questions on input stage

paba · 08:49 PM

Hi all,

I am think about making a change to the input stage of my mono bloc amps.
Changing from cathode bias to a battery on the grid as shown in the two PDF below of before and after.

My question is what would be the impact on gain and output impedance of the stage between the two options?

I have not been able to find much text book ref material on the battery grid bias. The first one of course is well documented with gain and impedance formulas including the case with no bypass cap.

Oh and feel free to give your opinion on this change (excellent, waste of time, etc..) with some back up reasoning or experience. Or corrections / additions of course.

thanks
paba

piano3 · 4th September 2010, 09:20 PM

Decrease in gain/increase in ra are normally only experienced when there is feedback from the cathode, ie. when you remove the cathode bypass capacitor. I have never used battery bias(although I did once toy with the idea of putting a rechargeable battery in the cathode of the 2nd stage of a preamp) but I don't see any mechanism for feedback here. I find feedback to be the most difficult topic in audio and I feel that the day that I totally grasp it will be the day I cease to be a newbie! Hopefully one of the big guns will answer your post.

piano3 · 4th September 2010, 09:21 PM

Oh!, getting rid of a cap in the cathode is certainly desirable

DF96 · 4th September 2010, 10:33 PM

Assuming the bias voltage is correct, then it will have no effect on gain or output impedance.

You may have to look in very old books to learn about battery grid bias, as it is rarely used now. An alternative is LED bias in the cathode circuit, which is now quite popular. Both of these have the disadvantage that they don't compensate as the valve ages, and you need to think about what happens when the battery ages. I have not tried either.

DF96 · 4th September 2010, 10:40 PM

Something I forgot to say: you get rid of an electrolytic, but add a cell, so you just swap one type of electrochemistry for another.

piano3 · 4th September 2010, 10:58 PM

Quote:

Originally Posted by DF96

Something I forgot to say: you get rid of an electrolytic, but add a cell, so you just swap one type of electrochemistry for another.

Yes, I hadn't thought about that! At least,though, a battery couldn't cause motorboating,could it?

DF96 · 4th September 2010, 11:08 PM

No, because the battery goes down to DC - unlike a capacitor.

It is CCS bias which can sometimes cause motorboating, because when bypassed it converts a lead-lag into an LF rolloff.

rman · 6th September 2010, 03:24 AM

Hi.

How about using a rechargeable battery on the cathode instead of on the grid?
This works very well if the voltage is what you need. About 1.3 v. for nimh. cell.
As a bonus, the cell has no load on it, it only acts as a voltage reference and is actually kept charged by the cathode current.

I have an amp like this and it works great

Cheers.
Rolf.

rman · 6th September 2010, 03:32 AM

PS.

No bypass capacitor needed. gain and output impedance are the same as for a fully bypassed cathode resistor.

Rolf.

paba · 6th September 2010, 06:10 AM

I must admit, I expected either the gain or the impedance to change... if they are both the same then, I am really tempted to try this.

As for LED or rechargeable battery in the cathode, I considered it, but is just a different flavour of what is there now, sure probably better than the cheap polypro cap that is there now.

Anyway I will probably try this in the next couple of weeks and report back.

Thanks
paba

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4th September 2010, 09:20 PM	#2
piano3 diyAudio Member Join Date: Aug 2010 Location: London,UK	Decrease in gain/increase in ra are normally only experienced when there is feedback from the cathode, ie. when you remove the cathode bypass capacitor. I have never used battery bias(although I did once toy with the idea of putting a rechargeable battery in the cathode of the 2nd stage of a preamp) but I don't see any mechanism for feedback here. I find feedback to be the most difficult topic in audio and I feel that the day that I totally grasp it will be the day I cease to be a newbie! Hopefully one of the big guns will answer your post.

4th September 2010, 09:21 PM	#3
piano3 diyAudio Member Join Date: Aug 2010 Location: London,UK	Oh!, getting rid of a cap in the cathode is certainly desirable

4th September 2010, 10:33 PM	#4
DF96 diyAudio Member Join Date: May 2007	Assuming the bias voltage is correct, then it will have no effect on gain or output impedance. You may have to look in very old books to learn about battery grid bias, as it is rarely used now. An alternative is LED bias in the cathode circuit, which is now quite popular. Both of these have the disadvantage that they don't compensate as the valve ages, and you need to think about what happens when the battery ages. I have not tried either.

4th September 2010, 10:40 PM	#5
DF96 diyAudio Member Join Date: May 2007	Something I forgot to say: you get rid of an electrolytic, but add a cell, so you just swap one type of electrochemistry for another.

4th September 2010, 11:08 PM	#7
DF96 diyAudio Member Join Date: May 2007	No, because the battery goes down to DC - unlike a capacitor. It is CCS bias which can sometimes cause motorboating, because when bypassed it converts a lead-lag into an LF rolloff.

6th September 2010, 03:24 AM	#8
rman diyAudio Member Join Date: Oct 2007	Hi. How about using a rechargeable battery on the cathode instead of on the grid? This works very well if the voltage is what you need. About 1.3 v. for nimh. cell. As a bonus, the cell has no load on it, it only acts as a voltage reference and is actually kept charged by the cathode current. I have an amp like this and it works great Cheers. Rolf.

6th September 2010, 03:32 AM	#9
rman diyAudio Member Join Date: Oct 2007	PS. No bypass capacitor needed. gain and output impedance are the same as for a fully bypassed cathode resistor. Rolf.

6th September 2010, 06:10 AM	#10
paba diyAudio Member Join Date: Sep 2004 Location: Montreal	I must admit, I expected either the gain or the impedance to change... if they are both the same then, I am really tempted to try this. As for LED or rechargeable battery in the cathode, I considered it, but is just a different flavour of what is there now, sure probably better than the cheap polypro cap that is there now. Anyway I will probably try this in the next couple of weeks and report back. Thanks paba

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