output impedance of the speaker output of the Kenwood TH-F7

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neazoi

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Hello, I need a bit of help here.
I would like to roughly find out what is the output impedance of the audio out (speaker connection) of the Kenwood TH-F7.
The service manual is here www.repeater-builder.com/kenwood/pdfs/th-f6a-and-th-f7e-svc-man.pdf
I would like to see if I will use a 600R:600R isolation transformer to connect this to the sound card of the PC, or a 8R:1K transformer I have.
Thanks
 
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The amplifier is designed to run into no less than 4 Ohms load but here are some additional components between the output pin and the speaker socket. I would not expect the load impedance to be critical, the actual loudspeaker impedance isn't in the manual but I would guess it's probably 8 Ohms. Personally, I would use the 600:600 transformer and add a resistor of about 22 Ohms across the TH-F7 side. It should give an adequate match and the resistor will help to dampen any resonances. If you use the 8:1K transformer you will probably find you get excessive voltage at the output and a horrible frequency response. Some experimentation may be necessary, you might find the 8:1K with a low value resistor across the output (1K side) gives good results too.

Brian.
 
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    neazoi

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The speaker output is designed to be a voltage source ( low Z) to dampen resonant load variations in the speaker from modulating the signal.

Like in DC power supplies , we call this , % Load regulation, but in audio amps we use the inverse and call it Dampening Factor. DF values range from 50 to 1000 in high power systems. DF of 100 is like 1% load regulation.

The speaker signal will be too high. thus step down 1k:8 can work.

The line output ought to match the Aux In level needed in the PC (1V).

If you decide to direct couple from Aux Out to Line Input, a CM choke around the stereo cable may reduce some noise and a shielded cable also.

Connecting a grounded PC to an ungrounded frame of an Audio Amp may affect the noise levels and depends on many factors depending on ground loops of other peripherals and SMPS noise from each.

600R balanced lines are used for long cables where compatible interfacing is needed with best common mode rejection. But this is not used for consumer stereo's. Consider the fiber interface option and above.
 
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    neazoi

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By looking at the datasheet of the amplifier IC, TA7368F that is used, I see that in less than 100mW of output the distortion is much reduced. Having that in mind, I thought that it would be better to keep the audio volume of the radio very low and benefit from a better matching between the radio and the PC sound blaster. I mentioned sound blaster but I did not specify it, I mean to connect this to the microphone input of a thinkpad x60s. I think the microphone input impedance in sound blasters is 1K or more.
Having all these in mind, I thought the 8R:1K transformer to be good.
These resistors you propose, confuse me a bit, won't they introduce signal loss? What are these resonances you refer and how do these appear on the DSP software?
 

Don't think in terms of audio power, only in the voltage because the power that can be delivered in to the PC's high impedance is only one or two mW anyway. What you are really interested in is the voltage across the PC input which typically only needs to be < 250mV, depending on the mixer settings and which input you use.

The other factor to consider is the load on the TH-F7 audio output, I'm not saying there would be a problem but there is certainly a risk of one, if you don't load it properly. Simple audio amps, especially with potentially non-linear loads like the switching arrangement, can go unstable.

If you use the 8:1K transformer in reverse, you will probably step up the voltage many times more than you need, it is probably more than adequate even without a transformer at all but obviously you want one for isolation. From the radio perspective you ideally want a resistive load, you might get that with a 1K resistor across the transformer secondary but if the PC impedance is higher than 1K, and I suspect it will be, you risk there being uneven output in the audio spectrum due to the inductance of the primary and secondary windings.

If you use the 600:600 transformer and add the parallel primary resistor, you bring it closer to a radio's prefered load, the load looks more resistive than inductive and the heavy damping effect will help to prevent uneven frequency response. It reduces the 'Q' if you like. The input and output voltages will be virtually identical so there will still be plenty to drive the sound input.

As I said, try it both ways. I strongly suspect the 600:600 solution will work better but there are unknown factors on the primary and secondary sides so see which works best.

If it helps - I use 600:600 isolators on my IC-756 but they connect to an external USB sound interface.

Brian.
 
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    neazoi

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Thanks Brian!
There is another factor I did not initially thought.
All these transformers have limited frequency responce. Typical is +/-3dB, 300Hz-3.4KHz @ 1KHz 0dB. This means that the lower frequencies are attenuated. Ok for radio amateur use but maybe not for SWL where better audio may be required.
Maybe I should better capacitor couple the two systems (which I can control better using different values of capacitors) and use the transformer if there are ground loop problems?


If I finally use the 600:600 transformer you mention "I would use the 600:600 transformer and add a resistor of about 22 Ohms across the TH-F7 side". Why not a 8.6R so the transceiver side is well matched to 8R?
Also do I need to use a resistor in the PC side of the 600:600 transformer?
 
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neazoi said:
.
Having all these in mind, I thought the 8R:1K transformer to be good.
Click to expand...

Dont use your transformers. make an R pad. ( small resistor divider)


A transformer impedance ratio of 8:1k is proportional to turns ratio n² while voltage ratio is n or 1:11.2 in this case.

Since a mic input is low level ( 0.05V) and speaker output is high level 1~5V you need to attenuate the signal by 100:1 so the reverse of 1:11 is not enough.

You can attenuate with any Resistor pad for each channel and worry less about the impedance as the mic input impedance will not load your signal pad.

e.g. Consider 1KΩ:10Ω for 100:1 voltage attenuation.or 100Ω:1Ω
 

I think Neazoi's intention was to use the 8:1K transformer with the '8' at the TH-F7 to match the speaker outlet and the 1K side to match the PC.

If I finally use the 600:600 transformer you mention "I would use the 600:600 transformer and add a resistor of about 22 Ohms across the TH-F7 side". Why not a 8.6R so the transceiver side is well matched to 8R?
Click to expand...
There would be no harm in doing that but you might be able to reduce the current in the audio amp by using a slightly higher impedance load. It's a compromise, lower resistance=more current, higher resistance=possibility of instability.

For SWL use I wouldn't be over concerned by those frequency responses. The upper response will be set by the IF bandwidth anyway. If you are using it for data modes it should be fine. Those are -3dB points, it will continue lower in frequency for a long way before having significant effect on reception.

Tony: I would advise against a pure resistive option. I'm not sure if Neazoi want's to transmit as well as receive using this interface and I've had bad experiences of RF flowing where it shouldn't. In fact last week I killed the PIC in a Signalink box because of a test done with bad SWR. I would be inclined to keep some kind of voltaic isolation, even if as poor at HF as an audio transformer.

There is another option if you really need wide bandwidth, a linear optocoupler.

Brian.
 
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    neazoi

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If the Kemwood output level is turned down then noise (hiss) and crossover distortion might be unbearable. So keep the output level from the Kenwood up as high as possible without any clipping distortion.
 
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