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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 20th Mar 2018, 3:13 pm   #21
Philips210
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Join Date: May 2007
Location: Redruth, Cornwall, UK.
Posts: 2,562
Default Re: OA79 - Modern Replacement

Quote:
Originally Posted by mike g3zii View Post
much better however is the ALD110900 this is a zero threshold dual gate mosfet
in an 8 pin package. details are on google and they are sold on the usual auction site.
Hi Mike.

That sounds very interesting and I shall be trying out that IC in some experiments. It would be good to see your tapped coil set up. If anything can better an OA47 for the detector then that's well worth experimenting with. Thanks for the info.

Regards
Symon.
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Old 20th Mar 2018, 3:25 pm   #22
Philips210
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Location: Redruth, Cornwall, UK.
Posts: 2,562
Default Re: OA79 - Modern Replacement

Hi

Whilst looking on eBay for the ALD 110900, I came across a surface mount MOSFET type 3SK143-Q 3DQ which also seems to be used as a detector in crystal sets. I wonder how the two devices compare with each other.

Regards
Symon.
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Old 20th Mar 2018, 11:12 pm   #23
Argus25
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Join Date: Oct 2016
Location: Maroochydore, Queensland, Australia.
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Default Re: OA79 - Modern Replacement

The great thing about diode detectors is their wide dynamic range, only limited by the voltage ratings of the diode. Active detectors saturate and depending on the design are sometimes limited to levels below the supply voltages.

However, germanium detectors start to become non linear below a few hundred milli-volts peak and essentially become square law devices. The distortion from this is actually very easy to hear on low level signals.

The help ameliorate this problem many radios use a small forward bias of 150mV or thereabouts. Some early Japanese transistor radios cleverly did this using the base-emitter voltage of one of the IF transistors as a temperature compensated source for this bias voltage.

One way to get around this non-linearity for diodes at low levels is to drive the diode with a current source rather than a voltage source and heavily load the diode output. This was done in a circuit by Horowitz & Hill. This way, even for small signals the drive voltage rises very high pushing the diodes into conduction and the heavy loading gives a flat response.

Another way to visualize this is to create a large impedance mis-match, by driving the diode/s by a high voltage & low current (high impedance) signal source and relatively heavily loading them with a low impedance.

I have attached a circuit that I made to drop in place of a germanium diode and a graph of its performance. I posted this on another thread once.

The transistor, with its un-bypassed emitter resistance turns the input signal into a current proportional to the input voltage which drives the inductor, which behaves similarly to a current source. So it is a bit deceptive looking at it, because it appears on the face of it that it is just another typical amplifier stage. But as you can see from the graph the low level signal performance is much better than the diode. I use this detector on one of my home grown short wave radios to improve the detection and reduce the distortion of low level signals.
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Old 22nd Mar 2018, 4:03 pm   #24
mike g3zii
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Join Date: Jun 2015
Location: Southport, Lancs. UK.
Posts: 31
Default Re: OA79 - Modern Replacement

I will post more on the ALD chip xtal set when I have had another look at it.
I had it lashed together with croc clip leads and it's all piled loose into a box
at the moment. along with quite a few other roundtuit projects.
I have found a piece of plywood to mount the main parts then I will be able to try different circuits to find the best. there is a bench book somewhere with
previous work in it if I can find it. I've got some brass terminals somewhere
for aerial and earth etc so it may end up looking posh in varnished case or maybe not.
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