent.

the network compensates for the lack of gain.

That is indeed a very interesting point, that I unfortunately cannot investigate as I don't know the originally used part BC149B. Was there really so much gain improvement for small signal bjts since the late 70ies? By the way, D. Self does not mention anything with respect to RIAA accuracy for this specific circuit (he mentions +1.6dB@20Hz for the simpler version without the follower though).

The Lipshitz article is really great to read and full of interesting points.

After all it seems, that the RIAA of the posted circuit is indeed not correct. I didn't calculate yet the finite open loop gain corrected constants, but simulation said right from the beginning the RIAA was inaccurate and the Lipshitz-values immediately bring the accuracy back. Still I might be wrong, and prototyping it will give the definitive reply in any case.

As even Self's version of the little circuit dates back to November '76 (almost 3 years before Lipshitz' paper) and he likely soon turned to the NE5534 based preamp (published '83), this error might have gone unnoticed. Especially as it appears to me after his NE5534 pre he never looked back at the original circuit.

These phono preamps are really fascinating little beasts

Hannes

[Bonsai]

"By the way, thanks for the hint to your article. I know it already and find it an interesting writeup! However, I find it irritating that your spreadsheet claims to work with R3=0, but always displays non-zero (and even negative) values for R3, which is inconsistent."

Thaks for raising that - it's a valid point. I don't know that I can neccessarily change it, but I may put a note on the spread sheet to highlight the fact that this may happen, and you just need to ignore it.