Studiophone shows two pads (vol control) between each UB set.
Are there any pros or cons to using the UBs with a single (SE) volume control, or to using a balanced volume control--you know--the two series resistor and shunt pot type thing?
That is assuming balanced in and out.
Will a balanced volume control work with balanced in and SE out--or vice versa?
thanks
t
Are there any pros or cons to using the UBs with a single (SE) volume control, or to using a balanced volume control--you know--the two series resistor and shunt pot type thing?
That is assuming balanced in and out.
Will a balanced volume control work with balanced in and SE out--or vice versa?
thanks
t
I'm very happy to report successful use of a Universal Buffer in a system including a Metrum Hex DAC and Linkwitz LX521.4 loudspeakers.
The UB is used to interface between the non-floating (ground referenced) balanced output of the Hex, and the single-ended line input of the Symfonia Reference preamp used in the system (a rare and expensive preamp made in Australia almost 20 years ago). The UB has replaced a pair of Jensen input transformers (PI-2XX) in this position. The owner had found he preferred the Jensens to the optional floating output on the Hex, which uses output transformers, and now prefers the UB to the Jensens.
The system's owner had tried other preamps with on-board balanced inputs which did not need the Jensens to receive the Metrum Hex output, but found he preferred the Symfonia plus Jensens to any of them. He is now satisfied that the UB offers better results than the Jensens in this position, so the Symfonia plus UB gets him the best results as a preamp so far.
Best regards.
The UB is used to interface between the non-floating (ground referenced) balanced output of the Hex, and the single-ended line input of the Symfonia Reference preamp used in the system (a rare and expensive preamp made in Australia almost 20 years ago). The UB has replaced a pair of Jensen input transformers (PI-2XX) in this position. The owner had found he preferred the Jensens to the optional floating output on the Hex, which uses output transformers, and now prefers the UB to the Jensens.
The system's owner had tried other preamps with on-board balanced inputs which did not need the Jensens to receive the Metrum Hex output, but found he preferred the Symfonia plus Jensens to any of them. He is now satisfied that the UB offers better results than the Jensens in this position, so the Symfonia plus UB gets him the best results as a preamp so far.
Best regards.
If your use case is balanced in, balanced out, balanced volume control, I would use one Universal Buffer on the output of the volume control in a "Basic" setup, and buffer both in and out in a "Deluxe" setup. I.e.:
"Basic": Balanced in -> Balanced Volume Control -> UB -> Balanced out
"Deluxe": Balanced in -> UB -> Balanced Volume Control -> UB -> Balanced out
In my experience, there are no drawbacks of going with a single-ended volume control as you have full control over the ground routing within the preamp chassis. Some may argue that one drawback is that a single-ended volume control forces you to take the "Deluxe" route as you need to convert from balanced to unbalanced:
Balanced in -> UB -> Unbalanced/SE Volume Control -> UB -> Balanced out
Fantastic! Thank you for sharing.
Tom
Very useful, thanks! I am looking for an alternative as to using the usual transformers (cinemag/jensen/Lundahl) on the dddac outputs. Can anyone suggest whether this buffer works well for this specific application? Balanced signal of dddac both have 2.7dc on the output. Can the buffer cope with this dc when converting to single ended?
Yes. As long as the absolute voltage on each input does not exceed the rail voltage, the Buffer will be fine.
The Buffer output is: Vout = Vin+ - Vin-
If you have DC on the input, you'll get: Vout = (Vin+ + DC) - (Vin- + DC) = Vin+ - Vin-.
Or Vout = Vin+ - Vin- + DC/CMRR, if you want to be pedantic about it. CMRR is in the 10s of thousands at DC, so the common-mode contribution can safely be ignored.
Tom
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Not sure of your circuit configuration, but for balanced output you must be using half of each op amp as a unity gain voltage follower. The only gotcha is if fed from a non-zero resistance source is common mode distortion.
So if using a balanced pot at the input, the effective maximum common mode impedance is a quarter the pot resistance. If 10k, the common mode impedance on such a voltage follower is 2.5k.
Anyhow, this might be an issue, and suggest a supplementary measurement might be distortion as a function of frequency and voltage input level for a range of feed resistances from zero to (say) 10k.
Craig
So if using a balanced pot at the input, the effective maximum common mode impedance is a quarter the pot resistance. If 10k, the common mode impedance on such a voltage follower is 2.5k.
Anyhow, this might be an issue, and suggest a supplementary measurement might be distortion as a function of frequency and voltage input level for a range of feed resistances from zero to (say) 10k.
Craig
@craig: D. Self has done such measurements on the lm4562 which is used on that buffer, found here: https://www.eetimes.com/op-amps-in-small-signal-audio-design-part-3-selecting-the-right-op-amp/ , scroll down to fig. 4.24
Not unmeasurable but not very worrying under 10k
Not unmeasurable but not very worrying under 10k
Yes - I'm very aware of Self's measurements. But common mode distortion has been known about for way longer than Self re-publicising it.
My point however is that preceding a voltage follower with a volume control pot in a buffer with exceptionally low distortion significantly compromises the distortion - and not just under 10kHz or 10k-ohms (not sure what your 10k refers to).
For an LM4562 at 10kHz, 10Vrms and 10k-ohms source resistance the distortion of an LM4562 follower is nearly 0.005%.
Basically this all kicks in seriously at around 4Vrms, although it is gaining ground at significantly lower levels. And it gets rapidly worse with increasing source resistance. So putting a pot on the input is perhaps not the best news for distortion.
But Tom is the ultimate arbiter on this - he knows his design better than your or my ideas.
Craig
My point however is that preceding a voltage follower with a volume control pot in a buffer with exceptionally low distortion significantly compromises the distortion - and not just under 10kHz or 10k-ohms (not sure what your 10k refers to).
For an LM4562 at 10kHz, 10Vrms and 10k-ohms source resistance the distortion of an LM4562 follower is nearly 0.005%.
Basically this all kicks in seriously at around 4Vrms, although it is gaining ground at significantly lower levels. And it gets rapidly worse with increasing source resistance. So putting a pot on the input is perhaps not the best news for distortion.
But Tom is the ultimate arbiter on this - he knows his design better than your or my ideas.
Craig
Oh, I certainly agree with your point. I actually suggested such a measurement in post 163 
I meant 10K Ohms. But point taken on the follower case, the measurements in that link are only done at a gain of 3, at which point there is virtually no degradation with a 10k impedance mismatch. The follower case is indeed much worse.
I meant 10K Ohms. But point taken on the follower case, the measurements in that link are only done at a gain of 3, at which point there is virtually no degradation with a 10k impedance mismatch. The follower case is indeed much worse.
Or with properly programmed relays. I am all for you to sell as many boards as you can, more people need to hear how they “do not” sound. But a full pre with XLR and SE ins and outs can be done “deluxe” with two boards and an attenuator in the middle.
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True. It's also been around since before the LM4562/LME49720 was released. However, the discovery of common-mode distortion does not change the fact that the LM4562/LME49720 does not exhibit much common-mode distortion. You can see that from Fig. 4.24 in the EE Times article that 00940 linked to. I've attached the figure to save you the trouble of clicking the link and scrolling.
10 kΩ.
Douglas Self measured 0.0004% at 10 V RMS output with 10 kΩ in series with the non-inverting input. The THD+N trace for 10 kΩ is line-on-line with the trace for 0 Ω in his figure.
I'll be measuring my upcoming buffer for the Purifi 1ET400A and Hypex NC500 within the next two weeks. I plan to measure more on the Universal Buffer at that time. Unfortunately, measuring these things at the precision levels I'm dealing with is not exactly plug-n-play. If it was, I would have provided the measurements long ago. Also, please note that I have many tasks and priorities completing for my time.
Tom
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Tom, a question. What type of power supply do you recommend? Would Jan's Silent Switcher be a good choice?
I built a a pair of Pass F4 amps and have been running them bridge mostly and to convert from a SE preamp, I am using a pair of Jensen transformers. No doubt they change the color of the sound.
I would like to try your UB.
Thanks
David
I built a a pair of Pass F4 amps and have been running them bridge mostly and to convert from a SE preamp, I am using a pair of Jensen transformers. No doubt they change the color of the sound.
I would like to try your UB.
Thanks
David
@Tom: the 0.005% quoted by Craig comes from further measurements by Self on the lm4562 in voltage follower mode. They can be found in his crossover book, some sections of which are free on google books: The Design of Active Crossovers - Douglas Self - Google Livres
It's doing it job
I put this buffer together with parts I recycled or had on hand. It's dead quiet and sounds nice. I'm using LT1963a regulators and a couple of single secondary transformers I picked up from a flea market in Guangzhou for a buck or two and I've been waiting for a chance to use them.
I will sit down later to compare it with my direct coupled B1, I don't expect to hear any differences but it's always fun to do.
I'm using Millet uniamp's and don't need any gain, I'm using a Topping D50s with 10 ohms output impedance and my amps have 47k input impedance. It does work fine directly from the DAC but I prefer using it with a buffer. I'm using the volume control of the DAC and it's set between -25 and -30 db for normal listening.
Anyway, thanks Tom
I put this buffer together with parts I recycled or had on hand. It's dead quiet and sounds nice. I'm using LT1963a regulators and a couple of single secondary transformers I picked up from a flea market in Guangzhou for a buck or two and I've been waiting for a chance to use them.
I will sit down later to compare it with my direct coupled B1, I don't expect to hear any differences but it's always fun to do.
I'm using Millet uniamp's and don't need any gain, I'm using a Topping D50s with 10 ohms output impedance and my amps have 47k input impedance. It does work fine directly from the DAC but I prefer using it with a buffer. I'm using the volume control of the DAC and it's set between -25 and -30 db for normal listening.
Anyway, thanks Tom
Attachments
Just opened for pre-orders on the promised Purifi 1ET400A / Hypex NC500 Input Buffer. You can find a vendor thread for it here: https://www.diyaudio.com/forums/ven...00a-hypex-nc500-input-buffer.html#post6127335
Tom
Tom