EL84 Push-Pull ccs bias

Tobruk · 28th April 2007, 06:21 AM

Hi,
about diyparadise.com El84 amplifier schematic ,what sound difference between resistor chatode bias and LM317 ccs.Have you an idea about LED CCS ? Thanks..
regards

Shoog · 28th April 2007, 07:52 AM

He says that the CCS gives a lot cleaner stronger sound, with more bass . This would be my experience of CCS bias as well.

Shoog

Johan Potgieter · 1st May 2007, 10:24 PM

Me again, Tobruk,

I don't want to lead you astray, but there was comment about the frequency response of the LM317 elsewhere. You must remember that such a device would have to be a CCS over at least the whole audio band. It is not clear that all power chips could do that. But I regret not recalling where I read that. Perhaps others could help.

Then to repeat, I would suggest as I did on another one of your threads that you ask a moderator to group the lot (if I am correct that it is about the same project) for your benefit.

Regards.

Shoog · 2nd May 2007, 08:07 AM

Quote:

I don't want to lead you astray, but there was comment about the frequency response of the LM317 elsewhere. You must remember that such a device would have to be a CCS over at least the whole audio band. It is not clear that all power chips could do that. But I regret not recalling where I read that. Perhaps others could help.

If adequately bypassed this is a none issue.
I think in the case of the diyparadise amp he uses no bypassing - the frequency response then becomes a real issue. I think possibly that he has the LM317 set up as a differential pair - which makes the quality of the CCS less of an issue.

Shoog

Yvesm · 2nd May 2007, 09:49 AM

Quote:

Originally posted by Johan Potgieter
Me again, Tobruk,
. . .
Then to repeat, I would suggest as I did on another one of your threads that you ask a moderator to group the lot (if I am correct that it is about the same project) for your benefit.

Regards.

Let me add that the best way to DO NOT SUCCEED is to "cut and paste" parts of schemos from -may be valuable- different sources but will lamentably fail to match together.

Look at your project as a whole.

Yves.

Brian Beck · 2nd May 2007, 01:39 PM

The DIYparadise EL84 amp drives the output push-pull pair from one side only. So the output stage MUST behave as a differential amplifier to work properly. To cause diff-amp behavior, there has to be a large tail resistor at the EL84 cathodes. The DIYparadise schematic shows just a 135 ohm resistor, which in my view, is inadequate. He later suggests using a CCS. And, in this case, you would not want to bypass the CCS with a cap, because one EL84 cathode must be able to "talk" to the other cathode for push-pull behavior to occur, when only one side is driven.

As Johan notes, an LM317 used as a CCS will fall down at higher frequencies. AudioXpress magazine recently published a very interesting two-part article by Walt Jung (April and May 2007) about CCS circuits and measurements. In effect, the raw LM317 has a roughly 400K resistance in shunt with a 350pF capacitance, and both components will be non-linear to a degree. A CCS in the cathode side of a tube circuit can be less critical than in the plate side because the impedances are much lower, but I would prefer to see the LM317 cascoded with a BJT or FET to reduce the effective capacitance (the resistance will go higher too in cascode, but 400K at low frequencies was already plenty good enough for this situation). Jung shows a circuit with an LM317 cascoded by a DN2540, giving outstanding performance, close to measurement floor across the entire frequency band.

Yvesm · 2nd May 2007, 01:46 PM

IMHO, the DIYparadise EL84 is just a single ended one.
The second EL84 can hardly do more than cancelling DC current in the OPT, wich is already not so bad.

I've not the schemo at hand, but I hope that the EL84 are not triode strapped.

Yves.

Brian Beck · 2nd May 2007, 01:49 PM

Quote:

Originally posted by Yvesm
IMHO, the DIYparadise EL84 is just a single ended one.
The second EL84 can hardly do more than cancelling DC current in the OPT, wich is already not so bad.

Without a CCS tail, it is indeed a partial SE amp. With a CCS, the other side comes alive and you have true push-pull.

Shoog · 2nd May 2007, 07:42 PM

Brian,
Your suggestion about caps between the cathodes, and then the earth reference lifted with a 1meg resistor seems appropriate here. Worked for me with the LM317 and avoids the need for bias adjustment.

With this circuit I always wondered what was the mechanism by which the second valve was driven - now I know. I wouldn't dismiss it as a design as a differential output stage has outstanding performance.

Shoog

Brian Beck · 2nd May 2007, 09:12 PM

Shoog,

I guess you're referring to the the two-CCS circuit we discussed in another thread. Yes, it should work here too, keeping each tube separately DC biased, while at the same time connecting the cathodes with caps for diff-amp behavior.

For a very simple design, I agree that this one-sided-drive diff-amp output stage amp should not be dismissed. But the CCS(s) has to be stout.

28th April 2007, 06:21 AM	#1
Tobruk diyAudio Member Join Date: Oct 2005	EL84 Push-Pull ccs bias Hi, about diyparadise.com El84 amplifier schematic ,what sound difference between resistor chatode bias and LM317 ccs.Have you an idea about LED CCS ? Thanks.. regards

2nd May 2007, 01:39 PM	#6
Brian Beck diyAudio Member Join Date: Dec 2004 Location: South Florida, USA	The DIYparadise EL84 amp drives the output push-pull pair from one side only. So the output stage MUST behave as a differential amplifier to work properly. To cause diff-amp behavior, there has to be a large tail resistor at the EL84 cathodes. The DIYparadise schematic shows just a 135 ohm resistor, which in my view, is inadequate. He later suggests using a CCS. And, in this case, you would not want to bypass the CCS with a cap, because one EL84 cathode must be able to "talk" to the other cathode for push-pull behavior to occur, when only one side is driven. As Johan notes, an LM317 used as a CCS will fall down at higher frequencies. AudioXpress magazine recently published a very interesting two-part article by Walt Jung (April and May 2007) about CCS circuits and measurements. In effect, the raw LM317 has a roughly 400K resistance in shunt with a 350pF capacitance, and both components will be non-linear to a degree. A CCS in the cathode side of a tube circuit can be less critical than in the plate side because the impedances are much lower, but I would prefer to see the LM317 cascoded with a BJT or FET to reduce the effective capacitance (the resistance will go higher too in cascode, but 400K at low frequencies was already plenty good enough for this situation). Jung shows a circuit with an LM317 cascoded by a DN2540, giving outstanding performance, close to measurement floor across the entire frequency band. __________________ Brian

2nd May 2007, 09:12 PM	#10
Brian Beck diyAudio Member Join Date: Dec 2004 Location: South Florida, USA	Shoog, I guess you're referring to the the two-CCS circuit we discussed in another thread. Yes, it should work here too, keeping each tube separately DC biased, while at the same time connecting the cathodes with caps for diff-amp behavior. For a very simple design, I agree that this one-sided-drive diff-amp output stage amp should not be dismissed. But the CCS(s) has to be stout. __________________ Brian

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28th April 2007, 07:52 AM	#2
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	He says that the CCS gives a lot cleaner stronger sound, with more bass . This would be my experience of CCS bias as well. Shoog

1st May 2007, 10:24 PM	#3
Johan Potgieter diyAudio Member Join Date: May 2005 Location: Pretoria, South Africa	Me again, Tobruk, I don't want to lead you astray, but there was comment about the frequency response of the LM317 elsewhere. You must remember that such a device would have to be a CCS over at least the whole audio band. It is not clear that all power chips could do that. But I regret not recalling where I read that. Perhaps others could help. Then to repeat, I would suggest as I did on another one of your threads that you ask a moderator to group the lot (if I am correct that it is about the same project) for your benefit. Regards.

2nd May 2007, 01:46 PM	#7
Yvesm diyAudio Member Join Date: Jul 2004 Location: Ardeche	IMHO, the DIYparadise EL84 is just a single ended one. The second EL84 can hardly do more than cancelling DC current in the OPT, wich is already not so bad. I've not the schemo at hand, but I hope that the EL84 are not triode strapped. Yves.

2nd May 2007, 07:42 PM	#9
Shoog diyAudio Member Join Date: Aug 2002 Location: Eire	Brian, Your suggestion about caps between the cathodes, and then the earth reference lifted with a 1meg resistor seems appropriate here. Worked for me with the LM317 and avoids the need for bias adjustment. With this circuit I always wondered what was the mechanism by which the second valve was driven - now I know. I wouldn't dismiss it as a design as a differential output stage has outstanding performance. Shoog

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