Confirming this.
I'm now building my third example, the first one came with the book, and the other two from a mgroup buy.
Regards,
Braca
Has any Member that has the previous PCB running given any indication of the maximum gain and maximum attenuation that they have for their build/s?
I figured post #317 was the immediate reply putting those concerns to rest..
Sounds like software making the final Gerbers appear more complicated for the sake of it's layout logic. And if the human doesn't know the full story of what the software has done, it might at first look to be wrong (only to find at closer inspection that it's absolutely fine).
Sounds like software making the final Gerbers appear more complicated for the sake of it's layout logic. And if the human doesn't know the full story of what the software has done, it might at first look to be wrong (only to find at closer inspection that it's absolutely fine).
Last edited:
Hi Folks,
If I want to replace the potentiometer in the Group Buy (http://www.diyaudio.com/forums/grou...tzeys-balanced-preamp-group-buy-part-2-a.html) kit with a remote controlled one, what is the model, make I should use?
My understanding is that I must choose a motorized potentiometer. I also gather that it need not be of an extremely high audio quality, as we are not using this potentiometer for its attenuation properties.
My knowledge of options out there is pretty poor - hence this request for help.
Cheers
If I want to replace the potentiometer in the Group Buy (http://www.diyaudio.com/forums/grou...tzeys-balanced-preamp-group-buy-part-2-a.html) kit with a remote controlled one, what is the model, make I should use?
My understanding is that I must choose a motorized potentiometer. I also gather that it need not be of an extremely high audio quality, as we are not using this potentiometer for its attenuation properties.
My knowledge of options out there is pretty poor - hence this request for help.
Cheers
There was an information as to this subject in post #231 at this link:
http://www.diyaudio.com/forums/group-buys/279981-bruno-putzeys-balanced-preamp-group-buy-24.html
With a 10K pot in the middle position, the preamp has then a gain of -6.8dB.
The unit I'm currently building for a friend will have a maximum gain of 6dB, and the volume will be controlled by a relay-switched resistor ladder attenuator.
Regards,
Braca
I don't get the reasoning of changing the gain of the middle section from 0dB gain to -6,8dB gain.
To get the same overall voltage gain, the third section will now have to amplify twice as much.
And since most noise is generated by this last section, you will roughly end up with twice as much noise!
Exactly the opposite to what Doug Self did in the same Linear Audio vol 5.
To reduce noise even further he used a 5K pot instead of 10K, and he placed two output amps in parallel, the combination of these two steps reducing the output noise by ca. 6dB.
To get the same overall voltage gain, the third section will now have to amplify twice as much.
And since most noise is generated by this last section, you will roughly end up with twice as much noise!
Exactly the opposite to what Doug Self did in the same Linear Audio vol 5.
To reduce noise even further he used a 5K pot instead of 10K, and he placed two output amps in parallel, the combination of these two steps reducing the output noise by ca. 6dB.
As a non-technical person on a DIY site - what does the value of the pot do to the circuit? If it was say a 1K pot what would happen?
There are no figures for maximum gain, nor for maximum attenuation in that post.
Last edited:
The poteniometer in stage 3 is the voulme control. It is being used for it's attenuation properties.
It has the same requirements as any volume control, except that this preamp uses a linear potentiometer instead of an audio/log law.
From Bruno's white paper Page 10.
"Long story short. Instead of operating as an attenuator
the potentiometer is used as the sole
feedback element in an inverting amplifier. Linearity
of the volume control now only hinges on the
linearity of the divider ratio. This is almost guaranteed
in linear pots. The track resistance can be very
very non-linear before this becomes an issue. Just
to make a point I decided to use a cheap 9mm “car
stereo” pot. Distortion performance is top notch.
The only drawback is that the control law follows an
S-curve. At both extremes control becomes very
sensitive. Since preamps are rarely used beyond
unity gain you'll mostly find the it to be a bit fiddly
at quiet settings. We’ll have to live with that because
adding external resistors to modify the control
law will immediately put the linearity of the
track resistance back into the equation. As it is,
channel matching is surprisingly good even down
to moderately low settings."
"Long story short. Instead of operating as an attenuator
the potentiometer is used as the sole
feedback element in an inverting amplifier. Linearity
of the volume control now only hinges on the
linearity of the divider ratio. This is almost guaranteed
in linear pots. The track resistance can be very
very non-linear before this becomes an issue. Just
to make a point I decided to use a cheap 9mm “car
stereo” pot. Distortion performance is top notch.
The only drawback is that the control law follows an
S-curve. At both extremes control becomes very
sensitive. Since preamps are rarely used beyond
unity gain you'll mostly find the it to be a bit fiddly
at quiet settings. We’ll have to live with that because
adding external resistors to modify the control
law will immediately put the linearity of the
track resistance back into the equation. As it is,
channel matching is surprisingly good even down
to moderately low settings."
I have read the full artecle 4 times.
I have read that paragraph at least two more times.
I just don't understand what "special" feature of a linear pot gives the volume control
It seems to me that the track needs to be linear to give the goodness he has designed in or it can be "very non linear" and still is a non issue.
It all mixed up.
In my mind BOTH of these can be solved or at least reduced by adding a limiting resistor either side of the vol pot, but B.Putzeys says this will not work with
If one chooses the limiting resistors sensibly, then one finds that the "sensitivity" to small movements/rotations can be removed and with that comes the big advantage that the whole rotation can be used for effective (almost) constant dB/degree rotation.
By removing the sensitive regions from the rotation one also removes the extreme need for exact channel to channel matching where very small errors between channels would give very big changes in gain/attenuation and thus differences in channel balance (not balanced impedance interconnections).
I have read that paragraph at least two more times.
I just don't understand what "special" feature of a linear pot gives the volume control
and then he adds
which confuses me even more.
It seems to me that the track needs to be linear to give the goodness he has designed in or it can be "very non linear" and still is a non issue.
It all mixed up.
this highlights the excessive gain and excessive attenuation that becomes evident near the two extremes of rotation.
In my mind BOTH of these can be solved or at least reduced by adding a limiting resistor either side of the vol pot, but B.Putzeys says this will not work with
Again, I just don't understand why he makes this statement.
If one chooses the limiting resistors sensibly, then one finds that the "sensitivity" to small movements/rotations can be removed and with that comes the big advantage that the whole rotation can be used for effective (almost) constant dB/degree rotation.
By removing the sensitive regions from the rotation one also removes the extreme need for exact channel to channel matching where very small errors between channels would give very big changes in gain/attenuation and thus differences in channel balance (not balanced impedance interconnections).
Changing the pot from 10k to 5k, would reduce generated noise from the last stage by -2.5dB and going to a 1K pot would even reduce this noise further with -6.7dB.
However, the two stages before the last stage are also producing noise adding to the noise of the last stage.
For a 5K pot in the 0dB and in the -20dB position, overall noise will go down by resp. -1.4dB and -0.7dB, not a big deal.
A 1K pot in the 0dB and -20dB position will produce resp. -3.1dB and -1.4dB less overall noise compared to the 10K pot.
Be aware that the last stage also has the task of driving the external load, but 1K is still well within the specs of the LM4562 Op-Amp and worth considering.
Hans
I would use a 5k1 limiting resistor before the pot and use a 5k pot.
In the maximum gain setting (+0dB) these two resistances add up to virtually the same as the 10k pot when it is at unity gain.
i.e. the noise contribution from the pot plus limiting resistor is near enough identical.
In the maximum gain setting (+0dB) these two resistances add up to virtually the same as the 10k pot when it is at unity gain.
i.e. the noise contribution from the pot plus limiting resistor is near enough identical.
Think of an extreme non linear resistance track of a linear pot.
The first part being two resistors X in series with two times 2 anti parallel diodes.
The second part of the pot having one resistors X and just 2 anti parallel diodes.
When using this as an attenuator connected to ground, the way a pot is mostly used, non linearity from the "taper"will be dreadfull.
Using this setup in a feedback loop, will result in a full cancellation of all non linearities, giving a distortion free gain of -6dB.
So as long as the ratios in the pot have the same ratios in non linearity, everything will be cancelled.
BUT, putting an extra resistor in series to reduce maximum gain, will immediately disturb the ratio in non linearity, leading to unwanted distortion.
Hans
Under the assumption that the third section would amplify most of the time, I would agree with your observation.
However, and at least in my situation, this is not the case. A preamp with this modification has been a permanent part of my stereo system for the last twelve monthts, attached to a DAC and a RIAA preamp. The preamp drives a power amp with 26dB gain.
With both sources having 4.2Vrms balanced outputs, and assuming loudspeaker sensitivity of 85dBSPL/W and 4 Ohm impedance, a preamp without the above modification would have to attenuate the input signal by some 38dB in order to produce an SPL of 85dB. This attenuation value represents simultaneously the resistance ratio of the linear pot in the circuit, i.e. the feedback to the input resistance ratio, resulting in the feedback resistance of 125R with a 10K pot.
It takes a high-precision linear pot to comfortably set resistance values of this order of magnitude; and by reducing the second stage gain by 6.8dB the above value becomes 268R, which may be somewhat less "fiddly", as Bruno put it in his article. The price to pay is reduced CMRR (but still about -60dB worst case), and a somewhat higher noise due to the 2K2 resistor instead of the original 1K.
Referring to Bruno's Mola-Mola blog (Volume control sores paragraph), he considered a high-precision P&G pot for his commercial preamp, but since there was no motorized version of it, he found a linear relay-switched resistor ladder solution for the feedback network. The latter is almost certainly a proprietary one - at least I'm not aware of such a device being offered, both commercially and in the "open hardware" spaces.
Regards,
Braca
Thanks for taking the time to explain that.
But I don't understand what your added diodes are doing.
What about using errors in resistance to explain the error cancellation when in the feedback loop?
eg when three different parts of the track are used to define the gain we can introduce a resistance error.
I'll take 1k:9k nominal as the gain setting.
5k:5k nominal as the unity gain setting.
9k:1k nominal as the attenuation setting.
At these three settings introduce a ±10ohm error to the track resistances.
at the 1:9 gain we have max gain = +19.18dB (20log(9010/990)) and min gain = +18.99dB, the difference in gain for a ±10r error is 0.19dB
at the unity gain setting 5:5, the difference is down to 0.0695dB for a ±10r error
at the attenuation setting, 9:1, the difference is even lower at 0.019dB for a ±10r error
The 10r error is 0.1% of track resistance from truly linear law.
It is clear that the 10r error has less effect in channel to channel matching at high attenuation than the same error has at high gain.
But what does that have to do with generating, or reducing "unwanted distortion"?
Last edited:
I would expect many Members using this balanced impedance pre-amp to often be using less gain than unity gain. There will be some Members who require to use a gain a bit higher than unity gain some of the time.
There will be a few Members who require more than unity gain most of the time. But I wonder how many of these will be using a balanced output that has a signal so low that it needs amplifying to feed a balanced impedance power amplifier.
I think it would be unbalanced outputs that require extra gain when their outputs are lower than our "standard" sources.
I know that even with typical 86dB to 90dB speakers and a +28dB amplifier that I rarely get as high as unity gain, never mind needing amplification.
The follow up letters in Linear Audio should also be read.
" I'd certainly not recommend anyone use this dirt cheap pot
in a serious design, for the precise reasons you mention. You'd at least have to use a decent multi-finger
wipered one, or perhaps a relay network like I do in my commercial preamp"
Quote from Bruno Putzeys.
Full details here, courtesy of Linear Audio.
http://linearaudio.net/sites/linearaudio.net/files/LTE to BP V5(1).pdf
" I'd certainly not recommend anyone use this dirt cheap pot
in a serious design, for the precise reasons you mention. You'd at least have to use a decent multi-finger
wipered one, or perhaps a relay network like I do in my commercial preamp"
Quote from Bruno Putzeys.
Full details here, courtesy of Linear Audio.
http://linearaudio.net/sites/linearaudio.net/files/LTE to BP V5(1).pdf
Last edited: