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 Post subject: Question on building a speaker cabinet
PostPosted: Oct Wed 24, 2018 8:01 pm 
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Joined: May Mon 14, 2018 6:14 pm
Posts: 212
I know it's not an antique discussion, but it will be going to a modified antique RCA sound projector amplifier.

If I want to make a 8 ohm to 16 ohm cabinet, would it be possible to just add a switchable inductor in series with my existing speaker?

For example, I have an Eminence Legend 1518. The nominal impedance is 8 ohms. DC resistance is 7.16 ohms. Resonance is 82 Hz. A 13.9 mH inductor has a resistance of 7.162 ohms at 82 Hz. So shouldn't I be able to add a DPST on/on switch in series with the inductor to double my impedance? In doing so, would I be able to keep the dynamics of my original speaker intact?


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Wed 24, 2018 8:11 pm 
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Location: Littleton, MA
DCW wrote:
So shouldn't I be able to add a DPST on/on switch in series with the inductor to double my impedance?

No. Add an 8 ohm resistor instead of an inductor.
DCW wrote:
In doing so, would I be able to keep the dynamics of my original speaker intact?

No. You are effect increasing the output impedance of the amplifier, so the damping factor will be decreased.

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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Wed 24, 2018 8:40 pm 
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I am trying to stay away from a purely resistive load because speaker impedance is reactive to the frequency passing through the voice coil. I may have to just live with a secondary 8 ohm speaker.


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Wed 24, 2018 10:11 pm 
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DCW wrote:
I am trying to stay away from a purely resistive load because speaker impedance is reactive to the frequency passing through the voice coil. I may have to just live with a secondary 8 ohm speaker.

Yes, the speaker impedance is not constant over frequency. But its impedance doesn't match an inductor, either.

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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 12:08 am 
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DCW wrote:
I am trying to stay away from a purely resistive load because speaker impedance is reactive to the frequency passing through the voice coil. I may have to just live with a secondary 8 ohm speaker.


Your speaker impedance is going to be much higher than 8 ohms at resonance. Generally speaking a 7 ohm dcr will give a nominal impedance closer to 10 ohms. Keep in mind that impedance will vary substantially over the frequency range, usually having its minimum an octave or so above resonance and then slowly rising after that. I would suggest just leaving it alone and living with the mismatch. It's not that big a deal, just a loss of some power transfer and perhaps a small increase in distortion. A 14mh inductor will cause a substantial rolloff in the higher frequencies and change the sound character considerably.

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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 8:18 am 
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Joined: Sep Thu 23, 2010 6:37 am
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Location: Powell River BC Canada
You might be better to use a transformer to double the impedance seen
by the amplifier. The damping factor seen by the amplifier is governed by the
DC resistance. However there is a lot of argument with this.

The impedance of your speaker around resonance is altered by the cabinet size
and porting.

A 13 mHy choke has a reactance of 100 ohms at 8000 Hz, and a highly damped
amplifier just might oscillate if the load reactance goes way up at some sweet spot.
-------
How to make a 8 to 16 ohm transformer:

Wind 10 turns for the 8 ohm side. Wind 14 turns for the 16 ohm side.

Core ? how many volts will your
amplifier produce at the
wattage you run ?

How many volts per turn
needed to get the core
to run around 1 Tesla ?

Wire size ? How many watts do you
want to loose at the current
flowing at your chosen
wattage.

Fiddlesticks.
Just grab an old 20 watt
70 volt line transformer
and wind coils with a turns
ratio of 1.4 :1 Use the
biggest wire that will
fill the space.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 11:33 am 
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Location: S. Dartmouth MA 02748-1225 USA
Quote:
Fiddlesticks.
Just grab an old 20 watt
70 volt line transformer
and wind coils with a turns
ratio of 1.4 :1 Use the
biggest wire that will
fill the space.
WAG: Forget the primary, use the secondary as an auto transformer. Most of these line transformers have at least four or more taps...

Chas

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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 3:40 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11420
Location: Powell River BC Canada
Chas wrote:
Quote:
Fiddlesticks.
Just grab an old 20 watt
70 volt line transformer
and wind coils with a turns
ratio of 1.4 :1 Use the
biggest wire that will
fill the space.
WAG: Forget the primary, use the secondary as an auto transformer. Most of these line transformers have at least four or more taps...

Chas


If you google 8 to 16 ohm transformer, that is how they market that concept. Sell you a 25, 70 or
100 volt line transformer.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 5:33 pm 
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Joined: May Mon 14, 2018 6:14 pm
Posts: 212
I actually have an old Jensen 70 volt line transformer laying around not being used, and I never thought of it. For most of my adult life, I worked in a garage, and apparently "Keep It Simple Stupid" did not stick!


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 5:56 pm 
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Joined: Aug Sat 01, 2009 2:23 pm
Posts: 1654
DCW wrote:
I actually have an old Jensen 70 volt line transformer laying around not being used, and I never thought of it. For most of my adult life, I worked in a garage, and apparently "Keep It Simple Stupid" did not stick!


What kind of amplifier are you using to drive this speaker?

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Antique Radio Club of Illinois


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 6:45 pm 
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Joined: Sep Thu 23, 2010 6:37 am
Posts: 11420
Location: Powell River BC Canada
There are a number of issues with 70 v line transformers. The high resistance
primaries might partially shorted. If a 1 watt tap is the lowest, the line
impedance is 5000 ohms.

A way of testing these transformers is given in Sound System Engineering by
Don and Caroline Davis. The reason it was included was to show how the
cheaper transformers can suck up available wattage. Line voltage systems
operating at 25, 70, and 100 volts are often used as safety announce devices

As such the sound system designer is obliged to name the transformer or
a specification which then is certified by the architect.

Why you might use only the core, and wind a new bobbin, is that ultimately
you can get the lowest dc copper resistance into the winding space.

Those who rewind old microwave transformer by knocking out the shunt,
make better isolation transformers by losing the high voltage windings.

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de
VE7ASO VE7ZSO
Amateur Radio Literacy Club. May we help you read better.
Steve Dow
ve7aso@rac.ca


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Thu 25, 2018 7:01 pm 
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Joined: May Mon 14, 2018 6:14 pm
Posts: 212
jmsent wrote:
DCW wrote:
I actually have an old Jensen 70 volt line transformer laying around not being used, and I never thought of it. For most of my adult life, I worked in a garage, and apparently "Keep It Simple Stupid" did not stick!


What kind of amplifier are you using to drive this speaker?
It's an RCA 16mm projector amp modified for guitar. It's power section is two 6L6 in push-pull.


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Sat 27, 2018 5:40 am 
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Joined: Nov Fri 10, 2006 12:24 am
Posts: 2313
Location: Thornhill, Ontario, Canada
DCW wrote:
... The nominal impedance is 8 ohms. DC resistance is 7.16 ohms. Resonance is 82 Hz. A 13.9 mH inductor has a resistance of 7.162 ohms at 82 Hz. So shouldn't I be able to add a DPST on/on switch in series with the inductor to double my impedance?
No, that would cut highs like a series X/O coil. Match the speaker properly with a 1:1.4 transformer, or, best, use an amplifier that drives 8 ohm speakers.
Cheers,
Roger

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Thornhill, Ontario
Ontario Vintage Radio Assoc. http://www.ovra.ca


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 Post subject: Re: Question on building a speaker cabinet
PostPosted: Oct Sat 27, 2018 10:07 pm 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 4242
Location: Boston, MA USA
Check your output transformer — some RCA projectors had an additional 8-ohm tap for driving two speakers in parallel. Not sure about the PG-201 (your amplifier).

-David


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