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Best Opamps to replace TL072's in TAC Scorpion-II
Old 24th October 2013
  #121
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Quote:
Originally Posted by Aerophone View Post
Be careful with lowering the rc pole in feedback networks. You will introduce lots of phaseshift in the audio band and will lower bandwidth. As an example my D&R was designed in a way that they could pass a 40khz square wave without overshoot to keep it 100% clean in the audioband.
200-250khz its just about right, although my D&R consistently uses 600khz -3db point to achieve what i described earlier.

Jim Williams recommended 22pf for values up to 20Kohm and 10pf for values between 20k and 30kohm. You can use 8,2pf and 6,8pf onwards.
Seem like good practice to me.

I have successfully used Lm6172's, LME49720's, LT1358's, ADA4610-2's, AD8512's and opa2134's without stability issues. I just added local 0.1uf bypasses from power rails to ground as near as i could from the Opamp power pins.
hmm, thanks for the tip. I figured it should be OK, 250K sounds awefully high for such an old board. I know API's used something around 80-100KHz, and from what I've read here and elsehwere, the TL072 are very "easy going" ICs, in that they don't oscillate as much as newer faster chips. Anyway that was my line of thought with lowering the LPF. Will let you know how it works out. The ICs you mention, are these specific to the TAC or to your D&R? I'd be interested to see a schemo of the input channel of your board if you have one to post.
thanks!
thanks!
Old 27th October 2013
  #122
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I would suggest just changing things the same if you like the way it behaves.
otherwise:
there is something to say about op amp feedback bypass cap.

People don't understand "why" it is there in the first place. second they think just simply calculate it. it does't work like that. this is why interfaces suck till they get modded. (wow, I found 2700pf bypassing op amps in motu, what magic I found changing it to the value usually seen with that op amp )

op amp change without changing the cap can give varying results.

so experiment, use: not installed vs. 6pf, 8pf, 10 pf, and/or 22pf.
Old 27th October 2013
  #123
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I for got to mention.

there are sever factors that this cap is sized. and some of them I seen is questionable. because it can compromise performance

its because of:

gain vs positive feedback (oscillation) due to too much bias for low +/-Vcc

gain vs loading effect of op amp in/out vs overall noise floor vs impedance differentials.

a lot of mistakes I see lately:

Relying on computer simulation for this value instead of selecting due to physical perimeters not available virtually: 1. stray inductance and capacitance in devices causing paralleled circuit across the whole die, leads and circuit lands. 2. power filtering and the sum of the differential noise and whether or not it was integrated as an internal noise reduction circuit via feedback networks. 3. Differential level error causing bias change 4. the linear response of the circuit or lack there of that over compensate and shifts the response in a logarithmic fashion in the wrong direction.
Old 31st October 2013
  #124
Here for the gear
 

Thanks for the tips. Drtechno, I'd be interested in hearing more about the relations between bias/loading/impedance. If you have any interesting links to share, I'd appreciate it.

We ended up changing the feedback networks to give between 100-130KHz -3dB points, and it's working just fine for now. There is a weird noise in one channel, but I suspect that it is something oddball, as all the rest are fine.

Also, I can confirm, the big TAC PSU (the one with the fan), recapped and modded to remove the regulator sockets(regulators soldered directly to PCB) can handle the extra current from the LME opamps in a 32/8/2 Scorpion. This is the fader amp in every module, 2 opamps in the groups, the auxes, 4-5 in the master channel, etc. Also, the input channels were changed to a line only using the LME chip, so that's 32 more. main filter caps are 15000 uF (originals were 10000), all module filter caps were upped from 220uF to 470 as well.

thanks to all for the help!
Old 23rd January 2015
  #125
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Hi
I have a doubt
which brand of capacitors are good for recap the tac scorpion?
I thought change by elna silmic II

many thanks
Old 23rd February 2015
  #126
Low offset opamps allow for removing most all of the el caps in the signal path. Stereo mix to main outs can be direct coupled, I use a 22.5k feedback resistor in the fader recovery opamp to set the master gain fader so zero is at the top of the fader throw.
Outside the 2015 input mic/line preamp, the rest of the input module can be direct coupled as well.
Old 24th February 2015
  #127
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Quote:
Originally Posted by heavyllegas View Post
Hi
I have a doubt
which brand of capacitors are good for recap the tac scorpion?
I thought change by elna silmic II

many thanks
I never tried the elna caps, they do sound intriguing. would be interesting to compare and contrast with panasonic FM series.
The caps are part of the harmonic signature as well as the dc isolation. changing these cap do effect that, but its nice to compare some different ones and different combination to bring out a character. removing the caps will make it sterile and could cause unforeseen failures. yes you could change some stuff to dc couple the board, but you remove its personality it has.

@ Jim Williams: Dude, I thought you were dead! Especially when another electronics engineer died in a car crash on the way to your funeral...... Pretty interesting your response is btw.

Last edited by drtechno; 24th February 2015 at 09:19 PM.. Reason: added the harmonic thing
Old 27th February 2015
  #128
Gear Nut
I have a TAC Scorpion II fully recapped with Elna Silmic II caps. Very happy with the results. There are a couple of difficult fits on the Master Buss (S4001) but nothing that can't be overcome with some Teflon on the leads.

It is difficult to make a comparison to the old caps as they were about 20 years old. The old caps had a mid-range crunch that was useful for tracking rock guitars, but everything else suffered. Basically, hard to know if the old caps sounded anything like the intended design or if they were just simply old.

The Elna's are much more smooth and pleasant, a clear (pun intended) improvement over the old caps. The 12k - 15k range was previously quite harsh and is now detailed and clear. Low end is vastly and immediately improved. Transient response is less blurred and the noise floor is lower. Subjectively, the console is more laid back; still rock and roll but less harsh in a good way.

I have two input channels that I re-capped with Cornell Dubilier caps that came with the AML mic pre mods. These channels sound a bit more forward and clinical, better for rock guitar cabinets in my opinion. Still very good, but my preference is for the smoothness of the Elna caps on most things.The remainder of the channels have the AML mic pre mods and Elna caps. I did the AML mods after the Elnas had been installed on the input channels, so to be specific the AML mod board still has the Cornell Dubiliers that came pre-installed.

I would be happy with having used the Cornell Dubilier part on the whole board, and the caps needed for an entire channel do come included with the AML kit. Using either part would have yielded immense and necessary improvements over the 20 year-old caps. I think the Elna part is a very good sounding capacitor, and would again make the choice to use it in the circuits over the Cornell Dubilier part. I used to be a parts buyer and built world-class tube amplifiers for a living; I've have tinkered with nearly every cap available on the market. I have to say that the AML mic pre mod is worth every single penny and really modernizes the board. It is the highest bang for your buck ready-made circuit mod I have ever come across. If you are at all thinking about doing the AML mods just use the Cornell Dubiliers that come with the kit and you will not be disappointed.
Old 27th February 2015
  #129
The master mix/fader amps don't need caps in that design. I remove them. I do install a ferrite bead in place of the cap feeding the mix opamp inverting input to trap rf. Change the fader amp feedback resistor to 22.1k and the fader will zero out all the way up. It sounds better that way, out of circuit. Add local .1 uf mono ceramics from pin's 4 and 8 to ground. That improves stereo crosstalk. Ditch the el caps in the signal path and you hear more music.

Opamps can be LME49720, OPA1612, AD8599, etc.
Old 27th February 2015
  #130
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synthRodriguez's Avatar
Quote:
Originally Posted by Jim Williams View Post
Ditch the el caps in the signal path and you hear more music.
What about protecting / eliminating aberrant DC in the audio signal? Isn't that the function of el caps in the audio path?
Old 28th February 2015
  #131
Gear Nut
The more electrolytic caps that can be removed the faster and more open the circuit will sound, or as Jim says, "more music". As a design philosophy the less in the signal path the better. Caps are particularly tough on an audio signal. It is a worth while exercise to design them out of an existing circuit to hear the difference as it is pretty significant. It can be done with careful and smart design.

A lot of what Jim does is beyond the normal musician, end-user or electronics tinkerer. I, for my own knowledge, would love to see Jim "connect the dots" between some of the why and how in many of his mods to better understand his philosophies. It would be a great help to the many that frequent these boards to break down the tips to a more rudimentary level and work through a single mod or circuit. Any interest Jim in a step-by-step pictorial? Something like what people post for car repair, but with a focus on learning some electronics basics. Not just a paint-by-numbers mod, but something more about how to use a scope to see how component level changes make improvements in a circuit? Maybe just the input channel EQ circuit from start to finish. See the phase shift changes, etc...

Anyways, without going as "far" as Jim takes the circuit and radically changing what the board is capable of the AML mods are step-by-step simple. A recap on any equipment as long in the tooth as the TAC boards are is required.

Last edited by Noisy1; 28th February 2015 at 01:33 AM..
Old 28th February 2015
  #132
Quote:
Originally Posted by scottrod View Post
What about protecting / eliminating aberrant DC in the audio signal? Isn't that the function of el caps in the audio path?
DC is blocked from the stereo buss due to local blocking caps on the input modules. If there is any DC component generated by any device inserted, caps should be retained. On the outputs, there should not be any DC present from the destination device. That is rather rare to find. My entire console has 2 caps in the signal path due to the discrete transistor trans-amp design.

Nothing cleans up an audio signal like removing several hundred electrolytic caps from your music.
Old 1st March 2015
  #133
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lame pseudonym's Avatar
 

Quote:
Originally Posted by drtechno View Post
I would suggest just changing things the same if you like the way it behaves.
otherwise:
there is something to say about op amp feedback bypass cap.

People don't understand "why" it is there in the first place. second they think just simply calculate it. it does't work like that. this is why interfaces suck till they get modded. (wow, I found 2700pf bypassing op amps in motu, what magic I found changing it to the value usually seen with that op amp )

op amp change without changing the cap can give varying results.

so experiment, use: not installed vs. 6pf, 8pf, 10 pf, and/or 22pf.
Here is what you need to know about the little capacitor across the feedback resistor:

Allen Burdick on RF Gain Compensation | Orion Gateway

Scroll down to the feature "Taming Op Amp Distortion".
Old 17th August 2015
  #134
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You´re joking, aren´t you?

I need a break from gearslutz.
Old 21st August 2015
  #135
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S2udio's Avatar
 

Audiophiles on RF

Nasty addiction that !........take a few months in rehab.
Old 22nd August 2015
  #136
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Hi
I wonder if the fact that the old 'broadcast' amplifier designs with frequency responses curtailed to 40KHz or less, but having properly designed 'filters' for extreme HF and usually 'extreme LF' seem to be pretty popular around here. Think older Neve and German 'Daner' type modules.
It seems the 'newer' generations might need to speak with their grandmothers about how to suck eggs.
Matt S
Old 22nd August 2015
  #137
Bandwidth restricted audio devices can soften the harsh harmonics of lower end digital productions. Restricting the low end has no benefit other than to keep the old needle in the record grooves.
Old 23rd August 2015
  #138
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lame pseudonym's Avatar
 

Quote:
Originally Posted by jensenmann View Post
You´re joking, aren´t you?
Is who joking about what?
Old 25th August 2015
  #139
Install sockets and play around with diffrent kinds. Not always is the best spec. component the best sounding.
Use 1458 or some japanese ones,5532 etc...let your ears descide not the price or specs.
Old 25th August 2015
  #140
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JohnRoberts's Avatar
 

Quote:
Originally Posted by lame pseudonym View Post
Here is what you need to know about the little capacitor across the feedback resistor:

Allen Burdick on RF Gain Compensation | Orion Gateway

Scroll down to the feature "Taming Op Amp Distortion".
I feel bad about responding to a post from several months ago, but....not that bad.

That discussion about the design decision related to using/selecting feedback caps across a feedback resistor is only one aspect of the design (maintaining loop gain margin).

Also significant for stability is that any stray capacitance on the - input PCB trace will form a one pole low pass filter with the feedback resistor. This phase lag makes the op amp even less stable, so enough lead capacitance across the feedback R to swamp out this stray capacitance lag will improve stability.

Op amps that are not unity gain stable (like widely used ne5534) generally do not tolerate feedback caps because in theory they reduce the closed loop gain to unity at some very high frequency.... but when input capacitance is lagging the NF signal, some small feedback C can cancel out that lag. In that case the feedback cap should be sized to deliver the minimum closed loop gain stability criteria, so for a 5534 with 10 dB min, the feedback cap should be 1/3rd the stray input capacitance... (we're talking low single digit pF).

Finally the feedback cap, lowers the effective op amp output impedance. The op amp output impedance is reduced by the loop gain margin. A feedback cap that reduces the closed loop gain at HF will likewise increase the loop gain margin and therefore reduce the output impedance at HF.

All subtle stuff perhaps but that link was only looking at one aspect. Circuit design is a simultaneous equation with multiple parts. Good design involves juggling these multiple parts against each other.

JR
Old 26th August 2015
  #141
Quote:
Originally Posted by VerreyckenGerd View Post
Use 1458 or some japanese ones,
The 1458 is a dual LM741, one of the original unity gain stable IC opamps. I remember them from the early 1970's. 22 nv noise, 40 db gain at 10k hz and plenty of slew distortion at 1/2V us.

I keep one on my bench with some of the other variations. If a circuit works/sounds pretty good with that chip, the good stuff usually sounds great.

You will find them in some 1970's era gear and consoles.
Old 18th December 2017
  #142
Gear Head
 

Quote:
Originally Posted by soundmugician View Post
I just recently purchased a 28 channel Amek Angela Blue, first thing i did was put in 6 channel strips and listened to them with a condenser and dynamic mic, through a mono Genelec monitor. I did notice some pretty audible distortion on hotter signals that was consistent through all the channels. I just thought that was how the console sounded. I decided to purchase caps / faders / relays from Audiomantinence and rebuilt 12 channels so far, including disassembling every switch cleaning them internally and "slightly" bending the contact bridges ( i did this because when I tested the strips originally the switches seemed pretty dodgey). I also cleaned out all the pots with a small amount of Deoxit. Upon plugging in these modules, to my ears the distortion was extremely lower, It definitely feels like the channels have a lot more headroom, and are handling signals much better. I did notice that a lot of the electrolytic caps I replaced had some blue / green corrosion on them, usually only one of the leads, and was on either the positive or negative side. Im assuming that is the fluid in the caps leaking out? In my opinion, the difference in sound in just cleaning and getting the channel strips back up to spec made a huge difference in the overall functioning and distortion levels I was getting before ( only tested by my ears that is ). The caps I got were also a higher voltage rating, not sure if this had anything to do with the noticeable distortion decrease.

Anyone have any opinions on balancing of these modules? I do have a lot of outboard gear, and would like to use some stuff as inserts, but am kinda concerned with all the outputs of this console being unbalanced. I have had instances in the past where I didn't like the results of interchanging balanced / unbalanced gear. Maybe possibly due to improper wiring of connectors? Any thoughts on this?
Ancient post I know but I have uploaded schematics & components list for "the balencing mod" performed on my scorpion II - aux, groups & master modules here (in case anyone's still interested in balancing their Scorpion II).
It does make a BIG difference

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