K3 Harmonic Distortion

[N8LP]

This discussion reminds me of some of the design choices I had to make when designing LP-PAN last year. I wound up using the SP-70 MILSPEC xfmrs, each driven by a pair of op-amps in a push pull configuration with "zero ohm" source. My original goal was to reduce 2nd harmonic distortion and improve response at frequencies approaching 0 Hz without the need for large capacitors in the output. The xfmrs are quite expensive, even in large quantities, but I have been very happy with the results.

Larry N8LP

> Date: Thu, 04 Sep 2008 14:33:19 -0400
> From: Jack Smith <[hidden email]>
> Subject: Re: [Elecraft] K3 Harmonic Distortion
> To: David Cutter <[hidden email]>
> Cc: [hidden email], "Leigh L. Klotz, Jr WA5ZNU"
> <[hidden email]>, "Julian, G4ILO" <[hidden email]>
> Message-ID: <[hidden email]>
> Content-Type: text/plain; charset=ISO-8859-1; format=flowed
>
> David:
>
> Even a rather poor transformer improves significantly when driven by a
> "zero ohm" source such as an op amp. The typical modem line transformers
> that are spec'd at 0.5% THD, for example, come in one to two orders of
> magnitude better when driven by a low impedance source. In fact, it's at
> the point where my lowest distortion audio generator (HP 200CD) isn't  
> good enough to measure a $4 transformer when driven by an op amp. I'm
> generator limited at -63 dB at the moment.  I hope to improve that
> measurement floor in the next week or so.
>
> Driving the same transformer with a 600 ohm source Z brings you back to
> the 0.5% THD range.
>
> It's all quite interesting and reminds me again that "it's not what you
> don't know that's the problem; rather it's what you think you know that
> bites you in the backside."
>
> Jack

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[Jack Smith-6]

I've completed a series of detailed measurements trying to understand
why my K3's LIN OUT port exhibits greater distortion and anomalies than
found in the headphone and speaker ports.

Details are at
http://www.cliftonlaboratories.com/elecraft_k3_receive_audio.htm

What I found is that the 604 ohm resistor between U29's output amplifier
stage and the TTC-108 transformer primary winding forces the TTC-108
isolation transformer to operate in a regime where the performance
improving effects of a zero ohm driving source are not available. Hence,
the TTC-108's 0.5% THD performance significantly limits the LIN OUT
ports performance.

I've also provided a  theoretical parts value change that should improve
the LIN OUT distortion and intermodulation. I have not implemented this
change in my K3, but the external circuit mockup I used for testing
strongly supports the change.

I've supplied my measurements to the gurus at Elecraft for their
consideration. If my data and  recommendations make sense, I would
expect confirmation from Aptos, after they have time to think it over
and consider possible side effects.

Jack K8ZOA
www.cliftonlaboratories.com

 
>
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[Jim Brown-10]

On Fri, 05 Sep 2008 21:22:33 -0400, Jack Smith wrote:

>What I found is that the 604 ohm resistor between U29's output amplifier
>stage and the TTC-108 transformer primary winding forces the TTC-108
>isolation transformer to operate in a regime where the performance
>improving effects of a zero ohm driving source are not available. Hence,
>the TTC-108's 0.5% THD performance significantly limits the LIN OUT
>ports performance.

Your advice is consistent with mine, except that I would eliminate the
transformers completely. They are not needed if all the interconnected
gear is bonded together, and they are susceptible to magnetic fields.  

73,

Jim K9YC



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[David Cutter]

Jack

The obvious question is: what is the purpose of the resistor?  Perhaps to
create a driving source to comply with the manufacturer's data?

David
G3UNA

> What I found is that the 604 ohm resistor between U29's output amplifier
> stage and the TTC-108 transformer primary winding forces the TTC-108
> isolation transformer to operate in a regime where the performance
> improving effects of a zero ohm driving source are not available. Hence,
> the TTC-108's 0.5% THD performance significantly limits the LIN OUT ports
> performance.
> >
> Jack K8ZOA
> www.cliftonlaboratories.com
>
>

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[Jack Smith-6]

You'll have to address that question to the Elecraft design staff.

My guess is that it was to protect the DAC's output amplifier from short
circuits a user might inadvertently apply across J22, the LIN OUT port,
but that can be done with much less  resistance. In fact, the winding
resistance of the TTC-108 is probably adequate for this purpose without
any additional resistance.

Question: You are given a  transformer with no specifications. How do
you determine the transformer's rated operating impedance? The answer to
this question should suggest that there is no inherent operating impedance.

Jack


David Cutter wrote:

> Jack
>
> The obvious question is: what is the purpose of the resistor?  Perhaps
> to create a driving source to comply with the manufacturer's data?
>
> David
> G3UNA
>
>
>> What I found is that the 604 ohm resistor between U29's output
>> amplifier stage and the TTC-108 transformer primary winding forces
>> the TTC-108 isolation transformer to operate in a regime where the
>> performance improving effects of a zero ohm driving source are not
>> available. Hence, the TTC-108's 0.5% THD performance significantly
>> limits the LIN OUT ports performance.
>> >
>> Jack K8ZOA
>> www.cliftonlaboratories.com
>>
>>
>

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[Jim Brown-10]

On Sun, 07 Sep 2008 11:27:39 -0400, Jack Smith wrote:

>Question: You are given a  transformer with no specifications. How do
>you determine the transformer's rated operating impedance? The answer to
>this question should suggest that there is no inherent operating
impedance.

Only for Alice in Wonderland. It could also suggest that the manufacturer
doesn't know any better and hopes you won't either. A transformer is a
passive circuit element with a rather complex equivalent circuit that
includes resistance in each winding, stray capacitance in each winding and
between windings, and leakage inductance. There may also be stray
capacitance to a frame or enclosure. The magnetic material contributes
more resistance and non-linearity, which means that some values in the
equivalent circuit vary with the applied signal.

Transformer performance can be optimized for a given circuit configuration
and set of operating conditions by many design decisions made by the
transformer manufacturer. What core material, how much of it, what form?
How many turns, how are they wound, to what degree of uniformity and
balance? Is there a magnetic shield? Is there a Faraday Shield? Some
manufacturers are far more aware than others of these parameters and how
they can be optimized for a given application. If you look at the Jensen
catalog (www.jensen-transformers.com) you will see transformers for use as
output transformers and for use as input transformers. You will see
transformers rated for a range of levels and frequency response. Ya pays
yer money and ya takes yer choice.

Fundamental circuit analysis dictates that for optimum performance, the
complex equivalent circuit and its non-linear properties must be
considered in the design of circuit. Deane Jensen's 1978 applications
note, referenced below, addresses the function of a build-out resistor for
a modern op amp. In essence, the op amp needs to be protected from
instability that can occur with a capacitive load. Most pro products use a
value on the order of 80-100 ohms. As Jack observes, there is already
series resistance in the equivalent circuit, and this may be sufficient.

http://www.jensen-transformers.com/an/an001.pdf 

600 ohm build-out and termination has not been used in pro audio for at
least 40 years. Among other things, it wastes 6 dB of headroom, and as it
is used here, inhibits damping of transformer non-linearity by the output
stage.

73,

Jim Brown K9YC


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[P.B. Christensen]

> You'll have to address that question to the Elecraft design staff.
> My guess is that it was to protect the DAC's output amplifier from short
> circuits a user might inadvertently apply across J22, the LIN OUT port,
> but that can be done with much less  resistance.

I can't speak on behalf of the designer either, but more often that not, a
series build-out resistor equal to the specified primary Z is oftentimes
seen in professional audio applications when driving the transformer from a
near zero Z source -- like an op-amp.  *Some* designers feel an absolute
need to match the source Z with the transformer primary.

In reality, the best value for Rs is what produces the best compromise
between distortion, transient response, protection to the driving source,
noise, and secondary output level.

Speaking of transient response, I have ordered the TTC-108 and Triad TY-145P
transformers and will be testing them against a high quality Jensen type.
High quality transformers like those made by Jensen are generally uniform in
phase and amplitude as a function of audio frequency.  A Tektronix TM series
audio test set will be used compare square wave response under varying
degrees of source and terminating Z across all three transformers.

A square-wave test through a transformer can be important in this
application since an improper termination of the secondary can lead to
severe overshoot and ringing into relatively Hi-Z bridging devices like a PC
sound card.  While sine-wave level testing may appear to be acceptable,
bizarre results can occur when trying to feed a demodulated CW signal (a
square wave-like source) into a PC sound card for applications like CW
Skimmer.

Paul, W9AC

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