In my case the pot has no effect at all at the cart termination impedance because the two 1K resistors to gnd are replaced by a 2k pot (here a 1K pot), but CM impedance can be increased quite a bit which may be favourable to suppress mains radiation.
Sometimes a picture tells more than a thousand words, that's why I included an example of what I did to nihilate current through the Cart.
Hans
.
Sometimes a picture tells more than a thousand words, that's why I included an example of what I did to nihilate current through the Cart.
Hans
.
Attachments
All of these approaches to base current equalization trim can work reasonably well. IME working with mic pre's where the problem is exacerbated due to
presence of phantom power blocking caps allowing more DC offset to occur it's worth running the IP pair through max / min temp conditions as the
current gain and VBE match between devices can shift.
This is where a/ dual package and b/ higher beta devices can have an advantage.
That PBSS4350SS dual device looks really good. I'd guess the noise is higher than ZTX851 and closer to the ZTX690B at probably between 0.4 to 0.5nV/rt
Hz purely based on capacitances and current but there's no RBB so just a guess. It also has nice high gain.
If you want absolute best gain / VBE matching / tracking and can sacrifice some noise the MAT14 in 2 + 2 parallel config should be really good. The
noise will be about 0.45 at the sweet spot. Also noise spec is guaranteed.
Just a general question Joachim, from reading back in this thread looks like you have heard many very good phono preamps. Subjectively which did you
prefer and could you correlate that to any particular topology?
TCD
Terry,
Good question about the sound of different preamps.
However IMO it is not just topology but also the sort of realisation like PCB, used components, power supply etc.
Like the head of design of Cambridge Audio once said, “give two designers the same circuit diagram, let them make their own PCB and you will end with two different sounding amps”.
On top of that we have only discussed voltage amplying topologies so far.
Because it’s impossible to compare a voltage amplfying amp from one make to a current mode amp from another brand, just because of all subtle differences mentioned between them in execution, I have made a pre-pre with a mother board with small switchable modules on top, going from voltage to current mode while keeping everything meticulously the same.
Designing a universal diff-in/diff-out Head Amp
For my Cart the current mode came out as superior but according to some magazines, not all MC Carts like it to being played in current mode, especially the ones with a higher internal impedance.
Hans
Good question about the sound of different preamps.
However IMO it is not just topology but also the sort of realisation like PCB, used components, power supply etc.
Like the head of design of Cambridge Audio once said, “give two designers the same circuit diagram, let them make their own PCB and you will end with two different sounding amps”.
On top of that we have only discussed voltage amplying topologies so far.
Because it’s impossible to compare a voltage amplfying amp from one make to a current mode amp from another brand, just because of all subtle differences mentioned between them in execution, I have made a pre-pre with a mother board with small switchable modules on top, going from voltage to current mode while keeping everything meticulously the same.
Designing a universal diff-in/diff-out Head Amp
For my Cart the current mode came out as superior but according to some magazines, not all MC Carts like it to being played in current mode, especially the ones with a higher internal impedance.
Hans
The newest one is always the best one (;
Not really. When I build prototypes I do not use the best parts available but Standart through hole 1% metalfilm resistors, Wima caps, Pannasonic elcaps and so one.
When I am satisfied with the performance I sometimes build the same circuit with better parts like Styroflex and silver mica caps, 0.1% resistors with low tempco,, Elna Silmik and so on.
Usually I hear a difference so better parts make a audible difference. It can be of cause that tighter tolerances make part of that difference.
Other factors are better RCA jacks, solder, better copper or silver cable.
PSU is important too and I always find double mono PSU better then single PSU.
Of cause I have personal preferences and one of my favorite phonostages is a tube unit.
There is more but I would say :
1: I do not like hum
A Phonostage that has virtually no hum always sound better in the bass
2 : I do not like noise like hiss. A low noise Phonostage always has better transparency and
resolution then one that produces noise.
3: RIAA should be better then + - 0,2dB.
4 : generous overloadmargin is a good thing
5 : RIAA should be exact also over 20kHz.
6 : I have not made up my mind about subsonic filters
7 : Distortion should be low but how low is still audible I can not say.
What I can say is that I very much like my tube Phonostage that has much more distortion then my solid state counterparts , but most of it relatively harmless second harmonic
Not really. When I build prototypes I do not use the best parts available but Standart through hole 1% metalfilm resistors, Wima caps, Pannasonic elcaps and so one.
When I am satisfied with the performance I sometimes build the same circuit with better parts like Styroflex and silver mica caps, 0.1% resistors with low tempco,, Elna Silmik and so on.
Usually I hear a difference so better parts make a audible difference. It can be of cause that tighter tolerances make part of that difference.
Other factors are better RCA jacks, solder, better copper or silver cable.
PSU is important too and I always find double mono PSU better then single PSU.
Of cause I have personal preferences and one of my favorite phonostages is a tube unit.
There is more but I would say :
1: I do not like hum
A Phonostage that has virtually no hum always sound better in the bass
2 : I do not like noise like hiss. A low noise Phonostage always has better transparency and
resolution then one that produces noise.
3: RIAA should be better then + - 0,2dB.
4 : generous overloadmargin is a good thing
5 : RIAA should be exact also over 20kHz.
6 : I have not made up my mind about subsonic filters
7 : Distortion should be low but how low is still audible I can not say.
What I can say is that I very much like my tube Phonostage that has much more distortion then my solid state counterparts , but most of it relatively harmless second harmonic
In the real world discs are warped and a rumble filter is very handy to take out the subsonic signals from riding the warped-disc rodeo and from the tone-arm oscillations and deck suspension oscillations.
These can easily be large enough to push woofers around visibly, especially for ported enclosures.
Also if the rest of your system is DC-coupled a rumble filter will kill the big DC offset you'll get otherwise (could trigger protection even - a quick search shows examples of protect operating with phono inputs).
[ edit: you can get nice results with a 10 pole filter using just 2 opamps, see: https://www.diyaudio.com/forums/analogue-source/338801-modular-phono-preamp-2.html#post6811503
you can just see on the very left hand side how the noise drops off at 20Hz (green trace - expand the image)
The extra signal processing doesn't seem to affect the nice distortion performance ]
These can easily be large enough to push woofers around visibly, especially for ported enclosures.
Also if the rest of your system is DC-coupled a rumble filter will kill the big DC offset you'll get otherwise (could trigger protection even - a quick search shows examples of protect operating with phono inputs).
[ edit: you can get nice results with a 10 pole filter using just 2 opamps, see: https://www.diyaudio.com/forums/analogue-source/338801-modular-phono-preamp-2.html#post6811503
you can just see on the very left hand side how the noise drops off at 20Hz (green trace - expand the image)
The extra signal processing doesn't seem to affect the nice distortion performance ]
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I have got very well matched high quality 15n and 47n caps in stock
Can I use them like this ?
O.k. here is my comment.
The first cap from 1n5 to 15n is perfect.
However going to 47n instead of 470nF forced you to increase gain of the middle stage, thereby reducing overload margin by 14dB, nor very nice.
At the same time overall gain has been reduced by 3dB and A-weighted equivalent input noise worsened by a factor 2 or 6dB, a total waste.
So keep the gain of the second stage unaffected with 2K35.
If you don't have 470nF, use five 47n in parallel in the last stage with resp 1354, 12k18 and 121k8 resistors.
Now your OLM is unaffected and EIN-A worsens by just 0.94dB.
Hans
The first cap from 1n5 to 15n is perfect.
However going to 47n instead of 470nF forced you to increase gain of the middle stage, thereby reducing overload margin by 14dB, nor very nice.
At the same time overall gain has been reduced by 3dB and A-weighted equivalent input noise worsened by a factor 2 or 6dB, a total waste.
So keep the gain of the second stage unaffected with 2K35.
If you don't have 470nF, use five 47n in parallel in the last stage with resp 1354, 12k18 and 121k8 resistors.
Now your OLM is unaffected and EIN-A worsens by just 0.94dB.
Hans
Ricardo,
Still a small tip.
When reducing the two 1K resistors from the two op-amp outputs to 500R and at the same time lowering the resistor par. to the 10R pot from 35 to 6.4R, overall gain remains the same but you improve EIN-A by 0.9dB to 0.23nV/rtHz when using 5x47nF instead of 470nF.
Hans
Still a small tip.
When reducing the two 1K resistors from the two op-amp outputs to 500R and at the same time lowering the resistor par. to the 10R pot from 35 to 6.4R, overall gain remains the same but you improve EIN-A by 0.9dB to 0.23nV/rtHz when using 5x47nF instead of 470nF.
Hans
