will this work as an attenuator

[walkman]

Hi

Picked up an vintage TV Aid at a market



Heres my tracing of the circuit .



I did'nt know which lead was the centertap , so i guessed

How could i modify this to be an attenuator for a 5w amp?

The circle is a globe , probably 6v that could be run off heater circuits

Want some advice before i wire it up with jacks and place it between my amp

and speakers

thanks in advance

[walkman]

bump?

[Steve Conner]

Sorry, I have no idea what in heck this thing is and how your schematic relates to it, and I doubt anyone else does either. But since I don't see any 4, 8 or 16 ohm resistors in there, I can only conclude that it's not safe for use as an attenuator.

[walkman]

Oh yer ....

Now i look at it again ... what i've drawn makes no sence .

It appears to be a volume control for a hearing aid or a earphone that you could connect to your tv ... most likly a valve black and white.

So the signal going in would be from the back of the tv ( internally from a 6bm8 or whatever to a 16 ohm oval rola speaker).

What i'd meant to draw .. was the circuit inside the box.

Speaker cable goe into a center -tapped choke or transformer ?, then one end goes to a light bulb and the other to ground. there a 100 ohm pot that is bypassed by 47 ohms..... when i measure the resistance at the output plug ( for th earphone) regardless of the position of the pot it is 16 ohm.

will take a gut shot later

doe that make sence?

[Regis]

Even if that thing is an L-Pad (doubtfull), a 5 watt amp would probably fry it. Doubtful it is the correct impedance anyway.

Build one of these, If you use a cheap case it will cost about $25-30.

[Steve Conner]

If the transformer were an audio isolating transformer, then it would make sense as a device to add an earphone output to a TV. Old TVs often had hot chassis and the transformer might be needed to save you from electrocution.

As an autotransformer or tapped choke, it doesn't really make any sense.

[walkman]

So 15 years later i dig this box out again… and trace a schematic

After some testing today.. it appears to me that this device must connect in series with an internal TV speaker for a 1950s TV set.

———————————————————————
The resistance measured at the input is 1 ohm. The resistance at the output measures between 3.2 and 13.2 ohms.

I connected the input to a 12v 300ac wall wart at the input , and a speaker to the output following the Lpad. Measuring the voltage across the input it was around 5.5v and the voltage on the output would vary between 0.1v and 0.01v depending on the position of the pot and the speaker load. The output was loud enough to hear a hum at a the speaker.

The globe would light up either with a speaker attached or without, though brighter with a speaker attached.

———————————————————————

As the input resistance was constant regardless of the pot or L-pad, so i have concluded that the device must be meant to be placed in series with an internal speaker for two reasons.

Firstly: 1ohm constant load in series will not affect the response of the internal speaker or the refelected load to the output transformer and pentode out put tube of the TV set.

Secondly: the load is constant with the device in series, the operator could unplug any hearing aid / headphone / speaker into the unit without affecting the loading of the power amp.

Not sure if it was due to the wall warts that i was using ( most likely in regulated) or due to circuits inductor /coil /choke , but it appears that the voltage to across the input and hence to an internal speaker following would also be halved. (This may have been a ‘feature’ , reducing the volume from the internal speaker (having been turned up loud for a hard of hearing listener) while also directing the sound to a unit that can provide a personal level control.)

The size of the tapped inductor / centre tapped coil / choke ( not sure what to call it ) is similar to a small radio OT meant for 3 or 4 watts or similar.
Any voltage reduction, along with the lamp and Lpad will reduce the power to the output and protect any attached speaker or the listener ear!

—————————————————————-

So i guess this could be modified to incorporate a load resistor ( or reactive load ) across the input to create an attenuator.

To be on the safe side it may be best to incorporate a 1/4 or 1/10 Rob Robinette attenuator at the input to the circuit.
I have attached a suggested connection diagram based on the ideas above.


Hope someone can take advise if this is makes sense.




Thanks

Walkman

[Chuck H]

What the amp will see is predominantly between the input and CT of that transformer. You'll need to make sure that's ok for the amp. Then parallel all the other stuff, the rest of the transformer, the lamp, the parallel 47 ohm, and then the final load, whatever that is at the output. Probably not intended to be a speaker. Doesn't make sense. Unless you know what this was actually used for and need to use something for THAT I'm not sure how useful this this thing is. It's not an attenuator for a speaker load. Maybe a buffer/attenuator for headphones or something. Lacking many specs for that transformer makes it impossible to say if it's safe to use for anything other than it's original implementation.

[walkman]

Hi Chuck,

Thanks for your reply.

The impedance that the amp would see is definitely too low at 1ohm if the device were the only thing connected to a tube amp OT / speaker output!
Like wise to attach the TV Aid circuit in parallel to an existing speaker (like a line out ) would load down the output too much … down to less than 1 ohm.

If there is a speaker or load beforehand or ‘following’ the device in series with it ( not considering anything attached to the Lpad output ), wont that also be seen by the amp?

Like in the suggested connection I drew up?
(Not sure if you saw my first post today , but looks like your response was around the time i finished editing it after adding further information and the second attachment)

[Helmholtz]

The inductor thing is shown as a transformer (both coils on same core).
Your input resistance measurement of 1R is irrelevant as it is impedance which matters to the amp.
Impedance will be larger than DCR.

Without knowing essential data of the transformer like inductance, turns ratio and current rating it is not possible to give good advice on wiring.
From your AC measurements it seems that the transformer is stepping down.
Your original schematic might even work with a low power amp (the second schematic doesn't make sense to me), but input impedance and power limit are unclear. Input impedance depends on turns ratio and reflected load.
I like the lamp which acts as a compressor.
Input impedance can be found with an AC input voltage and measuring AC current.

OTOH, you don't need a transformer for an attenuator.

[walkman]

Hi Helmholtz,

Thanks for your explanation, I will measure the turns ratio, and see if i can find any data for it , have had no luck yet.



I’ve seen https://dalmura.com.au/static/Choke%20measurement.pdf , though don’t have a bench supply with multiple voltages.

The ‘suggested connection’ / second attachment was due to my misunderstanding of inductors.
And thanks again Chuck and to the other previous posters…

[Helmholtz]

Hi Helmholtz,
Thanks for your explanation, I will measure the turns ratio, and see if i can find any data for it , have had no luck yet.

I’ve seen https://dalmura.com.au/static/Choke%20measurement.pdf , though don’t have a bench supply with multiple voltages.

Turns ratio can be calculated from primary and secondary AC voltages.

The dalmura method with DC bias is not needed here as there's no DC current.
You only need an AC voltage and an AC ammeter to find primary impedance.

Judging from the size of components the device might be good for an amp power of 2W to 5W.

What do you want to connect, phones or speaker?

[walkman]

I was hoping to connect it to a speaker