Forced Class A opamp

[Guest]

Would I be a total lameass if I thought that what this thread is actually talking about is turning a push-pull output circuit into a single ended type circuit? Like pulling one tube out of a Super Reverb? The "Class A" thing is throwing me.

You can bias a tube p-p output for class A. It gets hot! But, more power is available than a single tube class A output. There's bothering to do this w/ opamps because that's what everybody's got in thier consoles (including me, for now). Yeah, a single device out can work but would require even greater and more unpleasant re-design for most retrofits.
regards,
D.Bock

 

[CJ]

Of course. Now I get it.
Yes, the Class A push-pull has less power but sounds a lot nicer for non crunch music.

 

[Svart]

C37? that's another 50$. I ONLY accept beer.


:green:

 

[bushwick]

Svart, you should do the resistor mod to anything that passes audio. Depending on the circuit you will find big improvements with certain high end opamps, especially if yours is fitted with TL072's.

Learner, the class A resistor mod (coupled with everything else we did) solidified the soundstage and gave instruments more space around them, most likely because the image was smeared less. I am not able to positively articulate this in technical terms. As well, and like Svart said, there was much more depth to the instrument. The top end was much more natural and open sounding (where do I send my $50) probably becuase the artifacts of crossover "whatever" are no longer there.

Best,
joshua

 

[Svart]

i have OPA2604, OPA404, opa627, opa637 and ne5534s(only two channels of SSL9k preamp) in my equipment. no tl072/tl082/tl084 in the audio chain anymore.

The results i am getting definatly justify the purchase of resistors and the time spent doing it.. :green:

 

[NewYorkDave]

So, while everyone's oooh-ing and aaaahh-ing over the newfound soundstage, did anybody remember to take some actual before and after measurements? It would be interesting to compare the amount and spectra of the distortion products before and after the mods.

 

[bushwick]

I believe eikoow might have them in on his laptop via the AP 2500 analyzer. Not sure what he plans to do with that data. He did this gratis for me, to answer some questions and amuse himself. I'll find out.

 

[Svart]

I'll have a/b clips after this weekend too.

 

[bcarso]

I've seen before and after AP scans of an LM833 and there was a dramatic difference in the amount of distortion. Unfortunately at that time there was no fourier function so the relative strengths of the residual components were not shown.

 

[Guest]

It may sound like mumbo-jumbo since it's not measurable

It may actually be a little bit measurable with sophisticated enough gear. I don't know if AP does this, but we have a machine here that will really analize low level high order harmonics, repeatably. It definitely provides some insights unavailable from more rudimentary test gear.
What about this "spectra foo" I've heard about, does that measure full bandwith harmonic distortion?

I've seen before and after AP scans of an LM833 and there was a dramatic difference in the amount of distortion.

Assuming you mean before and after "FCA", which way increased it?

 

[bcarso]

Distortion with the forced class A was lower, significantly.

 

[Svart]

i have been doing some comparisons between channelstrips with all opamps running in the forced class A to others with no modifications and there is a signifigant difference. One thing i have noticed that has really made me happy is the need for less EQ. yep, I left my EQ settings on the two strips i modified in the same positions as my last session and used the same tracks that i used those channels on before the mods. I noticed right away that the EQ now sounded strange and had to back off the settings quite a bit. the audio is now much more intelligable without needing to eq the snot out of it for it to stand out of the mix.

:thumb:

 

[NewYorkDave]

Did you also perform the mod to the chips in the EQ circuit? Did you measure afterwards to see if the center frequencies and the amplitudes of cuts/boosts has remained the same? The change that you're ascribing to "enhanced clarity" might be partly due to the EQ circuit now behaving differently after the mods.

The problem with doing mods in an uncontrolled fashion, without measurements, is that you can never be quite sure WHY it sounds better afterward, and therefore don't know if the results are repeatable on different pieces of equipment.

 

[bushwick]

I'll add that before, I only used my board as a split out box from PT and Studer to mix from and for auxing and minor bussing. Bussing clearly sounds better (drum sub for ex.).

But one of the big wow's for me is the eq. I never used the eq before. I only used outboard and some PT eq if at all. Now I can actually use the eq on the board and I don't feel like the audio is getting squirrely. I really am still beside myself at how much of a change the mods we did made in everything to do with the console.

If some of you out there have 5532's in your board "eikoow" made pc adapter boards for putting a 627 and a 604 where there was only room for the eight pins before. He would probably sell you some to offset his troubles. (assembly required)

Best,
joshua kessler
www.bushwickstudio.com

 

[Svart]

NYD, I have not done any tests involving analyzation with equipment. Yes i agree we need to do testing of that nature and would gladly do so if i had the equipment needed. However, the EQ does not sound different in terms of peaky-ness/nonpeaky-ness. the difference seems to be along the same lines as the bussing channels, enhanced "depth". the tracks in question now sound more realistic, less boxy and somewhat less harsh.

 

[Flatpicker]

Is it safe to assume that this trick would work equally as well on a MC33078P?

 

[Svart]

i would think so, however this hack is just that and you might want to test it a bit before doing a whole bunch of those parts.

 

[bcarso]

[quote author="Flatpicker"]Is it safe to assume that this trick would work equally as well on a MC33078P?[/quote]

Yes.

Besides looking at the output stage circuit, you can look at the difference between the quiescent amplifier power supply current and the maximum output current. If the latter is substantially larger in magnitude than the former, then it is safe to say the output stage is class B or AB, and that the loading stratagem will work.

 

[Flatpicker]

Thanks. I'll give it a try and let you know what I find. This might be a great mod for channel strips like Joe Meek, Focusrite, Mindprint, etc...

 

[Gus]

Has anyone with the CAD opamp gain stage microphones done this?