I decided to take my amp apart to clean it. Its really dusty inside. Bought it 2nd hand a few years ago.... Hmm.... Also wonder if some of the bits could be replaced, esp. the caps!! Its also over 10yrs old. Not sure if the caps are still within spec.
The preamp board is using 3 x OP177G. There is also 1 x SST44 (not sure what it is). The caps are mostly rscon. Btw, I just realise the power amp board has also 1 OP177G and another SST44 SF.
For the power, there are Sanken 4 x LATP 4886 and A1860. Each is rated for 80W....hmm...I am wondering if I could use something better. Capacitors are SY VENT....Rscon and REC....Those red blocks are wima caps. Yellow ones are unknown, there are only values.
There is also a burnt area. I guess its due to bad repair job by previous owner. Since the amp is still working, I think I will leave it as it is. The cable from pre to power is well.....pretty much computer ribbon cable with a ferrite core (should be meant for noise) Speaker ones are 14AWG "fushen" ones....
The preamp board is using 3 x OP177G. There is also 1 x SST44 (not sure what it is). The caps are mostly rscon. Btw, I just realise the power amp board has also 1 OP177G and another SST44 SF.
For the power, there are Sanken 4 x LATP 4886 and A1860. Each is rated for 80W....hmm...I am wondering if I could use something better. Capacitors are SY VENT....Rscon and REC....Those red blocks are wima caps. Yellow ones are unknown, there are only values.
There is also a burnt area. I guess its due to bad repair job by previous owner. Since the amp is still working, I think I will leave it as it is. The cable from pre to power is well.....pretty much computer ribbon cable with a ferrite core (should be meant for noise) Speaker ones are 14AWG "fushen" ones....
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When seeing the dirt I can not help to think SMD technology must be way more sensitive to that compared to TH parts. I dip SMD boards in slow curing epoxy resin and let it harden. This is only possible when cost of replacement is not too high and when not much heat is dissipated. When done right one can dip the board in water and it will work perfectly. Radical but no problems with moist, dirt etc. Too bad the stuff is now too old to use and I can not find it in small quantities anymore.
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Oh, sounds like a good idea!! However, the resistors do get very hot. I am thinking maybe thats why it was repaired. Perhaps a few of them were fried.
Btw, how about coating them with clear laquer? Then put on some thermal pads and then a heatsink to help dissipate the heat?
The smell of the epoxy is terrible but after curing it is practically gone. I would not coat heat generating parts. Since I design/build low loss low heat stuff and other low power stuff I used the epoxy quite often. When regulator IC's were used then I used a cotton swab to wipe the epoxy away from the upper surface of that IC just to let it be less warm.
Repair is virtually impossible when using epoxy. There is another coating that can be soldered through but I forgot its name. Polyurethane based.
Repair is virtually impossible when using epoxy. There is another coating that can be soldered through but I forgot its name. Polyurethane based.
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Hi could it be the conformal coating?? I google and found on wiki a list of coatings. Btw, what do you think about enamel? The same coating thats used for wires? I think acetone can remove it.
Conformal coating - Wikipedia
Acrylic
- Ease of rework
- Simple drying process
- Good moisture resistance
- High fluorescence level
- Ease of viscosity adjustment
- Useful to about 150C [302F]
- Harder durometer, abrasion resistance
- CTE closer to epoxy PCB substrate
- Higher Tg (Glass transition)
- Good dielectric properties
- Good dielectric properties
- Good moisture resistance
- Solvent resistance
- Less reversion potential
- Abrasion resistance
- Stable over wide temperature range (in general, -40C to 200C)[-40F to 392F]
- Flexible, provides dampening and impact protection
- Good moisture resistance
- High dielectric strength
- Low surface energy for better wetting
- Excellent uniformity regardless of part geometry
- Chemical inertness
- Minimal added mass and low outgassing
- Low environmental impact process
- Low dielectric constant
- Low dielectric constant
- High glass transition temperature
- Low surface energy
- Low water absorption
- Solvent resistance
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It is the polyurethane stuff. Still have a can but since it is coloured red I don't use it much. Only on red boards 🙂 I tried a lot of this in the past and some stayed sticky which is not OK. Some become a kind of hard plastic that is easily cleaned.
SST is USA company Microchip of Alabama but nowhere can I find =SST44 not in my large German equivalents book nor US company Digikey.
Why? Unhappy with the sound and want to improve it in some direction? What?
The caps are an obvious place to begin, only the amp was probably voiced with these particular caps and better ones may or may not make it better.
It is also worth investigating if there are any coupling caps. Untypical for Krell, but who knows?
From experience with Krell preamps the Cmos volume control is the main sound obstacle. Not much can be done about that.
I know the OP177G although its still in production Analog Devices a well known company advise a modern replacement of ADA4077-1-
ADA4077-1 Datasheet and Product Info | Analog Devices
ADA4077-1 Datasheet and Product Info | Analog Devices
I am happy with the sound. But curious about what minor improvements I could do to it. I too believe caps should be replaced given its age. Maybe I will think of it as "unlock" the potential in the amp??
Thanks for the advice!!!
MG Chemicals has transparent and quite fluid Encapsulating Epoxy in 375mL kits, not too much waste if used at home hobbyist levels:
832WC - Optically Clear Epoxy - MG Chemicals | MG Chemicals
If you make a custom fit disposable cardboard box where PCB occupies most of its internal volume, you can mix a very small amount, the unmixed part stays in the bottles for up to a year.
I keep my Epoxy cans in a lower shelf in the fridge, only thawing them to refill "working" 250cc "daily use" bottles when needed, say once a Month, waste is minimized and main cans last over a year with no degradation.
Most paid, career amplifier designers know a bit about designing amplifiers so you're unlikely to be able to improve on their work by randomly replacing parts with boutique bits.
If the electrolytic caps are showing signs of strain, replacing them makes good sense for reliability.
Voicing? You'd have to do something pretty radical to change it from a 'flat from 20 to 20k' response. Why would you 'voice' a hifi amp anyway? All you're doing is making it less accurate. Musical instrument amp maybe, hifi amp, no.
If the electrolytic caps are showing signs of strain, replacing them makes good sense for reliability.
Voicing? You'd have to do something pretty radical to change it from a 'flat from 20 to 20k' response. Why would you 'voice' a hifi amp anyway? All you're doing is making it less accurate. Musical instrument amp maybe, hifi amp, no.
Hi! What differences do you think it will make if I replace the OP177G with the ADA4077-1? It sounds like a fun experiment to try out different ones.
Thanks, that is good info. As usual it is difficult to find in the EU (except for the usual suspects that charge too much).
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It sounds like you are not even sure what the OP177 does, right? Why don't you draw a partial circuit first?
It probably is a servo and the BP caps are integrators, which makes them at least as important for both reliability and audibility.