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Re-Cap OP24 Pre-Amp?

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arumako
Posted 2020-07-16 9:33 AM (#552537)
Subject: Re-Cap OP24 Pre-Amp?



Joined:
October 2012
Posts: 932

Location: Yokohama, Japan
Hey OFC/BFLG. Hope you're all doing okay. It's been a crazy unpredictable season globally, and I pray safety for all the OFC Family. Despite all that's going on I've been able to organize 5 short 60 minute outdoor praise/prayer concerts this summer with stipulations for full face masks for all but the lead singer; a strict no-stop, no gather policy for passers by, no ads, no posters policy to ensure gatherings aren't encouraged. Short concerts for no one in particular and whoever happens to come by. We finished our first two events safely and preparing our 3rd and 4th events with hopes to live stream our 5th event.

Because of unpredictable weather, I've opted to use my BFLG 1868, and I'm thoroughly enjoying this SSB workhorse. I noticed the high-end sounded a bit muffled through the PA. Not a big problem, as adjusting the on-board EQ compensates more than adequately. However, it occured to me that I'd never replaced the electrolytic capacitors in the OP24 pre-amp. I've done re-caps on my Peavey Session and Mesa Boogie Mark I, and the results were astounding, but those are high output AC powered guitar amps. Do low output 9V DC powered pre-amp caps need to be replaced? I've never heard of anybody doing this and couldn't find anything on the web. I suppose replacing the few caps would do no harm, but wasn't sure how the tone of the pre-amp would be affected. Of course, the EQ adjustment "as is" gets the job done, but the treble response of the pre-amp is definitely not what it used to be.

Have any of you done a pre-amp recap or heard of any such thing? Your insights would be greatly appreciated! Thanks a bunch and stay safe!
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keldon85
Posted 2020-07-16 10:09 AM (#552538 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?


Joined:
July 2019
Posts: 61

Location: northern Georgia
Certainly in the hi-fi audio world changing out caps in stereo preamps and power amps was common. Walt Jung published several articles in the late 70s-early 80s on swapping out electrolytic caps for film types, especially polystyrene and polypropylene film caps. Also, in the electric guitar pedal world, swapping out cap types is a common mod. I actually bought a kit to re-do a Boss pedal equalizer, but haven't done it since it really does work well stock - some of the mods in the kit are replacements for the original caps. In many of these mods, electrolytic coupling capacitors (especially tantalums) were replaced with film caps. Unfortunately, film caps tend to be much larger than electrolytics of the same capacity. The main reasoning behind these mods is that film capacitors have much lower dissipation factors than electrolytic caps, but in most cases, the dissipation factor for the electrolytic is low enough to have little effect in how it is used. Electrolytics also work best when they have an appropriate bias voltage across them. For a preamp circuit powered by a 9 V battery (guitar preamp) that condition is almost certainly met. One issue with electrolytic caps, is that they can dry out over time, especially if they are heated significantly during use. From my experience with old audio equipment, that is actually a rare occurrence with well designed amplifiers.

Now for tone control circuits, such as your equalizer in your guitar preamp, you have a capacitor within a negative feedback loop for a gain stage. I suspect that might be a bit more susceptible to capacitor problems than for coupling capacitors or power supply bypass caps. I would be a bit surprised to find electrolytic caps in those locations. Most tone control circuits need tighter tolerances than affordable electrolytic caps provide.
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alpep
Posted 2020-07-16 1:23 PM (#552539 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?


Joined:
December 2001
Posts: 10473

Location: NJ
id like to know how this turns out
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arumako
Posted 2020-07-17 6:08 AM (#552540 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?



Joined:
October 2012
Posts: 932

Location: Yokohama, Japan
Thanks keldon85. Sounds like you've got some good experience along these lines. Think I'll open the OP24 up and verify the caps based on the schematics available at the Tribute site. Should be a simple summer project. I assume adjusting cap values/types could provide additional headroom and fuller tonal characteristics too. My Peavey Session 400 is a solid state pedal steel guitar amp, and the recap I did (with lots of help from others with Session 400 mod experience) mellowed the tone and gave it more headroom. It's an amazing sounding 200Watt acoustic guitar amp. Will need to do some homework, but I'll post details once the project gets underway. Wish me luck!
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Love O Fair
Posted 2020-07-17 8:30 PM (#552546 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?



Joined:
February 2016
Posts: 1310

Location: What week?
@arumako - >>>with hopes to live stream our 5th event.<<<

Hello, Ken! Glad to hear you are well and, obviously, staying pretty busy. Please keep us posted on how to view your live stream for the 5th show. I'd love to tune in!
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tpa
Posted 2020-07-18 3:33 AM (#552548 - in reply to #552540)
Subject: Re: Re-Cap OP24 Pre-Amp?


Joined:
December 2004
Posts: 450

Location: Denmark
arumako - 2020-07-16 1:08 PM
... Will need to do some homework, but I'll post details once the project gets underway. Wish me luck!

Looking forward to another well documented thread from you. I have had similar considerations with a FET3, electrolytic capacitors do change over time. :-) Good luck, indeed.
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keldon85
Posted 2020-07-18 9:34 AM (#552549 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?


Joined:
July 2019
Posts: 61

Location: northern Georgia
I looked on the Tribute site and did not find a schematic for the plain OP24 preamp, there are schematics for the various follow on versions that have some additional tone shaping options. The OP24 was mentioned as being opamp based in promotional literature, so I suspect the basic design followed into the OP24 variants. The later OP24 variants used a TL064 quad opamp chip, as did some of the later preamp types; in the schematics I looked at there are several 10 microfarad electrolytic coupling capacitors (probably tantalum, to minimize space, common practice) in between the opamp stages. I suspect these would be candidates for replacement. The caps in the treble/bass and mid-range circuits are probably some type of film cap, and are probably good.

Edited by keldon85 2020-07-18 9:37 AM
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arumako
Posted 2020-07-19 9:59 AM (#552552 - in reply to #552537)
Subject: RE: Re-Cap OP24 Pre-Amp?



Joined:
October 2012
Posts: 932

Location: Yokohama, Japan

Thanks for taking the time to investigate keldon85. I found a few pics on a Japanese website where a guy cleaned and maintenanced his OP24. The underside of the PCB looks like this...

op24 pcb1

And the upper side with the sliders looks like this...

op24 pcb2

Interestingly, no aluminum electrolytic capacitors. The Mother Ship wasn't skimmping on parts for sure. Check out the LN4250 op amp chips. There's six of them in there. If these pictures are any indication the re-cap should be fairly simple with only a few caps to replace. I wonder if the Mother Ship used proprietary programming in the op amps. Very cool! Won't get to this for a couple of weeks, but it's good to know that the OFC's got my back!

@Al - Thanks! If we get the live streaming to work, I'll send you the link! Yeah, craxy busy!

Stay safe everybody!

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keldon85
Posted 2020-07-20 9:23 AM (#552554 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?


Joined:
July 2019
Posts: 61

Location: northern Georgia
Well so much for my guess about OP24 circuit design From looking at the spec sheet for this chip, its "programming" is done by selecting a resistor between one of the pins of the chip and ground to give a desired current; that selects the performance parameters needed. This involves a tradeoff between gain-bandwidth product and minimum current needed for the opamp to work. Too high a gain bandwidth product and you can get oscillations and a greater current draw, so I suspect the designers have it set on the lower end for stability, better battery life and a bit lower input noise level. The TLO64 used in the OP24 follow-ons was a variant of the TLO7x design optimized for low current use, it is also a quad opamp instead of the unitary LM4250C chips shown in these pictures, so the following OP24 versions were probably a little less expensive. I still suspect that the capacitors used will be similar to the later OP24 variations, but the layout on the pc-board will be different.
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keldon85
Posted 2020-07-20 11:26 AM (#552557 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?


Joined:
July 2019
Posts: 61

Location: northern Georgia
I also note that the op-amps are socketed. The pinouts on the LM4250C are standard, the only difference is that it uses pin 8 to set the current for the op-amp configuration set up, most single op-amps don't use pin 8, it is N-C or not connected. That would open up the world of op-amp upgrades, just make sure the trace to pin 8 is cut and a standard pinout single op-amp is used. From looking at the 4250 schematic, it is a bi-polar transistor input rather than an FET -- usually FET inputs are used for piezo transducers because they have a high input impedance that work better with piezo transducers. I suspect the 4250 was chosen (and later the TLO64) because of its low current draw. Some of the web-sites I visit opine that that is less an issue than it used to be because of improvements in 9V batteries.

It may be fun to play with this, but for me, the OP24 does work pretty well as is. My 1763 uses one, and it has both low noise and distortion plugged into my Ultrasound acoustic amp. I sort of wish the tone controls were set to a little bit different frequencies to work better with nylon strings versus steel, but I don't know that I would risk making the mods for that.

Edited by keldon85 2020-07-20 11:49 AM
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alpep
Posted 2020-07-20 8:45 PM (#552558 - in reply to #552537)
Subject: Re: Re-Cap OP24 Pre-Amp?


Joined:
December 2001
Posts: 10473

Location: NJ
all great information
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