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BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)
BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)
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Old 13th December 2015, 09:18 AM   #261
jan.didden is offline jan.didden  Europe
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You get all the advantages of the balanced system, including the unbalanced level control that doesn't have the sensitivity to noise and hum pickup of a 'traditional se level control' from the point you enter the balanced input stage.

Jan
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Old 13th December 2015, 09:32 AM   #262
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by TNT View Post
Max gain = 1 for this design?

//
R5A/B and the opamp U2/7B controls the attenuation and gain.

If the vol pot is set to 50% rotation, then the left side of R5 has 5k and the right side of R5 has 5k.

that gives the vol pot gain = 5k/5k = 1times (+0dB).

advance the vol pot all the way to the right to leave 0k to the right and 10k to the left and the gain is 0/10k = silent, or -infinity dB. This would by convention be obtained by turning the vol pot fully anticlockwise.

If your required range of volume adjustment is from silent (~-120dB) to +0dB then you use 50% of the rotation.
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Last edited by AndrewT; 13th December 2015 at 09:44 AM.
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Old 13th December 2015, 09:41 AM   #263
AndrewT is offline AndrewT  Scotland
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If one needs gain from the R5a/b+U2/7B stage, then one advances the vol pot to the left to use the other 50% of the rotation.
If one turns to 75% rotation one gets 2k5 to the left side and 7k5 to the right side.
The gain is 7k5/2k5 = 3times (+9.54dB)
If you need more gain, then turn the vol pot a bit further.
At 80% rotation the gain becomes 8k/2k = 4times (+12dB)
at 90% rotation the gain becomes 9k/1k = 9times (+19.08dB)
at 95% rotation the gain becomes 9k5/0k5 = 19times (+25.58dB)
at 98% rotation the gain becomes 9k8/0k2 = 49times (+33.80dB)
at 99% rotation the gain becomes 9k9/0k1 = 99times (+39.91dB)
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Old 13th December 2015, 09:47 AM   #264
billshurv is online now billshurv  United Kingdom
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BPPBP - Bruno Putzey's Purist Balanced Preamp (well a balanced volume control really)
If my memory serves me, you still get about 30dB CMRR with an imbalance of 1K. Obviously using shielded twisted pair for the interconnect is the smart thing to do hear, but I only see benefits from using single ended sources with a balanced reciever. A waste of time its not.

Last edited by billshurv; 13th December 2015 at 09:48 AM. Reason: Tautology
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Old 13th December 2015, 06:56 PM   #265
Eldam is offline Eldam  France
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Thank you Andrew and Jan for your answers.
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Old 20th December 2015, 01:22 PM   #266
DNi is offline DNi  Switzerland
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Originally Posted by jan.didden View Post
Andrew, I wonder how that would impact the tracking itself between the end-stops. I haven'really looked at it, but someone should do a list of attenuation versus angle with your numbers, in Excel or something and graph the list. Hopefully it will be linear, but I am doubtful.

jan
Here are the plots for a Vishay P11S2F0GGSY00D0063, nominally 2x10K conductive plastics pot. I've got this pot in both preamp samples that I have.
I did the measurements because I noticed channel imbalance at high volumes when measuring the preamp performance, and I wanted to know why this happened.
I measured the resistance (Rx) between the CCW end and the taper at 27 approximately equidistant angular positions. Having the exact angle values is not important in this case because I'm comparing the tracking and gain at the same wiper position for both potentiometer elements on the same shaft.
I entered the data obtained into an Excel sheet and analysed the tracking and gain errors for the Bruno's preamp for both the "Pot only" and "Pot + 2K2 resistor" configurations. The front element of the pot is assigned to the left channel, and the rear one to the right channel.
The first plot displays the tracking error between the front (L) and rear (R) pot elements, defined as the quotient between the respective Rx/R ratios, and expressed in dB. Discounting the first two positions, the mean tracking error is approx. 0.25dB for the respective pot element resistances R of 9102 and 9021 Ohms.
The volume control gain curve that this pot produces in the preamp is shown on the second plot. The useful control range is between approx. -40 and +26dB; as mentioned elsewhere, it becomes "fiddly" at both extreme positions. Observe that the gain law slope increase past position 17, the repercussions of which will be dealt with later. Gain differences between the channels are also clearly visible in this region.
Using the data measured to calculate the gain control law if a 2K2 resistor were connected in series with the pot, I obtained the volume control gain curves of the third Figure. The gain law between positions 5 and 26 has an almost perfect "linear in dB" characteritics, and the gain differences at high pot settings are not pronounced. Note that the addition of the 2K2 resistor limits the volume control stage gain to about 12dB.
Plotting now the gain differences between the channels for both configurations (the fourth Figure), it can be seen that the gain difference in the configuration "Pot only" increases as the stage gain is increased, reaching values that become audible from the pot position 19 (resistance approx. 6.5K out of 9K total for this pot) onwards. The configuration "Pot + 2K2" does not change its channel imbalance as much in this region, staying roughly around 0.5dB on the average. The reason for the latter behaviour is that the additional resistance diminishes the gain sensitivity with regard to the pot imperfections at high CW pot settings. This can be proved mathematically by taking the gain derivative with respect to the resistance ratio; I did it, but I omit the proof here in order not to overload the post.
On the basis of these results, I can conclude that the addition of a series resistance of 2K2 to this particular pot has improved the channel imbalance at high volumes; the price to pay is the reduction of the maximum volume control stage gain. In my case, it was the effect I sought anyway.

Regards,
Braca
Attached Images
File Type: jpg PotTrackingError.jpg (381.7 KB, 520 views)
File Type: jpg Gain_PotOnly.jpg (357.3 KB, 519 views)
File Type: jpg Gain_Pot&2K2.jpg (369.7 KB, 505 views)
File Type: jpg Gain_Difference.jpg (416.9 KB, 503 views)
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Old 21st December 2015, 08:08 AM   #267
AndrewT is offline AndrewT  Scotland
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Thanks Dni, that confirms my experiments with an active inverting opamp gain/attenuation control.

Adding in the limiting gain resistor at one end significantly reduces the balance error where the curve steepens. That's the series 2k2 you used.

Reducing maximum gain to 0dB:
I also looked at swapping out the 10k VR and replacing it with a 5k VR then adding in 5k1 of series resistor. This gives a maximum gain of ~0dB, i.e. virtually a no gain Buffer.
The total resistance before the -IN becomes ~10k and does not introduce significant noise. The standard arrangement uses the 10k vol pot as the total resistance before the -IN and it too equals 10k.

I also looked at adding in a resistor inside the out to -IN feedback loop.
This too has the effect of reducing the balance error as well as reducing the maximum attenuation.

The clue to balance errors are the very steep parts of the curve as one moves from the "constant slope" in the middle 80% of rotation.
It's the "steepness" at the end of travel that gives rise to the balance errors.
Reducing this slope to nearer the constant slope makes for a low error balance and gives a nice controlalble gain through the whole rotation range of the vol pot, that is more tolerant of small track to tracks resistance differences.
These two added resistors need to "match" the actual resistance of the vol pot track. They can be trimmed to get an exact channel balance when the contol is at either extreme rotation. It may be better to trim the added resistors when the rotation is just off the end stops and both wipers are on the resistive track.

I attach the spreadsheet that shows the effect of these two added resistors (4 in a two channel vol pot).
remove the .txt to leave a .ods open spreadsheet.
I think excell will open an .ods file.
If not, I can export an excel style and attach that as well.

The .ods file is not opening.
I will do an excel .xls version. The .txt has to be renamed to .xls
It is not opening either.
Attached Files
File Type: txt activeVolPot.ods.txt (26.3 KB, 34 views)
File Type: txt activeVolPot.txt (19.0 KB, 31 views)
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Last edited by AndrewT; 21st December 2015 at 08:37 AM.
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Old 21st December 2015, 08:32 AM   #268
AndrewT is offline AndrewT  Scotland
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How does one attach a spreadsheet?

I have attached spreadsheets with Open Office and it worked.
Today I have Libre Office and it does not work.
Is Libre Office the problem?
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Old 21st December 2015, 09:11 AM   #269
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by DNi View Post
Here are the plots for a Vishay
Braca
Braca, I understand these plots are from measuring the various pot parts versus position.

What I would like to see is the percentage or ratio of Rx/R for each position for each individual pot. While the pot may have different total values end-to-end, do the ratios track with position?
Maybe I didn't read your post correctly, but I think what you show is something different, rather values of Rx and R between pots. For this volume control, what is determining is the Rx/R ratio tracking versus angle between pots.

In other words, if one pot is 10k, the other is 11k, but in the 50% position one is 5k/5k and the other 5.5k/5.5k, that would still be perfect tracking in this context.

Jan
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Old 21st December 2015, 09:14 AM   #270
jan.didden is offline jan.didden  Europe
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Quote:
Originally Posted by AndrewT View Post
How does one attach a spreadsheet?

I have attached spreadsheets with Open Office and it worked.
Today I have Libre Office and it does not work.
Is Libre Office the problem?
Andrew it might be that the Libre office file extension is not known/accepted by the forum.
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