Some of you, especially visitors to Planet10's annual audio fest, may recall mild discussion about an amplifier design I've been working on for a few years which is quite plainly referred to as the "Trans-amp".
It's really a very simple embodiment of some well-known control systems engineering; a translation of the control loop as applied to virtually all modern solid state audio amplifiers from a simple voltage node to a variable mix between voltage and current. This has the effect of making the output impedance of the amplifier continuously variable over a considerably large range simply by operating a knob.
This has been in development for a number of years now since I first got some ideas from reading various articles on the internet. I was curious just to see what it sounded like to change damping factor on the fly while listening to music. With my curiosity now satisfied I have come to love having "That curious extra knob" available in my system.
A PCB/parts kit is in the making now, as with further development for some expansions on the principle which I will introduce later. I'd like to release some of these out into the audio community as I believe many will appreciate its abilities both as an excellent sounding amplifier and one with a very uncommon but highly useful feature. (Those who design speakers might come up with some creative ideas given the adjustability of the Qes parameter). This is a well-known principle with tube amplifiers, especially those without global feedback, which exhibit fairly high output impedance compared to those with the feedback.
I've been using my design for a few years now, day in and day out with excellent results on just about every speaker system. Most listeners agree that it is a very good amplifier sonically and in terms of usability. Test and measurement have also shown that the amplifier performs quite well electrically.
I'd be glad to answer any questions you have. If enough people would like to try this, please let me know and I will release a batch.
It's really a very simple embodiment of some well-known control systems engineering; a translation of the control loop as applied to virtually all modern solid state audio amplifiers from a simple voltage node to a variable mix between voltage and current. This has the effect of making the output impedance of the amplifier continuously variable over a considerably large range simply by operating a knob.
This has been in development for a number of years now since I first got some ideas from reading various articles on the internet. I was curious just to see what it sounded like to change damping factor on the fly while listening to music. With my curiosity now satisfied I have come to love having "That curious extra knob" available in my system.
A PCB/parts kit is in the making now, as with further development for some expansions on the principle which I will introduce later. I'd like to release some of these out into the audio community as I believe many will appreciate its abilities both as an excellent sounding amplifier and one with a very uncommon but highly useful feature. (Those who design speakers might come up with some creative ideas given the adjustability of the Qes parameter). This is a well-known principle with tube amplifiers, especially those without global feedback, which exhibit fairly high output impedance compared to those with the feedback.
I've been using my design for a few years now, day in and day out with excellent results on just about every speaker system. Most listeners agree that it is a very good amplifier sonically and in terms of usability. Test and measurement have also shown that the amplifier performs quite well electrically.
I'd be glad to answer any questions you have. If enough people would like to try this, please let me know and I will release a batch.
The combined use of voltage and current feedback in amplifiers to modify their output impedance behavior has been around for a long time, but not used as much as it might be. I recall in the 60's and perhaps earlier some amplifiers, like the Leak, using this technique to achieve continuosly variable damping factor.
Over the years there have also been appproaches to using this to effectively modify loudspeaker parameters actively, including things like Qts. By making the output impedance of the amplifier negative by an amount approaching the voice coil resistance, one can effectively apply a form of motional feedback, which at the same time converts the speaker from constant acceleration to constant velocity, requiring equalization.
The loudspeaker and its iteraction with the amplifier is one of the biggest listening variables, and this approach can sometimes be used to advantage in that regard. Consider this question. What amplifier did the loudspeaker designer use when he voiced his loudspeaker? The way things are usually done today, one might expect that the voicing was done with an amplifier with very high damping factor across the full audio range, as most people consider the standard ideal amplifier to be one that behaves as a voltage source.
But what if the loudspeaker designer was a tube amp guy, and voiced his loudspeakers with a tube amp with a DF of only 15? One could argue in that case that accurate reproduction from that loudspeaker in accordance with the designers intention requires an amplifier with a relatively low DF. If drive that same loudspeaker from an amplifier with very high DF, the bass may be thin and the mids and highs may be shrill. DF playes a big role in how tube amps sound different.
In some cases, amplifiers without NFB may sound different because of lower DF as well. Some might foolishly attribute this to bad effects of NFB when it is just a matter of coloration that is different as a result of different DF.
I must admit that on a couple of occasions I have taken measures to deliberately reduce DF of a solid state amp to make it sound more like a tube amp, and sometimes the results were quite pleasing. Bear in mind, however, that such differences will tend to be highly dependent on the particular loudspeaker being used. This is why a continuous adjustment like you mention can be useful and revealing.
Thanks for bringing up this interesting topic.
Cheers,
Bob
Over the years there have also been appproaches to using this to effectively modify loudspeaker parameters actively, including things like Qts. By making the output impedance of the amplifier negative by an amount approaching the voice coil resistance, one can effectively apply a form of motional feedback, which at the same time converts the speaker from constant acceleration to constant velocity, requiring equalization.
The loudspeaker and its iteraction with the amplifier is one of the biggest listening variables, and this approach can sometimes be used to advantage in that regard. Consider this question. What amplifier did the loudspeaker designer use when he voiced his loudspeaker? The way things are usually done today, one might expect that the voicing was done with an amplifier with very high damping factor across the full audio range, as most people consider the standard ideal amplifier to be one that behaves as a voltage source.
But what if the loudspeaker designer was a tube amp guy, and voiced his loudspeakers with a tube amp with a DF of only 15? One could argue in that case that accurate reproduction from that loudspeaker in accordance with the designers intention requires an amplifier with a relatively low DF. If drive that same loudspeaker from an amplifier with very high DF, the bass may be thin and the mids and highs may be shrill. DF playes a big role in how tube amps sound different.
In some cases, amplifiers without NFB may sound different because of lower DF as well. Some might foolishly attribute this to bad effects of NFB when it is just a matter of coloration that is different as a result of different DF.
I must admit that on a couple of occasions I have taken measures to deliberately reduce DF of a solid state amp to make it sound more like a tube amp, and sometimes the results were quite pleasing. Bear in mind, however, that such differences will tend to be highly dependent on the particular loudspeaker being used. This is why a continuous adjustment like you mention can be useful and revealing.
Thanks for bringing up this interesting topic.
Cheers,
Bob
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Joined 2009
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I think it's a great idea, well suited for DIY. Perhaps it's the only place it's suited because most commercial amplifier manufacturers are shy to provide customers with controls that are difficult to explain.
I've also read quite a few articles on the internet but it seems a topic that attracted a lot of interest in the past and then went silent. I'd be interested to see how you've decided to implement this control, especially given that it's been 'proven' out.
There are also some controversial comments about the relevance of DF. If I remember, some people feel it's a bit of a red herring.
I've also read quite a few articles on the internet but it seems a topic that attracted a lot of interest in the past and then went silent. I'd be interested to see how you've decided to implement this control, especially given that it's been 'proven' out.
There are also some controversial comments about the relevance of DF. If I remember, some people feel it's a bit of a red herring.
There are a number of ways I've implemented the control in my designs; the most simple being a potentiometer between two feedback nodes. There is a more complex system which I've developed that allows the output impedance to be changed via an input voltage control. For example, let's say the control voltage is between 0V and 5V. You could put a potentiometer, a computer signal, etc onto this input and use it to control the amplifier's output impedance. The main reasons I designed this were 1. To allow tracking of multiple amplifiers/channels for output impedance. 2. To allow noise-free control (no pot in the signal path). There are some possible extensions of this system and I intend to exploit them in the future.
The first release of my amplifier will be the basic version with the potentiometer since this have been proven to work very well and give outstanding results. The voltage controlled version will appear later on once I feel I've tested and developed it thoroughly enough.
The first release of my amplifier will be the basic version with the potentiometer since this have been proven to work very well and give outstanding results. The voltage controlled version will appear later on once I feel I've tested and developed it thoroughly enough.
Please have a look at Nelson Pass' websites
Pass Laboratories Manufacturing high end audio equipment and Pass DIY: Home of DIY Audio, Amplifiers, Preamps and Speakers
Especially this article:
http://www.passdiy.com/pdf/cs-amps-speakers.pdf
And read at Rod Elliot's site:
Elliott Sound Products - The Audio Pages (Main Index)
And especially this article:
Variable Amplifier Impedance
Cheers.
Pass Laboratories Manufacturing high end audio equipment and Pass DIY: Home of DIY Audio, Amplifiers, Preamps and Speakers
Especially this article:
http://www.passdiy.com/pdf/cs-amps-speakers.pdf
And read at Rod Elliot's site:
Elliott Sound Products - The Audio Pages (Main Index)
And especially this article:
Variable Amplifier Impedance
Cheers.
I can attest that this amplifier sounds really quite good. And that with an amp made with no particular consideration to parts (and originally not an amplifier aimed at audio).
It is really cool how you can twist the dial to optimize the output impedance for the particular speaker under audition (we have quite a few that are happiest when the DF is not very high)
We are currently developing a loudspeaker specifically aimed at taking advantage of the high output impedance end of the dial. We hope to have it together long before this years VI diyFEST in Aug.
dave
Hi bob,
What the best DF for speaker driving from your experience? I know that tube amps has different DF one and another. Which one is better, or there should different DF each different speaker? I need to know it much from experienced people, since there many issue and disagreement about DF.
Hi DUO,
Please explain more about how you drive the speaker, with the voltage-current mixing. Local feedback or general? like Carver do to their first amp.
It is nice topic indeed.
Thanks.
What the best DF for speaker driving from your experience? I know that tube amps has different DF one and another. Which one is better, or there should different DF each different speaker? I need to know it much from experienced people, since there many issue and disagreement about DF.
Hi DUO,
Please explain more about how you drive the speaker, with the voltage-current mixing. Local feedback or general? like Carver do to their first amp.
It is nice topic indeed.
Thanks.
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In terms of driving a speaker; different speakers work better with different damping factors. This depends on many variables in the design of the driver and the completed speaker as a system.
Planet10 may have some things to say elaborating on why this is so.
There is definitely no specific damping factor that's good for everything; though given typical amplifiers one would be led to believe that the highest possible DF is best. This is a highly unstable basis from which to assess system performance though, since some speakers simply do perform better with less DF; especially if this was intended in the original design.
onto aban: In terms of the feedback necessary to control output impedance; the feedback is global. This defines the output impedance of the amplifier by allowing the differential input stage to compare input with output voltage, current, or some arbitrary mixture of the two. There are of course ways of implementing finite damping factors without the use of any global feedback: consider a single ended triode amplifier with no feedback. This will naturally have a very non-zero output impedance, although nowhere near as high as a similar amplifier using a pentode. This has to do simpl with the plate resistance of the tubes being used.
Local feedback is used in whatever places are felt necessary by the designer in his/her amplifier in order to achieve desired levels of distortion, both temporal and harmonic. General feedback still defines the major distortion characteristic of the amplifier as a whole, but in my amplifier of course there are two general feedback loops in play and the user is selecting how much of either one goes into the mix. This opens up new issues with distortion control also.
I am unfamiliar with Carver's implementation so if anyone with knowledge on the subject would speak about it I'm sure it will add fruitfully to this discussion.
Planet10 may have some things to say elaborating on why this is so.
There is definitely no specific damping factor that's good for everything; though given typical amplifiers one would be led to believe that the highest possible DF is best. This is a highly unstable basis from which to assess system performance though, since some speakers simply do perform better with less DF; especially if this was intended in the original design.
onto aban: In terms of the feedback necessary to control output impedance; the feedback is global. This defines the output impedance of the amplifier by allowing the differential input stage to compare input with output voltage, current, or some arbitrary mixture of the two. There are of course ways of implementing finite damping factors without the use of any global feedback: consider a single ended triode amplifier with no feedback. This will naturally have a very non-zero output impedance, although nowhere near as high as a similar amplifier using a pentode. This has to do simpl with the plate resistance of the tubes being used.
Local feedback is used in whatever places are felt necessary by the designer in his/her amplifier in order to achieve desired levels of distortion, both temporal and harmonic. General feedback still defines the major distortion characteristic of the amplifier as a whole, but in my amplifier of course there are two general feedback loops in play and the user is selecting how much of either one goes into the mix. This opens up new issues with distortion control also.
I am unfamiliar with Carver's implementation so if anyone with knowledge on the subject would speak about it I'm sure it will add fruitfully to this discussion.
I honestly don't think there is a good answer to this. As I said, it would seem to depend a lot on what kind of amplifier the speaker designer voiced his speakers with (assuming you have the same subjective voicing preferences as the designer did).
Cheers,
Bob
I remembering this at Dave's place also. Daniel wasn't there but Dave, Chris and I were. I was impressed with the control it gave over the damping and how different speakers definitely sounded better with low damping. This is something I would not have guessed. But as a caveat, I did notice it was those little speakers that seem to sound better with the "apparent" bass augmentation.
This is simplified schematic of carver M1.0T but later, seem carver migrate to usual voltage amplifier with very large DF as the others.
I do have to built my own V+I amplifier, and just find something about two different kind of sound, both is very nice. The first, I am using higher damping factor, the sound just very interesting and then I change the amplifier overall gain almost twice, and it is another nice sound as result. I need to hear my amp with the first sound and also I don't want to change it (the second) back to the first condition.
The speaker is still same 2x AUDAX AX-12200 MKII in single box parallel connection.
I do have to built my own V+I amplifier, and just find something about two different kind of sound, both is very nice. The first, I am using higher damping factor, the sound just very interesting and then I change the amplifier overall gain almost twice, and it is another nice sound as result. I need to hear my amp with the first sound and also I don't want to change it (the second) back to the first condition.
The speaker is still same 2x AUDAX AX-12200 MKII in single box parallel connection.
Attachments
The schematic is typical of most examples of current-mode amplifiers at least in a very basic sense. There are some drawbacks to these but they can perform very well and still be quite simple.
There are a number of ways of combining this topology with the more often-seen voltage feedback topology to create an arbitrary output impedance. Further; you can make them variable and that's what I have done.
There are a number of ways of combining this topology with the more often-seen voltage feedback topology to create an arbitrary output impedance. Further; you can make them variable and that's what I have done.
Hi DUO,
Have you found something or any differences between two different way that has same damping factor but different in sound? or is there any different sound of two system same speaker, that both amplifier has the same THD, DF, IMD, slewrate, noise figure, and any other quality meter is just same, but different topology.
Sorry if its difficult to answer, but I have some problem with this.
Have you found something or any differences between two different way that has same damping factor but different in sound? or is there any different sound of two system same speaker, that both amplifier has the same THD, DF, IMD, slewrate, noise figure, and any other quality meter is just same, but different topology.
Sorry if its difficult to answer, but I have some problem with this.
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Hello Onto:
It is very difficult to quantify an amplifier in terms of sonic quality based on empirical measurements (hence the amount of foolery that is known to occur in audiophile circles).
My general rule is that one must take care of all loose ends before the amplifier really sounds good; though in some circumstances circuits that perform terribly in tests sound quite nice, albeit often colored. An example is an inductively coupled single ended transistor design I put together using a PNP germanium doorknob transistor from the late 50s. The circuit had relatively high amounts of distortion and an extremely limited bandwidth, yet it was quite pleasing to listen to!
The differences between amplifiers in terms of damping factor and terms of other figures are different in manifestation. For example: two very different amplifiers with the same damping factor still do exert the same damping on the speaker in a general way and will both produce nearly the same net effect on the sound if their damping factors are both changed the same amount. The inherent qualities of their individual circuits remain the same, but the effects of changing parameters is usually seen as quite similar from circuit to circuit. It's like saying that altering the power output of car A will have the same effects in general as altering the power output of car B; though both cars will still maintain their own often different characteristics in terms of ride quality, noise level, comfort, torque-steer, etc.
The real trouble is that there are a huge number of parameters at work and so this becomes more of an art form than a science; but you must remain scientific in your strategy if you are to constantly advance your art.
It is very difficult to quantify an amplifier in terms of sonic quality based on empirical measurements (hence the amount of foolery that is known to occur in audiophile circles).
My general rule is that one must take care of all loose ends before the amplifier really sounds good; though in some circumstances circuits that perform terribly in tests sound quite nice, albeit often colored. An example is an inductively coupled single ended transistor design I put together using a PNP germanium doorknob transistor from the late 50s. The circuit had relatively high amounts of distortion and an extremely limited bandwidth, yet it was quite pleasing to listen to!
The differences between amplifiers in terms of damping factor and terms of other figures are different in manifestation. For example: two very different amplifiers with the same damping factor still do exert the same damping on the speaker in a general way and will both produce nearly the same net effect on the sound if their damping factors are both changed the same amount. The inherent qualities of their individual circuits remain the same, but the effects of changing parameters is usually seen as quite similar from circuit to circuit. It's like saying that altering the power output of car A will have the same effects in general as altering the power output of car B; though both cars will still maintain their own often different characteristics in terms of ride quality, noise level, comfort, torque-steer, etc.
The real trouble is that there are a huge number of parameters at work and so this becomes more of an art form than a science; but you must remain scientific in your strategy if you are to constantly advance your art.
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