Forced Class A opamp

[eikoow]

[quote author="Svart"]what was your estimated current draw increased by and how many opamps did you try this to?[/quote]

I don't have the schematics in front of me, but here's a rough count:
160 - OPA2604
40 - OPA604
24 - OPA627
16 - AD797

All of these have a 3k resistor from output to negative supply, except for 9 - OPA2604's which drive the VU meters.

Subsequent tests in my lab have convinced me that an OPA-627 sounds slightly better if the resistor goes to the positive supply. However, by the time I figured this out, Joshua had already installed all the resistors to the negative supply, and the improvement was slight enough that we weren't tempted to go back and change them.

Before re-chipping and class-A mod, current draw was about 1.5 amps at +/- 16.5 volts. After mods, current draw is close to 4 amps at +/- 17 volts. Fortunately, I had a spare Amek power supply rated at 15 amps, so we used that. It runs nice and cool at 4 amps.

The interior of the console does get noticeably warmer, but everything is still well within rating. If we wanted to use a lower resistor value, we'd have had to install some air vents in the console frame, and beef up the internal power supply distribution system, so we didn't do that.

 

[chrissugar]

Thank you John and Joshua for the info.
I think I will mod one of my SSL comps and compare with the unmoded ones.

chrissugar

 

[Svart]

Awesome. I'll be trying a few channels to compare. Thanks!

 

[Flatpicker]

This is a great thread - Thanks to all! :thumb:

 

[bushwick]

Without getting too sappy, I have to say that I really deserve very little credit for the job, say, other than brut labor, supplying a testing environment, providing a problem and a pair of ears. Eikoow really deserves the kudos.

He solved what to me was a problem of mixes that seemed to fold under heavy mixing and a top end that had an air of falseness to it. The sound of the board had been cleaned up immensely before we got to these mods, yet there was still something that did not satisfy.

Between the reworking of the power supply distribution and powersupply, and the rechipping and mods to the ics, the board is to my ears astounding. It would be unfortunate if the credit did not go largely to the dude behind all this cool ideas and implemenation. So thanks for the 'thanks for sharing' but the gratitude should sit with Mr. Chester. He's the one who agreed to do this. (He also scrapped and built a discrete LR bus and scrapped and redid the grounding scheme - the prior two mod sessions to the board and also major improvements)

I invite any of you in the NY area to come out and here the console if you wish.

Very Best,
joshua kessler
www.bushwickstudio.com

 

[Svart]

Bushwick:

Your last post was as honest and humble as one searching this forum for true information could ever ask for and I offer my sincere gratitude!

I have just completed modding a couple of buss channels into class A via a 3.3k R to -18v and so far my testing is indeed inline with your findings. there is something different about the actual audio that i can't put my finger on. I am going to try to do more testing shortly and should come up with a few example clips too.

again, I personally appreciate your honesty and your helpfulness!

:thumb:

 

[Svart]

you are very right Butta, now that you mention it, depth does seem to describe it. i wouldn't say that the audio is any "cleaner" or that their is more detail or anything like that, but it just sounds a little bit more *natural*. I've been playing with this on a couple of buss channels that get a bit crowded in a heavy mix but i ran out of resistors before i could do some input channelstrips. more on this later.

:green:

 

[Svart]

i wonder what the 9K preamps would do if i hung some resistors on them..

:green:

 

[Svart]

i'm wondering if sections such as EQ need to be biased to class A..?

might as well do the GSSL compressors too eh?

man i'm going to need a lot of resistors..

 

[Flatpicker]

[quote author="Svart"]i wonder what the 9K preamps would do if i hung some resistors on them...[/quote]Same for the Green Pre.

 

[Svart]

Ok, almost done with a pair of channels.. should i do the EQ sections too? I've done the EQ summing sections but not the gyrators themselves..

 

[NewYorkDave]

opens up the sound stage.

Butta, you are hereby fined $50 for using such language here. Keep it on the audiophile messageboards where it belongs. See the court officer on your way out--the fine is payable in cash, by credit card, or in beer.

:razz:

So, is there gonna be a field trip on the L train out to Bushwick during AES? :wink:

 

[bcarso]

A fascinating idea for an output stage that has essentially no crossover distortion without operating in full class A is described on this website, under "An Improved Class-B Design":

http://www.angelfire.com/ab3/mjramp/index.html

I read this briefly in Electronics World in 1998 and didn't grasp what was going on then. There is a remote resemblance to a Malcolm Hawksford idea for local linearization of output transistors that appeared in the AES journal and was adopted by Sumo, but this is original I think. The guy's other amp designs are pretty interesting as well---see the minimalist MOSFET one for example. A number of highend folks will cringe or faint dead away, but his discussions and justifications are pretty compelling to me.

It's tempting to try to extend it to a symmetrical design but my intuition says the dish runs away with the spoon if you do.

Now what was I looking for when I stumbled across this site? :?

 

[CJ]

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. If so, why not just use a single ended output to begin with? I think that's what the early Neve's are. But then, I'm a tube head so I better get the H out. Sorry! Carry on. Ignore this as always. I do not exist. This was merely a fig-newton of your imagination.

 

[Svart]

mmm fig newtons

 

[Learner]

Just out of curiosity, what does "better" sounds like in this case? Can someone had the experience care to elaborate?

 

[Svart]

I think butta summed it up nicely however I have to add a personal experience.. I always had trouble with heavy mixes where the individual tracks sounded fantastic but the sum of the tracks sounded dead and rather boxy. just doing the upgrades/mods to the summing/buss channels has cleaned that up quite a bit. I'll call it more *natural*. it's not like other mods that change the tonality of the audio, it seems to just make everything sound a bit more natural.. i know that doesn't say much but I am going to post some A/B clips once i get everything in place.


:thumb:

 

[magicchord]

[quote author="CJ"]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. If so, why not just use a single ended output to begin with? I think that's what the early Neve's are. But then, I'm a tube head so I better get the H out. Sorry! Carry on. Ignore this as always. I do not exist. This was merely a fig-newton of your imagination. [/quote]

Well, yeah, you're taking one-half of your IC opamp output circuit, tackling it, tying its hands behind its back and sitting on it.

 

[skipwave]

[quote author="magicchord"][quote author="CJ"]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. If so, why not just use a single ended output to begin with? I think that's what the early Neve's are. But then, I'm a tube head so I better get the H out. Sorry! Carry on. Ignore this as always. I do not exist. This was merely a fig-newton of your imagination. [/quote]

Well, yeah, you're taking one-half of your IC opamp output circuit, tackling it, tying its hands behind its back and sitting on it.[/quote]

You make it sound like abuse. It's more like tough love. As long as the IC is still operating within its acceptable limits and producing a desirable result, I consider this a design feature, albeit, an unintentional one.

 

[CJ]

I see.

Just a side note, did you know that they are starting to make Class "D" opamps?
Some chopping going on at the output. Stay away from those!