Quick question to Bruno on volume attenuation. We covered this a little way back when the UCD180 was just out, but I wanted to double check before I built anything
Basically, with the UCD we have a differential input, but as you point out, this is differential and we don't need a mirror image signal on each leg.
So assuming I want to attenuate a balanced source component, can I just wire up a standard log pot, pretty much as normal, but using the -ve input leg wired to the ground on the pot (rather than wiring the log ground to audio ground). This way the -ve signal to the amp will be the -ve signal from the input, and the +ve signal to the amp will be an attenuated version of the +ve input signal.
OK, we reduce the amount of common mode rejection that we achieve, and the amp will see a signal that is effectively a DC offset from the original signal (ie not symmetrical about ground level), and the +ve amp input may well be below ground potential at low listening levels. However, is this design viable for a passive attenuator for the UCD?
I would prefer to use a shunt design with two resistors on each input leg, and the variable resistor tied between them (which would give a symmetric input signal), but using a log pot as the variable part means that it is no longer linear in volume, but gets louder much too quickly (obvious when you think about it). However, it's hard to get anything of reasonable quality in anything other than log pot style (goldpt.com do a few).
...and I also just bought a big DACT attenuator, so I would like to make something work using that..!!
Confirmation or suggestions for a new design appreciated
Basically, with the UCD we have a differential input, but as you point out, this is differential and we don't need a mirror image signal on each leg.
So assuming I want to attenuate a balanced source component, can I just wire up a standard log pot, pretty much as normal, but using the -ve input leg wired to the ground on the pot (rather than wiring the log ground to audio ground). This way the -ve signal to the amp will be the -ve signal from the input, and the +ve signal to the amp will be an attenuated version of the +ve input signal.
OK, we reduce the amount of common mode rejection that we achieve, and the amp will see a signal that is effectively a DC offset from the original signal (ie not symmetrical about ground level), and the +ve amp input may well be below ground potential at low listening levels. However, is this design viable for a passive attenuator for the UCD?
I would prefer to use a shunt design with two resistors on each input leg, and the variable resistor tied between them (which would give a symmetric input signal), but using a log pot as the variable part means that it is no longer linear in volume, but gets louder much too quickly (obvious when you think about it). However, it's hard to get anything of reasonable quality in anything other than log pot style (goldpt.com do a few).
...and I also just bought a big DACT attenuator, so I would like to make something work using that..!!
Confirmation or suggestions for a new design appreciated
You've opened a can of worms there. The use of a single pot to attenuate a balanced signal is not trivial.
When the pot is at zero volume, the common mode is not DC, it's half the audio signal (the half which is riding on the negative side). Secondly, the impedance balance is gone which indeed degrades CMRR. So this option would only make sense if the source ties cold to gnd (asymmetrical source).
The second option (2 fixed resistors, one variable) works fine for a symmetrical source, but... if the source is asymmetric, the common mode voltage is half the audio signal, even if the volume is at zero.
So far the best method is two ground-referred precision attenuators per channel (and I mean Precision, because the matching will be what determines CMRR).
I'm still looking hard for something more elegant, but the above is the best I've been able to come up with so far.
When the pot is at zero volume, the common mode is not DC, it's half the audio signal (the half which is riding on the negative side). Secondly, the impedance balance is gone which indeed degrades CMRR. So this option would only make sense if the source ties cold to gnd (asymmetrical source).
The second option (2 fixed resistors, one variable) works fine for a symmetrical source, but... if the source is asymmetric, the common mode voltage is half the audio signal, even if the volume is at zero.
So far the best method is two ground-referred precision attenuators per channel (and I mean Precision, because the matching will be what determines CMRR).
I'm still looking hard for something more elegant, but the above is the best I've been able to come up with so far.
Bruno Putzeys said:
Hi Bruno,
I searched the net a bit and found the j511. So it is a current source that forces a constant current between output and negative rail. So you turn off one of the output transistors (the one to the negative rail) and the other one is now biased in CLass A, so you have a single ended Class A output stage.
How much bias makes sense, the J511 gives you about 4.7mA. You want to stay always in the class A bias state, so worst case, when we see about 1kOhm input impedance at one of the UcD inputs (was at the negative one?) (when biased symmetrically), we could go to max output voltage of -4.7V, if more, then the current source can not deliver enough current and the transistor in the opamp to negative rail has to contribute to the current. So with 4.7V on the + input and 4.7V on the - input (of the UcD of course, not on the opamps), we have 9.4V input voltage and with a gain of about 4.5 for the UcD module itself, we would get 42.3V (peak). For my UcD180 that would just be OK, however, for UcD400, you may want to have a bit more current (two of them in parallel?) to make sure you always stay in Class A.
Does the above all make sense or am I mistaken somewhere?
Best regards
Gertjan
Hi Gertjan,
Here is one article on it for you
http://tangentsoft.net/audio/opamp-bias.html
You probably found that one already.
It seems the key is to have the bias current just greater than the max possible load current in order to always keep it properly biased in order for this to work, like you were saying.
I think you have it figured out anyway, I have seen some say they've used up to 10mA and others 2mA, as long as it's more than the load current and within the SOA of the op amp it should work right?
Yet another edit:
Every single reference to this I've found online always say "tie it to the negative rail"
Bruno say's go with the rail that has the worst PSRR though, I'd trust him. So if I recall correctly opa134 and opa627 both would tie to the positive rail as they both have around 20db worse PSRR on them.
Here is one article on it for you
http://tangentsoft.net/audio/opamp-bias.html
You probably found that one already.
It seems the key is to have the bias current just greater than the max possible load current in order to always keep it properly biased in order for this to work, like you were saying.
I think you have it figured out anyway, I have seen some say they've used up to 10mA and others 2mA, as long as it's more than the load current and within the SOA of the op amp it should work right?
Yet another edit:
Every single reference to this I've found online always say "tie it to the negative rail"
Bruno say's go with the rail that has the worst PSRR though, I'd trust him. So if I recall correctly opa134 and opa627 both would tie to the positive rail as they both have around 20db worse PSRR on them.
Hi Bruno, I don't quite understand a few of the points you are making (yeah, I get the big picture, just not a few details)
Firstly, assuming I have a source which is providing a symmetric output (It's a pro audio soundcard, an RME9632). The approach of using a standard log pot only has nearly full CMR at full volume. The CMR rejection reduces as the volume is lowered, but arguably this is at least still a benefit and not a hinderance? It is surely still better than just using one leg of the source component and going single ended?
Secondly though I am not sure what you mean by "impedance balance is gone"? Are you referring to the fact that we have different impedence on each input line?
However, my problem with using my *log* pot in a double-shunt style circuit is that of course it is only "log" when you compare the resistence of the output relative to the resistance of the other part of the potential divider. If I "cheat" and just use the variable part of the device, then it actually changes faster than "log" (if you like). Now there is a "law faking" idea for using linear pots as a kind of log type circuit, but I guess I need the opposite here!
(Yeah, I could use a linear pot, but I have already ordered this log thing and it's quite expensive, so I just need to do the best thing possible with it really)
We are somewhat off-topic here I know, but I think there are others who are probably thinking about the same kinds of thing
Any thoughts on the best circuit I can build for my asymmetric output using this log stepped attenuator device?
Thanks, Ed
Firstly, assuming I have a source which is providing a symmetric output (It's a pro audio soundcard, an RME9632). The approach of using a standard log pot only has nearly full CMR at full volume. The CMR rejection reduces as the volume is lowered, but arguably this is at least still a benefit and not a hinderance? It is surely still better than just using one leg of the source component and going single ended?
Secondly though I am not sure what you mean by "impedance balance is gone"? Are you referring to the fact that we have different impedence on each input line?
However, my problem with using my *log* pot in a double-shunt style circuit is that of course it is only "log" when you compare the resistence of the output relative to the resistance of the other part of the potential divider. If I "cheat" and just use the variable part of the device, then it actually changes faster than "log" (if you like). Now there is a "law faking" idea for using linear pots as a kind of log type circuit, but I guess I need the opposite here!
(Yeah, I could use a linear pot, but I have already ordered this log thing and it's quite expensive, so I just need to do the best thing possible with it really)
We are somewhat off-topic here I know, but I think there are others who are probably thinking about the same kinds of thing
Any thoughts on the best circuit I can build for my asymmetric output using this log stepped attenuator device?
Thanks, Ed
Whoa. That's what happens when correct observation (=listening test) is incorrectly interpreted (simply looking at the output stage).peranders said:
The hidden truth revolves around PSRR. An op amp always uses one rail as an implicit reference. The PSRR of this rail is zero, with only loop gain standing in for rejection. Thus, great improvement is to be had if you can make sure that rail only sees a constant current (thus making it cleaner). Tying a current source across the output transistor that ties to the "bad rail" turns it off and replaces it by the constant current source.
This has nothing to do with "class A" operation, even though for reasons of simplicity we call it that.
Most bipolar IC processes have p-jfets but no n-jfets. For this reason, the majority of IC jfet op amps has the VAS on the negative rail.
When the pot is fully ccw the amplifier sees the audio signal on both inputs common-mode. Common-mode rejection is not frequency-independent though - CMRR always degrades at higher frequencies. This means you'll get a weak signal that no longer has a flat frequency response.ewildgoose said:
UcD's CMRR is limited by resistor tolerances at low frequencies and by capacitor tolerances at high frequencies. One module may have 70dB CMRR up to a few kHz, then drop to 55dB at 20kHz. Another may have 60dB throughout. The phase of the small residual signal is can be 0 or 180 degrees.
A CMRR of typ 60dB is good for insuring hum and other interconnection problems are eliminated (this is why the amps have a balanced input), but it is not intended or sufficient to correctly extract a differential-mode signal riding on a common mode signal that is much larger.
I'll keep you posted on how I fare with my own balanced preamp project.
Bruno, you may be right here but I wonder if anyone has done any real tests, both with instruments and by hearing? I have this current source to V- and used LT1028 in my QSXM3 phono amp and OPA627, AD8610 and OPA134 in my headphone amps and I can't here a thing in difference. Some people claim they can really pick this but I'm skeptical.Bruno Putzeys said:
Bruno, hmm ok, I think you are saying that I am going to be much better off going single ended then? However, I suspect that in practice the volume control will be usually at 9 oclock ish, and only rarely turned all the way down. So if I am only clipping off, say, 20db from max signal levels then I might hope that this will be enough to give a useful boost to signal quality?
In practice, low level listening will only be useful in rare conditions (my wife for example likes some music on quietly if having 40 winks on the sofa.) So I think having lower quality audio at these levels will be fine.
What is your basic design for pre-amp then? PGA?
Oh, one other last question which is probably relevant here: earthing through the passive pre... Assuming balanced cabling still, multiple physical amplifiers in seperate earthed chassis, and a single multichannel source component. Now in the pre-amp is it best to just connect earth wires from each input direct to each output, or should all the input and output earth lines be tied to the chassis earth in the preamp? I think from what you said before that tying them all together is the correct technique, but I am anxious to avoid earth loops and later problems after what will probably be a long soldering session...
Thanks
In practice, low level listening will only be useful in rare conditions (my wife for example likes some music on quietly if having 40 winks on the sofa.) So I think having lower quality audio at these levels will be fine.
What is your basic design for pre-amp then? PGA?
Oh, one other last question which is probably relevant here: earthing through the passive pre... Assuming balanced cabling still, multiple physical amplifiers in seperate earthed chassis, and a single multichannel source component. Now in the pre-amp is it best to just connect earth wires from each input direct to each output, or should all the input and output earth lines be tied to the chassis earth in the preamp? I think from what you said before that tying them all together is the correct technique, but I am anxious to avoid earth loops and later problems after what will probably be a long soldering session...
Thanks
I don't have the answer 
peranders said:
Hi, I just double checked the data sheets for opa134 and opa627, I don't know about the others you've mentioned but these show worse PSRR on the positive rail, so if this trick indeed works as discussed here it wouldn't have helped being tied to the negative rail anyway.
Even though there's a thread on this forum which raves about the improvement with the opa627 being tied to the negative rail via a simple resistor.. I think we should probably look towards the instrumentation for proper test results on this one.
Jan-Peter said:
Hi Jan-Peter,
The box well arrived this morning.
Yes, the coupling caps (juste after the NE5532).
I have some BG PK serie 47UF 25V in stock. They are 6,3mm diameter. I should have some 47UF 35V ELNA Silmic II somewhere, but their are biggest (10mm).
I plan to test the modules first without mod...
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