That curious extra knob, or: "Trans-Amp"

[Duo]

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.

[Bob Cordell]

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

[Bigun]

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.

[Duo]

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.

[Gordy]

If it is a low power amplifier is it possible to simply switch in extra series resistors at the output?

[Duo]

Gordy: I assume you're referring to existing amplifiers without a variable impedance functionality?

It is possible to switch resistors in to effective increase the output impedance. You just start wasting a lot of power as you increase the resistance.

[Gordy]

Originally Posted by Duo

Gordy: I assume you're referring to existing amplifiers...

Yes. Thanks for the reply.

I'm looking forward to learning more about your amp, and I'm going to read-up on DF!

[Duo]

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.

[Sch3mat1c]

Originally Posted by Bigun

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.

Nahhh, that's easy: label the ends of rotation "MAGIC" and "MORE MAGIC".

Tim

[planet10]

Originally Posted by Duo

Most listeners agree that it is a very good amplifier sonically and in terms of usability.

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