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Old 28th Jun 2020, 9:29 pm   #21
mole42uk
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Default Re: Help! JLH RIAA pre-amp

I'm interested that Baxendall designed an RIAA pre-amplifier. I wonder if you have a reference for the WW issue? Mrs. Google can't find it and Doug Self doesn't reference it.
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Old 28th Jun 2020, 11:09 pm   #22
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Default Re: Help! JLH RIAA pre-amp

Not in my head. I seem to recall it was in a letter not an actual article. Tried Doug Self's website? He's got most of the best bits of WW archived or indexed.

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Old 29th Jun 2020, 1:08 am   #23
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Default Re: Help! JLH RIAA pre-amp

There is a 3 transistor series feedback design by HP Walker in the May 1972 copy of Wireless World.
Component values are shown.

It uses a 30V supply. The author has provided plenty of supporting mathematics.

You can view and download a copy at the americanradiohistory.com website.
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Old 29th Jun 2020, 6:42 am   #24
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Default Re: Help! JLH RIAA pre-amp

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Originally Posted by Radio Wrangler View Post
It would be easy to just support a friend in an automatic, unthinking way, but the maths is there spread across the letters columns: John Linsley Hood and Hugh P Walker. If you follow the derivations through to the conclusion, Hugh's point is correct.
The H.P. Walker articles and the ensuing debate with Linsley Hood are definitely well worth reading.

H.P. Walker calculated that a JLH-style shunt feedback RIAA preamplifier at best had a signal-to-noise ratio of 58.5 dB relative to 2 mV at 1 kHz. On the other hand a series feedback circuit like his own design (which was of the Bailey type) reached 72 dB relative to 2 mV at 1 kHz.

I think that the Baxandall NE5534-based circuit might have been first published in the AES Journal, rather than Wireless World. But here it is:

Click image for larger version

Name:	Talbot-Smith p.2-141 Baxandall Disc Input Amplifier-Equalizer.jpg
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Old 29th Jun 2020, 8:07 am   #25
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Default Re: Help! JLH RIAA pre-amp

Thanks.

JLH's point was that the shunt feedback circuit (inverting structure) gives a gain/frequency characteristic equal to the ratio of the impedance of the dual RC network to a single resistor. While the series feedback circuit (non-inverting structure) gives a gain equal to one plus the ratio of the impedance of the dual RC network to a single resistor.

Now, the RIAA curve is of diminishing gain at increasing frequency, and it just keeps on diminishing past the top of the audible range.

JLH's shunt feedback circuit can do this, but the series feedback circuit cannot, it floors out at unity gain, due to that added plus one term. Considering the values and gains these people used, this gives a small error in fitting the RIAA curve at the top end for the series feedback circuit, which the shunt feedback circuit avoids. Bailey and Walker in turn considered this to be small and not likely to be noticed in use. The higher the gain the RIAA stage is designed to give, the less significant the error becomes.

But if you've got your perfectionist goggles on, no level of predictable error is acceptable. JLH went for the shunt feedback approach, and he didn't appreciate its noise disadvantage.

Peter Baxandall's circuit uses the series feedback arrangement and gets the advantage in noise, bu he's grafted on an extra pole scaled to come into play at the frequency the 'plus one' term would have flattened the fall in gain off. This corrects the deviation from the RIAA curve. It assumes that the following stage has a high input impedance.

By using series feedback, Baxandall avoids the effect of cartridge impedance on feedback loop stability and he can afford to use a higher gain amplifier. By this time the NE5534/TDA1032 opamp had come along and was starting to be recognised as a jolly good thing.

JLH's choice of shunt feedback does have its open-loop gain limited by considerations of cartridge impedance affecting stability margins and closed-loop overshoot. The limited open loop gain means that the impedance ratio of the two RC network to the fixed resistor is less dominant. You have to stop assuming the feedback network alone sets the gain and use Black's feedback equation with the finite gain of the amplifier section, and what do you get from this? Well, an error in following the RIAA curve at the low end, where the curve requires extra gain and the transistor amplifier section is limited and causes a shortfall. To reduce this effect the whole RIAA stage has to be designed for low overall gain! so we have another reason why the JLH circuit is set for low gain in use. Sometimes you just can't win.

So the devil is in the detail. You'd have to get very specific over component values and then do a thorough analysis of each three-transistor circuit, and then you'd have to make a value judgement about whether you prefer the error from RIAA curve to be at high or low frequency.

Or you just build the Baxandall circuit

You can beat the noise performance of the NE5534 with a discrete circuit, but a lot of care is needed and you don't beat it by much. Careful scrutiny of transistor noise versus effective source-Z data is needed. You'd need the series feedback circuit and then add the pole like Baxandall put in to correct the high frequency error.

That sounds like we've tied the ribbons on the business, but we haven't. There is still an elephant in the room.

Back in the day people... reviewers... designers etc were a little fixated on overload margins and the low gain option for the RIAA stage looks good in these terms. This factor did steer some people into circuit design placing high importance on headroom. Some British designers went for passive RIAA networks ahead of flat amplifiers and sort of threw noise considerations out the window, some Japanese designers went for surprisingly high voltage supply rails.

Pragmatism won out in most cases in the end. If something created the scale of transient to make a good but not extreme RIAA stage clip, then something traumatic had happened at the end of the stylus. How accurately and unclipped do you feel you need to reproduce record scratches? and won't it also put the power amp into its end stops if you're listening at normal levels? How far can your speaker cones move?

Recognition that some form of clipping was inevitable in some circumstances made life easier. But not all forms of cliping are created equal. Eventually there was recognition down all the amplifier chain that transients would happen and had better be handled gracefully. Some ungraceful circuuits once rammed to their end stops stuck there for a while and made the effect of the transient much greater than the damned thing was in itself. Better design made circuits that did not elongate transients when they were clipped, and the need that had been felt to try to follow them to infinity and beyond eased.

The simple discrete RIAA stage amplifiers are not too good if overloaded this way. The 5534 recovers quite well. Balanced circuitry helps.

This is also one of the rationales behind those people trying to DC couple everything. It's a crazy pursuit leaving all sorts of messy failures possible, but it avoids the issue of clipping upsetting quiescent settings and charge getting stored in DC blocking capacitors. However, careful design for symmetrical clipping and avoiding rectification in overload modes does the job quite nicely.

So, the overall winner is definitely Peter Baxandall.

Phew, typing fingers raw!

David
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Old 29th Jun 2020, 8:50 am   #26
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Default Re: Help! JLH RIAA pre-amp

Thank you David for that clear explanation.

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Old 29th Jun 2020, 9:41 am   #27
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Default Re: Help! JLH RIAA pre-amp

Thank you David, and Peter.

A couple of years ago when I first started this project by buying a couple of JLH Class-A power amplifier kits, I was exploring his own pre-amp designs and the discussion between himself and Walker. I think I was persuaded to go along the JLH shunt feedback route because he appeared to claim that listening tests had decided that low-frequency artefacts are more significant than high-frequency ones. As a musician, I was persuaded by the ears rather than the slightly less penetrable mathematics of Mr. Walker.

Since most of my electronics working life has been involved with glassware (except for the couple of years playing with early microprocessors), making an acceptable audio amplifier using discrete transistors would be in interesting and educational project. I perhaps forgot to include in my thoughts that there might be some frustration along the way.

Right now I think I'll dig out a couple of 5534's and see if I can refresh the RIAA section since the remainder of the amplifier works quite well.
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Old 29th Jun 2020, 9:58 am   #28
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Default Re: Help! JLH RIAA pre-amp

Regarding the extra pole at the output of the series-feedback RIAA stage to correct the inherent HF error , it is curious that Walker left it out, considering that it was already an established approach; for example Quad had used it in the 33 control unit. That said, as far as I know Quad did not produce a paper or article on the 33 circuitry (although it did on the 303), so that detail may not have been all that well known in the industry. Similarly, Quad’s use of a bootstrapped stage for the tone control appears to have gone unnoticed, Quilter getting the credit for that a few years later.

Here is the curve for the Walker 3-transistor RIAA stage:

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Name:	from WW 197105 p.223 Walker Stereo Mixer.jpg
Views:	18
Size:	56.4 KB
ID:	209794

Even if the HF error as such is small enough to be negligible in an audio sense, would not it be preferable to continue the 6dB/8ve rolloff beyond 20 kHz, to help suppress any unwanted ultrasonic stuff?

JLH changed his tune somewhat with his 1982 modular preamplifier, using what he called a series-shunt circuit. That comprised a multiple-transistor discrete series feedback amplifier to do the LF equalization, followed by an IC with shunt feedback to do the HF part.


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Old 29th Jun 2020, 10:40 am   #29
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Default Re: Help! JLH RIAA pre-amp

Thanks for the heads up about the AES paper, which I now have downloaded (I'm a member of the AES). It has Baxandall's usual clarity of thinking. There is also a reply by Stanley Lipschitz with corrections to his seminal paper on RIAA.
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Old 29th Jun 2020, 11:09 am   #30
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Default Re: Help! JLH RIAA pre-amp

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Even if the HF error as such is small enough to be negligible in an audio sense, would not it be preferable to continue the 6dB/8ve rolloff beyond 20 kHz, to help suppress any unwanted ultrasonic stuff?
Certainly. For the cost of one R and one C why not do it right? You might not hear a difference but you'll sleep easier knowing it's there.

Adding the pole on the tail end seems to me to miss one trick. A general problem with a lot of audio gear is the ingress of any RF picked up on the cables. Fridge clicks, GSM burps etc. The mechanism involves self-rectification usually in the first semiconductor junction it hits.

So the added pole might be more beneficial stuck in front, provided the impedances around it are favourable.

On the other hand, there is a Baxandall NE5534 RIAA stage in my preamp in the lounge, and in an adjacent room there is an ICOM 200W PEP SSB transmitter which gives no trouble. Careful layout and a good groundplane are sufficient.

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Old 30th Jun 2020, 3:33 am   #31
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I've been measuring the remainder of the design, in particular the ac voltage levels. To achieve an output of about 550mV which is required to drive the 1969 Class-A power amplifier design it seems that much more gain is needed in the RIAA stage.
Bear in mind though that JLH provided the required extra gain (about x 50) in the filter/output stage of his modular design. As he mentioned, the tone control stage (unity gain) was designed to operate at a level of 60 -100 mV, not atypical for control units of the era. That kind of level was delivered by his RIAA stage.

The tone control stage could probably be deleted in its entirety without affecting overall gain, but if you also wanted to delete the filter stage, then you would need to replace it with a flat stage of about the same gain. But as already noted, a modern IC-based RIAA stage would probably get to the desired output level without assistance. If extra gain is needed, then the IC-based Baxandall negative feedback volume control (Wireless World 1980 November) would probably well complement the Baxandall RIAA stage.


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Old 1st Jul 2020, 2:49 pm   #32
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Default Re: Help! JLH RIAA pre-amp

Looking at the 5534 and the Baxandall RIAA design, I wonder if I can use it on a single-ended supply by adding a series capacitor on the output? Does the Shure cartridge care?
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Old 1st Jul 2020, 3:20 pm   #33
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Default Re: Help! JLH RIAA pre-amp

You'll need to split the supply as a bias reference point and take care to filter it to remove hum... any on there will appear on the output.

Take care to not use too big an input blocking capacitor because it will have to charge to half-rail voltage every turn-on and almost all of the current will go through the cartridge and you wouldn't want to magnetise the bits of if which aren't supposed to be.

The dual rail approach is a lot more elegant and will act to minimise turn-on and turn-off thumps.

David
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Old 1st Jul 2020, 5:28 pm   #34
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Default Re: Help! JLH RIAA pre-amp

Some bedtime reading here.
https://pearl-hifi.com/06_Lit_Archiv...%20Silence.pdf
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Old 1st Jul 2020, 8:07 pm   #35
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Just had a quick look , I now need to lay down in a darkened room . Mick.
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Old 1st Jul 2020, 8:14 pm   #36
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It going to be a late one tonight!
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Old 1st Jul 2020, 10:49 pm   #37
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Default Re: Help! JLH RIAA pre-amp

I've had a scan through part of it.

Page 19, diagram at the top assumes the impedance of the cartridge is purely resistive (only the resistive part makes noise, but the reactive part can influence its transfer into the following circuitry.

Page 21 onwards, resistor (Johnson) noise is OK but applies only to resistor types free from 'excess' noise. Carbon film types suffer from this. Carbon comp are very bad and can reach 20dB above simple Johnson. Motchenbacher's early book covered this (Motchenbacher & Fitchen)

If you want Johnson in power terms it becomes simpler: = kT watts per hertz power density. But then you have to remember that's the max available and a mismatched load does not take it all. So you have to do mismatch losses.

Page 28 Be careful, he doesn't explain that noise factor nd noise figure are degradations in signal to noise ratio in passing through a stage. The degradations to make sense need you to know what the signal to noise ratio of the incoming signal (with its associated noise) is. There is a covert assumption here, that the associated noise is that of RS at a standard temperature. The rest of the maths cannot be followed without knowing this temperature. 290K is normally assumed in RF work. It's hidden in the definition of noise figure and not at all obvious.

But he then gives noise figure contour plots of transistors versus Rs and Ic. These are great tools for showing the interaction between transistor type and how you choose I quiescent so suit different sources. I's why the JFET input RIAA stage was a bit of a joke.


Not that when a cartridge quotes an optimum load of, say 47Kilo Ohms, that is precisely what it means: "Please load me in 47k for the flattest response" . It does NOT mean Rs for this cartridge is 47k. The real cartridge Rs will be appreciably lower and will change with frequency.

We could get better noise behaviour by matching the real Rs, but that would upset flatness and transient response.

Page 34 AH! Excess noise of resistors!

His calculation shows 38% more noise in an metal film resistor with 100V across it. In real life transistor applications, it becomes negligible. Not so with carbon types especially if there are leakage currents around also thermoelectric potentials can drive excess noise generation mechanisms.

Enough for today. It hasn't been a good day. A few of us got made redundant. Not entirely unexpected, but I'd enjoyed that job. An amicable parting, at least.

David
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Old 2nd Jul 2020, 6:04 am   #38
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David, I’m really sorry to hear that. It happened to me twice and isn’t easy. I wish you success in the next endeavour.
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Old 2nd Jul 2020, 7:59 am   #39
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I am also very sorry to hear that David, I have also been made redundant twice but it must be especially hard in these circumstances.

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Old 2nd Jul 2020, 9:14 am   #40
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Default Re: Help! JLH RIAA pre-amp

I'm OK financially and not at all upset. I just need to find something to do to make sure I don't get bored. Number two for me too. I would have preferred to taper off over several years rather than retire. Anyway, no discussion, please.

Terrybull's bedtime reading is excellent. Noise is an important subject to have an interest in, on the lines of 'Know thine enemy'. It forms an ultimate limit on just about everything we do or try to do. There are a few places where I think he's missed a point, but the overall outcome is good.

There is also the law of diminishing returns to take into account. So long as you don't commit any cardinal sins in designing an RIAA stage, you can get within a few dB of the theoretical ultimate in performance, but if you try to close the gap, the effort tends towards infinity. For some reason, the expensive end of the hifi world think this principle doesn't apply to them.

How small a change is perceptible?

Well, that depends on circumstances. College lecture notes say 3dB is just noticeable, but is that someone listening to something they heard how long ago? People acclimatise to smells, colours, sounds. No sense seems to be absolute, but in careful comparisons smaller differences in the frequency response of a system can definitely be noticed.

But random noise includes what we perceive as fluctuations and these do tend to disguise level changes.

Mathematically, it can be shown that there is inevitable uncertainty in measuring the power of a noise signal. Repeat the measurement and the results will show scatter and you can plot the probability density function and also get the standard deviation. If you average your measurement over a longer period, the scatter narrows and the standard deviation reduces. To get a perfect result would take infinite time. This actually is fair. If noise is random, the only time you can ever achieve certainty over it is at the end of time when all is history and there is nothing left to fail to predict

Considering audio bandwidth and normal attention span, white, random noise sounds lumpy. This makes it very difficult to compare two levels by ear.

I was involved in an instrument whose job was to add noise to a signal in a controlled fashion, so that the error rates in microwave radios handling data links could be characterised over incoming signal/noise ratio.

We had a noise generator running with a long-term-averaging power detector monitoring it. So we knew its level with relatively low uncertainty. This noise signal then went through a variable attenuator to vat its level to track the result of measuring the incoming signal. Then the noise was added to the wanted signal.

So what's the point in describing this? Well it's not only untestable, you can also mathematically prove that it's untestable. The attenuator could be changed so quickly that if the outgoing noise were measured it would have to be in such a short time-frame that the inherent uncertainty is much greater than the precision of the attenuator and the long-term uncertainty of the noise going into the attenuator.

So we could make noise whose level we could calculate an expectation of, but it never lingered long enough to verify the uncertainty we thought it would have to have. We could prove that it ought to be right because of the way we constructed it, but the final result could never be checked directly. HP3708A.

Noise can be evil stuff!

David
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