Pacific Recorders & Engineering ABX control room monitor upgrades?

[plumsolly]

I am in the final stages of refurbishing a PR&E ABX-18, which is a pretty neat beast - more to follow.

I would like to use it's CRM in place of my current outboard one - It will save me some money and the console's solo functionality only works in the context of monitor path - It is not destructive solo, but has a separate solo bus.

I have attached the specs and the schematic. The specs are nothing to sneeze at: 0.5db down at 20khz, and .004% THD.

However, I would like to make this path even cleaner. I realize that the improvements will most likely be inaudible, especially in the context of my amp/speaker/room system. Consider it an academic exercise.

I plan to replace the volume pot with a rotary switch to maintain L&R matching.

I am going to experiment with replacing the CMOS switches with DIP plugin relay cards made from these: http://capi-gear.com/catalog/product_info.php?cPath=102&products_id=195

I ordered some fancier chips to replace the 5532/4s -LM4562 and LME49710.

I think the bandwidth is set by the feedback caps - (C6, 9 14 & 20 for the left channel) - I calculate 0.5db down at 20khz for the current values, which is in line with the specs. I'd like to shoot for something like -0.1db at 100khz which would mean replacing the 100pf with 5pf. 

Will I be able to get away with this? Is this the sort of thing that depends on layout? RF present? Specific opamp?

I see here: http://www.soundskulptor.com/docs/mc624-schematic-02.pdf, no feedback c.

Lastly, I am wondering whether it would be worthwhile to try to change the instrumentation input amps with THAT chips - I'm thinking/hoping that with the fancier opamps, the distortion of this stage as is will be vanishingly low.

Thanks for your input,

Ben

 

[plumsolly]

Specs

 

[plumsolly]

CMOS switches are Harris HI1-307-5. See attached datasheet.

 

[PRR]

> C6, 9 14 & 20

Consider C2 C3.

The 6 9 14 20 set is at 160KHz, 2 pole, so will be -2dB at 80Hz, -1dB at 40KHz.

The C2 C3 set is a third pole, still near 160KHz. I'm too tired to work that out.

It smells like they expected large supersonic signals. Was this a broadcast machine that might set right under an AM transmitter? You could probably use much smaller caps.

 

[plumsolly]

Thanks PRR

It smells like they expected large supersonic signals. Was this a broadcast machine that might set right under an AM transmitter?

Yessir, exactly.

I'll have a look at C2, C3 and  will experiment with some smaller caps.

Ben

 

[westernskystudios]

Thanks PRR



Yessir, exactly.

I'll have a look at C2, C3 and will experiment with some smaller caps.

Ben

I'm trying to get some info on the ABX-18...is this console transformer in/out with Jensens like I'm seeing on some older reverb listings? I know PR&E employed a 990 type op amp too. Any info you can share I appreciate.

 

[Dreams]

Kinda hard to find much info on their product line or history, though I've been seeing wider availability of schematics and manuals "recently." I'm still not even clear about all the name changes!

As far as i can tell, when transformers were used, they were jensens. The only DOAs I've ever seen in any of their products (line amps out of ??? console) were 918s, but most of their stuff used monolithic opamps

 

[westernskystudios]

There's a video of a BMX on YouTube with discrete amps on what looks like console outs. Wondering if the ABX came later or before, thinking that perhaps older models used the 918, circa '79 or so?

I think this is a pic of the 918?

 

[Dreams]

Cool, never seen that. Looks like an LM394 on there, probably 990

 

[waltzingbear]

That is a 990, only one console used the 918, and they only made a handful of them. That was the System One, pretty much their first console. All Jensen transformers I/O, all discrete. All too expensive for the market.

The 990 was used later as a summing amp and output amp in their consoles. They also used a 5534 front end on an output stage as a plug-in module.

The principals at PR&E were friends with Deanne and Jensen Transformers, many having worked on Quad Eight together.

 

[Dreams]

Here's a 918

 

[Dreams]

Closer

 

[AngryAudio]

I'm trying to get some info on the ABX-18...is this console transformer in/out with Jensens like I'm seeing on some older reverb listings? I know PR&E employed a 990 type op amp too. Any info you can share I appreciate.

The ABX was built in different sizes. The suffix -18 indicates the number of input channel slots in the frame. Note it doesn't necessarily mean there are 18 input channels installed. You may have fewer with the empty slots (hopefully) covered by blank panels. IIRC, ABX-14 was the smallest and ABX-34 was the largest, however there were at least two ABX-42 consoles built. Those were on the secret menu.

An ABX-18 is a charmingly small production console. These were designed primarily for radio production, but they were multipurpose. I have seen them used in production, TV and on the air. Note they are broadcast consoles, so some of the features were different from what you would see on recording consoles. Notably, the multiple output stereo mixes.

The ABX used a 990 for the summing amplifiers on the main stereo output mixes. Program, Audition and Utility all used the 990. There was an option for an output transformer, but those were rarely ordered. It's more likely this console will not have output transformers. It will have a very robust output amplifier however, on the main outputs. We called it a DA output and it was capable of driving multiple 600ohm loads.

The line input channels did not have transformers on ABX. It was an instrumentation type opamp circuit. If you get ahold of the schematics, you'll see some odd circuit values, at least they won't be what you would expect. We used a lot of techniques to keep RFI out of the circuitry. Remember, these were used for broadcast, sometimes installed near a transmitter/tower. Keeping the audio clean was not easy.

If you have any questions about PR&E consoles, I was a console designer there for more than a decade. I'm happy to answer any specific questions you have.

Catfish

 

[AngryAudio]

I'm still not even clear about all the name changes!

The company was originally Pacific Recorders & Engineering Corp, often referred to as PR&E or Pacific Recorders. The company was originally a distributor of MCI tape machines and also offered studio design services to area recording studios. Later, it migrated into radio broadcast where it found its niche.

The company continued to offer design services (furniture, wiring, infrastructure products) to radio broadcast studios. And it also manufactured consoles and cart machines and some other peripheral products. The original meaning of "Recorders" in the name which referred to MCI tape machines faded, but you could make the case that it later applied to cart machines.

Eventually cart machines gave way to computer-based audio playout and the "Recorders" part of the name just didn't fit any longer. Founder Jack Williams wanted to maintain the PR&E acronym so changed the name to Pacific Research & Engineering. It had this name right up until the end in the late 1990's.

The company was purchased by Harris for their broadcast division and I lost track of how they used the brand. I think they mostly called it PR&E by Harris or some such. Later Harris spun off the broadcast division which was then called GatesAir. Did it then become PR&E by GatesAir? I don't know. And finally, what was left of PR&E (not much) after GatesAir was done with it got sold to Wheatstone who used the brand for a minute or two on a new product. I don't think they even use it now, but maybe.

Catfish

 

[Dreams]

Thanks for clearing that up for me!

 

[Mihobro]

Hi, I was wondering, what kind of capacitors are being used for C15, 115, C16, C116, C1,101, C4, C104, C7, and C107? They may be ok, but two things would lead to changing them out. The first is the current quality capacitors available like the Nichicon Muse series, and the second is that many of these audiofile NP caps have shorter lifecycles and may have aged. As all signal passes through them, they immensely affect the sound. I have heard this on an early 80's Harrison and other gear from that period and what a difference. By the 90's though they may have had better components to work with. But it may be worth considering as they not very expensive to change out.

 

[AngryAudio]

There are multiple caps in the signal chain, quite frankly more than are needed, but I’ll give you the design intention before venturing into mods. Take a gain stage after a volume control for example. It might have 10dB or 15dB of gain.

There will be a cap between the pot wiper and the non-inverting opamp input. This is to keep the very high Ibias of the 5532 from generating a voltage across the pot and introducing noise when changing volume.

The relationship between Rfb and Rshunt sets the gain. There will be a cap between Rshunt and ground. That’s there to prevent any DC offset from being amplified. The gain at DC will be unity, the AC gain will be higher.

Finally there will be a cap at the output to keep any offset off the next stage. We didn’t ever want scratchy pots or popping switches. Generally, we tried to keep the Vos everywhere below 5mV.

In my later designs, I eliminated many caps in the signal chain using DC servos, better opamps, and better circuits. This came after the realization that coupling caps were not as harmless as I had thought, even the quality bipolar caps used in these consoles.

If you want to make a dramatic improvement, in the same circuit, replace the 5532 opamp with an OPA1678 (not a DIP so you would need a SMD to DIP board). I've been using this part a lot lately and as a general purpose dual audio opamp, it's my new favorite. It will drop into every PR&E console circuit that uses the 5532 and work perfectly.

Because it is a FET input opamp, it doesn't have the massive bias current of the 5532 bipolar input. So you can eliminate the cap between the non-inverting input and the potentiometer wiper with a short. The capacitor at the output may in some cases also be eliminated. This opamp has ≤ 2mV of Vos. If you leave the capacitor between Rshunt and ground, the output will have no more than 2mV of offset.

That's what I would do. But if you want to change caps, you certainly can do that. Will it sound better? I don't know. I kind of skipped the whole "this cap sounds better than that cap" adventure and went straight to DC coupling wherever possible. If I recall correctly, we used a lot of Nichicon Bipolar capacitors for those audio stages.

Catfish

 

[Mihobro]

Very good!
Does anybody remember Focusrite? They made large consoles that eliminated most caps favoring DC coupling between stages. They sounded great.
Their faders and switches also became really noisy.
One of the first lessons I had in electronics was to build an engraver that used a DC spark to create a carbon deposit on metal. This taught me the origin of deposits on switches and relays. etc. that get dirty and later the advantage of not having DC run through them.
I always valued experimentation, the pushing of the envelope. But, equipment that is working is is always better that a exotic design that is out of repair or needs constant attention.
I may be prudent to consider leaving your coupling options open.

 

[AngryAudio]

I always valued experimentation, the pushing of the envelope. But, equipment that is working is is always better that a exotic design that is out of repair or needs constant attention.
I may be prudent to consider leaving your coupling options open.

Not bad advise at all. If I wasn't the designer, I'd not be advising the mods. And I would not remove all caps. Just some strategically. He did ask how to make improvements.

 

[orphanaudio]

Does anybody remember Focusrite? They made large consoles that eliminated most caps favoring DC coupling between stages. They sounded great.
Their faders and switches also became really noisy.

Ok, the Focusrite comments below are off topic, and should not be chased further (hopefully the poster is not trolling
-Focusrite consoles (both Forte' and Studio Console types) had cap coupling throughout, in quantities no less than any other high-end console of that time (and no servo circuits, other than perhaps if you qualify their output transformer tertiary feedback winding as a servo function rather than feedback).
Remember these were both Rupert Neve designs from the 80's and built with more than a just a tip of the hat to the well known V and VR console technology.
-Although their momentary-contact illuminated pushbutton switches do have a fairly short lifespan (about 6-9 years if in constant use), they are NOT in the audio path & only drive logic which actuates sealed relays for all audio path switching (no FET,s for audio switching here either).
These relays are still clean to this day, 35 years out of the factory sitting in the middle of the African plains, & I think I replaced only five in the 116ch BOP console.
-Faders (GML or Flying Faders used exclusively) get dirty only if poorly maintained or contaminated.
-Focusrite console's amazing sonics low noise/THD and headroom are far more attributable to fully balanced/cascaded summing, thoughtful pcb layout, conservative circuit design and quality components, than anything else.


On Control-Room section design: (back on topic)
PR&E built gear to a level of "unreasonable quality" (that's a compliment) because they had to for broadcast applications, where ON-THE-AIR, (as in rock climbing) "You never get to make the same mistake once". A single preventable on-air failure could end a companies fortunes.

Less is often more when attempting to emulate a "straight wire with gain", with most control room sections only trying to emulate a straight wire with no loss, (and a few routing options).
The question becomes, how far do you want to go inside an already solid design before you are doing more work than starting from scratch, and this design is already at nearly the minimum parts count.
-Servo,s added where they will do the most good are still a lot more work than simply subbing-in a very low offset opamp. Also consider that board layout of a new amp module becomes an issue when dealing with distortion figures this low.
-Your initial idea of replacing the IN-Amp circuit with a That or TI DIff-in part is probably the simplest path forward and can certainly eliminate the previously necessary RF rejection filtering, but with .004% thd already achieved, I am not sure that a .003% thd I was able to get using 2143, 2142, 5532 parts in a recent Electrodyne Summing & Control Room product, for a total of three amps in the CR signal path total,... (you can look the product up,... not ok to flog my retail stuff here), will be audible even if measurable. Hopefully you own or have access to an Audio Precision or equivalent test system to confirm your results.
-Mechanical relays in place of the Harris parts could help with headroom (maybe) and would of course replace lots of semiconductor devices in the audio path with simple metal contacts, but as Catfish mentioned, this could require adding buffer amps ( and maybe blocking caps) for CR and DIM pots.
The CAPI part should eliminate the need for transistor level shifters/current buffers to drive relay,s vs semiconductor switches, but you should consider how to adjust available 16v rails to avoid excess drive/heating of the 12v relay coils.
-The schematic ends at the output buffers, so no way to know if it was fitted with transformers or just exited to speaker feeds un-balanced. If unbalanced, you might pick up some noise immunity from output to speakers with active balancing.

Thank you,

Ken Hirsch / Director of Engineering
Orphan Audio www.orphanaudio.com
Quad-Eight Electronics www.quadeightelectronics.com
Electrodyne Audio www.electrodyneaudio.com (a division of Orphan Audio)

"Education is the cure for everything"