A Low Noise Balanced Input Moving Coil Preamp Using the ZTX851

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Hans
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans »

JR. wrote:I am not arguing for using electrolytic caps in the MC preamp input. I advocate designing such caps out of audio paths when possible (see Wayne's floating mic preamp to eliminate input caps), but it is worth trying to understand the application before making such a broad sweeping condemnation.

Bateman's excellent series on capacitors is stressing the electrolytic caps at high enough current to generate significant terminal voltage across the capacitors. Capacitor non-linearities express only when there is terminal voltage due to significant charging and discharging current. In a simple DC blocking application the capacitor is passing very little current, and developing almost no terminal voltage at any audio frequency. JR
Concerning the "high enough current" flowing through the cap: In most measurements Bateman shows the AC voltage and DC Bias voltages applied to the cap, just as the load resistance.
When testing for instance a 10uF tantalum cap, the recorded graph mentions 0.3Volt applied to the cap in series with 10 Ohm. This cannot really be regarded as a significant voltage.
With the used frequency of 100Hz, only 1 mA rms is flowing.
Distortion however is huge.

A 100uF cap was tested with 0.1Volt/10 Ohm load , resulting in that case in a very moderate 3.5 mA rms, not exactly the kind of current flowing in "filter poles in a passive loudspeaker crossover".
However also in this case distortion is much too high to be used in the signal path.
Your accusation of "broad sweeping condemnation" is therefore completely unfounded and rather over the top.

Hans
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JR.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. »

Hans wrote:
JR. wrote:I am not arguing for using electrolytic caps in the MC preamp input. I advocate designing such caps out of audio paths when possible (see Wayne's floating mic preamp to eliminate input caps), but it is worth trying to understand the application before making such a broad sweeping condemnation.

Bateman's excellent series on capacitors is stressing the electrolytic caps at high enough current to generate significant terminal voltage across the capacitors. Capacitor non-linearities express only when there is terminal voltage due to significant charging and discharging current. In a simple DC blocking application the capacitor is passing very little current, and developing almost no terminal voltage at any audio frequency. JR
Concerning the "high enough current" flowing through the cap: In most measurements Bateman shows the AC voltage and DC Bias voltages applied to the cap, just as the load resistance.
When testing for instance a 10uF tantalum cap, the recorded graph mentions 0.3Volt applied to the cap in series with 10 Ohm. This cannot really be regarded as a significant voltage.
With the used frequency of 100Hz, only 1 mA rms is flowing.
Distortion however is huge.

A 100uF cap was tested with 0.1Volt/10 Ohm load , resulting in that case in a very moderate 3.5 mA rms, not exactly the kind of current flowing in "filter poles in a passive loudspeaker crossover".
However also in this case distortion is much too high to be used in the signal path.
Your accusation of "broad sweeping condemnation" is therefore completely unfounded and rather over the top.

Hans
And how much current is flowing through a cap in series with phono cartridge?

Evaluating components requires understanding the context how they are used.

NOTE: I am not advocating using electrolytic caps in premium audio paths. I spent many years avoiding them when it matters, and trying to better understand when.

JR
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ricardo
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by ricardo »

Hans wrote:This is not meant to criticize the nice circuit design in this thread, having only 0.33nV/rtHz or 12.5dB less noise.
It is only to make aware that in case of using Fet's instead of bipolars, generating slightly more noise, does not at all render the circuit into a less valuable design.
There is still enough margin to negotiate.
As it so happens, I have conducted Listening Tests on this in Jurassic times with a 0.28nV/rtHz device. I quote
Does ultra low noise on vinyl playback matter?

I think so. From analysing my listening test results and ignoring any obvious Golden Pinnae raving, there is a sense of less grittiness and also the system seems to disassociate clicks, pops & record noise from the music. Maybe a noise modulation effect. But I'm probably pontificating from the wrong orifice and it was 30+ yrs ago.
There's a couple of other nice characteristics of my circuit that might explain its good performance in Listening Tests .. eg its THD behaviour :mrgreen:

But it's unlikely I'll re-visit the design as I no longer have a MC cartridge(s) ... or even a record player :(

My record collection is in my sister's shed at the other end of the continent and will probably never see the light of day again.
_________________________

Hans, I don't think you grok the full implications of Bateman on Electrolytics. He shows their EVILs but also how to avoid them.

... the usual disclaimer ... I think they're EVIL too but won't hesitate to use them if their EVILs can be avoided bla bla ricardo
Hans
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans »

ricardo wrote:Does ultra low noise on vinyl playback matter?

I think so. From analysing my listening test results and ignoring any obvious Golden Pinnae raving, there is a sense of less grittiness and also the system seems to disassociate clicks, pops & record noise from the music. Maybe a noise modulation effect. But I'm probably pontificating from the wrong orifice and it was 30+ yrs ago.
30 years is a very long time ago. MC cartridges were a far cry from what is on the market today, and nobody at that time would have believed in a revival of Vinyl.
You did not mention to what your low noise preamp was compared, but there is not one single physical reason to explain that having less preamp noise, but still having the same much larger surface noise will make a system less sensitive to clicks etc.
Noise can be very well modulated in a digital environment when dithering is not applied, but noise modulation in a phone-preamp is unlikely to happen in a well designed circuit.
The tested very low noise preamp might be just a better design with a higher overload margin and maybe even a balanced design with a high CMRR compared to the other one in the test.

Apart from two ZTX bipolars, being steadily improved with modern production techniques, all active components in Wayne´s circuit can be modern components from today´s industry, not comparable what was available some 30 years ago.
The circuit is balanced, I agree with Wayne the "magic" advantages of that topology, and CMRR will be very high too.
There is just one thing of concern. The overload margin being only ca. 16dB ref 5mV@20kHz, exactly in the high frequency region where scratches causing sharp clicks, may cause a serious overload.
Reducing the gain of the head amp quite a bit, would inversely increase the overload margin at the higher frequencies.

Last but not least, I'm amazed by the ease you accept positive listening results from an ultra low noise preamp, yet having great difficulties in accepting that A/D/A in the signal line of a Pick-Up may change the listening experience.
Just adding some extra dB white noise to an extremely low noise amp that is producing noise ca. 30dB below surface noise , is most unlikely to influence sound.
Whereas 1) anti alias filtering, 2) sampling at 24bits/n*44.1Khz, 3) digital filtering before 4) downsampling and adding dithering noise to 16bit/44.1kHz in the A/D, followed in the D/A by 5) upsampling to m*44.1kHz plus digital filtering again and correcting with sinx/x all in one go, 6) converting to analog stairsteps, followed by 7) analog filtering to remove the sample stairsteps, a process with many high speed and complex operations, resulting in an audio signal that when subtracted from the original audio signal is evidently different in amplitude and phase.
It can only be regarded as astounding how music still sounds after all this "deformation" of the original sound, especially because of all the steep digital filters.

Hans
Gold
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Gold »

Ricardo,

When you were conducting these listening tests did you measure or characterize overall system noise? I know I keep harping on this but RF/EMI is always the dominant noise source when I setup a phono system. I know you can hear well into the noise floor. I guess I'm wondering how far into the noise floor you have to hear and how much the noise spectrum matters.

I'm less concerned with Hans argument that the noise floor of a record is a limiting factor. it may be in theory but the noise floor of a particular record depends on so many moving targets that I would be hesitant to state a metric and have it be right. Other than the theoretical technical limit.
Hans
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans »

Gold wrote:Ricardo,

When you were conducting these listening tests did you measure or characterize overall system noise? I know I keep harping on this but RF/EMI is always the dominant noise source when I setup a phono system. I know you can hear well into the noise floor. I guess I'm wondering how far into the noise floor you have to hear and how much the noise spectrum matters.

I'm less concerned with Hans argument that the noise floor of a record is a limiting factor. it may be in theory but the noise floor of a particular record depends on so many moving targets that I would be hesitant to state a metric and have it be right. Other than the theoretical technical limit.
Hi Gold,

I respect your opinion on surface noise, is it because you have information or measurements that can substantiate this a bit more?
And what do you suppose the theoretical technical limit is?

Hans
Gold
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Gold »

Hans wrote: Hi Gold,
I respect your opinion on surface noise, is it because you have information or measurements that can substantiate this a bit more?
And what do you suppose the theoretical technical limit is?
I base my opinion on cutting lacquer masters and checking test pressings for over 15 years. I know when I cut an acetate reference or lacquer it will approach the theoretical noise floor of around -70dBm. As long as my cutting stylus is in good shape and I have the stylus heat set correctly. I actually never get it that good. The Neumann VG66 cutterhead drive electronics have a stated noise floor of -65dBm. The later SAL74 has a lower noise floor. Also a DMM cut will usually have a lower noise floor than a lacquer cut.

IME the manufacturing process adds at least 6db of noise in the best case scenario. If the plating goes even a little wrong it can bring up surface noise and groove echo by a lot.

Then it's on to manufacturing. Each individual record is different. Depending on conditions one record could significantly noisier than the next.

There are so many variables in the manufacturing process that any measurements are only valid for the record under test. I would use the theoretical noise floor number and stick to it because it won't be better than that but it could be worse. You never know how much worse though.
Hans
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Hans »

Gold wrote:I base my opinion on cutting lacquer masters and checking test pressings for over 15 years. I know when I cut an acetate reference or lacquer it will approach the theoretical noise floor of around -70dBm. As long as my cutting stylus is in good shape and I have the stylus heat set correctly. I actually never get it that good. The Neumann VG66 cutterhead drive electronics have a stated noise floor of -65dBm. The later SAL74 has a lower noise floor. Also a DMM cut will usually have a lower noise floor than a lacquer cut.

IME the manufacturing process adds at least 6db of noise in the best case scenario. If the plating goes even a little wrong it can bring up surface noise and groove echo by a lot.

Then it's on to manufacturing. Each individual record is different. Depending on conditions one record could significantly noisier than the next.

There are so many variables in the manufacturing process that any measurements are only valid for the record under test. I would use the theoretical noise floor number and stick to it because it won't be better than that but it could be worse. You never know how much worse though.
Interesting to know this in more detail.
When playing a 1kHz test tone at 0dB, it means that theoretically the noise should be -70dB below this 0dB signal, or should the noise from a blank trace be -70dB below the 0dB signal?

1) Is this noise un-weighted or A-weighted, and
2) What frequency range is concerned, is it from 20Hz to 20KHz, and
3) Is this noise before or after applying the Riaa correction, I guess before ?

It would be nice to compare these theoretical figures to the signal coming from my system.
Sorry for asking that much questions, but it helps a great deal in finding relevant noise figures for a phono-preamp.

Hans
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JR.
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by JR. »

Gold wrote:
Hans wrote: Hi Gold,
I respect your opinion on surface noise, is it because you have information or measurements that can substantiate this a bit more?
And what do you suppose the theoretical technical limit is?
I base my opinion on cutting lacquer masters and checking test pressings for over 15 years. I know when I cut an acetate reference or lacquer it will approach the theoretical noise floor of around -70dBm. As long as my cutting stylus is in good shape and I have the stylus heat set correctly.
If you'll indulge me "pampering" some more old memories. Back in the 1950s while my dad (RIP) was a recording engineer for RCA Records in NYC, he took me, his <10 YO son into work with him on a Saturday. I'm sure I was a pest and asking everybody questions.

As I recall there was a tech in the studio working that Saturday morning. He was melting wax on microscope slides, that they would then use to assess the condition of the lathe cutting head by looking at the freshly cut wax groove under the microscope. I asked the tech why he didn't just weigh the wax first instead of adding a bunch of small pieces one at a time. :lol: For some reason he didn't appreciate my question.
I actually never get it that good. The Neumann VG66 cutterhead drive electronics have a stated noise floor of -65dBm. The later SAL74 has a lower noise floor. Also a DMM cut will usually have a lower noise floor than a lacquer cut.

IME the manufacturing process adds at least 6db of noise in the best case scenario. If the plating goes even a little wrong it can bring up surface noise and groove echo by a lot.

Then it's on to manufacturing. Each individual record is different. Depending on conditions one record could significantly noisier than the next.

There are so many variables in the manufacturing process that any measurements are only valid for the record under test. I would use the theoretical noise floor number and stick to it because it won't be better than that but it could be worse. You never know how much worse though.
Not to mention conducted mechanical noise from the turntable/platter (but I think I did mention that).

=========

Maybe if vinyl is really coming back they can develop some new quieter vinyl.

JR

PS: I was associated with a companded NR system for vinyl records, called CX promoted by CBS. The records were mastered with 2:1 compression applied before hitting the cutting lathe (Urie engineered the mastering compressor), then expanded back during playback. These used below threshold compression/expansion, similar to dolby B NR so playback level calibration mattered. The vinyl surface noise, even preamp input noise was all inside the NR encode/decode loop so reduced by the playback expansion (10-15 dB?). I have a bunch of CX records laying around that CBS gave me, I don't think I even played more than a couple of them to check that they worked.
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Gold
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Re: A Low Noise Balanced In Moving Coil Preamp Using the ZTX

Post by Gold »

Hans wrote: When playing a 1kHz test tone at 0dB, it means that theoretically the noise should be -70dB below this 0dB signal, or should the noise from a blank trace be -70dB below the 0dB signal?
Yes. -70dBm is the number I have seen in the literature most as the theoretical noise floor.
1) Is this noise un-weighted or A-weighted, and
2) What frequency range is concerned, is it from 20Hz to 20KHz, and
3) Is this noise before or after applying the Riaa correction, I guess before ?
All of it is a bit of a moving target. Pops and clicks wouldn't count I guess but crackle would. I think A weighted from 20Hz - 20K Hz is a good way to measure. That is probably the most often used way. The measurement would be after the RIAA decode.
It would be nice to compare these theoretical figures to the signal coming from my system.
Sorry for asking that much questions, but it helps a great deal in finding relevant noise figures for a phono-preamp.
The noise is nothing you can control and could vary a great deal. System noise should at least be kept below -70dBm. That's my point.
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