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Harshness, Clicks and Pops

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Graham Slee View Drop Down
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 03 Feb 2021 at 8:12pm
Another image where the effect will be seen by those with a smattering of technical understanding.

This is a single sided class-A amp with a collector load resistor (Rc, Ro is the "Early resistance"), and is virtually the same as the input valve in the RCA circuit.

So, you think your phono stage is reproducing sounds like real?

(it depends on its configuration)

This doesn't.

asymmetrical drive
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 03 Feb 2021 at 9:04pm
Take another look at the above image. If the instrument waveforms shown are in-phase, and the first gain stage inverts, the waveforms invert. Although this has no bearing on what comes out of the loudspeaker, the passive phono stage, driven class-A inverted, will give the instruments a different sound to a non-inverting first gain stage.

If the sharp "edges" point one way, and that way gets smoothed by the blunt side of the 2nd harmonic as well as the RIAA turning over at a lower frequency on that half-cycle, it might appear warm.

If the circuit was such that the opposite occurred, it might appear harsh!

Clicks and pops caused by hairline scratches, micro-bubbles in the vinyl, and debris, might well be "stretched" on one half-cycle and "narrowed" on the other.

The amount heard will depend on the loudness of the signal. The louder the signal, the more pronounced the difference is as far as the 2nd harmonic is concerned.

Now, if driven by a class-B output stage such as the humble 741 op-amp, there are no half-cycle differences. I'm not saying the 741 is good, just that there is an obvious difference.

We're not talking about component subtleties; we're talking about real things.


Edited by Graham Slee - 03 Feb 2021 at 9:05pm
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Sylvain Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2021 at 10:52am
I'd spotted a passive interstage preamp in a Tandy book about valves but could see any two gain stages could be used and set about making it with 741 op-amps. The result was revealing! Over the years, I had difficulty understanding how 741's were able to sound so good.

Pouring over valve datasheets many years later, the reason for the 741's success jumped out of the page. The RCA design was made for a cheap but low noise, twin triode valve (today's equivalent is the 12AX7). Remember that I previously remarked on triodes' capacitance limiting their high-frequency performance? This valve also had very poor distortion. By comparison, the 741 was light-years ahead.

Nearly everybody making phono stages today uses the same basic circuit, and some use three amplifier stages with two passive filters. I don't think they'd use 741 op-amps, but maybe the NE5534 or NE5532 dual cousin.

The discussion is of great interest but my ability to contribute to the Technical aspect is very limited ,,,However....I was advised by another Audio Design Engineer some 15 years ago to change NE5534 for the then modern OPA 2015!!!!!!! of a well known Texas instrument company .......and then The discussion between the WHY and the Merit of Passive and Discrete was still OP AMP were not fully arrived as yet .....BUT OP AMP development versatility seem to have overcome the argument over Discrete case specific application
Apologies for my interruption BUT teh subject is highly IMPORTANT to my Audio components upgrade and development 
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2021 at 11:49am
Another problem with the passive phono stage is the flat gain stage (how could it be any other?) between cartridge input and the passive filter.

The passive filter has an insertion loss of -19.3dB - as near as damn it, 1/10.

For a regular MM gain of 40dB (x100), each gain stage must share a gain of 1000 (60dB).

It makes sense for best noise performance to put most of the gain upfront, but you'll see the calamity in doing so as I continue to explain.

For the sake of this discussion, we'll share the gain between the two gain stages.

The square root of 1000 is 31.6227766, or we could divide 60dB by two. 30dB expressed linearly is also 31.6227766.

That's a bit cumbersome for a discussion, so let's call the input gain x30.

If we were to use op-amps, the supply is generally 30V (+/-15V dual-rail), and datasheets tell us the output swing into their rated load. Mostly that's 10V rms.

The cartridge output might be 5mV. Some output 6.5mV, others 3mV, and so on. A particular Stanton does 9mV. However, we'll stick at 5mV.

Vinyl can achieve 14dB headroom. However, it's doubtful if the highs go so loud, as most styli would skip on the inner groove, but perhaps we can use the studio PPM of +8dBu (x2.5).

The cartridge's rising output, helped by record EQ, means that the reference 5mV at 1kHz is 50mV at 20kHz, and multiplied by 2.5 is 125mV at 20kHz (merely taking the inverse of the RIAA filter as evidence).

Multiply 125mV by the gain of 30, and you get 3.75 volts. The maximum swing on a 30V rail op-amp is 10V, so the difference is x2.67 or 8.5dB. So, the headroom here is just 8.5dB. What does the spec of the advertising blurb say? I'd hazard a guess it doesn't admit what we just discovered.

Now, at the bass end, the cartridge outputs 0.54mV (5mV at 1kHz minus the 19.3dB of the RIAA curve). The gain stage outputs 30 x 0.5 = 15mV. The op-amp is "flying" at HF, and then there's the weedy bass signal at just 15mV - hardly linear gain.

But what about the scratch, or the microbubbles, the debris, or a little mistracking?

The scratch's leading edge will be in the upper-frequency end - a transient - and what voltage will that produce?

Hopefully, it won't result in the first gain stage outputting more than 10 volts?

What if it did try to output more than 10 volts? Well, it would clip, wouldn't it?

With low-frequency noise being a problem in most amplifier stages (op-amp or otherwise) - what the technical type understands as 1/f noise - we really should be making the first gain stage have more gain. But if we do, we drive the HF transients further into clipping. The problem is that we need to spec the lowest noise to the customer, so we're now stuck.

We could increase the power supply to give us more headroom, but to do so meaningfully, we'd need around 90 volts. That might seem crazy, but I've seen it. You cannot use op-amps anymore, so your circuit is discrete, and you're now competing with Silicon Valley.

But, continuing with our down-to-earth power supply voltage and accepting that clipping will happen at HF (but omitting to tell the customer), then perhaps the passive RIAA filter will help.

Well, yes, as in all RC filters, a square wave (the result of clipping) will change shape and become "more rounded," and so the passive EQ phono stage just about gets away with it.

Compared with the early solid-state feedback EQ, the passive interstage RIAA is far in advance. I made plenty of different passives, and my ears can vouch for that! But compared to the pentode feedback EQ, they may not fare so well. The difference here is due to the pentode valve's high-frequency performance and the fact that it had a lot more (HT) power supply headroom for a thing called overshoot.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote patientot Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2021 at 3:05pm
The stuff I see posted over and over on audio forums is that "passive RIAA is better" and "active RIAA is bad". A few manufacturers make a big deal of this and tout their passive RIAA phono stage as having "less prominent clicks and pops" and a "more natural sound" than anything with active RIAA. 

Honestly the worst phono stage I had in terms of sounding harsh generally and amplifying clicks and pops and surface noise was a passive RIAA one. It had great looking specs on paper but was plagued with problems in real life. 

It seems to me that whether a phono stage amplifies clicks and pops and harshness comes down to very particular aspects of the design, not whether it uses active RIAA or passive RIAA. 

Another thing of note - there is one newer company that has recently made the rounds on some forums that heavily touts their discrete, passive RIAA approach. I'm not out to bash this particular company or anything so I won't mention names here. 

I'm seeing people recommend this company's products over others solely due to the design approach and nothing else. Again, very few people have actually heard the units because they are so new. The product may well be decent or even excellent, but that seems a bit misguided to me. 
SL-1200 MK7 (modified) + Reflex M + PSU-1 used with AT150-40ML, AT VM95ML, Stanton 680mkII + Ogura, and Shure M35X cartridges.
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Post Options Post Options   Thanks (0) Thanks(0)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2021 at 4:22pm
Hi Sylvain, A word or few about op-amps.

Study the internal circuit of an op-amp from its datasheet (where they dare to show it), and you'll mostly see a common or garden transistor amplifier with a few enhancements.

To meet op-amp science requirements, it must have a "universal" power supply range. So you'll see a voltage reference that you'd not usually see in a proprietary amplifier design.

In a single op-amp, you'll also see offset null pins used to trim the device for some scientific applications. Otherwise, they generally follow the same path.

We hear a lot about Burr-Brown, but until 1983, little was known about this 1956 established company. It was taken over by Texas Instruments 17 years after making its second debut (the year 2000). It worked in precision digital signal processing, so I can see how the audiophile might associate with that.

Analogue Devices are similar to what Burr-Brown was, but Texas could see the value in milking the audiophile cow using the Burr-Brown tag.

Shall we take a technical peek? Perhaps the OP275? But wait a minute, that's an Analog Devices op-amp. Well, maybe Kevin Brown, whose company owns the patent, could be a relation of Thomas Brown, jr. but I'd be wasting my time finding out. So, what's the patent all about?

We read about the Butler input stage, so we take a look at the patent, and it uses two FET transistors to pad-out a Wilson current mirror. After it became apparent that a Wilson current mirror could be improved by adding emitter resistors, like fries, they came with everything. The patent states these are FET resistors, and that's sufficient novelty to grant a patent.

Its correct use is to reduce input noise, and compared with the nearest equivalent (LF351, in my opinion), the only main difference is that the LF351 uses 1k3 resistors instead of gate-drain shorted FETs.

It makes 7nV per sq. root of frequency range. That's a 6dB noise improvement for those capable of making use of it. Other circuit noise sources might drown out the improvement, and in which case, what difference is it going to make to your ears?

Skipping to the favourite pastime of chip-rolling, there are scores of partially technical proponents of high slew-rate op-amps all over the internet. The formula for the power supply capacitors in "Talkin' PSUs" Q = CV = IT also applies to slew rate, except, looking at the chip-rolling antics, it would seem that science has nothing to do with it.

If you don't know how an amplifier circuit works, you won't see what you're doing with an op-amp - it's that simple.
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Post Options Post Options   Thanks (1) Thanks(1)   Quote Graham Slee Quote  Post ReplyReply Direct Link To This Post Posted: 04 Feb 2021 at 4:28pm
Originally posted by patientot patientot wrote:

The stuff I see posted over and over on audio forums is that "passive RIAA is better" and "active RIAA is bad". A few manufacturers make a big deal of this and tout their passive RIAA phono stage as having "less prominent clicks and pops" and a "more natural sound" than anything with active RIAA. 

Honestly the worst phono stage I had in terms of sounding harsh generally and amplifying clicks and pops and surface noise was a passive RIAA one. It had great looking specs on paper but was plagued with problems in real life. 

It seems to me that whether a phono stage amplifies clicks and pops and harshness comes down to very particular aspects of the design, not whether it uses active RIAA or passive RIAA. 

Another thing of note - there is one newer company that has recently made the rounds on some forums that heavily touts their discrete, passive RIAA approach. I'm not out to bash this particular company or anything so I won't mention names here. 

I'm seeing people recommend this company's products over others solely due to the design approach and nothing else. Again, very few people have actually heard the units because they are so new. The product may well be decent or even excellent, but that seems a bit misguided to me. 


All I can really say is that I've pointed out the pitfalls as simply as I can translate them, and if they didn't exist, I'm sure I'd be making passive stages. And I've built plenty of them.

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