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revintage 19th January 2013 09:31 AM

Single stage line with anode-cathode feedback
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Have anyone tried the leftmost topology before? This one is with E88CC feedbacked to get 12dB gain. The opamp acts as inverter.

Another singletube alternative is anode-follower but you are then restrictied to low Zin and lower upper bandwith

In the middle example you must add a CF to get an acceptable Zout.

Note this is not yet a working cicuit, only a topology idea!

DF96 19th January 2013 11:26 AM

You do realise that the whole point of negative feedback is that a resistor has much less distortion than an active device? Putting active devices in the feedback chain seems perverse.

If opamps are good enough for audio, then use an opamp and ditch the valve. If not, ditch the opamp.

revintage 19th January 2013 12:00 PM


I ditched your answer instead of the idea ;).

Hopefully there are others that want to technically discuss the active feedback idea. Or has another solution to a single E88CC stage with 12dB gain.

0feedback 19th January 2013 03:04 PM

...simply apply anode/grid feedback via a serie resistor and condenser...sorry but i quite agree with DF96...:)

SY 19th January 2013 03:24 PM

At that point, what function does the tube actually have, other than increasing heat, distortion, and parts count? Just use the opamp.

If you want a 12dB ECC88 line stage, run the first stage as a CCS loaded voltage amp, use a high value resistor voltage divider, followed by a cathode follower (or your opamp configured as a unity gain buffer).

revintage 19th January 2013 06:14 PM

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As you seem to know the answer of what is wrong, why not give an explanation instead of just diss the idea?

Oooops, seems like you all get provocated by an OP-amp;).

Still noone have said what is so wrong with the topology if we assume an ideal inverter.

To make it easier for you, lets use an ideal inverter in the same circuit. Now with two outputs, before and after the "black box".

SY 19th January 2013 06:17 PM

The answer I was trying to give is, certainly, it will work, I've seen it done before, but it's pointless.

DF96 19th January 2013 06:27 PM

Could someone remind me what the Farnell part number is for an "ideal inverter"?

revintage 19th January 2013 06:43 PM

Off course it will work, anyone can tell. But as you seem to know it is pointless, why not explain?

Grumpy today, are we?

Loren42 19th January 2013 06:58 PM


Originally Posted by revintage (
Off course it will work, anyone can tell. But as you seem to know it is pointless, why not explain?

Grumpy today, are we?

It's like using a framing hammer to nail tacks. Of course it will work, but it is a waste of hammer.

In your case you add an op amp, but you also need a power supply for that amp. All that to replace a resistor? Why is an op amp superior?

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