Someone was recently trying to talk me around the idea of a balanced passive volume control using only a single attenuator. I think the idea was basically to use the attenuator to wire up the hot and cold signals rather than sinking each to ground using two attenuators.
Can anyone comment on this circuit? A sketch of what would be required would be nice?
Also, I need this to work with the output of a computer soundcard which has a pseudo balanced output, and for example seems to respond badly to inadvertent tying of -ve and ground lines together... Is this design likely to work well in this case?
For those wondering why bother... Well, I have 8 balanced output channels, and I can't afford a decent 16 channel attenuator, whereas an 8 channel one seems more reasonable... Any other suggestions for 8 balanced channels of volume control gratefully received...
Can anyone comment on this circuit? A sketch of what would be required would be nice?
Also, I need this to work with the output of a computer soundcard which has a pseudo balanced output, and for example seems to respond badly to inadvertent tying of -ve and ground lines together... Is this design likely to work well in this case?
For those wondering why bother... Well, I have 8 balanced output channels, and I can't afford a decent 16 channel attenuator, whereas an 8 channel one seems more reasonable... Any other suggestions for 8 balanced channels of volume control gratefully received...
Someone, a well known manufacturer, raised the issue of the input stage of the amp "looses" the damping factor when the resistors to GND is not left in the circuit.
I don´t think this is a problem, the balanced input see the shunt resistance between the + and - and in my mind a bleeding off of stored energy could as well take that path.
Another solution I have used is that first you use a series element and than you use two resistors to GND as usual. You scale these resistor for the max volume you need, I never need full ouput from source, especially running balanced.
This way you have a relatively low impedance to GND and then you use the stepped attenuator to shunt the signal between the + and -.
Try to use as low value of resistors as possible as long as your soundcard can handle it without loosing performance.
/Peter
I don´t think this is a problem, the balanced input see the shunt resistance between the + and - and in my mind a bleeding off of stored energy could as well take that path.
Another solution I have used is that first you use a series element and than you use two resistors to GND as usual. You scale these resistor for the max volume you need, I never need full ouput from source, especially running balanced.
This way you have a relatively low impedance to GND and then you use the stepped attenuator to shunt the signal between the + and -.
Try to use as low value of resistors as possible as long as your soundcard can handle it without loosing performance.
/Peter
Pan, thanks. I think this is the design that was being discussed.
I think the idea was two resistors in series, followed by the attenuator which joins +ve and -ve to each other and then on down to ground. Then I seem to remember that the sketch had two more resistors in series after the attenuator, but I'm not quite sure why?
Like you I won't need a full range signal, typically I am running about -40db down from max output for example.
I was going to get one of those 10K DACT stepped things. Could someone suggest suitable starting values for the other resistors in this scheme?
Also, rough finger in the air estimate here, but is this design going to be worth doing at all rather than simply using an unbalanced output into the DACT? Rest of the system is an RME card (50 ohm output imp), then preferably 1-2 feet of cable (yeah, I know, will try to minimise this), then the DACT-in-a-box, then straight down to the ZapPulse amp input (perhaps 3-9 inches since two amps, input impedence 17K ohm)
Any thoughts?
I think the idea was two resistors in series, followed by the attenuator which joins +ve and -ve to each other and then on down to ground. Then I seem to remember that the sketch had two more resistors in series after the attenuator, but I'm not quite sure why?
Like you I won't need a full range signal, typically I am running about -40db down from max output for example.
I was going to get one of those 10K DACT stepped things. Could someone suggest suitable starting values for the other resistors in this scheme?
Also, rough finger in the air estimate here, but is this design going to be worth doing at all rather than simply using an unbalanced output into the DACT? Rest of the system is an RME card (50 ohm output imp), then preferably 1-2 feet of cable (yeah, I know, will try to minimise this), then the DACT-in-a-box, then straight down to the ZapPulse amp input (perhaps 3-9 inches since two amps, input impedence 17K ohm)
Any thoughts?
Typically a passive sound best with as low resistance as posible, I don´t know what kind of load your card would like, but I´d guess the current capability isn´t all that great.
A series resistor for a shunt attenuator like this is bets kept at 1k or so, but that means an typicall output impedance of 1.1k or less including the shunt resistors. This might be to low, but again it might work. 10k is some kind of "standard" and will probably be ok, but lower is better.
One thing about the DACT I don´t like is that the resistors is soldered in a loooong serie (series atenuator). I prefer one only resistor to GND in the shunt position. You can always buy the elma switch and make your own balanced shunt attenuator, that I would recommend.
When using passives, it´s always wise to go balanced due to the noise issue in higher impedance circuits, especially since you have balanced out from source and I believe the latest Zapulse has balanced inputs as well, right?
/Peter
A series resistor for a shunt attenuator like this is bets kept at 1k or so, but that means an typicall output impedance of 1.1k or less including the shunt resistors. This might be to low, but again it might work. 10k is some kind of "standard" and will probably be ok, but lower is better.
One thing about the DACT I don´t like is that the resistors is soldered in a loooong serie (series atenuator). I prefer one only resistor to GND in the shunt position. You can always buy the elma switch and make your own balanced shunt attenuator, that I would recommend.
When using passives, it´s always wise to go balanced due to the noise issue in higher impedance circuits, especially since you have balanced out from source and I believe the latest Zapulse has balanced inputs as well, right?
/Peter
Hi Peter, thanks for the advice. So I will start with a 1K resistor and see how I get on. What are the likely signs of lack of current capacity though? Distortion increase? Failing that what I think you are saying is move up to perhaps 2K, 5K or 10K?
There is a picture of the card itself here if that gives any clues to opamps used and potential current output capacity:
http://www.wildgooses.com/downloads/HDSP9632mod.jpg
(FWIW: the notes on the jpeg were a friends suggestions on improvements to the standard card)
I agree about your comments on the series resistors not being ideal, but it's beyond my time and abilities right now to construct a better alternative, especially assuming an 8 way switch. (Anyone want to quote me to build one for me?)
Thanks for your advice, will see how it turns out. I assume that in practice this design will be sonically pretty similar to buying an extra 8 way DACT thing and doing it the obvious way with two attenuators per channel?
Zappulse has balanced inputs, and the source is all balanced, so yes, this seems to be the preferred option (and the source is a PC with all the obvious associated noise).
Cheers
There is a picture of the card itself here if that gives any clues to opamps used and potential current output capacity:
http://www.wildgooses.com/downloads/HDSP9632mod.jpg
(FWIW: the notes on the jpeg were a friends suggestions on improvements to the standard card)
I agree about your comments on the series resistors not being ideal, but it's beyond my time and abilities right now to construct a better alternative, especially assuming an 8 way switch. (Anyone want to quote me to build one for me?)
Thanks for your advice, will see how it turns out. I assume that in practice this design will be sonically pretty similar to buying an extra 8 way DACT thing and doing it the obvious way with two attenuators per channel?
Zappulse has balanced inputs, and the source is all balanced, so yes, this seems to be the preferred option (and the source is a PC with all the obvious associated noise).
Cheers
Guess you´ll have to try and see how it sounds, adjusting from there. Remember that if you use a 10k series attenuator as a shunt with a low, say 1-5k fixed series resistor, then it is possible the values are to big in the attenuator. In practice this means you´ll end up with very few volumesettings as you run out of attenuation range.
Not familiar with the components on your card but the advices you allready have been given seems good if your in tweaking mode
.
Using the approach we discuss here will reduce the "bad" with a series attenuator, since the fixed three resistor voltage divider are good quality and soldered. Even if a component is in paralell with the signal it effects the signal in the end, but less so in this instance.
A variation of this for a two channel rig is to use the balanced three resistor voltage divider and then add a plastic pot, a good high performance passive for those on a budget.
/Peter
Not familiar with the components on your card but the advices you allready have been given seems good if your in tweaking mode
.Using the approach we discuss here will reduce the "bad" with a series attenuator, since the fixed three resistor voltage divider are good quality and soldered. Even if a component is in paralell with the signal it effects the signal in the end, but less so in this instance.
A variation of this for a two channel rig is to use the balanced three resistor voltage divider and then add a plastic pot, a good high performance passive for those on a budget.
/Peter
Thanks for all the notes on this. Very helpful. Just digesting it all right now.
OK, a new twist on this question:
I have a couple of spare amps now that all have different gain... Can I tweak this design to use all of these amps without having a problem that as I adjust the volume, one amp will get a lot louder than the others..?
Any suggestions for the best way to accomplish this?
Thanks
OK, a new twist on this question:
I have a couple of spare amps now that all have different gain... Can I tweak this design to use all of these amps without having a problem that as I adjust the volume, one amp will get a lot louder than the others..?
Any suggestions for the best way to accomplish this?
Thanks
Passive balanced volume control using a single attenuator? Possible
Hi ewildgoose,
This princible has been used in professional consoles since the dawn of audio. I have included a schematic demonstrating what is required (have include details of mute as well.)
The transformer is there just to show the way things used to be done. The princibles are similar with solid state technology. The important thing to bear in mind are your grounds. Keep chassis and audio grounds seperate except for one point only.
Hope this is of some help.
res
Hi ewildgoose,
This princible has been used in professional consoles since the dawn of audio. I have included a schematic demonstrating what is required (have include details of mute as well.)
The transformer is there just to show the way things used to be done. The princibles are similar with solid state technology. The important thing to bear in mind are your grounds. Keep chassis and audio grounds seperate except for one point only.
Hope this is of some help.
res
Hi, thanks for the thought, but I'm not really sure what I am looking at here?
I see an unbalanced input, going via a transformer (presumably to decouple), then via an attentuator and through a mute switch to output? I don't really see a) how you do channel balance, and b) how you take a balanced input?
Thanks
I see an unbalanced input, going via a transformer (presumably to decouple), then via an attentuator and through a mute switch to output? I don't really see a) how you do channel balance, and b) how you take a balanced input?
Thanks
ewildgoose said:
Hi ewildgoose.
I hope I've read this right. Basically you have 8 pseudo balanced outputs and you want to control the volume from each of those outputs with one big (ideally) 8way attenuator.
If that is the case then the circuit should work for you. The transformer was left in just to demonstrate the unbalanced to balanced path. Sorry for the confusion. I have included a sch without the transformer for clarity.
Please explain what you mean by channel balance. In a studio channel balance would be the levels that each attenuator would be set to achieve a "mix" If you mean L-R balance then things start to get a bit more complicated. Please let me know.
regards
res
Hi, ok right I get what you mean now. Thanks!
Yes, the input is a pseudo balanced output from a PC sound card (well 8 output channels potentially).
However, potentially several of the amps have a different gain, and also the speakers have different sensitivity levels. So for each pair of speakers I need to level adjust, and the big question is can I simply tweak this circuit to handle the amps with different sensitivities..?
I can't quite get my head around whether this is easily possible with a passive amp? Seems that although I can match the levels on the different amps, the one with the higher sensitivity amp will always get louder quicker as the input levels rise?
Any thoughts? It's not the end of the world if I have to use the same amps throughout, but it would be nice to be able to reuse stuff, and I don't really have the time to investigate an active amp of this scale.
By the way, input impedence into the power amp is 17K, and the output impedence of the source is 50 ohms. Is there some relationship that would allow me to pick good values for the resistors knowing this info? (Amp is a Zappulse 2.2SE, I could find out the gain levels if that were useful)
Thanks for any thoughts. Much appreciated.
Yes, the input is a pseudo balanced output from a PC sound card (well 8 output channels potentially).
However, potentially several of the amps have a different gain, and also the speakers have different sensitivity levels. So for each pair of speakers I need to level adjust, and the big question is can I simply tweak this circuit to handle the amps with different sensitivities..?
I can't quite get my head around whether this is easily possible with a passive amp? Seems that although I can match the levels on the different amps, the one with the higher sensitivity amp will always get louder quicker as the input levels rise?
Any thoughts? It's not the end of the world if I have to use the same amps throughout, but it would be nice to be able to reuse stuff, and I don't really have the time to investigate an active amp of this scale.
By the way, input impedence into the power amp is 17K, and the output impedence of the source is 50 ohms. Is there some relationship that would allow me to pick good values for the resistors knowing this info? (Amp is a Zappulse 2.2SE, I could find out the gain levels if that were useful)
Thanks for any thoughts. Much appreciated.
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