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Old 17th Feb 2019, 3:51 pm   #1
mole42uk
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Default 600Ω Radford LDO3?

I have a Radford LDO3 audio signal generator (thanks Chris!) that I would like to use for setting up some equipment that operates in the 600Ω environment.

Radford did offer the LDO3 in both 150Ω and 600Ω versions - unfortunately mine is the 150Ω one! The circuit diagram shows the alternate attenuator resistors that are needed to convert it to 600Ω but the values are 1706.9Ω, 1157.9Ω, 790.5Ω which I doubt I'll be able to find easily.

Is there another way that I can reliably drive a 600Ω load with the generator as it is?
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Old 17th Feb 2019, 4:03 pm   #2
barrymagrec
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Default Re: 600Ω Radford LDO3?

Put an additional 200 ohm load across the output and the output will be 6dB down on the nominal.
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Old 17th Feb 2019, 4:39 pm   #3
Dickie
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Default Re: 600Ω Radford LDO3?

I would have thought at those frequencies and impedances you could parallel up resistors quite successfully.E.G.
1800R and 33K, 1200R and 33K, and 820R and 22K get close to the values you are looking for.
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Old 17th Feb 2019, 5:22 pm   #4
mole42uk
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Default Re: 600Ω Radford LDO3?

Yes, Richard, but these are in the attenuator, so there are ten of the 1706.9 and eight of the 1157.9.....
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Old 17th Feb 2019, 6:00 pm   #5
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Default Re: 600Ω Radford LDO3?

How about a 1:2 transformer, accepting the limits to its performance ?

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Old 17th Feb 2019, 6:04 pm   #6
Dickie
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Default Re: 600Ω Radford LDO3?

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Originally Posted by mole42uk View Post
Yes, Richard, but these are in the attenuator, so there are ten of the 1706.9 and eight of the 1157.9.....
Aw that's just being lazy! - But I take the point.

A 1:2 transformer would work, maybe an AF output transformer with a number of seconday taps perhaps? (used as an autotransformer)

A minimum loss resistive pad would work, but you would suffer nearly 12 dB attenuation.
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Old 17th Feb 2019, 6:27 pm   #7
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Default Re: 600Ω Radford LDO3?

The minimum loss pad suggested would mean no mods to your LDO3 and do you need the top 12dB of its output capability?

Alternatively, for the funny resistor values, look up the E96 series. There are 96 values per decade and it's the natural series for 1% resistors, so this will get you within 2% worst case which ought to be close enough.



You'll find E96 series values at RS and Farnell without any bother and at reasonable prices. expensive resistors start at 0.1% types.

E96 values are ratiometrically spaced by the 96th root of 10, then rounded to 3 significant figures. Aren't calculators wonderful!

1150 farnell have 1% at 7.5p or 0.1% at 1.70
1694 farnell have 1% 0.5% and 0.1% at similar prices
787 farnell have 1% but 0.1% is no longer stocked

I've just looked


Alternatively, contact Holsworthy and have a chat. I know they made custom resistors on the exact values for 50, 75, 300 and 600 Ohm attenuators of round number values. They may have some lurking in stock. We used to buy them from them.

David
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Old 17th Feb 2019, 6:32 pm   #8
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Default Re: 600Ω Radford LDO3?

Farnell list the A262A2E transformer; 150 ohms in, 600 ohms out.


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Old 17th Feb 2019, 6:42 pm   #9
mole42uk
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Default Re: 600Ω Radford LDO3?

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Originally Posted by Radio Wrangler View Post
The minimum loss pad suggested would mean no mods to your LDO3 and do you need the top 12dB of its output capability?

<snip>
David
Actually I only need a -26dB output level, so I care not for the top 12dB at all.

I did try a 200ohm resistor as barrymagrec suggested, it drops the -20dB output close to 38mV rms which is not far from -26dB. I don’t yet know what will happen when I connect a load…
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