"Another Poor Man's" Fairchild 660/670

[emrr]

I recall rising 6db/octave for power rating.  So +35 dBm max at 20 Hz will do +41 dBm at 40 Hz.  etc etc. 

 

[rotheu]

I recall rising 6db/octave for power rating.  So +35 dBm max at 20 Hz will do +41 dBm at 40 Hz.  etc etc. 

The transformer is rated at 2.5W (+34dBm) at 20Hz. Does that mean that it can have 10W (+40dBm) output at 40Hz?
If this is true, then it's great, but my question is still, what happens to the frequencies below 40Hz, are they gonna
cause any problems? How the transformer will do?

 

[emrr]

Sounds basically correct.  I gotta think we don't have frequencies below 40Hz very much, at least not of any significant level.  Kick drums and pipe organs.  Anything down there will saturate and distort, and that signature will get into the compression CV.    So, reduced compression at those freq because of inability to pass accurate signal.  Creation of lots of third harmonics related to those signals, effecting compression.  Steady state overload may cook transformer.  Pipe organ?  Doubt a kick drum could ever do it.  What wattage down there can the amp stage actually deliver to the output transformer in the first place?  Is it even an issue in current design? 

 

[nielsk]

SC filter?

 

[rotheu]

Sounds basically correct.  I gotta think we don't have frequencies below 40Hz very much, at least not of any significant level.  Kick drums and pipe organs.  Anything down there will saturate and distort, and that signature will get into the compression CV.     So, reduced compression at those freq because of inability to pass accurate signal.  Creation of lots of third harmonics related to those signals, effecting compression.  Steady state overload may cook transformer.  Pipe organ?  Doubt a kick drum could ever do it.   What wattage down there can the amp stage actually deliver to the output transformer in the first place?  Is it even an issue in current design? 

In current design it's somewhere between 3-3.5W max and the transformer is getting slightly warm, but I'm thinking about
trying ECC99, which can deliver up to 5W, instead of 12BH7. How do you think the creation of third harmonics can affect the compression?

 

[emrr]

Harmonic effect on compression?  I don't really know.  Stands to reason it's minimal, yet audible.  Very crudely and possibly incorrectly interpreted, distortion would have to be 100% to create equal level in harmonics over an octave higher.  Lose some lower level, gain some amount back via harmonics? 

It's sort of a brute force destructive SC filter.  Could limit response within amp and keep it clean down at the bottom.  Many similar limiters limit bottom response a bit already, and that is sometimes the fix to problems with pumping in some types. 

I'd be more concerned with current flow and balance through transformer.  Might be worth taking one to meltdown and see where it gives it up.  Look at response throughput at increasing level past rating. 

 

[rotheu]

SC filter?

That's an option too. Reduction of coupling caps from 47nF to 22nF should take care of that. I'm not sure what will be
the exact corner frequency, but it's probably somewhere around 40Hz.

 

[Larrchild]

Hey rotheu,
The NFB just lowers the output z, albeit at the expense of some gain, yes.
Lower output z charges things up faster.
The 12AX7 following another just seems like overload potential with it's sharp cutoff.
12AU7 plate lines look more like a 12BH7.

 

[lolo-m]

Steady state overload may cook transformer.  Pipe organ?  Doubt a kick drum could ever do it.   What wattage down there can the amp stage actually deliver to the output transformer in the first place?  Is it even an issue in current design? 

We have to think that the push pull amp isn't a real push pull... I mean the sc amp is two amplifiers amplifying one half of the signal. If the output power of the amp is 5W in a real push-pull it will be only 2,5W in a Fairchild SC amp...
We have to think too that the SC-amp output is fonction of the compressed signal. Only peak will deliver full power in the SC-amp, then the signal will be compressed an the SC-amp will only deliver a fraction of it's power capacity... A Fairchild comp is a looped amplifier...
Maybe this helps, maybe not !!!

 

[rotheu]

Steady state overload may cook transformer.  Pipe organ?  Doubt a kick drum could ever do it.   What wattage down there can the amp stage actually deliver to the output transformer in the first place?  Is it even an issue in current design? 

We have to think that the push pull amp isn't a real push pull... I mean the sc amp is two amplifiers amplifying one half of the signal. If the output power of the amp is 5W in a real push-pull it will be only 2,5W in a Fairchild SC amp...
We have to think too that the SC-amp output is fonction of the compressed signal. Only peak will deliver full power in the SC-amp, then the signal will be compressed an the SC-amp will only deliver a fraction of it's power capacity... A Fairchild comp is a looped amplifier...
Maybe this helps, maybe not !!!

Lolo, the case you are describing is when the DC Threshold 100K pot is set high, which pushes the 1st 12AX7 in to Class B and it passes
only the highest positive peaks of the waveform. If the DC Threshold is low, the full waveform will be present in the SC amp. And I think
the SC amp is still real push-pull at any time, it will just push and pull whatever signal is passing through the detector (1st 12AX7) in to the SC, depending on the 100K pot setting. The transformer is rated at 2.5W RMS, which is the case of low DC Threshold and probably the most common setting for compression, unless you need some peak limiting. Transformer can handle a lot more peak power than rated RMS. Low DC Threshold is what could possibly cook the transformer.

 

[analag]

Hey rotheu,
The NFB just lowers the output z, albeit at the expense of some gain, yes.
Lower output z charges things up faster.
The 12AX7 following another just seems like overload potential with it's sharp cutoff.
12AU7 plate lines look more like a 12BH7.

I don't believe he heard you...that's one of the things that popped out at me. 12ax7 driving 12ax7, hmmm.

 

[SIXTYNINER]

COOL PROJECT
MAN

 

[rotheu]

Hey rotheu,
The NFB just lowers the output z, albeit at the expense of some gain, yes.
Lower output z charges things up faster.
The 12AX7 following another just seems like overload potential with it's sharp cutoff.
12AU7 plate lines look more like a 12BH7.

I don't believe he heard you...that's one of the things that popped out at me. 12ax7 driving 12ax7, hmmm.

I did, Analag, believe me, I did...I heard him and I heard you. I'm in the process, waiting for some tubes.
We all live and learn every day. I very much appreciate everybody's help and advise.

 

[veermaster]

Timing network with independent attack and release controls and true stereo linking (two linked networks are isolated and don't affect each other)

Sorry people, I gotta ask:
Does this network also work in Analags version of the PM660?
I really would like to add an independant attack and release switch.
Thanks in advance!
emre

 

[nielsk]

are the big caps in the timing network non polar?

 

[rotheu]

Timing network with independent attack and release controls and true stereo linking (two linked networks are isolated and don't affect each other)

Sorry people, I gotta ask:
Does this network also work in Analags version of the PM660?
I really would like to add an independant attack and release switch.
Thanks in advance!
emre

If you are asking me, I don't know. I didn't see the schematic.

 

[rotheu]

are the big caps in the timing network non polar?

Big caps are polarized. 25V should work fine.

 

[veermaster]

Thanks for your reply rotheu

 

[lolo-m]

Sorry people, I gotta ask:
Does this network also work in Analags version of the PM660?
I really would like to add an independant attack and release switch.
Thanks in advance!
emre

Sorry to answer so late but this Rotheu's time network doesn't work with the PM660 without any mod. If you do my last mod (SCAmp buffer), it will give something close. Don't forget that the GROUND in Rotheu's TC will become the adjustable -4,5V in the PM660...
Have fun, this is a really interresting TC network !!! 

 

[veermaster]

Thanks lolo for your reply.
As soon as I find the time I will try your SC mods, will report back then...