Update for Elektor Crescendo Power Amplifier 1982

[HumNSmoke]

I don't believe in ghosts I can't ear . I'm just chasing 50 or 100Hz hum.
Esperado, is the drawing below a good understanding of your last advises ? And what is better: W1 like in original, or W2.
PS: whatever the changes, R3 remains a complete mystery to me.

[Esperado]

Originally Posted by HumNSmoke

Esperado, is the drawing below a good understanding of your last advises ? And what is better: W1 like in original, or W2.
PS: whatever the changes, R3 remains a complete mystery to me.

Not complicated. In the source (preamp) ground and cold wire connected to the ground output plug. On power amp, shield not connected, and cold used as ground.
Just a question, if you short cut your inputs, with no preamp connected, did-you still have hum ?

R3 is just to allow ground reference of the input stage to be connected directly to the star ground reference, without creating loop (current across the 10 Ohm will be near null) while allowing a good enough reference when nothing is connected.
Is is because the ground connection to the power parts are high current, so, even a low z of cabling can create a voltage in it. Spécially at HF, where wires present an inductance.

[Esperado]

Originally Posted by wahab

Below is the CLG without and with the said caps.

Did-you have a real Crescendo to measure ?

[wahab]

Yes , i did build one a few months after publication and experimented
oscillations with slightly reactive loads.
I got some help from a friend , a gifted Yougoslavian engineer specialized
in high frequency designs (!) who rapidly fixed the thing this way.

[Esperado]

Originally Posted by wahab

Yes , i did build one a few months after publication and experimented oscillations with slightly reactive loads....rapidly fixed the thing this way.

My amp, before modified to CFB was flat, with no overshoots on 20Khz sqare waves after derating. Sure, decreasing the cascode pole cutoff will solve any oscillation problem, but don't you find it is sad to use a cascode to avoid capacitive load on the previous stage and increase bandwitch, then add a cap to decrease bandwitch and add a capacitive charge to the input ? With some expected TIM increase ?
Too, i REALY wonder why you have so different sim results than mine.
Attached my sim for your 22pf. (with no input filter)
Just for understand, HumNSmoke had yet solved his 1MHz oscillation. And is talking about hum, not the typical noise when some hf osc.

[Esperado]

One thing is for sure, this amp can be marvelous or awfull. But because its poles are very near one from the other, it is has an open loop with a low pass filter of near 18dB/oct at his Hf limit. We flirt with Nyquist limits for stability, and it is not exactly the kind of amp for soldering and play. Need very often some derating in operating situation.

[wahab]

Originally Posted by Esperado

Sure, decreasing the cascode pole cutoff will solve any oscillation problem, but don't you find it is sad to use a cascode to avoid capacitive load on the previous stage and increase bandwitch, then add a cap to decrease bandwitch and add a capacitive charge to the input ? With some expected TIM increase ?

The miller compensation will decrease the Vas output Z , so
what is lost in gain is traded for better VAS linearity and driving capability, hence the distorsion will not rise , in the audio band at least.

Originally Posted by Esperado

Too, i REALY wonder why you have so different sim results than mine.
Attached my sim for your 22pf. (with no input filter)

Dont know , perhaps the fets spice models.

Originally Posted by Esperado

Just for understand, HumNSmoke had yet solved his 1MHz oscillation. And is talking about hum, not the typical noise when some hf osc.

Hum can be the result of oscillations or badly implemented ground circuit.
Often ground return using the main ground connection is the culprit ,
no wonder that symmetrical XLR connections are the rule in pro gear.

Originally Posted by Esperado

One thing is for sure, this amp can be marvelous or awfull.

Yes , at the time it was a breakthrough in amateur dedicated designs.

[Esperado]

Originally Posted by wahab

The miller compensation will decrease the Vas output Z

I was not talking about HD, but TIM, not on the cascode stage (with has enough current at 1Mhz anyway) but input stage, with has only ~300µa of collector current.

About the peak we get without miller comp, a little cap (around 3pf) can do the job in parallel with the FB resistance to flatten the bandwidth, and so, help to stability. Zobel is an other point where we can turn the open loop phase, with no negative effects. I would try those first, and use miller in last resort.
Of course, Miller cap would be a requisite for an industrially produced amp, where we want no trouble.
I believe, in a DIYer forum, each one prefer to tune his amp near the limit ?

[HumNSmoke]

Originally Posted by Esperado

...
Just a question, if you short cut your inputs, with no preamp connected, did-you still have hum ?

-1 °K hum with inputs shortened, this for each channel. When just both or only one unplugged, very very light hum and shuffle, but heavy hum when both-plugged-both-side (no matter the preamp is on/off/powercord removed). With no ground, earthquake but this is normal with such a powerfull amp isn't it?

R3 is just to allow ground reference of the input stage to be connected directly to the star ground reference, without creating loop (current across the 10 Ohm will be near null) while allowing a good enough reference when nothing is connected.
Is is because the ground connection to the power parts are high current, so, even a low z of cabling can create a voltage in it. Spécially at HF, where wires present an inductance.

Not sure I really understand as there is already a big gauge wire between ground board input and the star with nearly zero current inside. Do you mean this wire is say shortened by R3 at HF?
OK, I get the car now to Bricodépot to buy some copper bar for the supplies.
Added the earth link below.

[HumNSmoke]

Hi Esperado
We killed the hum . Thank you for your help. Two no-concession stars did the job.
One last question please: I'd like now beginning to play with my DCX2496, plugging it between the preamp and the amps. All my home made is unbalanced, and when shorting inputs pins 1-3 does not frighten me at all, I wander if I can/must do the same for the outputs. This is not very clearly stated in Behringer's doc. Shorting the so called "cold" pin3 to ground permanently make me afraid to kill the output OP-amp. Should I instead leave the pin3 unconnected?