Neg-Feedback into Grid of gain stage instead of cathode?

Alastair E · 05:58 PM

Building a very simple OTL amp.

Currently, I have two gain-stages which are common-cathode triode, (6SN7) which is DC coupled to the cathodyne phase-splitter (6J5) and the auto-bias output pair, 6C33C....
The design follows the 'inverted Futterman' idea.

As built it works very well, Sound is great, reliability perfect so far...

I would like to drop one of the gain-stages and still have the neg. FB...

Currently the FB is going into the Cathode of the first gain-stage.

Being a OTL, I cant invert the phase of the FB signal as there's no Tx.

Ive heard of neg FB into the grid of the gain-stage, but how best to implement it...?

Thanks.....

atmasphere · 24th January 2011, 09:59 PM

It sounds like your first stage is a differential amplifier? If so this is easy. I think a schematic might help too...

Bigun · 25th January 2011, 01:57 AM

If I understand correctly you want to apply the nfb to the input grid ?

The usual way is to connect your input to the grid via a resistor and your nfb to the same grid via another resistor (I assume you also have a shunt resistor + cap to signal ground to form a divider that defines your feedback ratio).

The input impedance will drop - instead of seeing a high resistance grid the input will also see the feedback signal. You have to be careful of this or your input resistance wil be too low. It usually means using high value resistors in the nfb network.

mastodon · 25th January 2011, 10:19 AM

This schematic by Wavebourn implements NFB into the input stage grid in an OTL headphone amp: http://www.wavebourn.com/forum/viewt...&t=998&start=0

24th January 2011, 05:49 PM	#1
Alastair E diyAudio Member Join Date: Feb 2004 Location: South Wales	Neg-Feedback into Grid of gain stage instead of cathode? Building a very simple OTL amp. Currently, I have two gain-stages which are common-cathode triode, (6SN7) which is DC coupled to the cathodyne phase-splitter (6J5) and the auto-bias output pair, 6C33C.... The design follows the 'inverted Futterman' idea. As built it works very well, Sound is great, reliability perfect so far... I would like to drop one of the gain-stages and still have the neg. FB... Currently the FB is going into the Cathode of the first gain-stage. Being a OTL, I cant invert the phase of the FB signal as there's no Tx. Ive heard of neg FB into the grid of the gain-stage, but how best to implement it...? Thanks..... Last edited by Alastair E; 24th January 2011 at 05:58 PM. Reason: addition

25th January 2011, 01:57 AM	#3
Bigun diyAudio Member Join Date: Jan 2009 Location: K-W Ontario Blog Entries: 2	If I understand correctly you want to apply the nfb to the input grid ? The usual way is to connect your input to the grid via a resistor and your nfb to the same grid via another resistor (I assume you also have a shunt resistor + cap to signal ground to form a divider that defines your feedback ratio). The input impedance will drop - instead of seeing a high resistance grid the input will also see the feedback signal. You have to be careful of this or your input resistance wil be too low. It usually means using high value resistors in the nfb network. __________________ "The test of the machine is the satisfaction it gives you. There isn't any other test. If the machine produces tranquility it's right. If it disturbs you it's wrong until either the machine or your mind is changed." Robert M Pirsig.

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24th January 2011, 09:59 PM	#2
atmasphere diyAudio Member Join Date: May 2007	It sounds like your first stage is a differential amplifier? If so this is easy. I think a schematic might help too...

25th January 2011, 10:19 AM	#4
mastodon diyAudio Member Join Date: Jan 2011	This schematic by Wavebourn implements NFB into the input stage grid in an OTL headphone amp: http://www.wavebourn.com/forum/viewt...&t=998&start=0

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