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Author Topic: Question on Stereomour II...  (Read 1006 times)

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Offline Checksix

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Question on Stereomour II...
« on: March 25, 2018, 05:06:17 AM »
Not a EE but understand most of the principles.

Trying to decide on some speaker cables for my Stereomour II. 

Stereomour II OT has inductance (and resistance/capacitance), The speaker has inductance (and resistance/capacitance).   The driver I'm using, is the Dayton PS180-8.   Le = 0.63 mH at 1KHz, Re is 6.4 Ohms.

With all the choices in speaker cables and some have their characteristics documented (re: inductance, capacitance & resistance), I'm wondering from a EE perspective, what speaker cable electrical characteristics are a better match, if one knew the OT characteristics and the PS180-8 numbers above?   I don't know the OT numbers but I figure the designers do.

Thanks!

Offline Paul Birkeland

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Re: Question on Stereomour II...
« Reply #1 on: March 25, 2018, 06:40:15 AM »
An ideal speaker cable will have no resistance, no inductance, and no capacitance.  In all practical senses, the amount of inductance, capacitance, and resistance that you're going to encounter in your average set of speaker cables is minuscule, so the choice will ultimately boil down to preferences and perceived differences.

Paul "PB" Birkeland

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Offline Checksix

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Re: Question on Stereomour II...
« Reply #2 on: March 25, 2018, 07:20:30 AM »
Right!  I know that no R/L/C is ideal.   What I'm wrestling with is, knowing that the OT has significant inductance, what is a preferred cable characteristics "match".... electrically.   There is Source Z, connector Z, cable Z, Load Z.   I can somewhat control Cable Z, by my cable choice.   (note: I'm also a Amateur Extra - Ham radio operator.   We deal with transmission feed line impedances and SWR all the time.)   So, I'm trying to approach this in a more scientific, structured way.

Speaker Cables have many different materials and constructions.   Some have higher/lower.... inductance / capacitance / resistance and various combinations of the three.   Some have low inductance, at the sacrifice of high capacitance and so on.

Unfortunately, a lot of cables are ridiculously priced, there seems to be a lot of snake oil / voodoo... and I'm trying to not shoot in the dark here.

Appreciate more thoughts and insight.   Thanks!

Offline Paul Birkeland

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Re: Question on Stereomour II...
« Reply #3 on: March 25, 2018, 08:06:21 AM »
knowing that the OT has significant inductance
The primary does, but the speaker driving secondary not so much.

There is Source Z, connector Z, cable Z, Load Z.   I can somewhat control Cable Z, by my cable choice. 
The source Z and load Z are quite high in this scenario, and the others are pretty tiny.
Speaker Cables have many different materials and constructions.   Some have higher/lower.... inductance / capacitance / resistance and various combinations of the three.   Some have low inductance, at the sacrifice of high capacitance and so on.
What I would suggest looking at is analyzing the highest capacitance speaker cable you can find and consider what kind of HF effect it will have on your particular loudspeaker driver, then do the same for high inductance.  If you need a little help number crunching, we are certainly here for you. 
Paul "PB" Birkeland

Bottlehead Grunt & The Repro Man

Offline Checksix

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Re: Question on Stereomour II...
« Reply #4 on: March 25, 2018, 08:57:07 AM »
Thanks Paul!

Would you happen to know the Stereomour II OT secondary inductance value?   It would help me to know this and put the output chain in electrical perspective.   I'm currently using some Monster XP speaker wire.   I think it is 4 milliOhm/ft, 24pf/ft and 0.21 uH/ft.    Then I can schematically diagram the network and might be able to run some numbers and see what the math says at various frequencies.   A fun mental exercise anyways.

Offline Paul Joppa

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Re: Question on Stereomour II...
« Reply #5 on: March 25, 2018, 10:28:20 AM »
My measurements are not very precise, maybe +/-20% or so, but the leakage inductance referred to an 8 ohms secondary is around 100uH. Capacitance referred to an 8 ohm secondary is around 60,000pF. Source resistance (plate resistance plus primary and secondary winding DCR, all referred to an 8-ohm secondary) is about 3500 milliOhms.

At 20kHz, the EM wavelength is about 150km, so any cable is "short" relative to a wavelength. SWR is not a useful concept in a domestic installation.
Paul Joppa

Offline Checksix

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Re: Question on Stereomour II...
« Reply #6 on: March 25, 2018, 01:49:34 PM »
Thanks Paul!   I'll use and try your data.   It should be fun to simulate in SPICE.   I'm just curious about what is happening and if there is a better choice on the speaker cable electrical parameters.

In the past, with some s.s. amp designs with really wide bandwidth, they would go into oscillation due to bad combinations of the cables and loads.   Sometimes, if it didn't blow fuses, it could affect the audio band performance with ringing (overshoot) on the waveforms.

You were close on the 20kHz wavelength, I come up with 15km.   (300/.02)   
Some Hams are experimenting with the new 2200 meter band (136kHz).  (300/0.136)  Costly though, this low radio frequency takes massively sized components.    ;)

Addendum:
I just found this Belden 5T00UP speaker cable...
ETP High Conductivity Copper with 65x28 stranding
PVC insulation
10AWG
1 milliohm per foot
0.15uH per foot
26pF per foot
Cost - about $1.20 per foot
Seems really reasonable and has good electrical characteristics.
https://catalog.belden.com/techdata/EN/5T00UP_techdata.pdf
« Last Edit: March 25, 2018, 02:57:46 PM by Checksix »

Offline Paul Joppa

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Re: Question on Stereomour II...
« Reply #7 on: March 25, 2018, 03:26:44 PM »
Hah! I must have slipped a digit on the wavelength...  :^)

Usually, the instabilities in SS amps (and P-P tube amps) are due to the combination of high open-loop output impedance and heavy negative feedback. With high output impedance, open-loop gain in proportional to the load impedance, so it goes nuts at high frequencies when the load is inductive. Often you will see a series RC shunted across the output for this reason. It's not a problem with triodes (low impedance) or with zero-feedback amps (most of ours). Also, an output transformer has shunt capacitance to serve that function, so it's less of a problem with tubes in general.
Paul Joppa

Offline Checksix

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Re: Question on Stereomour II...
« Reply #8 on: March 25, 2018, 04:35:27 PM »
Exactly!   an RC damper with C in the 0.05uF - 1uF range and R in the 5-6 Ohm range.

Somewhere I was reading this started to occur when bandwidth in amplifiers started getting up to something like 5GHz.   I realize that isn't an issue here but it is interesting.

Appreciate your inputs, all good stuff to know and I try to keep connecting the dots.  ;-)

By the way, I really enjoy my Stereomour II... even bone stock, it sounds really good.    Been running the Sophia 2A3 mesh monoplates in it lately.    ...and I just received the Shunt Regulator and DC Filament supply kits for it on Friday.   Time to warm up the soldering iron again.